Name:
Laura Green
I.D:
09005859
Supervisor:
Nathan Quill
Course:
Applied Physics MSc
DEPT:
Dept. of Physics and Energy
Project Title:
.
Formation of Nanoporous
Structures in n-InP
Date:
29/08/2014
Abstract
InP samples were galvanostatically and potentiostatically anodised in a variety of KOH
concentrations at a variety of temperatures. A porous layer, almost twice the thickness
(12.48 µm) of any porous layer previously formed, was observed in InP
galvanostatically anodised. Variations in layer thickness are proposed to be mainly
affected by temperature and KOH concentration. The ability of pits and pore tips to
carry increased quantities of current is proposed to increase with temperature for both
galvanostatic and potentiostatic experiments. In potentiostatic experiments, the peak
current per pit was observed to increase with increasing temperature and for
galvanostatic experiments the voltage required to pass 5 mA cm-2 decreased with
increasing temperature. The findings agree with previous work which indicates that the
rate of electrochemical reactions increases with temperature. Current density appears to
have the biggest affect of any variable on the width of the pores formed, with
temperature and KOH concentration not showing much of an effect in galvanostatic
experiments.
Acknowledgements
 I wish to express my sincere gratitude to my supervisor Dr. Nathan Quill for
his time, guidance, support, and encouragement over the course of this thesis,
I will always appreciate it.
 Many thanks to Robert Lynch who often weighed in with his extensive
knowledge and provided great insights into the work done in this thesis.
 A gigantic thank you to all of the people in the Physics Department, if I was to
list out how you have all helped me throughout the year, there would be
another ten pages in this document. You made me feel comfortable and
welcome in the department from the very beginning. I extend my gratitude
especially to Josephine, Joe, Andrea, Daniellea, and Catherine.
 Enormous thanks to my parents, who have always encouraged me to do my
best and have shown me their support, no matter the outcome. Without your
love and encouragement I would not be where I am today. I love you and am
forever grateful for all that you have done for me.
 To my sister and brother, thank you for being a welcome distraction when I
needed it and always being there for me.
 To the lunch time/tea break girls (Beulah, Danielle, Sarah O’D, Sarah C, Aine,
and Clare), it was you who really made this year enjoyable. There was always
a laugh to be had with you around and maybe a serious discussion or two. 
 Finally I would like to thank my boyfriend Daniel. Over the past year you
have been there for me. You encouraged me throughout this year to
accomplish my goals. I hope you realise how much I appreciate your support
and all that you have done for me.
ii
iii
Contents
Abstract ............................................................................................................................. i
Acknowledgements ..........................................................................................................ii
Chapter 1: Introduction .................................................................................................... 1
1.1 Semiconductor importance .................................................................................... 2
1.2 Semiconductor Porosity ......................................................................................... 2
1.3 Si versus InP .......................................................................................................... 3
1.4 Factors affecting nanopore formation in InP ......................................................... 4
Aim and objectives of project ...................................................................................... 4
Chapter 2: Literature Review ........................................................................................... 5
2.1 Properties of porous InP......................................................................................... 6
2.2 Possible application of porous InP ......................................................................... 7
2.3 Electrochemical Formation of Porous III-V Semiconductors ............................... 8
2.4 Pore growth in InP ................................................................................................. 9
2.5 Chemistry of Anodic Etching of InP ................................................................... 12
2.6 InP in KOH work ................................................................................................. 13
2.6.1 Initial pore growth and pit formation ............................................................ 13
2.6.2 Nanoporous domains and Directionality of Pore Growth............................. 17
2.6.3 The Three Step Charge-Transfer Model ....................................................... 23
2.6.4 Effect of Electrolyte Concentration .............................................................. 25
2.6.5 Effect of Temperature ................................................................................... 26
iii
2.6.6 Current-Line Oriented Pore Formation ......................................................... 27
2.6.7 Cessation of Porous Layer Growth ............................................................... 27
2.6.8 Effect of Current Density .............................................................................. 29
Chapter 3: Experimental ................................................................................................ 30
3.1 Introduction .......................................................................................................... 31
3.2 Electrochemical Apparatus .................................................................................. 31
3.2.1 Reference Electrode ...................................................................................... 32
3.2.2 Counter Electrode ......................................................................................... 33
3.2.3 Working Electrode ........................................................................................ 33
3.2.4 Ohmic Contact to the Working Electrode ..................................................... 35
3.2.5 Experimental Setup ....................................................................................... 36
3.2.6 Computer Control of Experiments ................................................................ 37
3.2.7 Experimental Procedure ................................................................................ 40
3.3 Electroanalytical Techniques ............................................................................... 40
3.3.1 Linear Potential Sweep ................................................................................. 40
3.3.2 Galvanostatic Anodisation ............................................................................ 41
3.3.3 Variations of Galvanostatic Anodisation ...................................................... 41
3.3.4 Potentiostatic Anodisation ............................................................................ 42
3.4 Characterization Techniques ................................................................................ 43
3.4.1 Scanning Electron Microscopy (SEM) ......................................................... 43
Chapter 4: Results & Analysis ....................................................................................... 46
iv
4.1 Linear Potential Sweep ........................................................................................ 47
4.2 Galvanostatic experiments ................................................................................... 48
4.2.1 1mA cm-2 Current Density ............................................................................ 52
4.2.2 Current Density Burst ................................................................................... 55
4.2.3 Current Density Sweep ................................................................................. 64
4.2.4 Varying Temperature .................................................................................... 66
4.2.5 Varying Concentration .................................................................................. 79
4.3 Potentiostatic Experiments................................................................................... 85
4.3.1 Varying Temperature .................................................................................... 85
4.3.2 Varying Concentration .................................................................................. 98
Chapter 5: Discussion .................................................................................................. 104
5.1 Linear Sweep Voltammogram ........................................................................... 105
5.2 Ambiguity in Pore Width and Pit Diameter measurements ............................... 106
5.3 Standardising Pit Density plus Mass Transport Effect ...................................... 107
5.4 Effect of Mass Transport versus Temperature and Concentration for
Galvanostatic Experiments ...................................................................................... 109
5.5 Effect of Temperature and KOH Concentration on the Potential applied to and
the Current passing through Pits and Pores ............................................................. 111
5.6 Other Trends in Temperature/Concentration Galvanostatic Experiments ......... 112
5.7 CLO Pores Present in Potentiostatic Experiments. ............................................ 113
5.8 Other trends in Temperature/ Concentration Potentiostatic Experiments ......... 114
Conclusions and Future Work ..................................................................................... 116
v
Main Conclusions of Thesis..................................................................................... 117
Future Work ............................................................................................................. 118
References .................................................................................................................... 119
Appendices ................................................................................................................... 124
vi
Chapter 1: Introduction
1
1
Introduction
1.1 Semiconductor importance
In the late 1950’s Jack Kilby and Robert Noyce both independently created an
integrated circuit (IC) on which the electronic devices were based on wafer comprised
of semiconducting material. Since this initial fabrication of the integrated circuit, there
has been a great deal of research carried out on semiconducting materials with the
emphasis on silicon. The research has been driven by Moore’s law which states that the
number of transistors on each chip doubles approximately every two years.[1] Today
IC products are used in a large variety of products, such as wireless communication
systems including printers, faxes, laptops, top computers, iPods and mobile phones
being big users. The semiconductor electronics field continues to be a fast changing
one in which the intense research on silicon has led to lower costs, higher fabrication
yields, and greater reliability of integrated circuits.[2]
1.2 Semiconductor Porosity
The detection of the luminescent properties of electrochemically etched nanoporous
silicon [3] has resulted in numerous studies focused on porous semiconductors. This is
principally due to the considerable differences between the diverse optical and
morphological properties exhibited by different porous semiconductors and their bulk
counterparts.
Pores
are
generally
formed
in
semiconductors
through
the
electrochemical etching of the semiconductor in an electrolyte solution. Pore formation
has been reported for GaP, InP and GaAs anodised in a variety of electrolyte solutions.
2
[4–18] While there have been numerous studies on porous silicon, there has been far
less research done on the porous III-V semiconductors mentioned above. In order to
utilize the utmost potential of nanoporous materials it is important to acquire an
understanding of the formation of nanopores in semiconducting materials. This may be
done by examining the characteristics of nanopore formation in these materials under
varying conditions.
1.3 Si versus InP
Although Silicon has been the basis for much of the research with regards to
semiconductors and the IC industry, other semiconducting materials have more
favourable properties with regards to communications. Porous InP has been discovered
to exhibit birefringence at wavelengths suitable for optical communication systems (l =
1.55 μm). [19] InP has a direct band gap unlike Si which has an indirect band gap. For
direct band gaps the maximum energy of the valence band lies directly below the
minimum of the conduction band in momentum (k) space. Therefore, for a direct band
gap semiconductor, phonon emission/absorption is not required to conserve momentum
when an electron moves from one band to another. [2] For more information on the
background theory of semiconductor read chapter two of reference 20. However there
are limitations to the alternative semiconducting materials such as InP, with respect
today’s semiconductor industry which has evolved around Si and has been developed
to produce Si in a very cost effective manner.
3
1.4 Factors affecting nanopore formation in InP
Pore formation has been described as an interfacial phenomenon controlled by the
solubility and evacuation rate of the dissolution products by Gonçalves et al. This
group discovered that the InP dissolution rate increases directly with current density.
[13] Thus increasing the current density applied in the electrochemical system
increases the rate of reaction of the anodization process. Quill et al. [21] observed a
variation in pore width when temperature and electrolyte concentration were varied
during the formation of porous InP. A three-step model of charge transfer was
developed to explain the variation in pore width seen with varying temperature and
concentration. This model is discussed in more detail on page 21. A relationship
between pore width and layer thickness was also observed and suggests that mass
transport may be the main factor limiting porous layer thickness.
Aim and objectives of project
The aim of this project is to significantly enhance the understanding of pore formation
thus leading towards greater control of porous structure. This aim will be focused on
the study of InP. The objectives of the study are: Investigate the mass transport effect
of layer thickness. Investigate the effect of temperature and KOH concentration on
pore morphology and the thickness of porous layer formed. Evaluate the three step
charge-transfer model developed by Lynch, et al.
4
Chapter 2: Literature Review
5
2
Literature Review
2.1 Properties of porous InP
Photoluminescence (PL) and cathodoluminescence (CL) measurements performed on
InP nanostructures exhibit a wide variety of results. The anodization of InP has been
discovered to result in a strong reduction of CL intensity. [7,8] The degree of porosity
was found to influence the CL intensity in InP, with increase of porosity, decreasing
CL efficiency was observed. The considerable decrease in CL intensity, with the
increase in porosity, suggests that energy states at the surface of porous InP contribute
to non-radiative recombination processes. Hidalgo, et al. [23] also observed a blue shift
of 60nm in porous InP relative to that of bulk material using cathodoluminescence.
Takizawa, et al. prior to this had reported a PL blue shift of approx. 20nm, to which he
accredited the quantum confinement to the nanostructures in porous InP as the cause of
this. [9] The different anodization conditions used in formation of the porous structure
was attributed to the differences in the blue shifts reported. [23]
A dependence on temperature of the photoluminescence spectra of InP was
reported by Liu & Duan. [24] They observed two peaks in the PL spectra of porous InP
compared to that of the bulk InP wafer. A blue shift of approx. 14meV was shown by
the peak that dominated at temperatures above 150 K, while a red shift of approx.
33meV was shown by the other peak that dominated at temperatures below 120 K.
Similar to above the quantum confinement effect was thought to be the origin of the
blue shifted PL. The red shifted PL was found to be very sensitive to chemical
6
treatment and this was ascribed as a radiative recombination via surface states. In
photoelectrochemical anodized InP intense red-shifted peaks were discovered which
were accredited to the formation of a set of well-defined new surface state levels on the
anodized wall surfaces or pores. [7]
2.2 Possible application of porous InP
As discussed in section 2.3 it is possible to grow two types of pores in InP by switching
the current applied during the formation of porous InP between high and low values.
This concept was used by a number to groups [24,25,26] to create a three-dimensional
porous InP crystal. By switching the value of current throughout the creation of porous
InP it was possible to form layers varying in porosity which in turn demonstrated
different optical properties. The possibility that waveguide structures could be created
by using porous InP was demonstrated by Langa, et al. [27,28] The selective nature
pore growth in the InP samples was controlled by the selective anodization of the
surface through the lithographic pattering of the InP surface before anodic etching.
The mask pattern consisted of strips which created 10µm wide exposed strips in
photoresist after lithographic process. Fig. 1 shows waveguide structures created
through this process. The principle behind the creation of the waveguide in porous InP
is that, the refractive index of the porous structure will decrease as the porosity
increases when considering the porous layer as an effective medium with a porosity
dependent refractive index. By the selective anodization outlined above, a waveguide
with a pronounced difference in the degree of porosity and thus refractive index (Fig. 1
a) was created, through the formation of a layer of CO pores with a low degree of
porosity first, followed by the radial growth of CLO pore with higher porosity. [27,28]
7
In this waveguide like structure, the low porosity CO layer was deemed to be core,
with the CLO layer acting as the cladding. To improve the guiding properties of the
waveguide like structure the ‘core’ was surrounded by a photonic crystal/Bragg
structure (Fig. 1 b). [29] This was done by varying the current density during the
anodisation process, however the periodicity of the Bragg structure created was not
normally distributed around the core. This was attributed to the difference in diffusion
behaviour of the electrolyte in the centre and at the edges of the photoresist.
b)
Figure 1 A waveguide a) structure with a step like porosity change, b) with a Bragg like
structure around it, obtained by modulating the current density during the etching.
2.3 Electrochemical Formation of Porous III-V Semiconductors
The formation of an collection of pores, extending into the single-crystal
semiconductor-anode bulk, was first discovered in 1956 during a study of
electrochemical etching of silicon in hydrogen fluoride solutions. [30] However, it was
not until much later that interest was taken into the similar electrochemical behaviour
of III−V crystals [31]. The interest in porous semiconductors was generated by the
8
discovery of visible luminescence in porous silicon in 1990 [2]. The creation of a subsurface layer in III-V semiconductors results from phenomena whereby the decay of
the continuous front of the electrochemical interaction i.e. selective dissolution, causes
the initial pore formation on the originally homogeneous surface of a semiconductor
crystal. Subsequently resulting in its transformation into an ensemble of discrete
macroscopic (on the atomic scale) regions, that steadily reproduce as they move away
from the outer phase interface. [32] Generally porous materials are produced by
anodizing the semiconductor in a suitable electrolyte under suitable conditions. Anodic
dissolution depends on the exchange of electrons and/or holes between the
semiconductor and the electrolyte. The process requires a minimum of one hole in
order to proceed. In anodic etching, positively biasing the electrode forces holes
present in the semiconductor to move to the surface, where they are expended in the
electrochemical reaction process. [33] For n-type semiconductor material, a higher
current flow and consequent dissolution can only occur at voltages above the Schottky
barrier breakdown potential. The Schottky barrier (i.e. potential barrier) is generated at
the substrate/electrolyte interface as a result of the depletion of the majority carriers
that takes place under anodic bias. [27]
2.4 Pore growth in InP
Pores may be obtained in InP by photoassisted anodization [7,9,10,33] but are more
commonly grown in solely electrochemical conditions. Sckmuki, et al. observed in the
photoassisted anodization of n-InP(100) that at areas of strong illumination intensity,
electropolishing occurred instead of pore formation. A decrease in the average pore
size with decreasing light intensity was noticed. This indicates that the morphology of
9
the dissolution process is strongly affected by illumination and that light generated
holes participate largely in the pore growth process. It was also observed in that study
that the finest pore structure was obtained in the dark. [10] Takizawa noticed that in the
anodization n-InP (100) in HCL under dark conditions, that <111> was the preferential
direction of pore growth. It was also demonstrated, on (111)A oriented InP with the
creation of triangular pores that the size and shape of pores could be fixed, through the
combination of a SiO2 mask with a periodic arrangement of holes and the anodization
process in the dark in HCl solution. [12] Pore growth morphology and structure in nInP has been shown to strongly depend on the electrochemical conditions. Tsuchiya et
al. presented the ability to produce highly defined and regular superlattices on (100) nInP in HCL, by periodically varying the applied current or potential. They observed for
low potential (1.5 V) the pore growth structure was facet-like with <111> the
preferential direction for growth, at increased potential of 3 V the pores consisted of
tree-like structures of random and tangled branches. At a high potential (5 V) regular
array of straight pores with wavy walls, with the pore running along the <001>
direction, were noticed. [27]
The growth of pores in the direction of the current line when exposed to high
anodization currents/potential has also been observed in other electrolytes containing
halides. [11] At high current densities, there is an initial CO layer growth at the start of
anodisation from which the CLO pores emerge at a certain depth. [32,19] CLO pores
formed in HCL were found to have an improvement of pore uniformity (nano-pore
arrays consisting of square shaped straight pores vs. random pore positioning and wavy
pore walls) upon the addition of HNO3 to the electrolyte. Fig 2 shows examples of
structures obtained in porous InP. [19] Image (a) shows the switching from CLO pores
to CO pores resulting from the periodic varying of the applied current density, image
10
(b) shows the transition from CLO pores to CO pores resulting from the switching of
the applied current density from high to low. CLO pores formed in nonaqueous
electrolyte (acidic liquid ammonia) can be seen in Fig 3. The surface and pore
morphology of porous InP was relatively different to the morphology of porous InP
formed in aqueous electrolyte solution. CLO pores were formed in curved domains
with layer depths of only a few µm. [13]
Figure 2 a) The depth modulation was achieved by switching between current line oriented
pores and crystallographically oriented pores by suitable current modulations. b) Transition
between current line and crystallographic pores in InP. [19]
11
Figure 3: Morphology of porous n-InP after galvanostatic treatments in acidic liquid ammonia
(1 M NH4Br) at 223 K. (a) SEM cross section (j = 2 mAcm−2, Q = 5 C cm−2). (b) Focused view of
the interface between a detached porous layer and the semiconductor bulk. (j = 4 mAcm−2, Q
= 6 C cm−2). (c) Details of the pore morphology (j = 2 mAcm−2, Q = 3.6 C cm−2). [13]
2.5 Chemistry of Anodic Etching of InP
Anodic treatments of InP generally involve the formation of some sort of surface oxide
which is either deposited on the surface or dissolved into the electrolyte. Reports on the
composition of the resulting oxide vary significantly with electrolyte, pH and anodic
treatment. [20] The bulk of work on the anodic oxidation of InP has been performed in
acidic or neutral electrolyte solutions. [33–37] O’Dwyer [40] examined the deposition
of anodic oxide layers on InP in KOH. Electron diffraction patterns of the resultant
oxide suggested that it was comprised of In2O3. X-ray photoelectron spectroscopy
(XPS) was conducted on an as received InP surface and it revealed the presence of a
native oxide layer comprised of In2O3 and InPO4. Only In2O3 was found, following
anodisation in KOH. Energy-dispersive X-ray spectroscopy (EDX) analysis on the
same oxide film exhibited that 95.4 % of the oxide encompassed indium whereas only
4.6 % encompassed phosphorous. Minute quantities of InP crystals in the film were
mostly accredited to the phosphorous signal in the EDX analysis. The anodisation was
presumed to be an eight electron process with a proposed reaction
12
InP + 11OH- → ½ In2O3 + 11/2 H2O + PO43- + 8e- (1)
It is thus apparent that anodic oxide layers of InP are very complex, with
electrolyte type, pH, anodic etching time, and applied potential/current density all
having an effect on the exact composition of the oxide. Anodic layers produced in
alkaline solutions are normally enriched in In2O3. Anodic oxide layers are revealed to
be comprised almost entirely of In2O3 when the KOH solution is extremely basic (pH =
14). It is not clear if this is a result of the increased solubility of the familiar
phosphorous oxides at high pH or the formation of previously unobserved phosphorous
oxides in this distinctive chemical environment. Unfortunately the chemistry of the
porous InP etching process is poorly understood. This is mainly due to the fact that the
etching surface is largely inaccessible, as it resides deep within the sample itself. [20]
2.6 InP in KOH work
2.6.1 Initial pore growth and pit formation
Linear sweep voltammogram (LSV) of n-InP electrodes in 5 mol dm-3 KOH showed
that a significant anodic process clearly occurs above 0.4 V and becomes self-limiting
at higher potentials. Little current was detected at potentials less than 0.3 V but
continued anodization to potentials greater than 0.4 V caused a rapid increase in the
current density to a peak value of 20 mA cm−2 at 0.48 V. [41] In the electrochemical
etching of GaAs Schumki et.al attributed the steep current increase at higher potentials
to the potential at which the first pores are formed in the material. [42] Schmuki termed
this potential value as the pore formation potential (PFP). This occurrence was ascribed
to Schottky barrier breakdown which provides the necessary holes (h+) to achieve
dissolution of the semi-conductor. [43] The LSV’s of n-InP with low and high carrier
13
concentrations showed one and two pronounced anodic peaks respectively. The single
peak for the low carrier concentration and the second peak for high concentration
correspond to the formation of the porous region. [41,44] In this study it was evident
from the use of TEM that the nanoporous layer in n-InP is formed by the penetration of
surface pits to form channels through the dense near-surface layer at particular points,
with pore propagation within the InP originating at these points. Such channels provide
the required connectivity between the bulk electrolyte and porous region, thus enabling
etching to proceed at the pore tips. The dense near-surface layer is believed to originate
from electron depletion at the semiconductor surface. [41]
In the absence of light, n-InP can etch anodically only if holes are supplied by a
breakdown mechanism, such as tunnelling. For pitting of the surface to occur, a
mechanism that limits current flow across the semiconductor-electrolyte interface is
necessary to establish the stipulation for localized preferential etching. [45] This
current limitation could arise due to either, the lack of available holes at the surface
(i.e. the electric field not being high enough for breakdown of the depletion layer) or
the formation of a passivating oxide film on the surface. [41] The breakdown of the
depletion layer at defect sites is recognized to occur at lower potentials, [40,46,47]
consequently enhanced etching will occur at these sites resulting in pitting. [42] A
similar effect may occur due to breakdown of an otherwise stable oxide film on the
surface. When there is pit initiation in the surface of the substrate, subsequently pitting
of the substrate occurs. This is due to the fact that as a pit is formed, the electric field is
greatly increased due to the small radius of curvature at the base of any given pit.
Preferential hole generation consequently occurs in the vicinity of the pit tip, leading to
an increased etch rate and consequent pore growth. [41]
14
Narrow channels are formed through the near-surface layer between the pits on
the surface and the pores propagating along <111>A directions. These channels are a
consequence of that; at initial pit formation, hole generation can only occur by
tunnelling paths at small angles to the surface normal, because of the geometry of the
depletion layer boundary as shown in Fig. 4 (a). Therefore the etching at the surface pit
is restricted to be normal to the surface, thus producing a very narrow channel, as
shown in Fig. 4 (b). When a pit has grown an adequate distance, carriers become
available towards the sides of the pit as depletion region bulges sufficiently inward.
Thus allowing{111}A-oriented facets to be exposed, and establishing a shape
resembling a truncated tetrahedron, the two vertices of which become the tips of
primary pores (Fig. 4 (c)). These tips then propagate along <111>A directions as
shown in Fig. 4 (d). [48] The truncated tetrahedron shape and propagation of pore
along the <111>A direction will be discussed in more detail in section 2.6.2.
15
Figure 4: Schematic representation of the progression of etching from a pit in the surface. (a)
The pit initially etches vertically due to availability of holes by paths x and y but not z. (b) The
channel lengthens in the [-100] direction. (c) Once the channel is deep enough, the availability
of carriers to the sides allows some lateral etching. This etching widens the end of the channel
into a truncated tetrahedral void that produces (d) two primary pores when the void is
sufficiently large.
AFM measurements have shown that the density of surface pits increases
progressively with time and each of these pits acts as a source for an individual
nanoporous domain. It is then apparent that the expansion of these domains beneath the
surface must also be progressive in nature. TEM images have been used to visibly
present individual domains at various stages of development. As the domains grow
larger, the domains ultimately meet and the merging of multiple domains eventually
leads to a continuous nanoporous sub-surface region. The rate of increase of pit density
is greatest in the vicinity of values that correspond to half of the peak current, however
above the potential of peak current the pit density begins to plateau. It has been
proposed that new pits form only in areas where bulk (nonporous) InP still remains
16
immediately beneath the surface, so that there is a high electric field at the electrode–
electrolyte interface. Thus, the plateau in pit density occurs when the underlying
nanoporous domains have grown large enough so that their bases begin to touch and
cover the area. [49]
2.6.2 Nanoporous domains and Directionality of Pore Growth
Gatos and Levine proposed a model where by the etch rate of the {111} surfaces of IIIV semiconductors with a zinc-blende are dependent on the relative reactivity of the
terminating atoms of these planes. [50,51] The dissolution of InP in a range of etchants
[52,53], by photochemical etching [54] and by thermal decomposition [55] has since
identified the {111}A planes as
slow etch planes, this is due to the preferential
removal of phosphorous in the {111} B planes. The fast etch rate of phosphorous is
deemed to be due to the full dangling bonds that extend from the surface oc
phosphorous atoms. [50,51] Lynch et. al [56] developed a model of porous structure
growth in InP in KOH based on propagation of pores along the <111>A directions. The
model, which is in excellent agreement with quantitative measurements, predicts that
pores originating at a surface pit and propagating at an instantaneously homogenous
growth rate, leads to porous domains with a truncated tetrahedral shape. The domain
were observed to have trapezium-, triangle-, and square-shaped cross-sections in the
(011), (01 ̅ ) and (100) planes respectively.
17
Figure 5: Isometric drawing of porous domains predicted for pore growth along the <111>A
directions. The Trapezium-shaped and triangular cross-section in the (011) and (01̅) cleavage
planes, respectively, are shown. Where the (01̅) cleavage plane intersects a domain near one
of its vertices, the resulting small triangular cross-section (X) appears at a distance from the
surface.
Figure 6: Cross-sectional SEM image of InP sample cleaved along (011) plane following an LPS
from 0 to 0.44 V (SCE) at 2.5 mV s-1 in 5 mol dm-3 KOH. Pore growth along the <111>A
directions and trapezium-shaped domain are visible.
18
Fig. 6 and 7 are cross sectional SEM micrographs of the (011) and the (01 ̅ )
planes respectively. Here the experimentally observed cross sections are seen to agree
with the predicted domain shape. An inverted triangle shape is seen in both Fig. 5 and
Fig. 7, and this triangle is mostly filled with pores growing perpendicular to the image
plane. A domain separated from the surface can be seen at position X in Fig. 5 and Fig.
7.
Figure 7: Cross-sectional SEM image of InP cleaved along the (01̅) plane (orthogonal to Fig.
B) following an LPS from 0 to 0.44 V (SCE) at 2.5 mV s-1 in 5 mol dm-3 KOH. Pore growth along
the <111>A directions and a triangular domain are clearly visible. A smaller domain crosssection at a distance from the surface is also visible (at X).
The propagation of pores is constrained jointly by the availability of charge
carriers at the pore tips and by the strength of the electric field that enables transport of
these carriers across the depletion layer surrounding the tips. SEM and TEM images
have shown that pore tips have a pyramidal form with {111}A internal facets.
Regardless of the tip shape, cross sectional images of pores show that the pores are
round and their width is constant with respect to distance from tip. The {111}A faces
19
near the tip are a consequence of the fast etching of slackly bound {111}B phosphorus
atoms. As result of this process, there is removal of the planes of atoms parallel to the
{111}A facets, initiating with an indium vacancy in {111}A face. Where the three
{111}A facets intersect, a vertex is maintained and the electric field near the tip of the
pore is thus sustained; this is due to the high surface curvature in this region. [57] The
electric field is adequately high near the pore tips to facilitate substantial quantum
tunnelling of carriers [58]; this with the dangling bonds of the phosphorous atoms
enables a higher etch rate, resulting in the propagation of pore tips. The pores have a
consistency in their width (there is no evidence that the pores are wider when closer to
the surface), which indicates that pore growth occurs in the vicinity of the pore tips The
cylindrical structure of the pores is a consequence the locus of the threshold electric
field on the {111}A facets; towards the rear of the tip, the electric field drops below the
threshold value required for etching. Hence it is clear that in the wake of the
propagation of the tip, a channel is therefore created, the walls of which do not etch
because of the lack of a suitable electric field. [57]
20
Figure 8: Schematic representation of the widening of pore tips (a) after the tip has extended
past the depletion region of the void from which it originated, (b) followed by the expansion
of these tips into tetrahedral voids. (c) This widening of the pore tips leads to branching of the
pores along the <111>A directions.
Although pores are known to propagate along a <111>A direction, it has been
observed that pores can deviate from the <111>A direction. Fig. 9 is an example of this
deviation, where pores are branching from a primary pore and are in close proximity to
regions that are depleted of carriers by the presence of nearby pores. [57]
21
Figure 9: Cross-sectional (011) SEM image of InP after an LPS from 0 to 0.383 V (SCE) in 5 mol
dm-3 KOH at 2.5 mV s-1.
Fig. 9 shows an SEM image of a (011) cross-section through a porous domain.
The primary pore can be seen at V; this pore propagated along a <111>A direction and
had rough but parallel edges. The parallel edges of this pore can be seen to end at a tip
with a rounded point at T. This figure also shows that not all of the pores were exactly
along the <111>A directions. This is particularly the case with the pores that branched
from the primary pore and were in close proximity to regions that were depleted of
carriers by the presence of nearby pores. An example can be observed at S, where a
pore diverged from the primary pore at V. At I the pore was restricted by several
neighbouring pores (e.g. at J) and thus it propagated into a bottle-neck resulting in a
tapered shape. Again, at K restriction of the development of a branching pore at V
resulted in narrowing, presumably due to the proximity of other pores not visible in the
micrograph. Despite this narrowing, the pore later re-established a normal width. [57]
22
2.6.3 The Three Step Charge-Transfer Model
Lynch et al. [59] presented a mechanism for pore formation which suggested that
charge transfer between the semiconductor and the electrolyte during anodisation
involves three key steps. The first step is rate limiting and is the supply of holes to the
semiconductor surface. As discuss above, this supply of carriers can only occur at the
pore tip. Upon arrival at the pore tip, the holes have some time to diffuse at the
electrode surface, via the valence band and surface states, before participating in the
electrochemical reaction with the active species in solution. This hole diffusion and
hole annihilation in the electrochemical reaction are the second and third steps in the
charge transfer model respectively. In this model it is suggested that the holes defusing
at the surface would tend to become ‘trapped’ at the {111}B surface states in the band
gap, leading to preferential etching at these sites. Here ‘zip like’ propagation of etching
depicted by MacFayden [60] is said to occur and this will ultimately reveal {111}A
facets. Subsequently the presence of an indium vacancy in the {111}A face initiates an
etch process in which the planes of atoms parallel to the {111}A facets are
removed.[59]
23
Figure 10: Schematic representation of the three-step charge transfer model at a pore tip. K1
represents the rate at which holes are supplied to the pore tip. K2 represents the
characteristic diffusion time for holes at the electrode/electrolyte interface. K3 represents the
rate at which the active species in the electrochemical reaction captures holes from (injects
electrons into) the interface
While the second and third steps in the charge transfer model do not affect the rate at
which CO pore etching transpires, these steps are however reputed with having an
effect on both the degree of preferential etching and the width of pores etched. [59]
With regard to preferential etching, if the kinetics of step three were fast comparative
to step two, then the diffusion distance of holes from the tip would be short as they
would be annihilated in the oxidation reaction close to where they were created. If that
was the case, etching would occur close to the site of hole generation rather than at
preferred crystallographic sites and thus the pore propagation would not be in the
preferred crystal direction. [48] The effects of steps two and three on pore width can be
24
seen, if the rate of the third step were to increase with respect to the second step (e.g.
due to increase in the activity of the electrolyte) then the pores would become
narrower. However, if the solutions activity was to decrease, the pores would widen.
[59] It was also highlighted that the width of an isolated pore is not controlled solely by
the charge transfer model. The width of the pores are determined by three factors; (i)
the ration of the radius of curvature ro and the space-charge layer thlickness xsc (i.e.
ratio ro/xsc), (ii) the rate of diffusion of holes at the semiconductor surface and (iii) the
kinetics of the electrochemical reaction (which governs the length of time the hole will
exist on the surface before being annihilated). [59]
2.6.4 Effect of Electrolyte Concentration
Some work using LPSs has been done to investigate the effect of electrolyte
concentration on the formation of porous layers in InP. [61][21] One investigation
observed the porous layer thickness increase significantly with decreasing
concentration of KOH reaching a maximum value at ~2.2 mol dm-3. However at
concentrations less than 1.8 mol dm-3 the layer thickness decreased sharply and no
porous layers were observed at concentrations of and lower than 1.1 mol dm-3. [61]
Another investigation observed both the porous layer thickness and pore width
decrease with increasing concentration between 2.5 mol dm-3 and 9 mol dm.3, however
at concentrations above 9 mol dm.3 the opposite trend was observed with both layer
thickness and pore width increasing with increasing concentration. The relationship
between porosity and varying concentration also follows the trend described above for
layer thickness and pore width. This indicated that the pore width increase is due to the
25
increased etching of pore walls and not due to a change in the characteristic length
scale of the pore structure. [21]
The three-step model of charge transfer was used to explain this variation in
pore width when the concentration was varied. It proposes that the pore width
variations are a consequence of a variation in the rate of hole capture by the
electrochemical reaction at different concentrations. With increasing electrolyte
concentration, there may be less time for the holes, which tunnel through the depletion
layer at the pore tip, to diffuse laterally at the interface before being removed
electrochemically. This is accredited to an increase in the rate of reaction as the
concentration of KOH is increased. The increase of the pore width as a result of
increase in concentration above 9 mol dm.3 suggests that the rate of the electrochemical
reaction decreases above this concentration.[21] The specific conductivity of KOH has
been reported to reach a maximum at approximately 7 mol dm-3 and then decreases
with increasing concentration thereafter. [62] This implies that there may be a decrease
in the kinetics of the relevant electrochemical reaction as a consequence of a change in
the structure of the electrolyte that occurs at higher concentration.[21]
2.6.5 Effect of Temperature
LPSs have also been used to examine the effect of temperature on the formation of
porous layers in InP.[21] In this study the pore width, layer thickness, and porosity
decreased as the temperature at which the InP was anodized increased. Similar to
concentration the three-step model of charge transfer is used to explain the variation in
pore width with varying temperature. The rate of the electrochemical reaction is
expected to be higher at higher temperatures, thus the amount of time a hole has to
26
diffuse at the interface is shorter than that at lower temperatures. Hence at higher
temperatures the etching is more spatially confined, as the effective diffusion length is
shorter.
2.6.6 Current-Line Oriented Pore Formation
The formation of current-line oriented pores (CLO) has been observed to occur at 10
°C in both 2.5 mol dm-3 and 17 mol dm-3 KOH between 0.6 and 1.0 V. To rationalize
the formation of CLO pores outlined above, a qualitative model based on a three-step
mechanism of charge transfer between semiconductor and electrolyte was used. It is
assumed at higher potentials, the rate of hole supply will increase and eventually may
no longer be rate limiting. In situations where the rate of hole annihilation by the
electrochemical reaction is reduced, this non rate limiting potential will be reached
sooner. This situation is possible at low temperature and at both high and low KOH
concentrations due to kinetic factors. In these situations, as holes become more
abundant at the interface, pores can grow continuously until they come into contact
with another pore. The widening pores eventually reach a packing density so high that
branching is impossible and holes are available only in the direction of the bulk InP.
Thus leading to wide pores and very thin walls as observed for CLO pores. Further
increases in hole availability (i.e. at potentials of 1.1 V or above) results in planar
etching, as carriers are available at all points on the electrode surface. [63]
2.6.7 Cessation of Porous Layer Growth
The formation of porous InP in KOH has seen the pore propagation spontaneously halt
in LVS, potentiostatic, and galvanostatic experiments. [20,52,55,56] Resulting in the
27
formation of porous layers of finite thickness unlike porous InP formed in acidic
solutions. [7,9,10] A study investigating this cessation of porous layer growth in KOH
indicated that the thickness limit is likely due to the low solubility of etching products
in alkaline solutions. [64] Findings from this study indicated that the precipitation of
etch products in merely a small number of pores may start the cessation of porous
etching via a domino effect. Facilitating this domino effect of precipitation could be
due to the lateral growth of domains, that neighbour domains which have ceased
etching and/or to the formation of a surface layer of precipitates over domains that have
ceased etching, which limits diffusion through the surface pits. These events lead to the
saturation of products near the pore tips of domains that are adjacent to domains that
have ceased etching, similarly causing the cessation of etching at these pore tips. The
dispersion of these precipitates ultimately obstructs the surface pits of all domains,
leading to greater mass transport impediments in the porous layer which brings about
the formation of precipitates firstly near the tips of each pore (where the concentration
of products is greatest) and eventually along almost all of the length of each pore. In
cases where a minute quantity of domains continue to etch, the lateral growth of such
domains will lead to saturation of etch products that precipitate, stopping all pore
propagation.[64] The influence of mass transport on the termination of porous layer
growth was demonstrated in an experiment at low current density where the current
was interrupted a number of times before the layer growth ended. The interruption, is
presumed to allow the reaction products to be transported out of the pores and for the
concentration of reactants at the pore tips to recover to the bulk electrolyte value. A
layer thickness of 6.9 µm was obtained, more than twice as deep as any CO porous
layers previously etched in InP in KOH by this group. [65]
28
2.6.8 Effect of Current Density
In InP with carrier concentration of 5.0 – 5.6 x 1018 cm-3 that was galvanostatically
anodised (i.e. constant current) the potential was seen to initially decay and the density
of pits on the electrode increased rapidly. Porous domain growth followed trends seen
before, with each pit giving rise to a porous domain which developed as pores
branched and the number of active pore tips respectively increased. The potential
reached a minimum when the porous domains began to meet, with a loss in active pore
tips. The potential increased then consequently resulting from the continuous decrease
in number of active pore tips due to domains merging to form a continuous porous
layer. The termination of porous layer etching was identified by a rapid increase in
potential (layer completion ~ 0.4 V). [65]
It was observed that pores formed at low current densities had sharp tips,
triangular cross-sections and large pore widths. Whereas in the pores formed at higher
current densities, both the pore tip and cross-sections were more rounded and the pore
width decreased. These differences were justified in terms of the effect that different
rates of hole supply (at various current densities) can have on the rate of indium
vacancy formation on the three {111}A faces that make up the pore tip. [65]
29
Chapter 3: Experimental
30
3
Experimental
3.1 Introduction
In this chapter, the experimental methodology, the apparatus and materials used will be
described. The particulars of each electrochemical technique concerning their direct
application to the formation and study of porous InP electrodes will be discussed. The
techniques utilized in the ex-situ microscopical examination and characterization of the
electrode following anodic treatment, namely scanning electron microscopy (SEM)
will also be briefly discussed.
3.2 Electrochemical Apparatus
Electrochemical sample preparation and electroanalytical experiments were carried out
using a conventional three-electrode cell, shown schematically in Fig. 12. The set up
employed of examining low current densities is shown schematically in Fig. 11.
Silver paint for
electrical contact
Figure 11: Variation of three-electrode cell which was used to examine the effects of low
current densities.
31
3.2.1 Reference Electrode
The role of the reference electrode is to provide a fixed potential reference, which does
not vary during the course of the experiment. Thus, differences in the potential applied
by the potentiostat, between the working electrode and the reference electrode, appear
across the working electrode/solution interface, since the potential of the reference
electrode is fixed and the counter electrode completes the circuit for current flow. A
good reference electrode should be able to maintain a constant potential even if a few
microamps of current are passed through it.
In all electrochemical experiments performed during the course of this work, a
commercially available saturated calomel reference electrode (SCE) was used. This
consists of a Hg/Hg2Cl2 electrode in saturated KCl. Such a reference electrode has a
potential of +0.241 V ± 1 mV versus the standard hydrogen electrode (SHE). All
potentials in this work are referenced to the saturated calomel electrode.
32
Figure 12: Schematic of three electrode cell used in experiments showing the working
electrode, counter electrode and reference electrode (in Luggin capillary) all immersed in the
electrolyte.
3.2.2 Counter Electrode
The counter electrode is an inert material, which is typically required to complete the
circuit thus allowing current to flow. In this work, a coil of platinum wire (99.99%
purity) was used as a counter electrode. The total surface area of the counter electrode
was ~1 cm2 and so was greater than that of the working electrode (~ 0.2 - 0.4 cm2).
3.2.3 Working Electrode
The working electrode used in this work was (100) oriented LEC grown polished
monocrystalline n-InP wafer. The InP was sulphur doped and had a carrier
concentration typically of 3 × 1018 cm-3. An ohmic contact to the working electrode
33
was formed by alloying pure indium onto the back-side of the InP on a hotplate at ~325
°C and subsequently the contact was connected to copper wire. The contact was
isolated from the electrolyte by masking it in a suitable varnish, which was resistive to
the electrolytes used. This varnish was also used to mask off the cleaved wafer edges,
i.e. the (110) oriented crystal faces. Electrodes had a typical surface area of 0.2 - 0.4
cm2. Prior to the formation of the ohmic contact, and again prior to immersion of the
sample in the electrolyte, the sample surface was etched to remove any native oxides.
A 3:1:1 H2SO4:H2O2:H2O (v/v) etchant was used in all etching procedures in this work
to remove native oxides. Electrodes were rinsed in doubly deionized water and blown
dry under nitrogen. Acidic etches are known to leave a thin P-rich layer whereas basic
etches leave residual In-rich layers [66]. Since photoluminescence (PL) measurements
of InP were found to result in a higher PL signal following an acidic etch as opposed to
a basic one [67], an acidic etch was used to remove the native oxides. The principal
components of native oxide films on InP are believed to be In2O3, InPO4, In(PO3)3, and
P2O5.
A H2SO4:H2O2:H2O solution etches the substrate in the following mode: the
H2O2 oxidises the surface and the H2SO4 etches the oxide. InPO4 is readily soluble in
H2SO4, P2O5 is highly soluble in water and the solubility of In2O3 is greatest in strongly
acidic etches [67]. Auger analysis of samples etched in H2SO4:H2O2:H2O solutions
revealed the lowest oxygen concentration on InP surfaces in comparison to other etches
investigated [68].
34
3.2.4 Ohmic Contact to the Working Electrode
The contact to the working electrode was examined by measuring its current-voltage
characteristics. Two ohmic contacts were formed on a slice of the InP wafer and
connected in series to with a 1 KΩ resistor to a potentiostat where a linear potential
sweep was imposed at a constant rate. The associated current was measured as a
function of the applied potential. A typical result is shown in Fig. 13 and ohmic
behaviour is observed. Current values observed in the experiments were less than 1
mA. Indium was chosen for creating the low resistance ohmic contact because its work
function is less than that of the semiconductor. Field emission is independent of
temperature, while thermionic field emission is temperature dependent. Both types of
quantum mechanical tunnelling can lead to ohmic behaviour.
1.5
Current (mA)
1
0.5
0
-1.5
-1
-0.5
0
0.5
1
1.5
-0.5
-1
-1.5
Applied Potential (V)
Figure 13: Plot of current versus applied potential resulting from testing of the ohmic contact
to InP wafer
35
3.2.5 Experimental Setup
The typical experimental setup is shown in Fig. 14. The cell is housed in a dark box.
This prevents unwanted light from interfering with the experiments. The dark box has a
number of small inlets cut through it to accommodate two water carrying rubber pipes
which pump heated water around the cell. This helps to maintain the temperature of the
electrolyte at 25 °C for the duration of the experiment. Additionally cables are passed
through this inlet which connect the electrodes in the cell to the potentiostat/galvanostat
outside the darkbox.
Figure 14: Schematic representation of the experimental arrangement showing dark box, cell,
thermostat, controlling computer, data acquisition unit, function generator, oscilloscope and
potentiostat/galvanostat.
36
For controlling the electrical parameters in the cell, a Princeton Applied Research
Model 363 Potentiostat/Galvanostat was used. The potentiostat is a controller circuit
that maintains the potential between the working electrode and reference electrode at a
value equal to a predefined control potential, which may be constant or time varying.
The controller reacts to the difference between the control and reference potentials
through a negative feedback system, and passes current through the counter electrode
circuit so as to reduce the difference to zero. This instrument can provide a constant
voltage or current, set by turning a dial on the front panel. The instrument also has an
external voltage input through which a set potential or current program can be input.
The function generator can generate functions of changing voltage with time. Standard
outputs include triangle, square and sine wave. The signal from the function generator
is fed into the potentiostats external voltage input, and the potentiostat matches that
signal and applies it to the electrodes. An oscilloscope is used to monitor the output
from the function generator. The final piece of equipment needed is the data acquisition
unit. This device has a multiplexer capable of measuring up to 16 potentials at once. It
is connected to the voltage and current monitor of the potentiostat in order to measure
voltage across, and current through, the cell.
3.2.6 Computer Control of Experiments
In this setup, both the function generator and the data acquisition unit are interfaced to
a personal computer. The devices are connected using the general purpose interface bus
(GPIB), which is an 8 bit parallel electrical bus, designed for instrument control a
Hewlett Packard in the late 1960s. In order to control the devices, a software user
interface was written in LabVIEW. For the function generator, a few different
37
programmes were used. The most commonly used was a triangle wave program. This
allowed the user to input the rate at which the potential should be swept as well as the
highest and lowest values of potential.
There was specific software written for the data acquisition unit. The program
asks the user what resolution is desired for the measurements. Once started the program
reads up to four potentials at a time from the data acquisition unit. Every second it
takes the measurements from the data acquisition unit’s output buffer and transfers
them to the computer. The program then makes a copy of the data and assembles it in a
file. The four values of potential are plotted in real time on the computer screen. There
is also a custom plot option which allows the user to plot any two of the potentials
against each other in real time. Once the user stops the scan, the program arranges the
data in csv format (comma separated value – this can be opened in any standard
spreadsheet program) and asks the user where they would like to save the data. It
should be noted that if the data is not saved in this instance then it is lost (some of the
data from experiments was not saved immediately and thus was lost). A screenshot
from the data acquisition unit control software is shown in Fig. 15.
38
Figure 15: Screenshot of program written to control and read data from the data acquisition
unit. The program written using LabVIEW.
39
3.2.7 Experimental Procedure
The experimental procedure is as follows:
1. The working electrode is prepared as detailed in section 3.2.3
2. The cell temperature is maintained at a constant 25 °C using the thermostat unless
otherwise stated.
3. The cell is placed in the darkbox and the electrodes are placed in the cell.
4. After allowing 20 s of quiet time for the reactions at the surface of the electrode to
stabilise, the potential or current program is switched on.
5. Immediately after the experiment has been performed, the sample is removed from
the cell, rinsed in deionised water and dried under nitrogen flow.
3.3 Electroanalytical Techniques
Linear sweep voltammetry, galvanostatic and potentiostatic experiments were the
primary electrochemical techniques used in the growth and analysis of anodic films and
in the formation and study of porous InP electrodes in KOH.
3.3.1 Linear Potential Sweep
Linear potential sweep was the initial electrochemical technique used in this work. The
linear potential sweep was employed to study the current-voltage response of the InP
electrode before carrying out further experiments. In this technique a linearly varying
potential is applied to the electrode while in solution and the current is measured as a
40
function of the applied potential. In linear sweep voltammetry, the potential is linearly
changed (swept) with time from the starting, Ei, to the end potential, Eu. The rate of
potential change with time is called the scan rate.
Commonly Ei is selected where no Faradaic processes take place and Eu is set to
a potential sufficiently past the E1/2 of the investigated redox reaction. In the case of an
oxidation (the analyte being present in its reduced state), Eu > Ei and the scan will
proceed to more positive potentials (anodic scan). When Eu < Ei, a reduction process is
usually being investigated the scan will go to more negative potentials (cathodic scan).
Linear sweep voltammetry is mainly employed in analytical chemistry. If slow scan
rates (< 10 mV s-1) are used, the potential-current curve obtained is similar to steadystate curves.
3.3.2 Galvanostatic Anodisation
For galvanostatic anodisation, the galvanostat passes a constant current between the
counter electrode and the working electrode. This means that the reaction rate at the
electrode surface is kept constant for the duration of the experiment. This is useful for
controlling the rate of the electrochemical reaction. It is also useful for passing a set
amount of charge through the electrode.
3.3.3 Variations of Galvanostatic Anodisation
Current density has an effect on the number of pits formed on the surface of an n-InP
electrode anodised in KOH. The effect of increased current density is that the surface
pit density also increases, with an apparently linear trend. [20] The density of surface
41
pits therefore may limit mass transport process in the anodisation of InP thus obscuring
thus observation of the true effects of mass transport on porous layer growth in InP. To
standardise pit density in the study, current burst experiments and current sweep
experiments were carried out (described below).
Current Burst Experiment
An initial current density burst was passed to standardise pit formation followed by a
rest period of 30 s. Then the selected current density (5, 10, 15 or 20 mA cm -2) was
passed until completion of layer growth. The current density burst consisted of 20 mA
cm2 for 5 s, or 5 mA cm2 for 45 s.
Current Sweep Experiment
An initial current density 20 mA cm-2 was passed for 5 s to standardise pit formation
following which the current density was swept down at a rate of 2 s per 1 mA cm-2 to
the selected current densities (5, 10, 15 or 20 mA cm-2) respectively and left to anodise
until completion of layer growth.
3.3.4 Potentiostatic Anodisation
For potentiostatic anodisation, the potentiostat maintains the potential between the
working electrode and reference electrode at a value equal to a predefined control
potential. This means that the potential applied is kept constant but reaction rate at the
electrode surface can vary at the different stages of the porous layer formation during
the experiment.
42
3.4 Characterization Techniques
Following the anodisation procedure, various experimental and analytical techniques
were used for studying the surface morphology of the electrode, film and porous layer
thickness, and the structures thereof. Cross sections of the electrode were prepared by
cleaving the InP substrate along the (110) crystal plane and allowing the porous layer
to cleave with the underlying substrate. These cross sections were studied
microscopically by scanning electron microscopy and values for the layer thickness,
pore width, pit diameter and pit density were measured respectively. Where alterations
to standard procedures were used, they are outlined appropriately.
3.4.1 Scanning Electron Microscopy (SEM)
Scanning electron microscopy was used for the examination of the anodically etched
porous films and for obtaining measurements of layer thickness, pore width and
interpore spacing. The technique was most useful for structures in the range of a few 10
s of nanometres to several microns in size. In scanning electron microscopy an electron
beam is focused to a small spot and scanned across the sample surface in a raster
fashion. This primary beam of electrons ejects electrons out of the surface of the
sample under investigation. These secondary electrons are then detected and when used
synchronously with the incident beam, a magnified image of the surface can be
obtained. The electron beam interacts with the solid via electrostatic forces, with the
interaction taking place chiefly in a pear-shaped region with an extent of about 1 μm
below the surface. This is illustrated in Fig. 16 [69] The secondary electrons produced
within the solid and escaping from it can only come from the surface regions, with the
actual escape depth depending on the energy of the electrons. Although secondary
43
electron images are obtained most frequently with the SEM, backscattered electron
images also provide important information.
Figure 16: Electron scattering in the near surface region leading to characteristic interaction
volume.
Backscattered electrons vary in their amount and direction with the composition,
surface topography, crystallinity and magnetism of the specimen. The contrast of a
backscattered electron image depends on (1) the backscattered electron generation rate
that depends on the mean atomic number of the specimen, (2) angle dependence of
backscattered electrons at the specimen surface, and (3) the change in the backscattered
electron intensity when the electron probe’s incident angle upon a crystalline specimen
is changed. The backscattered electron image contains two types of information: one
on specimen composition and the other on specimen topography. To separate these two
types of information, a paired semiconductor detector is provided symmetrically with
respect to the optical axis. Addition of them gives a composition image while
subtraction gives a topography image. And with composition images of crystalline
44
specimens, the difference in crystal orientation can be obtained as the so-called
“channelling contrast,” by utilizing the advantage that the backscattered electron
intensity changes considerably before and after Bragg’s condition. The generation
region of backscattered electrons is larger than that of secondary electrons, namely,
several tens of nm. Therefore, backscattered electrons give poorer spatial resolution
than secondary electrons. But because they have a larger energy than secondary
electrons, they are less influenced by charge-up and specimen contamination. Contrast
in the final image corresponds directly to variations in the number of electrons that
reach the detector from different regions of the specimen. The simplest source of
contrast is irregularity of the surface of the specimen. The number of secondary
electrons reaching the detector is closely related to the angle between the incident beam
and the surface at the point of impact. The resolution of the SEM depends on the probe
spot size and the probe current density. Charge build up on samples of low
conductivity can be induced by the SEM process and results in a distortion of the final
image. Sample tilting and variation of probe current densities were employed as
techniques for optimising the resolution, by enhancing the magnification and contrast
while still maintaining high image quality. All SEM images in this report were taken
by a Hitachi S-4800 Field Emission SEM.
45
Chapter 4: Results & Analysis
46
4
Results
4.1 Linear Potential Sweep
Linear potential sweeps were employed in previous experiments; they show how the
current requirements of anodic etching of InP changes throughout the formation of pits
on the surface of the electrode, growth of nano-porous domains and growth of a porous
layer.
40
Current Density (mA cm-2)
35
30
25
20
15
10
5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Potential (V)
Figure 17: Linear sweep voltammogram (LSV) of InP electrode anodised at 25 °C in 5 mol dm-3
KOH.
Fig. 17 shows the LSV of an n-InP electrode anodised at 25 °C in 5 mol dm-3 KOH. It
can be seen in Fig. 17 that there was little current at potentials less than 0.2 V.
47
However continued anodisation to potentials greater than 0.26V results in a rapid
increase in the current density to a peak value of ~ 35.5 mA cm-2 at 0.32V, this
corresponds to the pore formation potential described by Schmuki, et al. At this pore
formation potential pitting and pore growth from the surface commences. [16] The
peak in the current correlates to the merging of the porous domains, the pores can no
longer continue to grow laterally but must propagate downward through the electrode.
[49] [48] Above 0.32V the current density decreases rapidly with just a slight plateau
after the initial decrease. The slight plateau corresponds to the continued growth of the
pores downwards, while the number of active pore tips remains generally constant.
[70]. The rapid decrease in current density indicates the cessation of porous layer
growth.
4.2 Galvanostatic experiments
Galvanostatic anodisation of InP in KOH has been employed to a far less extent than
the anodisation of InP in KOH under controlled potentials. Interest in galvanostatic
anodisation has sparked from the fact that the rate of reaction can be controlled in this
type of experiment. [26,65] The ability to keep the rate of reaction constant, allows for
the investigation of the influences of other variables on the anodisation of InP in KOH.
As the current in constant in the galvanostatic experiments, the potential of the system
changes throughout anodisation. A typical plot of potential versus time, (Fig. 18)
shows a high potential at the start of the experiment, which then decreases rapidly and
reaches a minimum. The initial rapid decay in potential is described by the rapid
increase in the density of pits on the surface of the electrode and subsequent pore
48
growth. The growth of porous domains follow the same trend as outlined in the linear
potential sweep section and literature review, with each pit giving rise to a porous
domain which develops as pores branch and the number of active pore tips respectively
increases. [65] The potential reaches a minimum when the porous domains begin to
meet, resulting in a loss of active pore tips. Subsequent increases in the potential seen
in Fig. 18, are attributed to the decrease in the number of active pore tips due to
domains merging to form a continuous layer. The termination of porous layer is
identified by the rapid increase in potential.
0.6
Potential (V)
0.5
0.4
0.3
0.2
0.1
0
0
50
100
150
200
250
300
350
Time (s)
Figure 18: Potential versus time plot for InP electrode anodised galvanostatically (5 mA cm -2)
in 5 mol dm-3 KOH at 20°C
49
Cross sections of the surfaces of electrodes were studied microscopically using a
scanning electron microscope and values for the layer thickness, pore width, pit
diameter and pit density were measured respectively. Cross sections of the electrode
were prepared by cleaving the InP substrate along the (110) crystal plane and allowing
the porous layer to cleave with the underlying substrate. Fig. 19 shows a typical cross
section of an electrode, revealing: (a) the porous layer and (b) a pore to be measured.
Figure 19: (a) Porous Layer, (b) pore; formed galvanostatically (5 mA cm-2) in InP electrode in
5 mol dm-3 KOH at 20 °C.
The SEM images were measured using a digital micrograph from ‘Gatan_TEM
software’. The images taken were opened in this software and the measurement tool
was calibrated to the scale at the bottom of the images. The measurement tool consisted
of an extendable line which was dragged from a desired starting point to the end point.
The starting and ending points for this work were the edges of the porous layers, pores
and pits. Fig. 20 shows the digital micrograph being used to measure the thickness of a
porous layer. Five measurements were taken for each porous layer and pore images
taken. While, for pit diameter the number of measurements was dependant on the
50
number of pits present in the image. For diameter, the area of each image was
measured and the number of pits present was counted from printed copies of the
images. Generally speaking eight porous layer images, twenty-two pore images and ten
surface images (5 high magnification images for pit diameter and 5 lower
magnification images for pit density), were taken.
Figure 20: Software used to take measurements of layer thickness, pore width, diameter and
area (for pit density).
51
4.2.1 1mA cm-2 Current Density
During the galvanostatic anodisation of InP at low current densities the run time of the
experiments tends to exceed the length of time for which the nail varnish effectively
isolates the sides, back and connections of the InP electrodes. To try and overcome this
problem in low current density experiments a different set up was employed (Fig 11)
Unfortunately with this set up, to create a seal at the surface of InP, a good deal of
pressure had to be put on the fragment of the InP wafer. The majority of the time the
InP fragment cracked under application of pressure, however there was an occasion in
which it did not crack and there was a sufficient seal created at the InP surface. In this
case the InP was anodised in 5 mol dm-3 KOH at room temperature (set up could to be
connected to water bath). As there was only one result from this style of experiment,
the result was plotted with similar work done previously by others (Fig. 21) [65] and
the results for the current study which had similar conditions (Fig. 22).
Layer Thickness (µm)
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
Current Density (mA cm-2 )
Figure 21: Plot of layer thickness versus current density for InP electrodes anodised in 5 mol
dm-3 KOH. Black data points are results from previous work carried out at 25 °C [65], and red
data point is a result of the current study and carried out at room temperature ~20 °C.
52
Layer Thicknesss (µm)
8
7
6
5
4
3
2
1
0
0
5
10
15
Current Density (mA
20
25
cm-2)
Figure 22: Plot of layer thickness versus current density for InP electrodes anodised in 5 mol
dm-3 KOH. Black data points were anodised at 20 °C, while the red data point was anodised at
room temperature ~20 °C.
Examining Fig. 21 and Fig. 22, it can be seen that at 1 mA cm-2, a thicker porous layer
is formed in comparison to the porous layers formed at higher current densities (5-20
mA cm-2). The standard deviation for layer thickness formed at 1 mA cm-2 is also
greater than that for the other current densities examined. This is assumed to be
resulting from the lower pit density on the surface of electrodes formed at lower current
densities. Fewer pits mean, that fewer porous domains will be formed; the distribution
of these pits and domains throughout the electrode is not necessarily even. The uneven
distribution of the reduced quantity of pits and domains means, that some domains may
have a longer growth period before merging than other domains have. This, results in
large variations in the thickness of the porous layer observed throughout the electrode.
Comparing both Fig 21 and Fig. 21 it can also be noted that the average value of the
layer thicknesses formed 5 mA cm-2 and 20 mA cm-2 have similar values for the two
separate studies. The work done by Quill, et al. [65] recorded the thickness of the
53
porous layer formed at 5 and 20 mA cm-2 were 3.016 and 2.889 µm respectively, while
in this study the thickness of the porous layer formed at 5 and 20 mA cm -2 were
recorded as 3.035 and 2.833 µm respectively.
Layer Thickness (µm)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
Current Density (mA
20
25
cm-2 )
Figure 23: Data points from Fig. 21 in black and Fig. 22 in blue graphed on the same plot
excluding 1 mA cm-2 data point.
Looking at Fig. 23 and comparing the layer thickness measured in previous
galvanostatic work and the layer thickness measured in the present galvanostatic work,
it does not appear to change. There was a 5 °C difference between the experiments but
none the less the comparison of the results indicates that the results obtained for
galvanostatic experiments remain relatively constant.
54
4.2.2 Current Density Burst
Current density has been noted to have an effect on the density of pits formed on the
surface of an n-InP electrode anodised in KOH. The effect of increased current density
is that the surface pit density also increases, with an apparently linear trend. [20] The
density of surface pits therefore may limit mass transport (mainly diffusion), of
electrolyte and reaction products in the anodisation of InP, thus obscuring the
observation of the true effects of mass transport on porous layer growth in InP. To
counteract the limitations of the pit density on observing the effect of mass transport,
the current density burst experiments were devised to standardise pit density among all
electrodes. In these experiments, an initial current density burst was passed, to
standardise pit formation followed by a rest period of 30 s. Then the selected current
density (5, 10, 15 or 20 mA cm -2), was passed until completion of layer growth. The
current density burst consisted of 20 mA cm-2 for 5 s, or 5 mA cm-2 for 45 s. The
results of the 20 mA cm-2 burst experiment will be examined first, followed by the 5
mA cm-2 burst experiment results.
55
0.8
0.6
Potential (V)
0.4
0.2
0
-0.2
20 mA cm-2
0
50
100
150
200
250
15 mA cm2
-0.4
10 mA cm2
-0.6
5 mA cm2
-0.8
-1
-1.2
Time (s)
Figure 24: Potential-time plot for InP electrodes at 25 ° C in 5 mol dm-3 KOH anodised initially
at 20 mA cm-2 for 5 s followed by 30s rest period before further anodisation at current
densities specified above.
Above the change in potential during anodisation can be seen as an initial current
density burst of 20 mA cm-2 was passed for 5 s, followed by 30 s rest period and then
the application of the respective current densities until the completion of layer growth.
Previous work [34,65] has shown that a rapid increase in potential after a period of
relatively constant potential indicates the cessation of porous layer growth. This rapid
increase had been observed at ~ 0.4 V in most cases, therefore ~0.4 V was deemed the
termination point for the galvanostatic experiments.
56
Layey Thickness (μm)
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
Current density (mA cm-2)
Figure 25: Layer thickness of InP electrodes anodised at 25 °C in 5 mol dm-3 KOH with initial
current density burst of 20 mA cm-2 for 5 s followed by 30s rest period before further
anodisation at current densities specified above.
In Fig. 25 it can be seen that the layer thickness is increasing with current density from
5 to 15 mA cm-2 and is decreasing slightly from 15 to 20 mA cm-2. The 5 mA cm-2 and
10 mA cm-2 values lie within each other’s standard deviation error bars, while the 20
mA cm-2 value also lie within 15 mA cm-2 and 10 mAcm-2 standard deviation error
bars. On examination of the electrodes from this experiment it was though that not all
the pores continued to grow upon application of the respective current densities after,
the 20 mA cm-2 current density burst and rest period. Previous work has noted, pores
formed at lower current densities exhibit sharper pore tips, while pore tips formed at
higher current densities have a more rounded appearance. [65] The tip shape is
determined mainly by the radius of curvature at which the electric field reaches the
threshold for hole generation. [71,72] The pore tips after 20 mA cm-2 burst, would be
expected to correspond to a radius of curvature necessary for the electric field to reach
the threshold for hole generation, for a current density of 20 mA cm-2. However, the
57
radius of curvature associated with 20 mA cm-2, would be expected to be insufficient
for the electric field to reach the threshold for hole generation at lower current
densities. The suggestion that all pores might not continue to grow after 20 mA cm-2
creates problems in trying to investigate the true effect of mass transport. To resolve
this implied issue, 5 mA cm-2 burst was employed to standardise pit density. This is
because at low current densities such as 5 mA cm-2, the radius of curvature associated
with the sharp tip formed, would be expected to be sufficient for the electrical field to
reach the threshold for hole generation, for all the higher current densities applied after
current density burst.
0.6
0.4
Potential (V)
0.2
0
-0.2
0
100
200
300
400
500
20 mA cm-2
15 mA cm2
-0.4
10 mA cm2
-0.6
5 mA cm2
-0.8
-1
-1.2
Time (s)
Figure 26: Potential-time plot for InP electrodes at 25 °C in 5 mol dm-3 KOH; anodised initially
at 5 mA cm-2 for 45 s followed by 30s rest period before further anodisation at current
densities specified above.
58
In Fig. 26 the change in potential during anodisation can be seen as an initial current
density burst of 5 mA cm-2 was passed for 45 s, followed by 30 s rest period and then
the application of the respective current densities until the completion of layer growth.
In Fig. 26 it is observed, that the time taken for the rapid increase in potential to ~ 0.4
V to occur increases, with the decreasing current densities. This is in line with the fact
that, the current density controls the rate of reaction for etching [26,65], i.e. at lower
current densities the rate of reaction is slower and therefore etching of the electrode
takes longer to complete.
4
Layer Thinkness (µm)
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
Current density (mA
20
25
cm-2 )
Figure 27: Layer thickness of InP electrodes anodised in 5 mol dm-3 KOH with initial current
density burst of 5 mA cm-2 followed by 30s rest period before further anodisation at current
densities specified above.
59
In Fig. 27 the layer thickness can be seen to be slightly decreasing with increasing
current density (At 5 and 20 mA cm-2 the layer thicknesses are ~ 2.865 and 2.423 µm
respectively). It is clear that the points above do not lie outside of the standard
deviation of one and other. Below in Fig. 28 the pit density is observed to increase with
current density, similar to results found in other studies, [34,65] the pit density at 20
mA cm-2 is approximately 3.7 times greater than the pit density at 5 mA cm-2. Images
of the variation in pit density with current density can be seen below in Fig 29. Each
image was taken at the same magnification level; it can clearly be seen that the
electrode exposed to the 20 mA cm-2 burst, has a greater pit density than the electrode
exposed to the 5 mA cm-2 burst. This increase in pit density, with current density, are
the results that were expected to be obtained, as pervious work saw the pit density
increase proportionally with increases in the pit density applied. [65] This increase in
pit density in not unexpected, as at increased current densities, a large number of
current paths are requires at the beginning of anodisation. A small quantity of domains
at low current densities, will swiftly produce an adequate amount of pore tips, to fulfil
the current density requirements at a relatively low potential. A large quantity of
domains must form quickly at high current densities, so as to retain a potential within
the scope that results in porous etching. Each domain appears to reduce the overall
resistance of the circuit. As additional domains are added, larger current densities can
be achieved at the same potential. As substantial pit formation can only transpire in the
short period of time between the start of anodisation and the establishment of a
continuous porous layer, increased quantities of domains during the initial stages of
porous growth result in the measurement of greater surface pit densities at the end. [65]
60
0.00006
Pit Density (pits nm-2 )
0.00005
0.00004
0.00003
0.00002
0.00001
0
0
5
10
15
20
25
Current Density (mA cm-2 )
Figure 28: Pit density versus initial current density burst for InP electrodes anodised at 25 °C
in 5 mol dm-3 KOH.
(a)
(b)
Figure 29: Surface of InP electrode electrodes anodised at 25 °C in 5 mol dm-3 KOH with an
initial current density burst of (a) 20 mA cm-2 and (b) 5 mA cm-2.
61
40
Pore Width (nm)
35
30
25
20
Average
15
Median
10
5
0
0
5
10
15
20
25
Current Density (mA cm-2)
Figure 30: Pore width for InP electrodes anodised at 25 °C in 5 mol dm-3 KOH with initial
current density burst of 20 mA cm-2 (solid line) and 5 mA cm-2 (dashed line) followed by the
respective current densities.
Above in Fig. 30, the pore width appears to be decreasing with increasing current
density with a level trend between 10 and 15 mA cm-2. It would be assumed that the
initial current density burst would not have an effect on the width of pores formed at
the current densities outlined in the graph. Looking at Fig. 30, it would appear that the
initial current density burst does have a slight effect on the width of the pores formed.
However the standard deviation of these values for each experiment includes the points
from the other experiment. Standard deviation of pore widths measured at 5, 10, 15, 20
mA cm-2 were respectively 4.642, 3.755, 4.328, 3.467 nm for 5 mA cm-2 burst
experiment and 4.830, 4.620, 3.638, 5.676 nm for 20 mA cm-2 burst experiment.
Therefore the variation in pore width between the two experiments is unlikely to be a
result of the change in magnitude of current density burst, but more likely a
consequence ambiguity in pore width measurement and errors in measuring at the
nano-scale. The variation in pore width with current density can be seen in Fig. 31.
62
This image shows a pore which was initially formed at 5 mA cm-2 and then continued
to grow under an applied current density of 20 mA cm-2. The position on the dotted
line was calculated by getting the ratio of the charge passed for the initial 5 mA cm -2
burst to the total charge passed (including charge from initial current density burst) and
letting that equal the ratio of the depth of pores formed during initial burst (x) to the
total thickness/depth (in nm) of the porous layer. This calculation assumes that there is
a relationship between the amount of charge passed and the thickness of the porous
layer formed.
393.033 nm
5mA cm
-2
20mA cm
-2
Figure 31: InP electrode anodised initially by 5 mA cm-2 current density burst for 45s and then
by 20 mA cm-2 until completion of porous layer growth.
63
4.2.3 Current Density Sweep
3.5
Layer Thinkness (µm)
3
2.5
2
1.5
1
0.5
0
0
2
4
6
8
10
Current density (mA
12
14
16
cm-2 )
Figure 32: Layer thickness of InP electrodes anodised at 25 °C in 5 mol dm-3 KOH with initial
current density of 20 mA cm-2 for 5 s after which the current density was swept down to the
respective current densities at a rate of 2 s per 1 mA cm-2 decrease.
As all the pits may not have continued to grow in the 20 mA cm-2 current density burst
experiment, the experiment corresponding to the graph above was devised so that all
the pores would continue to grow after the initial exposure to 20 mA cm-2 for 5 s. After
exposure to initial current density, the current was then swept down to the current
densities specified on the graph above at a rate of 2 s per 1 mA cm-2 decrease. In the
graph of the resulting layer thicknesses (Fig. 32), the layer thickness increases with
current density from 3 to 10 mA cm-2 and it decreases slightly from 10 to 15 mA cm-2.
Both the layer thickness values for 10 and 15 mA cm-2 lie within the standard deviation
error bars for 5 mA cm-2, and the layer thickness value for 5 mA cm-2 lies within the
64
standard deviation error bars for 3 mA cm-2. Upon examination of the procedure of this
experiment, and the corresponding images and results, it was apparent that this
experiment did not fairly compare the layer thickness obtained to current densities
being examined even though the pit density of all samples had been standardised.
Sweeping the current density down at a rate of 2 s per 1 mA cm-2 decrease meant that
each electrode was exposed to varying current densities over varying periods of time.
Thus the layer thickness obtained was not directly comparable to the current densities
desired to be examined.
65
4.2.4 Varying Temperature
The results presented below correspond to experiments carried out galvanostatically
with no current density bursts at constant KOH concentration, varying the temperature.
There was no current density burst in these experiments, as the effects of temperature
and concentration wished to solely be examined. As the current was kept constant, the
change in potential with time for the 12 mol dm-3 set of experiments can be seen in Fig.
33. The potential throughout this set of experiments is observed to follow a similar
trend to that shown in Fig. 18 and discussed at the beginning of section 4.2. It is also
observed in Fig. 33, that there appears to be a shift in which the values for the
minimum potential and the relatively constant potential period, appear to decrease with
increasing temperatures. To examine the effect of temperature on the voltage in
galvanostatic experiments, initially the peak voltage versus temperature was
considered, however taking the value of potential at 100 s was deemed to be a more
reliable representation. The reason for this is as follows: The rate at which the peak
potential is reached in galvanostatic experiments occurs extremely rapidly and there is
a chance that the intervals at which the DAQ collects values may miss the ‘true’ peak
potentials for these experiments. Therefore, to compare the variation in the potential of
the experiment with temperature, the value of the potential recorded at 100 was used,
because for each experiment the potential at 100 s was not changing rapidly.
66
0.6
0.4
0.2
Potential (V)
0
-0.2
0
500
1000
1500
2000
10°C
-0.4
20°C
-0.6
30°C
-0.8
40°C
-1
-1.2
-1.4
-1.6
Time (s)
Figure 33: Potential versus time plot for InP electrodes anodised galvanostatically (5 mA cm-2)
in 12 mol dm-3 KOH at varying temperatures.
0.45
0.4
13.6 M
Voltage at 100 s (V)
0.35
0.3
12 M
0.25
7M
0.2
5M
0.15
4M
0.1
2M
0.05
0
-0.05
0
10
20
30
40
50
Temperature (°C)
Figure 34: Plot of voltage at 100 s versus temperature for galvanostatic experiments (5mA cm 2
) displaying the trend line for the 4 mol dm-3 KOH experiment.
67
According to Fig. 34 the potential at 100 s for galvanostatic experiments decreases
linearly with increasing temperature regardless of KOH concentration. This decreasing
trend is not surprising, as previously it has been indicated that the rate of the
electrochemical reaction is expected to be higher at higher temperatures. [21]
Therefore, at higher temperatures, the current carrying capacity of pore tips is assumed
to increase, which is observed as decrease in the potential required to pass the same
current density. The slopes of these linear trends for each KOH concentration are given
below in Table 1. The slopes of the linear trends of voltage dependences on
temperature have a relatively small range (-0.0043 to -0.0064 V/°C), suggesting that,
the relationship between temperature and its influence on the voltage required to pass a
constant current during anodization, is generally proportional for all KOH. The
experiment at 40 °C in 13.6 mol dm-3 KOH did not appear to run correctly (see
appendices Fig. 77 and Fig. 39 – Charge density uncharacteristically low), and did not
run to 100s so therefore it is not represented in Fig. 34. This could have been the result
of a sudden change in the surface area, maybe due to exposure of the electrode that had
been previously covered by nail varnish.
KOH Concentration (mol dm-3)
Slope of corresponding trend line (V/°C)
13.6
-0.0052
12
-0.006
7
-0.0064
5
-0.0054
4
-0.0043
2
-0.0062
Table 1: Slope of trend lines for each concentration plotted on Fig. 34 (voltage at 100 s versus
temperature plot for galvanostatic experiments)
68
14
Layer Thickness (µm)
12
10
13.6 M
8
12 M
7M
6
5M
4
4M
2
0
0
10
20
30
40
50
Temperature (°C)
Figure 35: Plot of layer thickness versus temperature for InP electrodes anodised at 5 mA cm2
. (Trend lines shown only for 4 and 5 mol dm-3)
The application of trend lines to Fig. 35, shows a linear trend for all concentrations for
all except 12 mol dm-3 KOH. The layer thickness increases with increasing temperature
for all KOH concentrations, except for 5 mol dm-3 where the trend in the layer
thickness decreases with increasing temperature. The largest layer thickness (~12.48
µm) of all the experiments run for this thesis was recorded in an electrode anodised at
40 °C in 12 mol dm-3 KOH (Fig. 36). Looking at Fig. 35, the porous layer formed at 40
°C in 12 mol dm-3 KOH is more than twice the thickness of the next thickest layer
(~5.71 µm formed at 30 °C in 12 mol dm-3) formed in the galvanostatic experiments.
The values for the layer thickness of electrodes anodised in 13.6 mol dm-3 KOH at
both 30 °C and 40 °C were not included in Fig. 35, because at 30 °C the experiment
was terminated before the completion of layer growth (See appendices – Fig 77). At 40
°C the charge was significantly smaller than that of other experiments (see Fig. 39),
indicating that the experiment did not run correctly, as expressed earlier this may have
69
been due to a change in the surface area. Measurements for 2 mol dm-3 KOH
galvanostatic experiments are not included in the results as they do not provide useful
information. At 2 mol dm-3 KOH for galvanostatic experiments the porous layer
formed was very shallow, the surface was rough with irregular surface structures
formed and in many cases a large amount of oxide was present on the surface of the
electrodes (Fig. 37 & Fig. 38). This may be due to change in etch mechanisms at lower
KOH concentrations. Previous work has seen the gradual transition from porous to
non-porous etching as the concentration of KOH electrolyte was decreased. [61] This
transition was observed for InP electrodes of unspecified carrier concentration,
anodised between 1.8 and 1 mol dm-3 KOH.
Figure 36: Porous layer galvanostatically formed in 12 mol dm.3 KOH at 40 °C.
70
Figure 37: Surface of InP electrode galvanostatically anodised at 10 °C in 2 mol dm.3 KOH
Figure 38: Porous layer galvanostatically formed at 10 °C in 2 mol dm.3 KOH.
71
Charge Density (mC cm-2)
9000
8000
7000
6000
13.6 M
5000
12 M
4000
7M
3000
5M
2000
4M
1000
0
0
10
20
30
40
50
Temperature (°C)
Figure 39: Charge density passed through InP electrodes anodised at 5 mA cm-2 varying
temperature.
The above graph (Fig. 39) shows the charge density passed through the electrode for
each galvanostatic experiment. Visually comparing Fig. 39 to Fig. 35 (Layer thickness
versus temperature plot), it would appear that the charge density follows a similar trend
to layer thickness at varying temperature for different concentrations. It would make
sense that the amount of charge density passed had a relationship with the thickness of
the porous layer formed, however upon further analysis of this relationship, there was
no apparent proportionality between the two. This analysis was done by getting the
ratio of the current density to the layer thickness and plotting these values versus
temperature for each KOH concentration (Fig. 40), this plot includes error bars of the
standard deviation of the layer thickness (m-1). These error bars are so small that they
cannot be seen for many of the data points. Assuming that wider pores require a greater
amount of charge to be formed, then, variations in the width of pores formed in the
porous layer may be accountable for the lack of proportionality between the charge
72
density and layer thickness. To analyse if this was the case, the ratio above (Current
Density/Layer Thickness) was divided by the corresponding pore widths (Fig. 41). A
clear relationship is not seen between charge density, layer thickness and pore width
for the galvanostatic experiments. This lack of a visible relationship may be caused by
the measurements not being sensitive enough to show the relationship. Considering
that the errors from each measurement (charge, layer thickness, and pore width) all add
up to give the errors for this relationship, then it is estimated that the error bars would
be quite large.
Charge Density/Layer thickness (mC
cm-2 m-1)
1.4E+09
1.2E+09
1E+09
800000000
13.6 M
12 M
7M
5M
4M
600000000
400000000
200000000
0
0
10
20
30
40
50
Temperature (°C)
Figure 40: Plot of the ratio of charge density to layer thickness for each InP electrode versus
the temperature at which anodisation was carried out
73
(Charge density/Layer
Thickness)/Pore Width (mC cm-2
m-1)
3.5E+16
3E+16
2.5E+16
13.6 M
2E+16
12 M
1.5E+16
7M
5M
1E+16
4M
5E+15
0
0
10
20
30
40
50
Temperature (°C)
Figure 41: Plot of the ratio of (charge density/layer thickness) to pore width for each InP
electrode versus the temperature at which anodisation was carried out
74
The variation of pore width with temperature as a result of galvanostatic anodisation is
shown in Fig. 42. The application of trend line (only 5 mol dm-3 trend line remains in
the graph so as not to clutter it), shows the pore width linearly increasing with
increasing temperature for all KOH concentrations, (for 13.6 mol dm-3 only a very
slight increase). With regards to the three step charge-transfer model, if the rate of the
second step increases with respect to the third step, then the pores are expected to
become wider. [59] The increase in temperature may increase the rate of supply of
holes to the tip, which is the rate limiting step in the three step charge-transfer model.
[59] The increase in holes supplied may result in a greater increase in diffusion time of
holes at the electrode surface than the increase in the electrochemical reaction (step 3),
which is also observed with increasing temperature (i.e. a greater number of holes
means the active species cannot annihilate all the holes at the same rate therefore some
holes may diffuse greater distances from the pore tip), thus leading to wider pores.
40
Pore Width (nm)
35
30
25
13.6 M
12 M
20
7M
15
5M
10
4M
5
0
0
10
20
30
40
50
Temperature (°C)
Figure 42: Plot of pore width versus temperature for InP electrodes anodised at 5 mA cm-2.
(Linear trend line shown only for 5 mol dm-3)
75
40
Pit Diameter (nm)
35
30
25
13.6 M
20
12 M
7M
15
5M
10
4M
5
0
0
10
20
30
40
50
Temperature (°C)
Figure 43: Plot of Pit Diameter versus Temperature for InP electrodes anodised at 5 mA cm-2.
The above graph (Fig. 43) shows no apparent trend between pit diameter and
temperature for all the KOH concentrations. It can be difficult to measure the diameter
of pits on the surface of an electrode accurately due to the presence of the near surface
layer and its relationship to the depletion layer. This will be discussed in more detail in
section 5.2. There is a clear difference in the pit diameter between the pit diameter at
the lowest and highest temperatures of anodisation for 5 mol dm-3 KOH. The images
below (Fig. 44), show the visual variation in pit diameter with temperature for InP
electrodes anodised in 5 mol dm-3 KOH.
76
(a)
(b)
(d)
(c)
Figure 44: High magnification images of pits formed galvanostatically anodised (5 mA cm -2) in
5 mol dm-3 at (a) 10 °C, (b) 20 °C, (c) 30 °C, (d) 40 °C
77
0.00005
Pit Density (pits nm-2)
0.000045
0.00004
0.000035
0.00003
13.6 M
12 M
0.000025
7M
0.00002
5M
0.000015
4M
0.00001
0.000005
0
0
10
20
30
40
50
Temperature (°C)
Figure 45: Plot of pit density versus temperature for InP electrodes anodised at 5 mA cm-2.
It is clear for Fig. 45 that the density of pits on the surface of electrodes decreases with
increasing temperature for all KOH concentrations. For most of the experiments the
biggest decrease in pit density is seen between 10 and 30 °C. This increase in pit
density in not unexpected, as current carrying capacity per pit and pore tip is assumed
to increase with increasing temperature as discussed earlier. Therefore at higher
temperatures a reduced number of pits and subsequent porous domains are requires to
satisfy the current density requirements set.
78
4.2.5 Varying Concentration
As highlighted in the last section experiments were carried out at constant KOH
concentration, varying the temperature. The results plotted below are the same values
as presented in the varying temperature section above. The plots below were created to
inspect if there was any obvious relationships to varying KOH concentration.
As the current was kept constant the change in potential with time for
experiments carried out at 30 °C can be seen in Fig. 46. The potential throughout this
set of experiments is observed to follow a similar trend to that shown in Fig. 18 and
discussed at the beginning of section 4.2. It is also observed in Fig. 46, that there
appears to be a shift, in which the values for the minimum potential and the relatively
constant potential period appear to decrease with increasing KOH concentrations. The
decrease in the potential may be due to an increase in the presence of available ions at
higher KOH concentrations, similarly higher potentials are required to pass the same
amount of current through an electrolyte of lower concentration.
0.7
Potential (V)
0.6
0.5
12 M
0.4
7M
0.3
5M
0.2
4M
0.1
2M
0
0
200
400
600
800
1000
Time (s)
Figure 46: Potential versus time plot for InP electrode anodised galvanostatically (5 mA cm -2)
in a variety of KOH concentrations (mol dm-3) at 30 °C.
79
0.45
Voltage at 100s (V)
0.4
0.35
0.3
0.25
10 °C
0.2
20 °C
0.15
30 °C
0.1
40 °C
0.05
0
-0.05 0
5
10
15
KOH Concentration (mol dm-3)
Figure 47: Plot of voltage at 100 s versus KOH Concentration for galvanostatic experiments
(5mA cm-2). Displaying the trend line for 10 °C
The potential at 100 s is seen to decrease, with increasing KOH concentration for
galvanostatic experiments. This is not as clearly linear as that of the potential versus
temperature plot (Fig. 34). There is however a clear linear decrease in potential at 100 s
observed between 2 and 5 mol dm-3 KOH. This clear decrease may be due to the
increase in the activity of the electrolyte at the higher concentration, which increases
the rate of the electrochemical reaction between the electrolyte and electrode. Or as
proposed above it may be due to the increased quantity of ions in electrolytes of higher
concentration.
80
14
Layer Thickness (µm)
12
10
8
10 °C
20 °C
6
30 °C
40 °C
4
2
0
0
2
4
6
8
10
KOH Concentration (mol
12
14
16
dm-3)
Figure 48: Plot of layer thickness versus KOH concentration for InP electrodes anodised at 5
mA cm-2. Displaying the trend line for 10 °C.
Apart from the 12 mol dm-3 data, there is little change in the layer thickness with KOH
concentration. This may be due to constant current, as the flow of ions through the
electrolyte should remain to satisfy this constant current regardless of KOH
concentration.
81
40
Pore Width (nm)
35
30
25
10 °C
20
20 °C
15
30 °C
10
40 °C
5
0
0
5
10
15
KOH Concentration (mol dm-3)
Figure 49: Plot of pore width versus KOH concentration for InP electrodes anodised at 5 mA
cm-2. Displaying the trend line for 10 °C.
Application of linear trend lines to Fig. 49, show the pore width to decrease linearly
with increasing KOH concentration. Considering the three step charge-transfer model,
the decrease in pore width with increasing concentrations, would indicate an increase
in rate of the third step (hole annihilation), relative to the second step (hole diffusion),
with increasing KOH concentrations. This suggests that the rate of electrochemical
reaction increases with increasing KOH concentration, even though the specific
conductivity of KOH has been reported to reach a maximum at approximately 7 mol
dm-3 KOH. [62]
82
40
Pit Diameter (nm)
35
30
25
10 °C
20
20 °C
30 °C
15
40 °C
10
5
0
0
5
10
KOH Concentration (mol
15
dm-3)
Figure 50: Plot of Pit Diameter versus Temperature for InP electrodes anodised at 5 mA cm -2.
Displaying linear trend line for 20 °C.
In Fig. 50, although a large degree of fluctuation of the pit diameter with KOH
concentration is observed, there is an overall trend of decreasing pit diameter with
increasing KOH concentration. The fluctuations are likely due to difficulties in pit
diameter which were highlighted above in Section 4.2.4 and discussed in Section 5.2.
83
0.00005
0.000045
0.00004
Pit Density (pits nm-2)
0.000035
0.00003
10 °C
0.000025
20 °C
0.00002
30 °C
0.000015
40 °C
0.00001
0.000005
0
0
5
10
KOH Concentration (mol
15
dm-3)
Figure 51: Plot of Pit Density versus Temperature for InP electrodes anodised at 5 mA cm-2.
The density of pits on the surface of the electrodes appears to increase at the highest
and lowest concentrations used and decreases towards intermediate concentrations. The
trend resembles a parabolic curve. This plot implies that the pits formed at intermediate
concentrations of KOH can carry more current per pit, than the pits formed at higher
and lower concentrations of KOH. This trend, unlike the pore width trend, appears to
correlate with the maximum specific conductivity values recorded for KOH. [62]
84
4.3 Potentiostatic Experiments
Potentiostatic anodisation of InP in KOH has been employed to a far less extent than
anodisation of InP in KOH under linearly sweeping potentials. Potentiostatic
experiments hold interest, as the reaction rate of the electrochemical reaction during
anodisation is able to change as the different stages of porous layer formation occur.
An additional benefit of potentiostatic experiments is that, the effects of increasing
potential that are associated with LPS’s do not have to be considered, because as
implied the potential remains constant throughout the experiment.
4.3.1 Varying Temperature
The results presented below correspond to experiments carried out potentiostatically at
constant KOH concentration, varying the temperature. As the potential was kept
constant, the change in current with time for the 12 mol dm-3 set of experiments can be
seen in Fig. 52. A similar trend was observed for a current-time plot of an InP electrode
anodised at a constant potential of 0.4 V in 17 mol dm-3 KOH at 10 °C. The gradual
increase in current up to a peak was affirmed to be indicative of the progressive nature
of surface pits. [63] Also observed in Fig. 52, is variation of the peak current with
temperature, with the peak current appearing to increase with temperature. As each
electrode used was not the same size, to obtain a clear relationship between the peak
current and the temperature, the peak current density for each concentration was
plotted against temperature.
85
0.25
Current (x10-5 A)
0.2
0.15
10°C
20°C
0.1
30°C
40°C
0.05
0
0
100
200
300
400
500
Time (s)
-0.05
Figure 52: Current versus time plot for InP electodes anodised at a constant potential of 0.3 V
in 12 mol dm-3 KOH at a variety of temperatures.
Peak Current density (mA nm-2)
8E-13
7E-13
6E-13
13.6 M
5E-13
12 M
4E-13
7M
3E-13
5M
4M
2E-13
2M
1E-13
0
0
10
20
30
40
50
Temperature (°C)
Figure 53: Plot of Peak Current Density versus Temperature for InP electrodes anodised at 0.3
V.
86
In Fig. 53, it can be clearly observed that the peak current density measured has a
generally increasing trend with increasing temperature for each KOH concentration.
The relationship between the peak current density and pit density (i.e. the peak current
ln(Peak Current Density (mA cm-2))
per pit) will be examined later in this section.
-27.5
0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035 0.00355
-28
-28.5
13.6 M
12 M
-29
7M
-29.5
5M
4M
-30
2M
-30.5
-31
1/Temperature (K-1)
Figure 54: Arrhenius plot for InP electrodes anodised at 0.3 V
Arrhenius plots are routinely completed for chemistry experiments with changing
temperatures. The Arrhenius plot is used to find the activation energy of reaction (slope
value) and pre-exponential factor (extrapolating the line back to the y-intercept). Upon
acquiring these values from the plot the rate constant at any temperature can be solved
using the Arrhenius equation. The slopes of the trend lines are given below in Table 2.
Unfortunately, we can’t interpret this analysis of the data in any useful way, because
87
reaction transpiring on the InP surface is irreversible, and the limiting rate occurs for
complex reasons that are not yet apparent.
KOH Concentration (mol dm-3)
Slope of corresponding trend lines
13.6
-4632.5
12
-4080.7
7
-3313.9
5
-4657.8
4
-2684.8
2
-2878.8
Table 2: Slope of trend lines for each concentration plotted on Fig. 54 (Arrhenius plot)
88
4.5
Layer Thickness (µm)
4
3.5
3
13.6 M
2.5
12 M
7M
2
5M
1.5
4M
1
2M
0.5
0
0
10
20
30
40
50
Temperature (°C)
Figure 55: Plot of Layer thickness versus Temperature for InP electrodes anodised at 0.3 V.
Displaying linear trend line for 12 mol dm-3 KOH experiment.
Application of linear trend lines to Fig. 55, exhibits the trend of decreasing layer
thickness with increasing temperature. In these potentiostatic experiments there was
both the presence of crystallography oriented (CO) pores and/or current line oriented
(CLO) pores in different samples. CO pores propagate along <111>A direction in the
electrode whereas CLO pore propagate generally in the direction of current flow. As
just stated not all the pores formed in this section of experiments were CO pores; in 7
mol dm-3 KOH at 10 °C, CLO type pores were formed (Fig. 56). As can be seen in Fig.
56, these pores were contained branching and did not grow completely perpendicular to
substrate, but they did propagate in the direction of current flow, unlike CO pores
which propagate in the <111> A direction. It should also be noted, that other electrodes
showed the tendency to CLO pores deeper in their porous layer. There is a clear
transition from CO pores to CLO pores in the later section of a porous layer formed at
89
10 ° C in 12 mol dm-3 KOH (Fig. 57). While other electrodes have exhibited what
appears to be CO pores initiating the transition to CLO pores at the base of the porous
layer, coupled with undercutting (7 mol dm-3 at 20, 30, 40 °C and 12 mol dm-3 at 20,
30, 40 °C), an example of this can be seen in Fig. 58.
Figure 56: CLO pores observed in InP electrode anodised in 7 mol dm-3 KOH at 10 °C.
90
Figure 57: CO pores transitioning into CLO pores in 12 mol dm-3 KOH at 10 °C.
Figure 58: Attempting transition from CO to CLO pores with the presence undercutting at the
base of the porous layer in 7 mol dm-3 KOH at 20 °C.
91
Charge Density (mC cm-2)
3500
3000
2500
13.6 M
2000
12 M
7M
1500
5M
1000
4M
2M
500
0
0
10
20
30
40
50
Temperature (°C)
Figure 59: Plot of Charge Density versus Temperature for InP electrodes anodised at 0.3 V.
Similar to the galvanostatic section previously, upon visually comparing the charge
density plot (Fig. 59), to the layer thickness plot (Fig. 55), it would appear as if some of
the data points follow a similar trend. To investigate the presence of a trend the same
analysis applied in the galvanostatic section was used here; getting the ratio of the
current density to the layer thickness and plotting these values versus temperature for
each KOH concentration (Fig 60), this plot includes error bars of the standard deviation
of the layer thickness (m-1). This analysis resulted in no apparent proportionality
between charge density and layer thickness. Again, to examine if the pore width was
responsible for the lack of proportionality between the charge density and layer
thickness, the ratio above (Current Density/Layer Thickness) was divided by the
corresponding pore widths (Fig. 61). The lack of a clear relationship is likely due to the
addition of all the errors for each measurement which would make a trend impossible
to see.
92
Charge Density/Layer Thickness
(mC cm-2 m-1)
3.00E+09
2.50E+09
2.00E+09
13.6 M
12 M
1.50E+09
7M
5M
1.00E+09
4M
5.00E+08
2M
0.00E+00
0
10
20
30
40
50
Temperature (°C)
(Charge density/Layer
Thickness)/Pore Width (mC cm-2 m-1)
Figure 60: Plot of the ratio of charge density to layer thickness for each InP electrode versus
the temperature at which anodisation was carried out
1.4E+17
1.2E+17
1E+17
13.6 M
8E+16
12 M
7M
6E+16
5M
4E+16
4M
2M
2E+16
0
0
10
20
30
40
50
Temperature (°C)
Figure 61: Plot of the ratio of (charge density/layer thickness) to pore width for each InP
electrode versus the temperature at which anodisation was carried out
93
60
Pore Width (nm)
50
40
13.6 M
12 M
30
7M
20
4M
2M
10
0
0
10
20
30
40
50
Temperature (°C)
Figure 62: Pore Width versus Temperature for InP electrodes anodised at 0.3 V. Displaying
linear trend line for 4 mol dm-3 KOH experiments.
Examining Fig. 62, with the exception of 7 mol dm-3 KOH at 10 °C, the pore width
remains relatively level with increases in temperature, The expected trend in pore
width with temperature will be discussed later, at the end of this section. The pores
formed in 7 mol dm-3 KOH at 10 °C as expressed already were CLO pores and are
characteristically wider in their nature than CO pores. There appears to be greater
variation in pore width with KOH concentration than with temperature. This variation
in pore width with KOH concentration will be examined later in Section 4.3.2. To
reiterate a point made previously, with pore width results there can be a degree of
uncertainty in pore width measurements, this is due to the ambiguity in pore width
measurement and errors in measuring at the nano-scale.
94
40
Pit Diameter (nm)
35
30
25
13.6 M
12 M
20
7M
15
5M
4M
10
5
0
0
10
20
30
40
50
Temperature (°C)
Figure 63: Plot of Pit Diameter versus Temperature for InP electrodes anodised at 0.3 V.
Displaying linear trend line for 13 mol dm-3 KOH experiments.
The diameter of pits on the surface of the electrode appears to follow a linear
decreasing trend with increasing temperature. There are no values included for 2 mol
dm-3 KOH as there was no uniformity in the pits on the surface of all the electrodes at
this concentration. As highlighted earlier, it can be difficult to measure the diameter of
pits on the surface of an electrode accurately, due to the presence of the near surface
layer and its relationship to the depletion layer. This will be discussed in more detail in
section 5.2.
95
0.000035
Pit Density (pits nm-2)
0.00003
0.000025
13.6 M
0.00002
12 M
7M
0.000015
5M
0.00001
4M
0.000005
0
0
10
20
30
40
50
Temperature (°C)
Figure 64: Plot of Pit Density versus Temperature for InP electrodes anodised at 0.3 V.
Peak Current per pit (A pit-1)
1.2E-10
1E-10
8E-11
13.6 M
6E-11
12 M
7M
4E-11
5M
4M
2E-11
0
0
10
20
30
40
50
Temperature (°C)
Figure 65: Plot of Peak Current per Pit versus Temperature for InP electrodes anodised at 0.3
V.
96
Looking at the relationship between the density of pits on the surface of the electrode
and temperature (Fig. 64), there appears to be no obvious trend or relationship between
them. There are no values included for 2 mol dm-3 KOH, as there were no uniform pits
on the surface of all the electrodes, at this concentration. The pit density for 4 and 12
mol dm-3 KOH appeared to be decreasing with temperature, while for 5 and 13.6 mol
dm-3 KOH, it appeared to be increasing with temperature. However upon calculating
the peak current carried per pit, a clearer trend was revealed (Fig. 65). Here it can be
noted that the peak current per pit tends to increase with temperature. Looking at
previous work, it would be expected that, the increased current flow, at higher
temperatures, would result in thinner pores, due to the increased rate of the
electrochemical reaction, that is expected at higher temperatures. [21] In turn, the
results from the previous work also saw a decrease in layer thickness, which was
correlated with a decrease in pore width. This correlation was used to suggest that mass
transport (in this case limited by the thickness of the pores), may be the main factor
limiting porous layer thickness. That work only looked at varying temperature for one
KOH concentration (9 mol dm-3), in contrast, the results being presented here examined
the effects of varying temperature on porous layer formation in InP, in a number of
different concentrations of KOH. The correlation between pore width and layer
thickness was not observed in the results presented earlier. The results suggest, that
varying temperature and KOH concentration have a greater effect on the thickness of
the porous layer formed, than the spatial limitation of pore width and pit density,
presumed to effect mass transport during anodisation, which in turn was assumed to
limit the thickness of the porous layer formed.
97
4.3.2 Varying Concentration
It should be noted that experiments were carried out at constant KOH concentration,
varying the temperature. The results plotted below are the same values as presented in
the varying temperature section above. The plots below were created to inspect if there
was any obvious relationships to varying KOH concentration. Current time plots for
the different KOH concentrations (see appendices) showed a similar trend and
expected trend, with a gradual increase in current up to a peak, which has been
affirmed to be indicative of the progressive nature of surface pits. [63] As each
electrode used was not the same size, in order to obtain a clear relationship between the
peak current and the temperature, it was essential that the peak current density for each
temperature was plotted against KOH concentration. It was then possible to examine if
Peak Current density (mA cm-2)
there was a variation of the peak current with KOH concentration.
8E-13
7E-13
6E-13
5E-13
10 °C
4E-13
20 °C
3E-13
30 °C
2E-13
40 °C
1E-13
0
0
5
10
15
KOH Concentration (mol dm-3)
Figure 66: Plot of Peak Current Density versus KOH Concentration for InP electrodes anodised
at 0.3 V.
98
According to Fig. 66 for potentiostatic experiments, there is an increasing trend in the
peak current density with initially with increasing KOH concentrations up to 5 mol dm3
, after which the peak current density appears to plateau with increasing KOH
concentrations. This peak current per pit will be examined later in this section.
4.5
4
Layer Thickness (µm)
3.5
3
2.5
10 °C
2
20 °C
30 °C
1.5
40 °C
1
0.5
0
0
5
10
KOH Concentration (mol
15
dm-3)
Figure 67: Plot of Layer Thickness versus KOH Concentration for InP electrodes anodised at
0.3 V.
Examining Fig. 67, it is noted that the layer thickness appears to increase at the highest
and lowest concentrations and decreases towards intermediate concentrations. Despite
fluctuations in the data, the trend resembles a parabolic curve. Again unlike
observation in previous work [21] the variation in layer thickness does not correlate to
the variation seen in pore width with varying KOH concentration.
99
60
Pore Width (nm)
50
40
10 °C
30
20 °C
30 °C
20
40 °C
10
0
0
5
10
15
KOH Concentration (mol dm-3)
Figure 68: Plot of Pore Width versus KOH Concentration for InP electrodes anodised at 0.3 V.
The pore width appears to decrease with increasing KOH concentration up to 5 mol
dm-3 and then plateau with increasing concentration after that. The ambiguity and
uncertainty in the measurement of the pore width was highlighted earlier in Section
4.2.4 and is discussed later in Section 5.2.
100
40
35
Pit Diameter (nm)
30
25
10 °C
20
20 °C
30 °C
15
40 °C
10
5
0
0
5
10
KOH Concentration (mol
15
dm-3)
Figure 69: Plot of Pit Diameter versus KOH Concentration for InP electrodes anodised at 0.3 V.
Examining Fig 69, the diameter of pits on the surface of the electrode is seen to
generally decrease with increasing KOH concentrations. There are no values included
for 2 mol dm-3 KOH, as there were not uniform pits on the surface of all the electrodes
at this concentration. The problems with getting a precise measurement for pit diameter
have been highlighted earlier and will be examined further in Section 5.2.
101
0.000035
0.00003
Pit Density (pits nm-2)
0.000025
10 °C
0.00002
20 °C
0.000015
30 °C
40 °C
0.00001
0.000005
0
0
5
10
15
KOH Concentration (mol dm-3)
Peak Current per pit (A pit-1)
1.2E-10
1E-10
8E-11
10 °C
6E-11
20 °C
30 °C
4E-11
40 °C
2E-11
0
0
2
4
6
8
10
KOH Concentration (mol
12
14
16
dm-3)
Figure 70: Plot of Pit Density versus KOH Concentration for InP electrodes anodised at 0.3 V.
Figure 71: Plot of Peak Current per Pit versus KOH Concentration for InP electrodes anodised
at 0.3 V.
102
The pit density for all temperatures tends to generally increase with KOH concentration
(Fig. 70). The peak current carried per pit was calculated for each temperature and
plotted in Fig. 71. Despite some fluctuations in data, the trend observed in this plot
resembles an inverted parabolic curve. The highest peak current per pit at all
temperature is observed at 5 mol dm-3. This suggests that the ability of pits to carry
current increases for intermediate concentrations of KOH and decreases at higher and
lower concentrations.
103
Chapter 5: Discussion
104
5.1 Linear Sweep Voltammogram
A single current peak that is associated with lower carrier concentrations [41] [44], is
observed in in Fig. 17. A significant anodic process is noted to occur at ~0.26 V in Fig.
17, this potential corresponds to pore formation potential at which pits on the surface
may be observed [16]. At this potential, pitting and pore growth from the pits begin to
grow from the surface. The current density after 0.26 V increases exponentially as the
voltage increases, this occurs as the domains grow, the number of growing pore tips
increases and consequently the current required to facilitate the process increases. [49]
Lynch, et al. [48], observed a trend similar to Fig. 17, in an LSP of an InP electrode of
like carrier concentration (n
3.4 x 1018 cm-3), anodised in 5 mol dm-3 at room
temperature. The current density reaches a peak, which corresponds to the merging of
the porous domains; the current density reaches its max, as there is no more space for
the pores to continue to grow in a lateral direction, or for more pits to form since the
porous domains have grown into each other. The current density is then observed to
slightly plateau (the number of active pore tips is more-or-less constant once a
continuous layer is formed [70]), we can observe here the pores are growing
downwards and the layer thickness is increasing. At 0.44V the current has fallen off
and is no longer varying, at this point the pores have ceased propagation and the layer
growth is completed.
105
5.2 Ambiguity in Pore Width and Pit Diameter measurements
Very few definite trends can be extracted from the pore width and pit diameter plots in
the results section. Difficulties in measuring the pore width arise as although pore
width is relatively uniform through the porous layer, branching and the presence of
other nearby pores results in coarseness of the pore structure. Even though similar
points on each pore were selected for each measurement, the unevenness of the pores
along with errors in human observation may have resulted in slight inaccuracies of the
measurements taken. For the galvanostatic experiments, there was no change greater
than 10 and 8 nm respectively for the varying temperature and varying concentration
plots. With the standard deviation for the data points contained in these graphs ranging
from 2.97 to 7.39 (Appendices: Tables 4-9), a definite trend cannot be acquired from
these plots. For the potentiostatic experiments it is obvious that CLO pores have a
greater pore width than the CO pores formed. Other than that the only distinct trend is
that the pore width increases in 2 mol dm-3 KOH. For all the other pore widths, again
none of the data points fall outside 10 nm of each other and the standard deviation of
the ranges from 3.28 to 6.77 (Appendices: Tables 10-15) excluding the standard
deviation of the CLO pores which was 8.63.
Complications in measuring pit diameter may be caused by the changing
thickness of the dense near-surface layer in the different experiments. To explain the
difficulty in measuring pit diameter, lets first look at how the pits channel through the
near surface layer. Etching at the surface pit requires a supply of holes; these holes are
generated by a tunnelling mechanism through the concomitant depletion layer at the
surface. The increase in availability of carriers with decreasing angle to the surface
normal, causes etching at the surface pit to be initially constrained to be normal to the
surface resulting in a very narrow channel. [59] This relates to the uncertainty in pit
106
diameter measurements, as the diameter observed may not be solely the pit, but may be
influenced by the diameter of the narrow channel. The thickness of the near-surface
layer is known to scale with thickness of the depletion layer. [41] Subsequently
changes in the depletion layer will affect the narrow channels formed through the nearsurface layer and therefore add further ambiguity to pit density measurements.
5.3 Standardising Pit Density plus Mass Transport Effect
Constant current allows for the control of pore propagation rate (etch rate) and for the
passage of a prescribed amount of charge through the electrode, [26,65] i.e. the current
density controls the rate of reaction for etching at pits and in pores. The current burst
and current sweep experiments were devised to further examine the concept that mass
transport, which may be limited by the number of pits present on the surface of an
electrode, limits the thickness of a porous layer formed. As highlighted in the results
section 4.2.2 for the 20 mA cm-2 all pores may not have continued to grow after the rest
period, due to irreversible changes at the pore tip, consequently the results obtained did
not fairly represent the limitations of pit density and mass transport on the thickness of
the porous layer formed. This in turn cannot be fairly compared to the 5 mA cm-2 burst
experiment. In Fig 28, it can be seen that the pit density of electrodes exposed to 5 mA
cm-2 burst is approximately 3.7 times smaller than that of the electrodes exposed to 20
mA cm-2 burst. The lack of change in the layer thickness, with current density for the 5
mA cm-2 burst, may be due to the fact that reduced number of pits posed limitations on
mass transport in the electrodes at all current densities.
As noted previously in section 4.2.3 the current sweep experiment was devised
so that all the pores would continue to grow after the initial exposure to 20 mA cm-2 for
107
5 s, to overcome the problem encountered in the burst experiment. After exposure to
initial current density, the current density was swept down to the current densities
specified on Fig. 32, at a rate 2 s per 1 mA cm-2 decrease. Unfortunately, it was realised
that the results from this experiment could not effectively examine the thickness of the
layer formed at different current densities. This was because sweeping the current
density down at rate of 2 s per 1 mA cm-2 decrease, meant that each electrode was
exposed to varying current densities over varying periods of time. Thus, the layer
thickness obtained was not directly comparable to the current densities desired to be
examined.
Work done by Lynch, et al. [64], indicated that the precipitation of etch
products in pores, which creates a knock on effect of mass transport difficulties in the
porous system, may cause the cessation of porous layer growth. This influence of mass
transport of porous layer growth was then demonstrated by Quill, et al. [65], in an
experiment at low current density, where the current was switched on and off a number
of times, before the layer growth ceased. A layer thickness of 6.9 µm was achieved,
which was more than twice as deep as any CO porous layers previously etched in InP
in KOH. The thicker porous layer formed by an applied current density of 1 mA cm-2
(Fig. 21, Fig 22 – red data point), suggests that mass transport has an effect on the
thickness of the porous layer formed. The lower current density means a slower etch
rate and therefore more time for the reaction products to diffuse out of the porous
system. Despite the large standard deviation in measurement at this current density and
the limited number of pores, the data hints that mass transport plays a role in the
cessation of pore growth in InP in KOH.
Although the experiments done in this current thesis study did not directly
examine the effect of mass transport on the thickness of porous layer formed, this
108
work, plus previous work by other people in this area, appears to indicate that mass
transport does have some effect, although the magnitude of this effect is not yet
apparent. To improve examining the effect of mass transport on the thickness of the
porous layers formed, experiments could be carried out where the current is
immediately switch from the initial burst value to the value corresponding to the
current density to be examined. In this case all the pores should continue to propagate
and any unknown processes occurring during the rest period would be eliminated.
5.4 Effect of Mass Transport versus Temperature and Concentration
for Galvanostatic Experiments
According to the discussion of results above in section 5.3, it would appear and was
suggested that mass transport, which is assumed to be limited by the density of pits on
the surface, has an effect on the thickness of the porous layer etched in InP in KOH.
The results for the varying temperature and concentration of KOH electrolyte, appear
to contradict this theory.
The thickest layer formed in the galvanostatic experiments was at 40 °C in 12
mol dm-3 KOH. The porous layer had a thickness of ~12.48 µm, which was more than
twice the thickness of the next thickest layer (~5.71 µm formed at 30 °C in12 mol dm3
), formed in this set of galvanostatic experiments. The layer thickness formed at 40 °C
in 12 mol dm-3 KOH is almost twice the thickness (~1.81 times) of the thickest porous
layer formed by Quill, et al. [65], by periodically switching on and off the current at a
low current density until the cessation of layer growth.
109
The reason why the thickness of the layer grown at 40 °C in 12 mol dm-3 KOH
appears to contradict the mass transport theory is because, the density of pits created on
the surface in these circumstances was ~ 6.017 x10-6 pits nm-2. This pit density is
approximately 7.5 times less that the greatest pit density recorded in the galvanostatic
experiments at 10 °C in 4 mol dm-3 KOH (~ 4.509 x10-5 pits nm-2). The thickness of
the layer formed at 10 °C in 4 mol dm-3 KOH was 3.157 µm, which adds to the
contradiction that mass transport has an effect on layer thickness in InP in KOH.
The reasons for the effects exhibited by temperature and KOH concentration on
layer thickness are not clearly understood. The fact that the etching surface for porous
InP resides deep within the samples and this surface is essentially inaccessible, means
that the chemistry of the porous InP etching process is poorly understood. [20] What is
known though is that at higher temperatures the rate of the electrochemical reaction is
expected to be higher. [21] The rate of diffusion is also expected to increase with
temperature. Values reported for the specific conductivity of KOH, show that its
specific conductivity reaches a maximum at approximately 7 mol dm-3 and decreases
with increasing concentration thereafter. [62]
The results from varying temperature and concentration for the galvanostatic
experiment would suggest that, the temperature of the system and the electrolyte
concentration have a greater effect on the thickness of the porous layer formed than
that of mass transport. This does not mean that mass transport has no effect on the
thickness of the porous layer formed, as previous results would suggest that it does.
Rather it demonstrates that the effect of mass transport is less influential than
temperature and electrolyte concentration. To further examine the relationship of mass
transport, temperature, and electrolyte concentration on the thickness of porous layers
110
formed, the varying temperature and concentration experiments should be repeated for
a variety of different current densities, as in this study.
5.5 Effect of Temperature and KOH Concentration on the Potential
applied to and the Current passing through Pits and Pores
Examining Fig. 65, it may be deduced that, at a potential of 0.3 V, the peak current
carried per pit increases with temperature. In other words increasing the temperature
increases the current carrying capacity of the pits and in turn it is assumed that there
would be a similar effect for the pore tips. With regards to concentration (Fig. 71), the
max peak current carried per pit for each temperature was at 5 mol dm-3 KOH.
Increasing or decreasing the concentration around this value reduced the peak current
carried per pit. According to Fig. 34, the potential required to pass a current
corresponding 5 mA cm-2 decreases, with increasing temperature. And examining Fig.
47, it can be seen that the potential required to pass a current corresponding 5 mA cm-2
decreases, with increasing KOH concentration. The steepest decrease in potential was
observed between 2 and 5 mol dm-3 KOH.
Evaluating both Fig. 65 and Fig. 71, it can be reasoned that the ease at which
anodisation can occur increases with temperature, which is in line with previous work
which indicates that the rate of the electrochemical reaction is expected to be higher at
higher temperatures. [21] The effect of varying concentration is not as clear. The
chemical composition
of the electrolyte may be effectively different, at
electrode/electrolyte interface at the surface and within the porous structure, as
chemistry of the porous InP etching process is not well known. This along with the fact
that, different batches of electrodes were used for different KOH concentrations,
(experiments were carried out at constant KOH concentration, varying the
111
temperature), means that no precise observation can be made for the effect of KOH
concentration on the electrochemical reaction occurring during etching.
5.6 Other Trends in Temperature/Concentration Galvanostatic
Experiments
There is a clear decreasing trend in pit density with increases in temperature, with the
biggest decreases in this trend seen between 10 and 30 °C generally (Fig 45). This
trend agrees with the idea above which suggests that, as the temperature of system
increases, the amount of current capable of being carried per pit also increases. This is
seen here as fewer pits are required at higher temperatures to carry the same amount of
current (5 mA cm-2). The density of pits on the surface of the electrodes appears to
increase at the highest and lowest concentrations of KOH used and decreases towards
intermediate concentrations.
The uncertainty in pore width and pit diameter measurements were outlined
previously in section 5.2, with that said, the following trends were observed: the pore
width was observed to slightly increase with increasing temperature and decrease with
increasing KOH concentration. These changes in pore width were discussed with
consideration of the three step charge-transfer model. There was no obvious
relationship between pit diameter and temperature, whereas with increasing KOH
concentration the pit diameter generally appeared to decrease.
It is worth noting that in galvanostatic experiments the anodisation of InP is
being forced to occur at a particular rate, as the current density controls the rate of
reaction for etching. [26,65] This is unlike potentiostatic experiments where the current
flowing can vary according with the different stages of layer formation. With constant
112
current, the pores are being forced to etch at the same rate regardless, which may make
it difficult to clearly identify the processes (i.e. compare to the three step chargetransfer model),, occurring during anodisation.
5.7 CLO Pores Present in Potentiostatic Experiments.
In this study a complete CLO porous layer was observed in InP anodised
potentiostatically (0.3 V) at 10 °C in 7 mol dm-3. The presence of CLO pores were
observed by Quill, et al [63], to occur in LPS’s between 0.6 and 1.0 V at 10 °C in high
and low concentrations of KOH (17and 2.5 mol dm-3). The formation of CLO in this
case was explained by a qualitative model, based on the three-step mechanism of
charge transfer between semiconductor and electrolyte which was outlined in sections
2.6.3 and 2.6.6. In this current work, unlike the work done previously by others, the
CLO porous layer was observed at a low potential, in an intermediate KOH
concentration.
Considering the three step charge-transfer model; while the overall etch rate of
pores is determined by the rate of hole generation at the pore tip, the principle factor
determining the average diffusion distance of holes is, the competition in kinetics
between hole diffusion (step 2) and electrochemical reaction (step 3). It is highlighted
that if the kinetics of step 3 are fast relative to step 2, holes are annihilated in the
oxidation reaction close to where they are generated (i.e. short diffusion distance of
holes) In this situation, etching transpires close to the site of hole generation rather than
at favoured crystallographic sites, so there is no preferred crystal direction for pore
propagation. [48] In the environment of 7 mol dm-3 KOH at 10 °C, it could be proposed
that the kinetics of step 3 are fast relative to step 2. This proposition is justified as KOH
reaches its maximum specific conductivity at approximately 7 mol dm-3 [62], therefore
113
the ability to annihilate holes would be increased. The charge transfer model however
requires an abundance of holes at the interface which is not necessarily characteristic of
0.3 V. Also, short diffusion distances means that pore widths should be smaller, which
is opposite of what occurs for CLO.
5.8 Other trends in Temperature/ Concentration Potentiostatic
Experiments
The thickness of the porous layer formed potentiostatically was observed to decrease
with increasing temperature. While with varying concentration the layer thickness was
observed to be generally at minimum value at 5 mol dm-3 KOH and increase in a
parabolic fashion at concentrations higher and lower than 5 mol dm-3 KOH. A similar
trend was observed by Quill, et al. [21], where the layer thickness showed a minimum
at 9 mol dm-3 and increased as the concentration was either increased or decreased
from this value. Like the galvanostatic experiments, the thickness of the porous layer
formed varied more with temperature and KOH concentration rather being affected by
the density of pits on the electrodes surface. There is also a lack of correlation between
the width of pores and the thickness of the porous layer formed. Therefore it would
appear that mass transport does not limit porous layer growth as much as the
consequences of varying the temperature of the system or KOH concentration.
The width of pores showed no clear variation with temperature and/or KOH
concentration apart from the decrease in pore width between 2 and 4 mol dm-3 KOH
and the distinct increase of pore width for CLO pores. This is unlike results from a
previous study, where the pore width was seen to clearly decrease with increasing
114
temperature, and to show a minimum at 9 mol dm-3 KOH and increase as the
concentration was either increased or decreased from that value. [21] Due to
uncertainties in the pore width measurement it is not feasible to evaluate the use of the
three-step charge transfer model to justify variations in width of pores formed in KOH.
115
Conclusions and Future Work
116
Main Conclusions of Thesis
Mass transport was found not to limit the thickness of the porous layer formed in InP in
KOH to the extent that was previously assumed. A porous layer almost twice the
thickness (12.48 µm) of any porous layer previously formed was observed in InP
galvanostatically anodised at 40 °C in 12 mol dm-3 KOH at 5 mA cm-2. The density of
pits on the surface of this electrode was approximately 7.5 times less than that of the
electrode in which the thickness of the porous layer formed was 3.157 µm. The lack of
a proportional relationship between the thickness of the porous layer formed and the
density of pits on the surface of the electrode was also observed for potentiostatic
experiments. The variation in layer thickness is correlated to be mainly effected by
temperature and KOH concentration.
The ability of pits and pore tips to carry increased quantities of current is
proposed to increase with temperature. The peak current carried per pit for
potentiostatic experiments was observed to increase with temperature and the potential
required to pass a current corresponding 5 mA cm-2 was noticed to decrease with
increasing temperature. This corresponds to previous work that indicates that rate of
electrochemical reactions increases with temperature.
Current density appears to have the biggest effect of anything on the width of
the pores formed. Evaluation of the three step charge-transfer model was unsuccessful
in this study. Uncertainties in pore width measurements meant that no clear analysis
could be carried out on this model although the appearance of CLO pores was
rationalised by this model.
117
Future Work
If mass transport was to be examined further via standardisation of pit density;
improvements on the current density burst experiment should be made as follows: the
experiment should be carried out with the selected current density burst to be followed
immediately by the current density to be examined (no rest period).
The effect of varying temperature and KOH concentration should be examined
galvanostatically at a variety of current densities. This would lead to a greater
understanding between the relationship of mass transport, temperature and KOH
concentration on the formation of porous layers in InP.
Efforts should be made to increase the understanding of the chemistry of the
porous InP etching process. Attempts could be made to model the interactions of
molecules, ions and reaction products within the pores to increase the understanding of
how the anodisation of InP can be affected.
118
References
119
[1]
Moore, G. E., Electronics, 38, (1965).
[2]
Neaman, D. A., Semiconductor Physics And Devices, 2012).
[3]
Canham, L. T., Appl. Phys. Lett., 57, 1046 (1990).
[4]
Erne, B. H., Vanmaekelbergh, D., and Kelly, J. J., J. Electrochem. Soc., 143,
305–314 (1996).
[5]
Wloka, J., Mueller, K., and Schmuki, P., Electrochem. Solid-State Lett., 8, B72
(2005).
[6]
Schmuki, P., Lockwood, D. ., Labb , . ., and raser, .
69, 1620 (1996).
[7]
Hamamatsu, A., Kaneshiro, C., Fujikura, H., and Hasegawa, H., J. Electroanal.
Chem., 473, 223–229 (1999).
[8]
Liu, X., Denker, M. S., and Irene, E. A., J. Electrochem. Soc., 139, 799–802
(1992).
[9]
Takizawa, T., Arai, S., and Nakahara, M., Japanese J. Appied Phys., 33, L643–
L645 (1994).
., Appl. Phys. Lett.,
[10] Schmuki, P., Santinacci, L., Djenizian, T., and Lockwood, D. J., Phys. Status
Solidi, 182, 51–61 (2000).
[11] Weng, Z., Liu, A., Sang, Y., Zhang, J., Hu, Z., Liu, Y., et al., J. Porous Mater.,
16, 707–713 (2009).
[12] Weng, Z., Zhang, W., Wu, C., Cai, H., Li, C., Wang, Z., et al., Appl. Surf. Sci.,
256, 2052–2055 (2010).
[13] Gon alves, A.-M., Santinacci, L., Eb, A., Gerard, I., Mathieu, C., and
Etcheberry, A., Electrochem. Solid-State Lett., 10, D35–D37 (2007).
[14] Oskam, G., Natarajan, A., Searson, P. C., and Ross, F. M., Appl. Surf. Sci., 119,
160–168 (1997).
[15] Faktor, M. M., Fiddyment, D. G., and Taylor, M. R., J. Electrochem. Soc., 122,
1566–1567 (1975).
[16] Schmuki, P., Fraser, J., Vitus, C. M., Graham, M. J., and Isaacs, H. S., J.
Electrochem. Soc., 143, 3316–3322 (1996).
[17] Tiginyanu, I. M., rsaki, V. V., Monaico, E., oca, E., and
Electrochem. Solid-State Lett., 10, D127–D129 (2007).
ll,
.,
[18] Gonçalves, A.-M., Santinacci, L., Eb, A., David, C., Mathieu, C., Herlem, M., et
al., Phys. Status Solidi, 204, 1286–1291 (2007).
120
[19] Föll, H., Carstensen, J., Langa, S., Christophersen, M., and Tiginyanu, I. M.,
Phys. Status Solidi, 197, 61–70 (2003).
[20] Quill, N., ormation of Nanoporous InP in KO and KCl : Electrochemistry and
Electron Microscopy, (2012).
[21] Quill, N., Lynch, R. P., O’Dwyer, C., and Buckley, D. N., ECS Trans., 50, 131–
141 (2013).
[22] Tiginyanu, I. M., Langa, S., Sirbu, L., Monaico, E., Stevens-Kalceff, M. A., and
Föll, H., Eur. Phys. J. Appl. Phys., 27, 81–84 (2004).
[23] Hidalgo, P., Piqueras, J., Sirbu, L., and Tiginyanu, I. M., Semicond. Sci.
Technol., 20, 1179–1182 (2005).
[24] Liu, A., and Duan, C., Solid. State. Electron., 45, 2089–2092 (2001).
[25] Langa, S., Christophersen, M., Carstensen, J., Tiginyanu, I. M., and Föll, H.,
Phys. Status Solidi, 197, 77–82 (2003).
[26] Tsuchiya, H., Hueppe, M., Djenizian, T., Schmuki, P., and Fujimoto, S., Sci.
Technol. Adv. Mater., 5, 119–123 (2004).
[27] Tsuchiya, H., Hueppe, M., Djenizian, T., and Schmuki, P., Surf. Sci., 547, 268–
274 (2003).
[28] Langa, S., rey, S., Carstensen, ., ll, ., Tiginyanu, I. M.,
al., Electrochem. Solid-State Lett., 8, C30 (2005).
ermann, M., et
[29] Langa, S., Lölkes, S., Carstensen, J., Hermann, M., Böttger, G., Tiginyanu, I.
M., et al., Phys. Status Solidi, 2, 3253–3257 (2005).
[30] Uhlir, A., Bell Syst. Tech. J., 35, 333–347 (1956).
[31] Belogorokhov, A. J., Karavanskií, V. A., Obraztsov, A. N., and Timoshenko, V.
Y., Lett. to J. Exp. Theor. Phys., 60, 274–279 (1994).
[32] Ulin, V. P., and Konnikov, S. G., Semiconductors, 41, 832–844 (2007).
[33] Föll, H., Langa, S., Carstensen, J., Christophersen, M., and Tiginyanu, I. M.,
Adv. Mater., 15, 183–198 (2003).
[34] Kikuno, E., Amiotti, M., Takizawa, T., and Arai, S., Japanese J. Appied Phys.,
34, 177–178 (1995).
[35] Franz, G., J. Appl. Phys., 63, 500 (1988).
[36] Robach, Y., Joseph, J., Bergignat, E., and Hollinger, G., J. Electrochem. Soc.,
136, 2957–2959 (1989).
121
[37] Besland, M. P., Robach, Y., and Joseph, J., J. Electrochem. Soc., 140, 104–108
(1993).
[38] Van de Ven, J., Binsma, J. J. M., and de Wild, N. M. A., J. Appl. Phys., 67,
7568 (1990).
[39] Yamamoto, A., Yamaguchi, M., and Uemura, C., J. Electrochem. Soc., 129,
2795–2801 (1982).
[40] O’Dwyer, C., Ph.D Thesis, niversity of Limerick, (2003).
[41] O’Dwyer, C., Buckley, D. N., Sutton, D., and Newcomb, S. B., . Electrochem.
Soc., 153, G1039 (2006).
[42] Schmuki, P., Fraser, J., Vitus, C. M., Graham, M. J., and Isaacs, H. S., J.
Electrochem. Soc., 143, 3316–3322 (1996).
[43] Schmuki, P., Erickson, L. E., Lockwood, D. ., raser, .
Labb , . ., Appl. Phys. Lett., 72, 1039 (1998).
., Champion, G., and
[44] Quill, N., O’Dwyer, C., Lynch, R. P., and Buckley, D. N., ECS Trans., 19, 295–
304 (2009).
[45] Vermeir, I. E., Goossens, H. H., Vanden Kerchove, F., and Gomes, W. B., J.
Electrochem. Soc., 139, 1389–1396 (1992).
[46] Yamamoto, A., and Yano, S., J. Electrochem. Soc., 122, 260–267 (1975).
[47] Theunissen, M. J. J., J. Electrochem. Soc., 119, 351–360 (1972).
[48] Lynch, R. P., Quill, N., O’Dwyer, C., Nakahara, S., and Buckley, D. N., Phys.
Chem. Chem. Phys., 15, 15135–45 (2013).
[49] O’Dwyer, C., Buckley, D. N., Sutton, D., Serantoni, M., and Newcomb, S. B., J.
Electrochem. Soc., 154, H78 (2007).
[50] Gatos, H. C., and Lavine, M. C., J. Electrochem. Soc., 107, 427–433 (1960).
[51] Gatos, H. C., J. Appl. Phys., 32, 1232 (1961).
[52] Notten, P. H. L., J. Electrochem. Soc., 138, 243–249 (1991).
[53] Vermeir, I. E., J. Electrochem. Soc., 142, 3226 (1995).
[54] Soltz, D., and Cescato, L., J. Electrochem. Soc., 143, 2815–2821 (1996).
[55] Chu, S. N. G., Jodlauk, C. M., and Johnston, W. D., J. Electrochem. Soc., 130,
2398–2405 (1983).
122
[56] Lynch, R. P., O’Dwyer, C., Sutton, D., Newcomb, S. B., and Buckley, D. N.,
ECS Trans., 6, 355–366 (2007).
[57] Lynch, R., O’Dwyer, C., Quill, N., Nakahara, S., Newcomb, S. B., and Buckley,
D. N., ECS Trans., 16, 393–404 (2008).
[58] Zhang, X. G., J. Electrochem. Soc., 151, C69 (2004).
[59] Lynch, R. P., Quill, N., Dwyer, C. O., Nakahara, S., and Buckley, D. N., ECS
Trans., 50, 319–334 (2012).
[60] MacFadyen, D. N., J. Electrochem. Soc., 130, 1934–1941 (1983).
[61] Buckley, D. N., O’Dwyer, C., Lynch, R. P., and Quill, N., ECS Trans., 35, 29–
48 (2011).
[62] Gilliam, R. J., Graydon, J. W., Kirk, D. W., and Thorpe, S. J., Int. J. Hydrogen
Energy, 32, 359–364 (2007).
[63] Quill, N., Lynch, R. P., O’Dwyer, C., and Buckley, D. N., ECS Trans., 50, 143–
153 (2013).
[64] Lynch, R. P., Quill, N., O’Dwyer, C., Dornhege, M., Rotermund,
Buckley, D. N., ECS Trans., 53, 65–79 (2013).
.
., and
[65] Quill, N., Lynch, R. P., O’Dwyer, C., and Buckley, D. N., ECS Trans., 58, 25–
38 (2013).
[66] Longère, J. Y., Schohe, K., Krawczyk, S. K., Schütz, R., and Hartnagel, H. L.,
Appl. Surf. Sci., 39, 151–160 (1989).
[67] Krawczyk, S. K., and Hollinger, G., Appl. Phys. Lett., 45, 870 (1984).
[68] Wilmsen, C. W., Physics and chemistry of III-V compound semiconductor
interfaces, (Plenum Press, 1985).
[69] Jenkins, T. E., Semiconductor science: growth and characterization techniques,
(Prentice Hall, 1995).
[70] Lynch, R. P., Quill, N., O’Dwyer, C., and Buckley, D. N., ECS Trans., 50, 191–
203 (2013).
[71] Beale, M. I. J., Benjamin, J. D., Uren, M. J., Chew, N. G., and Cullis, A. G., J.
Cryst. Growth, 73, 622–636 (1985).
[72] Zhang, X. G., J. Electrochem. Soc., 138, 3750 (1991).
123
Appendices
124
Galvanostatic experiments data
Charge density/Layer
Thickness (mC cm-2 m-1)
1.4E+09
1.2E+09
1E+09
800000000
10 °C
600000000
20 °C
400000000
30 °C
40 °C
200000000
0
0
2
4
6
8
10
12
14
16
KOH Concentration (mol dm-3)
(Charge density/Layer
Thickness)/Pore Width (mC cm-2
m-1)
Figure 72: Plot of the ratio of charge density to layer thickness for each InP electrode versus the KOH concentration
at which anodisation was carried out
3.5E+16
3E+16
2.5E+16
2E+16
10 °C
20 °C
1.5E+16
30 °C
1E+16
40 °C
5E+15
0
0
5
10
15
KOH Concentration (mol dm-3)
Figure 73: Plot of the ratio of (charge density/layer thickness) to pore width for each InP electrode versus the KOH
concentration at which anodisation was carried out
125
Charge density/Layer Thickness
(mC cm-2 m-1)
4 mol dm-3
800000000
700000000
600000000
500000000
400000000
300000000
200000000
100000000
0
28
29
30
31
32
33
34
35
Pore width (nm)
Figure 74: Plot of the ratio of charge density to layer thickness versus the pore width for all electrodes
galvanostatically anodised in 4 mol dm-3 KOH
126
(Charge density/Layer Thickness)/Pore
Width (mC cm-2 m-1)
Potentiostatic experiments data
1.4E+17
1.2E+17
1E+17
8E+16
10 °C
20 °C
6E+16
30 °C
40 °C
4E+16
2E+16
0
0
2
4
6
8
10
12
14
16
KOH Concentration (mol dm-3)
Charge Density/ Layer
Thickness(mC cm-2 m-1)
Figure 75: Plot of the ratio of (charge density/layer thickness) to pore width for each InP electrode versus the KOH
concentration at which anodisation was carried out
10 °C
1.6E+09
1.4E+09
1.2E+09
1E+09
800000000
600000000
400000000
200000000
0
0
1E-08
2E-08
3E-08
4E-08
Pore Width (m)
127
5E-08
6E-08
Figure 76: Plot of the ratio of charge density to layer thickness versus the pore width for all electrodes
potentiostatically anodised at 10°C
Data recorded from Data Acquisition Unit
1
Potential (V)
0.5
0
0
100
200
300
400
500
10°C
20°C
30°C
-0.5
40°C
-1
-1.5
Time (s)
Figure 77: Potential versus time plot for 13.6 mol dm-3 KOH and 5 mA cm-2 current density
0.25
Current (x10-5 A)
0.2
0.15
10°C
0.1
20°C
40°C
0.05
0
0
-0.05
100
200
300
Time (s)
128
400
500
Figure 78:Current versus time plot for 13.6 mol dm-3 KOH and 0.3 V potential
Data for 30 °C was lost
0.6
0.4
Potential (V)
0.2
0
0
200
400
600
800
1000
-0.2
10°C
20°C
30°C
-0.4
40°C
-0.6
-0.8
-1
Time (s)
Figure 79: Potential versus time plot for 7 mol dm-3 KOH and 5 mA cm-2 current density
1.2
Current (x10-5 A)
1
0.8
10°C
0.6
20°C
30°C
0.4
40°C
0.2
0
0
-0.2
100
200
300
400
Time (s)
Figure 80: Current versus time plot for 7 mol dm-3 KOH and 0.3 V potential
129
500
0.6
0.4
Potential (V)
0.2
0
0
100
200
300
400
500
-0.2
10°C
20°C
30°C
-0.4
40°C
-0.6
-0.8
-1
Time (s)
Figure 81: Potential versus time plot for 5 mol dm-3 KOH 5 mA cm-2 current density
0.25
0.2
Current (x10-5 A)
0.15
0.1
10°C
0.05
20°C
30°C
0
0
50
100
150
200
250
300
-0.05
-0.1
-0.15
Time (s)
Figure 82: Current versus time plot for 5 mol dm-3 KOH and 0.3 V potentenial
130
350
40°C
0.8
0.6
Potential (V)
0.4
0.2
10°C
0
0
100
200
300
400
500
600
700
-0.2
20°C
30°C
-0.4
40°C
-0.6
-0.8
-1
Time (s)
Figure 83: Potential versus time plot for 4 mol dm-3 KOH and 5 mA cm-2 current density
0.08
0.07
0.06
Current (x10-5 A)
0.05
10°C
0.04
20°C
0.03
30°C
0.02
40°C
0.01
0
0
50
100
150
200
250
-0.01
Time (s)
Figure 84:Current versus time plot for 4 mol dm-3 KOH and 0.3 V potential
131
300
350
0.8
0.6
Potential (V)
0.4
0.2
10°C
0
20°C
0
100
200
300
400
500
-0.2
30°C
40°C
-0.4
-0.6
-0.8
Time (s)
Figure 85: Potential versus time plot for 2 mol dm-3 KOH and 5 mA cm-2 current density
0.045
0.04
0.035
Current (x10-5 A)
0.03
0.025
10°C
0.02
20°C
0.015
30°C
40°C
0.01
0.005
0
-0.005
0
200
400
600
800
1000
1200
Time (s)
Figure 86: Current versus time plot for 2 mol dm-3 KOH and 0.3 V potential
132
1400
1600
Galvanostatic Experiment Measurements
Current Density Burst 20 mA cm-2
Current Density
(mA cm-2)
5
10
15
20
Layer Thickness (µm)
Average
1.530185
2.018143
2.490942
2.458383
Median
1.6502
1.98205
2.5045
2.4685
0.398142
0.328912
0.099646
0.077204
Standard Dev
Pore Width (nm)
Average
33.72422
29.35829
29.087
26.9283
Median
34.31
29.8
29.315
27.4
4.829904
4.619951
3.637593
5.675759
Standard Dev
Pit Diameter (nm)
Average
27.04767
Median
27.57
Standard Dev
N/A
6.137657
Pit Density (pits per nm2)
Average
4.29E-05
Median
3.95E-05
Standard Dev
N/A
Table 3
133
1.11E-05
Current Density Burst 5 mA cm-2
Current Density
(mA cm-2)
5
10
15
20
Layer Thickness (µm)
Average
2.865123
2.523707
2.488287
2.423429
Median
2.9324
2.74325
2.509
2.4775
0.639587
0.402607
0.094655
0.170249
Standard Dev
Pore Width (nm)
Average
31.138
30.45514
29.54578
25.43467
Median
31.11
29.96
29.9
25.105
4.642359
3.755128
4.327985
3.466648
Standard Dev
Pit Diameter (nm)
Average
21.12168
Median
20.79
Standard Dev
N/A
4.569653
Pit Density (pits per nm2)
Average
1.16E-05
Median
1.15E-05
Standard Dev
2.14E-06
N/A
Table 4
134
Current Density Sweep
Current Density
(mA cm-2)
3
5
10
15
Layer Thickness (µm)
Average
2.362366
2.711843
2.963973
2.737783
Median
2.3784
2.5766
2.9779
2.7463
0.267326
0.238433
0.075808
0.078324
Standard Dev
Pore Width (nm)
Average
38.151
33.9884
30.08314
Median
37.585
34.42
29.96
4.99587
3.788156
Standard Dev
Pit Diameter (nm)
Average
Median
N/A
Standard Dev
Pit Density (pits per nm2)
Average
Median
N/A
Standard Dev
Table 5
135
N/A
4.13584
13.6 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.4927925
4.269331
4.188693
1.499398
Median
3.4865
4.2684
4.1912
1.49605
0.071080411
0.068593
0.047024
0.04296
Standard Dev
Pore Width (nm)
Average
27.20128
26.01287
27.37336
18.28783
Median
27.31
25.84
26.7
17.95
3.544274
3.370032
4.12402
2.968242
Standard Dev
Pit Diameter (nm)
Average
19.34042254
15.94286
20.12875
13.70372
Median
19.85
16.54
19.85
13.24
3.796684432
2.516301
3.536503
2.071854
Standard Dev
Pit Density (pits per nm2)
Average
4.32E-05
2.22E-05
1.33E-05
4.53E-05
Median
4.27E-05
2.18E-05
1.4E-05
4.45E-05
Standard Dev
2.02E-06
1.81E-06
1.59E-06
4.07E-06
Table 6
136
12 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.349297143
3.987803
5.709935
12.48393
Median
3.3604
3.9699
5.7405
12.4435
0.073941895
0.294432
0.110742
0.329196
Standard Dev
Pore Width (nm)
Average
21.4535
29.32918
29.45164
31.01138
Median
21.455
28.95
29.66
30.71
3.034823
4.148811
3.969883
5.04894
Standard Dev
Pit Diameter (nm)
Average
19.18313725
24.22214
18.5113
19.80222
Median
18.75
24.86
17.65
18.34
3.331536276
5.249711
2.154875
5.351275
Standard Dev
Pit Density (pits per nm2)
Average
3.73E-05
1.51E-06
1.31E-05
6.02E-06
Median
3.88E-05
5.23E-07
1.22E-05
6.11E-06
Standard Dev
4.84E-06
2.22E-06
1.6E-06
9.45E-07
Table 7
137
7 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.3819225
3.58917
5.699025
4.22316
Median
3.38075
3.6329
5.57
4.15
0.081378582
0.27873
1.135163
0.569618
Standard Dev
Pore Width (nm)
Average
25.75352
28.39762
29.67236
32.3591
Median
25.67
28.06
29.29
32.91
3.16397
4.840787
4.881647
6.20932
Standard Dev
Pit Diameter (nm)
Average
20.61285714
25.23333
33.75111
19.5
Median
20.69
24.92
33.86
18.06
4.186559347
4.200622
7.945071
6.029776
Standard Dev
Pit Density (pits per nm2)
Average
2.99E-05
3.83E-06
2.88E-06
1.13E-06
Median
3.1E-05
3.05E-06
2.62E-06
8.72E-07
1.98E-06
1.33E-06
9.04E-07
5.85E-07
Standard Dev
Table 8
138
5 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.640208
3.03564
3.638635
1.617115
Median
3.7129
3.0396
3.73765
1.6622
0.21382259
0.263729
0.420697
0.283729
Standard Dev
Pore Width (nm)
Average
29.12957
32.09674
34.35095
37.80333
Median
28.585
32.76
33.76
37.515
3.57165
4.306828
5.50904
7.014776
Standard Dev
Pit Diameter (nm)
Average
25.4028125
23.21227
28.45625
37.16571
Median
25.365
22.61
28.125
35.64
5.887260842
6.055954
6.331037
7.196911
Standard Dev
Pit Density (pits per nm2)
Average
2.33E-05
1.2E-05
2.05E-06
2.33E-06
Median
2.18E-05
1.18E-05
2.18E-06
3.05E-06
Standard Dev
5.47E-06
2.36E-06
9.42E-07
1.65E-06
Table 9
139
4 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.157165
4.348345
4.859148
5.257248
Median
3.1683
4.3612
5.0154
5.4122
0.114759426
0.222247
0.462955
0.463919
Standard Dev
Pore Width (nm)
Average
29.04882
30.544
28.85648
34.36627
Median
28.95
30.585
29.37
33.65
3.934109
4.268489
4.938339
5.886489
Standard Dev
Pit Diameter (nm)
Average
27.13869565
30.25214
35.14733
29.21923
Median
27.57
29.78
36.4
31.99
4.140714704
5.000384
7.685129
6.562757
Standard Dev
Pit Density (pits per nm2)
Average
4.51E-05
1.67E-05
6.98E-06
4.36E-06
Median
4.54E-05
1.57E-05
6.98E-06
3.92E-06
140
Standard Dev
8.62E-06
2.14E-06
1.07E-06
1.54E-06
30
40
Table 10
2 mol dm-3
Temp (° C)
10
20
Layer Thickness (µm)
Average
1.5982325
1.688395
1.32812
1.39087
Median
1.60005
1.703
1.34655
1.4158
0.128365439
0.097679
0.106103
0.112848
Standard Dev
Pore Width (nm)
Average
39.14178
36.879
37.02645
35.01524
Median
39.075
37.15
36.695
34.46
7.395919
5.622612
6.456455
5.685734
Standard Dev
Pit Diameter (nm)
Average
Median
N/A
Standard Dev
Pit Density (pits per nm2)
Average
141
Median
N/A
Standard Dev
Table 11
Potentiostatic Experiments
13.6 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.0065275
4.207425
3.405725
2.36072
Median
3.009
4.1967
3.3739
2.3889
0.137411766
0.145436
0.191449
0.147479
Standard Dev
Pore Width (nm)
Average
25.22098
24.96227
22.38318
19.45891
Median
25.01
24.29
21.89
19.51
4.223375
3.820605
3.510551
3.284487
Standard Dev
Pit Diameter (nm)
Average
15.60533333
14.365
15.63588
10.965
Median
15.44
14.34
15.44
11.03
142
Standard Dev
3.01987181
2.556555
2.178133
1.271349
Pit Density (pits per nm2)
Average
1.6E-05
1.7E-05
1.28E-05
2.87E-05
Median
1.66E-05
1.7E-05
1.26E-05
2.44E-05
Standard Dev
2.24E-06
4.32E-06
1.59E-06
1.1E-05
Table 12
12 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.74854
3.015333
2.188548
1.530358
Median
3.74755
3.0792
2.0988
1.54235
0.107746973
0.346337
0.301258
0.126293
Standard Dev
Pore Width (nm)
Average
22.72409
22.95782
23.973
22.82924
Median
22.63
22.67
23.45
22.65
2.761551
3.045555
3.931124
3.92756
Standard Dev
Pit Diameter (nm)
Average
18.62307692
13.28944
15.37316
11.28474
Median
18.75
13.24
15.44
10.34
143
Standard Dev
3.155789308
2.282675
2.52877
1.776608
Pit Density (pits per nm2)
Average
1.55E-05
1.36E-05
1.06E-05
1.04E-05
Median
1.61E-05
1.31E-05
1.05E-05
1.05E-05
2.1E-06
2.43E-06
1.78E-06
3.25E-06
30
40
Standard Dev
Table 13
7 mol dm-3
Temp (° C)
10 (CLO)
20
Layer Thickness (µm)
Average
2.338845
3.31846
1.10737
0.866923
Median
2.32695
3.34235
1.0844
0.8692
0.109965518
0.136515
0.162922
0.100204
Standard Dev
Pore Width (nm)
Average
50.55556
22.3705
22.9299
23.12248
Median
47
22.12
22.65
23.36
8.633816
3.103055
3.22231
3.555714
14.89333
10.27933
Standard Dev
Pit Diameter (nm)
Average
18.39167
144
Median
18.34
14.58
10.34
3.952911
2.352763
1.257773
7.5E-06
6.36682E-06
7.59E-06
7.41E-06
7.41342E-06
7.41E-06
1.25E-06
1.70575E-06
1.53E-06
N/A
Standard Dev
Pit Density (pits per nm2)
Average
Median
N/A
Standard Dev
Table 14
5 mol dm-3
Temp (° C)
10
20
30
40
Layer Thickness (µm)
Average
3.00654
2.008497
1.479965
1.220725
Median
3.0135
1.955
1.46395
1.2027
0.182665815
0.370257
0.204099
0.178884
Standard Dev
Pore Width (nm)
Average
27.76375
28.34365
24.40955
23.65418
Median
27.485
28.12
24.29
22.95
4.634304
4.54711
5.323309
4.212271
Standard Dev
Pit Diameter (nm)
145
Average
36.79142857
20.15364
15.51
14.96857
Median
37.5
20.96
15.44
15.44
6.875987042
3.122189
2.29414
3.050467
Standard Dev
Pit Density (pits per nm2)
Average
2.76E-06
3.89E-06
5.67E-06
7.41E-06
Median
2.83E-06
3.92E-06
5.67E-06
7.85E-06
Standard Dev
1.09E-06
1.22E-06
1.19E-06
1.02E-06
30
40
Table 15
4 mol dm-3
Temp (° C)
10
20
Layer Thickness (µm)
Average
2.425
1.5769
1.149533
0.75811
Median
2.42075
1.5991
1.1216
0.7252
0.286132782
0.204643
0.22167
0.150748
Standard Dev
Pore Width (nm)
Average
28.79155
28.08581
27.87694
26.75632
Median
29.035
28.12
27.4
26.56
3.756695
3.977246
4.570409
4.462716
Standard Dev
146
Pit Diameter (nm)
Average
25.0025
17.52333
18.04727
16.7275
Median
26.47
16.54
17.65
17.095
4.558524548
4.390766
3.010691
2.435921
Standard Dev
Pit Density (pits per nm2)
Average
9.68E-06
3.23E-06
3.58E-06
3.31E-06
Median
1E-05
3.49E-06
3.05E-06
3.05E-06
2.38E-06
6.61E-07
1.32E-06
1.37E-06
30
40
Standard Dev
Table 16
2 mol dm-3
Temp (° C)
10
20
Layer Thickness (µm)
Average
3.7445
2.27054
1.335528
0.944991
Median
3.7723
2.24255
1.32845
0.9661
0.194458869
0.291167
0.284963
0.125202
Standard Dev
Pore Width (nm)
Average
37.41145
38.04747
37.99267
34.54314
Median
37.585
37.73
38.22
34.46
147
Standard Dev
6.345063
6.594554
Pit Diameter (nm)
Average
Median
N/A
Standard Dev
Pit Density (pits per nm2)
Average
Median
N/A
Standard Dev
Table 17
148
6.772987
6.286254