Investigation of Phase Separation in GaInAsSb
using Transmission Electron Microscopy
by
Sara L. Ransom
Submitted to the Department of Materials Science and Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Materials Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 1998
@ Sara L. Ransom, MCMXCVIII. All rights reserved.
The author hereby grants to MIT permission to reproduce and
distribute publicly paper and electronic copies of this thesis document
in whole or in part, and to grant others the right to do so.
A uthor ..........................
Department of Materials Science and Engineering
May 8, 1998
Certified by...
Christine A. Wang
)Lincoln Laboratory Staff
Thesis Supervisor
Certified
by..........
...
........
.............
Eugene A. Fitzgerald
Associate Professor
Thesis Supervisor
.
Accepted by ................................... )..... . . .
Linn Hobbs, John F. Elliott Professor
Chairman, Department Committee pn Graduate Students
,
4'
.L"
Investigation of Phase Separation in GaInAsSb using
Transmission Electron Microscopy
by
Sara L. Ransom
Submitted to the Department of Materials Science and Engineering
on May 8, 1998, in partial fulfillment of the
requirements for the degree of
Master of Science in Materials Science and Engineering
Abstract
The performance of semiconductor devices largely depends on the quality of their
composing materials. This study focuses on the improvement of GaInAsSb devices
for use in Thermophotovoltaics (TPVs). One important materials property for these
devices may be the uniformity of the material composition. Composition modulation
has been studied in similar alloys (GaInAsP, InGaAs) using the Transmission Electron
Microscope (TEM). However, this phenomena is still not well understood. Many have
attributed the compositional changes to spinodal decomposition.
Samples expected to spinodally decompose and samples not expected to spinodally
decompose were prepared with the methods developed in this study (comparison made
with G. B. Stringfellow's calculations [1]). Different sizes of contrast modulation were
observed in the TEM, with a coarse - 300nm size modulation giving the most distinct
indication of decomposition. The orientation of the phases in the material in the (100)
direction corresponds to the elastically-soft directions of the zinc-blende structure. A
surface roughness was also noted on this sample, although its orientation in the (110)
direction did not correspond to the microstructural orientation.
A mixture of thermodynamic and kinetic effects are occurring. Sample growth
temperature and composition seem to be the most influential variables, but there is
much future work to be done in understanding their correlation with the thermodynamic and kinetic effects of epilayer growth. A phase separation mechanism other
than spinodal decomposition may better explain the microstructure.
Thesis Supervisor: Christine A. Wang
Title: Lincoln Laboratory Staff
Thesis Supervisor: Eugene A. Fitzgerald
Title: Associate Professor
Acknowledgments
Many have given much time and effort to bring about the completion of this thesis.
First of all, I thank God for creating materials and their science and for giving me
the strength to complete this endeavor. I also thank my advisor, Christine Wang,
for her technical support and general advice about careers and life. Professor Eugene
Fitzgerald and many of his students, especially Mayank Bulsara, Sri Samavedam, and
Tom Langdo, also offered invaluable support and help. My gratitude also extends to
Kevin Lee, Christopher Vineis, Doug Oakley, Paul Nitishin, and Skip Hoyt for their
assistance and involvement in my experiments. Thanks to Matthew Farinelli and
Bryan Robinson for reading this work and lending a hand whenever it was needed.
I am grateful to MIT Lincoln Laboratory and the Department of Energy, which
provided the facilities and means that this could even be done.
Finally, I thank
all those who offered moral support and never tired of hearing me talk about my
thesis, especially the members of United Christian Fellowship and my parents, Jeff
and Annette Ransom.
This material is based upon work supported under a, National Science Foundation Graduate Fellowship. Any opinions, findings, conclusions or recommendations
expressed in this publication are those of the author and do not necessarily reflect
the views of the National Science Foundation.
Contents
9
1 Introduction
2
10
.. ...
......
1.1
Thermophotovoltaics ................
1.2
D evice M aterial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3
Device Growth
..
...........
....
11
..
.....
..
13
Spinodal Decomposition
13
..
2.1
General Spinodal Decomposition ...................
2.2
Spinodal Decomposition in III-V Materials . ..............
17
2.3
Investigating Spinodal Decomposition . .................
20
22
3 Previous Experiments and Results
4
11
.
. ..................
3.1
Analysis Preparation/Conditions
3.2
Results and Observations .....................
25
...
33
Experiment and Procedures
4.1
Wedge Polishing ..................
4.2
Sam ple Types . ...
4.3
Sam ple Set
33
............
. . . . . . . . . . . . . . . .
36
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
. . . ..
. . . . ..
39
5 Results and Discussion
5.1
5.2
23
Sample Preparation ...................
....
.
39
.
5.1.1
Wedge M ethod ..........................
39
5.1.2
Ion Milling Conditions ......................
41
M icrostructure
..............................
..
43
5.3
Surface Structure .............................
5.4
Comparisons ..................... .....
5.5
Diffraction Patterns ........................
6 Conclusions
50
.....
.....
54
......
..
55
57
List of Figures
. . . . .
1-1
Typical Ga-Sb based TPV structure. .............
2-1
Free energy curve. Cross-sections of this are plotted to make phase
. . . . .
diagrams. From source [2]. . ..................
2-2
Spinodal and phase boundaries from [2].
10
14
15
. ................
2-3 Amplitude figure from [2]. A, is the critical wavelength and Am, is the
maximum wavelegnth.
2-4
Spinodal isotherms calculated from Stringfellow [1].
16
..
......
.................
The innermost
dotted contour is for 10000C, the outter solid contour is for 600 0 C,
and the third contour is for 80000C.
........
. ..
. ..........
.
19
2-5
Calculated binodal phase boundaries from [3]. . .............
3-1
PV sample from Growers [4]. There is no particular orientation for the
24
contrast modulation. Magnification bar is 100nm. . ..........
3-2
Epilayer in xsec: bottom edge lines the substrate and top edge is the
24
sample edge. 1.3cm = 0.1 /1 m actual. From LaPierre [5] ........
3-3
Phases interpreted by LaPierre. Note dimension orientation (a) and
corresponding bandgap diagram (b) [5] ...............
3-4
. . . .
29
PV sample contrast modulation from Bulsara [6]. The contrast lines
are oriented in the [110] direction. . ..................
4-1
19
Comparison of conventional and wedge preparation methods. ......
.
31
35
4-2
The innermost
Spinodal isotherms calculated from Stringfellow [1].
dotted contour is for 10000C, the outter solid contour is for 525 0 C,
and the third contour is for 575C. The points plotted are: square .
.
392A, circle - 760, diamond - 269. . ................
40
. .......
5-1
PV sample of 392A prepared by conventional technique.
5-2
PV sample of 392A prepared by wedge technique. Features are similar
.
to those seen in conventionally prepared sample. . ..........
5-3
Speckle ion mill damage visible. Sample milled at 5kV for over 8 hours
5-4
Ion mill damage in wedge sample milled for 3hrs at 3kV. .......
5-5
Although dark, this picture shows a wedge sample with a fairly clean
edge . . . . . . . . . . . . . . . ...
5-6
38
.
. . . . . . . . . . . . . . . . . . .
41
42
42
43
Sample 269 (011) xsec. Note epi-substrate boundary crossing the picture diagonally. FCM is seen in the epilayer and the substrate does
.
not appear completely uniform. . . . . . . . ...........
45
5-7
Sample 392A, (011) xsec. Note fine and coarse modulation .......
5-8
Coarse and fine contrast noted in sample 392A pv sample. ......
5-9
Sample 269 xsec sample. Faint vertical line hints at features like those
seen in 392A related to noted coarse contrast modulation.
44
.
46
47
. ......
5-10 STEM micrograph of 392A. Note striations (narrow bands) crossing
picture diagonally.
..........
...
....
48
......
...
.
5-11 Surface of 269 smooth except for a few defects. . ...........
5-12 Surface of 392A rough and textured in [110] direction....... .
5-13 AFM plot of 269 with z scale = 10nm/div
.
51
52
. ..............
53
5-14 AFM plot of 392A with z scale = 50nm/div . .............
5-15 AFM plot of 760 with z scale = 10nm/div
50
54
. ..............
5-16 DP taken from the quaternary epilayer. Faint streaking is noticible..
55
5-17 DP taken from GaSb substrate. Streaking is more noticible than in
quaternary. .................
....
.
.......
56
List of Tables
3.1
Varied Decomposition Results . .................
4.1
Sample Set Information . ..................
5.1
EDX integrated intensity ratios. ...................
5.2
AFM Results
.........
.
.
. . . .
.......
.......
.........
27
.
37
. .
48
51
Chapter 1
Introduction
Materials are often the limiting factor in systems operation or development. Their
properties contain the secrets of improving or preventing any process. This study
is the beginning of a fundamental investigation into phase stability of the semiconducting alloy, GaInAsSb, to be specifially used in thermophotovoltaic (TPV) devices.
These devices have been made of various quality, giving a working knowledge of the
use of GaInAsSb in this application. The ultimate goal of TPV research is to increase
the efficiency of thermophotovoltaic devices. In order to do this, the material quality
must be optimized. Inhomogeneity of the material may result in efficiency reduction
due to decreased carrier lifetimes. One type of inhomogeneity is a change in composition where different phases or compositions of material may be present. Therefore,
the ability to characterize and predict phase separation is desirable for understanding
how to make efficient devices. Since the devices provide motivation for this project,
they will be described in this section. A description of spinodal decomposition, the
type of phase separation expected, follows. After a discussion of previous works to
provide a basis for comparison, the results of this study are presented. Conclusions
and suggestions for future research close this document.
0.025 pm
p-GaSb Cap
p-AIGaAsSb Window 0.1 pm
p-GaInAsSb Emitter
3 pm
n-GaInAsSb Base
1 pm
n-GaSb Buffer
0.1 pm
n-GaSb Substrate
Figure 1-1: Typical Ga-Sb based TPV structure.
1.1
Thermophotovoltaics
Thermophotovoltaics (TPV) are devices that convert heat to electrical energy. They
are similar to solar cells, except the thermal source operates in a different temperature range. There has been recent interest in the 1100--1500K temperature range,
which corresponds to a 1.9-2.6 pm cutoff wavelength in a photovoltaic cell [7]. These
wavelengths require materials with a bandgap between 0.65 and 0.48 eV. Since this
low-bandgap cannot be achieved with Si or GaAs based devices, III-V devices lattice mismatched to InP or lattice matched to GaSb or InAs substrates are being
investigated [7].
The typical structure of a TPV (shown in Figure 1-1) consists of a base, emitter,
and cap layer. The base, usually 1 pm thick, is the carrier generation site, while the
emitter is the generation and current collection site and is typically 3 fm thick. The
thin cap layer forms a protective over-layer. Sometimes a lattice-matched window
layer is grown over the emitter. Passing radiation through this layer has been found
to improve electrical properties of devices by reducing surface recombination [7].
Key performance indicators are the electrical and optical properties as well as the
quantum efficiency (QE) of the devices. TPV devices have been grown with external
QE up to 60 % at 2 um [7]. The electrical properties are characterized by electron and
hole mobilities and their concentration. Optical property characterization includes
making sure that devices absorb efficiently at the desired wavelength and inside a
close range around that wavelength (small full-width-half-maximum and high peak
intensity on optical absorption spectrum).
TPV devices have potential uses in co-generation where excess heat can be utilized for useful energy production. These devices are made lattice-matched to reduce
defects, and are thus assumed to be fully strained devices if lattice matched.
1.2
Device Material
The device material is chosen to produce the largest wavelength (and lowest energy
bandgap) possible while maintaining an efficient device. As mentioned above, material layers on InP, InAs, and GaSb have been grown. For various thermodynamic,
electronic and mechanical reasons [7], as well as the availability of GaSb substrates,
GaSb is used as the substrate for the devices in this project.
The device layers are made of the quaternary material Gal_,InAsySbly. The
advantage of quaternaries is that one element, Indium in this case, can be used to
adjust the bandgap of the material, while another element, Arsenic, can be used to
adjust the lattice-matching. In this way a lattice-matched device with the desired
wavelength can be produced. As a disadvantage, the Ga.Sb materials are very soft,
making specimen preparation for transmission electron microscopy analysis challenging.
1.3
Device Growth
There are many variables in device growth, all of which affect the device properties or performance to some degree. Adjustments made using the growth equipment
(Organo-Metallic Vapor Phase Epitaxy or OMVPE equipment in this case) include
growth temperature and gas flows (affecting growth rate, III-V ratio, and composi-
tion). Substrate misorientation is another variable that has an effect on layer quality.
A definite improvement in surface morphology, sometimes an indication of layer quality, has been shown for devices grown on a 60 towards the (111)B substrate offcut (B
indicates orientation towards the Group V (111) plane) as opposed to the 20 towards
the (110) substrate offcut [8].
Chapter 2
Spinodal Decomposition
One factor predicted to detrimentally affect device performance is decomposition of
the epilayers. Spinodal decomposition, the mechanism of decomposition expected
for these devices, is the separation of a material into lower energy regions of different
composition. Spinodal decomposition, then, is a type of phase separation. A material
may separate into regions of different composition by mechanisms other than spinodal
decomposition. If the material is unstable and spinodally decomposes, the optical
absorption spectrum can be broadened greatly (or disappear!) and the phases may
serve as recombination sites and traps for carriers. Both factors would decrease device
efficiency. To better understand this defect and how it forms, we look at spinodal
decomposition in bulk materials for an indication of what might happen in the more
unpredictable epilayers.
2.1
General Spinodal Decomposition
Below a critical temperature, spinodally decomposing material separates into two
homogeneous phases with the same structure but different compositions. The free
energy curve (Figure 2-1) shows the free energy at one particular temperature and
has two inflection points (where
2
G
= 0).
The equilibrium phase compositions at the designated temperature are the compositions that correspond to a common tangent on the free energy curve. If the
Fig u
Z
I I nflection
(
.
a
diagrams. From source [2].
equilibrium points from this graph are plotted as temperature changes in a composition versus temperature plot, a curve that marks the phase boundary is obtained.
By plotting inflection points, another curve called the spinodal is derived inside the
phase boundary curve (see Figure 2-2).
If a material has an undecomposed composition inside the spinodal it is unstable.
If it is outside the spinodal but inside the phase boundary it is metastable. Unstable
material may form composition fluctuations and phase separate into homogeneous
phases; this process is spinodal decomposition.
The probability of occurrence of
small fluctuations in a solid is high, creating areas of composition gradient. This
induces more change in composition as material tries to reach its lowest energy state,
achieved by components migrating to where their chemical potential is lowest.
For compositions in the spinodal, lower chemical potential is found when the
component is at a higher concentration [2]. Therefore, a flux from low to high concentration develops. Continuation of this leads to formation of small, evenly spaced
clusters of high concentration components.
Liquid
Phase Separation by
Nucleation and Growth
S/Mechanism
w
cr
a Tc
/
e
Spinodal
Phase
Boundary
/
Phose
Sepor tion
I
Ij
by Spinodol
/
c a2
Decomposition
Mechanism
X
X,Xs X
X%
2
COMPOSITION -
Figure 2-2: Spinodal and phase boundaries from [2].
If a composition is outside the spinodal, the flux direction is reversed. Components
migrate to low concentration areas and eventually the fluctuation is diffused-out. If
the fluctuation is large enough, the material may behave differently, instead experiencing nucleation and growth.
Thermodynamics predict when this decomposition occurs and kinetics determine
the extent. All the variables of growth affect which regime of each is appropriate to
apply to a sample. Thermodynamics is the focus of this study since the possibility
of decomposition occurrence is fundamentally important to understand. The kinetics
suggest what size these clusters and fluctuations may be. By relating the composition
modulation to the free energy of the flux, an expression for the dimension of the
fluctuation can be found. The composition may be related to the flux using a simple,
binary, 1-dimensional system diffusion equation:
dC
dt
-dJ
dx '
(2.1)
where C is composition and J is net flux. After a derivation described in [2], an
amplitude, R(), for the variation can be obtained (3 = 2~
t +
T
" P Xm
X-
Figure 2-3: Amplitude figure from [2]. A, is the critical wavelength and Am is the
maximum wavelegnth.
R()
2w 2
= (-2) (M/Co) f
(2.2)
Here, (A) is the wavelength of composition variation, M is a mobility expression,
Co is the number of atoms/volume and f" is the second derivative with respect to
composition of free energy/volume. For large A, R(@) is small and for small A, R(@) is
large. At some point, the amplitude decreases again and the net free energy becomes
positive so that a maximum amplitude wavelength, Am, and a critical wavelength, Ac,
above which spinodal decomposition occurs (R(3) > 0) are found. See Figure 2-3.
The simplified formula can be modified for inhomogeneous, or strained situations.
In theory, if a composition modulation dimension is given for a particular material,
this calculation can be made to determine whether the modulation is possibly from
spinodal decomposition by looking at the amplification factor or free energy. Kinetics calculations will be useful in a later phase of this project. It is important to
remember that both kinetic and thermodynamic calculations included here are bulk
theory calculations and their applicability to epilayers should not be unconditionally
assumed.
2.2
Spinodal Decomposition in III-V Materials
Spinodal decomposition is a perplexing issue in III-V semiconductors. We can look to
bulk thermodynamics to get an idea of where to start an investigation into epilayer
spinodal decomposition. As explained above, there is a difference between phase
boundaries and spinodal boundaries. If the phase boundary is plotted for x and y
at a particular temperature, this is called a binodal isotherm. Similarly, spinodal
boundaries are called spinodal isotherms. If the two types of isotherms were plotted
together, the binodal would probably lay outside of the spinodal. This can be seen
from Figure 2-2. Furthermore, if a spinodal isotherm is calculated, it makes sense that
layers grown farthest into the unstable region have the most probability of showing
spinodal decomposition. For the GaInAsSb system of concern here, this means a
higher Indium composition.
There are a few different methods used to calculate the isotherms. G. B. Stringfellow uses the Delta Lattice Parameter (DLP) method [1] and others have used methods
such as the Regular Solution Model [9]. Since Stringfellow's calculations are widely
accepted, conservative estimates, we use his results for reference.
He derives the
stability condition below based on the DLP model applied to the thermodynamics
of a quaternary system [9]. The general free energy equation expanded to include
quaternaries is:
G = N,(-Kao2 5 + RT[xlnx + (1 - x)ln(1 - x) + ylny + (1 - y)ln(1 - y)]).
(2.3)
This equation is from [10]. x represents the Ga, (1 - x) the In, y the As, and (1 - y)
the Sb concentrations. The second derivatives of this equation combine to make the
stability equation:
a2G (2G
( aX2 )(-)y
ay2
a2G 2
- (
aX1y
)
>
0.
(2.4)
If the above is true, the composition is "stable" [1]. The equations for the second
derivatives are:
02 G
RT
5 AaA2 )
- 8.75Kao-4.
= N,(_
- )
a2 G(1
(2.5)
- 8.75Kao-4.5 ac 2 )
N,(RT
y(1 - y)
(2.6)
Ox
22
y
G
-= N,(-8.75Kao-4.5
aAAac)
(2.7)
N, is the moles/volume, R is the gas constant, T is the growth temperature, K is a
constant (1.15 x 107 cal/molA 2 5 ), a, is the average lattice constant,
aA and Aac are
factors dependent on composition and constituent lattice constants, x and y are the
compositions if the formula is: GaInlxAsySbl_y. Equation 2.4 was taken from [1].
However, the actual derivative of equation 2.3 gives an additional term of +Nv 2 .D
The formula rewritten is:
02G
2.5KD
3
5 )
= N,(-8.75Ka,-4 5 AaAAac +
--
Oxay
a
(2.8)
D is another factor dependent on lattice constants of the constituent binaries (like
AaAandAac). If these equations are solved (using 2.8 instead of 2.7) and the stability equation is negative, the material is unstable. If it is positive, the material is
metastable or stable. This calculation is done for the samples of this study in the
Sample Set section of the Experiment and Procedures chapter. When the stability
equation is set equal to zero, the spinodal isotherms are obtained. Figure 2-4 shows
calculated isotherms for GaInAsSb. Figure 2-5 from [3] shows the binodal isotherms
for different temperatures. The stability criterion would be slightly different for these
but result in the same kind of stability equation equaling zero. Even though the
binodal isotherms mark the phase boundaries and are distinct from the spinodal
boundaries, they are still illustrative of tendencies to decompose or remain stable. In
comparing Figures 2-4 and 2-5, notice that the axes are different.
Again, these are the predicted isotherms for GaInAsSb bulk material. Stringfellow also claims that coherency strain of the epilayer may prevent the decomposition
Spinodal_Isothermsfor_GaInAsSb
1.
rJ.
,--~
0.8
'4
'L-1C
N N.c
X
AI
0.6
!Jii I
As
it
1A
0.4
'I
/
I
0.2
0
0:2
0.8
06
04
Ga
Figure 2-4: Spinodal isotherms calculated from Stringfellow [1]. The innermost dotted
contour is for 1000 0 C, the outter solid contour is for 600 0 C, and the third contour is
for 800C.
GaSb
0.7
1.0
0.6
0.4
0.5
0.2
0.3
inSb
0.1
Eg (eV)
Eg (e.8
0.8
0.9
0.4
Y
1.0
1.1
0.2
1.2
1.3
0
GaAs 0.2
GaAs
0.4
x
-.
0.6
0.8
1.0
InAs
Figure 2-5: Calculated binodal phase boundaries from [3].
described by these calculations [1]. In his DLP model, Stringfellow ignores the strain
between phase separated regions and strain between those regions and the substrate.
In reality, for many cases these strains increase the free energy so much that phase
separation is not energetically favored. In fact, calculations that take this strain into
account show that spinodal decomposition will not occur at all in several common
III-V systems because of this stabilization, and suggests that phase separation may
be determined by kinetic factors, not thermodynamic factors. This is a possibility to
consider during investigation.
2.3
Investigating Spinodal Decomposition
There are a few different methods researchers have used to characterize spinodal decomposition. Analyzing samples with the transmission electron microscope (TEM)
provides the most visual information. For TEM analysis, an electron beam passes
through a very thin sample (- 100nm) and the image produced is viewed on the opposite side of the sample. Irregularities in structure cause the beam to be diffracted
differently, creating regions of varying contrast on the image. Since a spinodally decomposed material will have regions composed of different phases (different structure
spacings), contrast changes can indicate the presence of this phase instability. Caution
must be exercised as other effects, such as strain, can also influence the contrast.
TEM contrast is also highly affected by the angle at which the sample is viewed.
The spacing between the layers that the beam intersects changes with orientation. By
tilting the samples and looking down the sample in various directions, the contrast
is enhanced or dulled. For zinc-blende structure samples (includes GaSb materials),
the (001) growth plane and (110) cross-section plane are often viewed down the [220]
and [400] type directions. For this study, two-beam conditions using these directions
produce good contrast.
The visual appearance of spinodal decomposition during microscopy can vary.
Elastic anisotropy is an important determining factor. If isotropic, a material is likely
to develop periodic fluctuations in all directions. However, if anisotropic, phases may
form preferentially in directions where elastic energy is minimized (soft directions).
The soft directions for the zinc-blende structure are the [100] directions [5]. This
indicates that the viewing direction may be important for seeing phases, depending
on the phase dimensions.
The TEM also has the capability of producing electron diffraction patterns (DP).
These patterns of spots on the back focal plane of the image beam indicate the
structure of a sample in reciprocal space. The pole directions, such as (001) and
(110) mentioned above, have distinct patterns that can be used to indentify different
phases. Tilt position off the pole can also be deciphered from these patterns. As the
structure influences spot position and spacing, spinodal decomposition may also have
an effect on DP's. Extra (satellite) spots surrounding expected spots or elongation
of spots are the effects of a structural change.
Besides TEM, there are a few other techniques that indicate structural changes in
materials which may be useful for spinodal decomposition analysis. X-ray diffraction
produces a spectrum that also depends on the spacing between atomic layers. As
the structure changes, the Bragg angle changes, giving a varying intensity in the
reflection collected. Therefore, broad peaks may indicate a variation in material.
Photoluminescence (PL) spectra can also reflect structural change. The bandgap of
the material varies with composition (and thus phase). Therefore, the characteristic
wavelengths of materials (as observed in PL spectra) may broaden or shift in the
presence of phase changes.
Chapter 3
Previous Experiments and Results
The potential damage of spinodal decomposition to device efficiency led to studies of
material microstructure during the 1980's and 90's. Much of the research done on
spinodal decomposition has focused on InGaAsP and InGaAs alloys. In-lGaxAsyP_-y
lattice matched to InP can have a bandgap corresponding to wavelengths between 0.92
to 1.65 ym. This wavelength range overlaps with the current optical fiber technology
needs at 1.3 and 1.55 Mm. It is therefore a material of great interest for detector and
emitter devices. In_GaAs also has potential optical applications. Therefore, these
materials are the focus of the articles discussed below. There is little published on
GaInAsSb, the material of study in this project.
The techniques mentioned in Chapter 2 (TEM, DP, X-ray, PL) are often used
to investigate spinodal decomposition. Different analysis conditions and samples are
used and the studies produce a variety of results. Contrast modulation of various
dimensions and orientations are noted in layer cross-sections or planview samples.
More than one size of modulation may also be present at one time. The explanations
for the different results reflect their variety. Explaining some of these previous results
will demonstrate the present understandings about spinodal decomposition in IIIV materials in general. This identifies the issues that concern the current work on
GaInAsSb and gives the context in which this study was conducted.
3.1
Analysis Preparation/Conditions
The preparation and conditions of analysis have a large impact on the results for these
studies. The type of sample and directions of viewing affect the image contrast on
TEM and thus how much can be learned about spinodal decomposition of a specimen.
Cross-section (xsec) samples allow viewing of layer interfaces and contrast throughout
epilayers. The nature of wafer cleaving creates easy viewing of the (110) poles with
a xsec sample. Samples are also often viewed down the (001) pole. This requires a
different specimen thinned parallel to the substrate (from the substrate side of the
wafer) instead of perpendicular to it, as in a planview (pv) sample.
Although xsec samples usually show contrast in at least one of these specific
directions, pv samples may not. In some cases, the pv samples have shown a significant
amount of contrast [11, 6], and in other studies they have not [12, 13, 14, 15]. The
lack of contrast could be explained by an absence of spinodal decomposition when it
was expected or by a difference in dimension of the phases present. The modulations
that would be visible in pv samples may be too small to resolve.
There is only one way to prepare a pv sample of a device since there is only
one growth direction. However, a family of (110) planes are possible for viewing in
xsec.
Cutting two xsec samples perpendicular to each other should represent the
possible results. It is possible that the contrast of the two (110) cut samples would
vary. In most previous works, only one xsec sample was reported. This adds another
variable to comparing the results of different works. Researchers may be viewing
a lesser contrast sample in one case and a sample with more contrast in another
and contribute the difference to another effect. Works that have analyzed both xsec
directions have found some discrepancy between the resulting micrographs [5]. (0il)
xsec samples showed a sharper contrast than (011) xsec samples. This is attributed
to the difference in adatom diffusion lengths in the two directions.
An example of a pv sample is shown in Figure 3-1. An example of a xsec sample
is shown in Figure 3-2. These figures show what might be seen in looking at samples
in pv and xsec.
4-.. 22
Figure 3-1: PV sample from Growers [4]. There is no particular orientation for the
contrast modulation. Magnification bar is 100nm
____ __
___
_____
.__ _
Figure 3-2: Epilayer in xsec: bottom edge lines the substrate and top edge is the
sample edge. 1.3cm = 0.1 fim actual. From LaPierre [5]
The reflection directions (220) and (400) are typically used in two-beam imaging
since they produce the best image contrast. It is interesting to note the difference
between the images and which of these two viewing directions produces the most contrast. In most cases, the (220) family of directions produces more contrast. For some
works, the contrast was limited for the (400) direction, producing contrast modulation
in only one direction [16]. The (220), on the other hand, produced modulation in two
directions. For the same study, xsec samples showed contrast in the (220) and (400)
reflections but no clear contrast changes in the (004) reflection. For other works, the
extent of contrast changed or disappeared, with the (220) usually containing more
contrast [5, 11, 13, 16, 6, 17].
Other possible small sources of variation having to do with preparation are method
of polish and mode of TEM analysis. Most samples were prepared by mechanical
polish followed by ion milling. However, a few were chemically prepared [4]. Samples
were either imaged in bright field or dark field mode. In cases where both types were
done, the images were similar but the dark field produced a little more contrast [17].
3.2
Results and Observations
Since contrast modulation is an indication of spinodal decomposition, researchers are
interested in fully understanding the characteristics of these modulations, including:
when they appear, their size, and their orientation. This is all part of an effort
to model and understand under what conditions spinodal decomposition occurs as
the means of limiting phase instability in devices. A summary of the results from
literature discussed here is found in Table 3.1. A dash indicates that the information
is unavailable.
One way to try to model the contrast modulation is to relate its appearance back to
the phase diagram. If the sample composition places it inside the spinodal unstable
region, or in the miscibility gap, does contrast modulation indeed occur? In most
cases a correlation has been noted. The contrast modulation appears most distinct
further inside the miscibility gap and disappears well outside of the miscibility gap
[4, 16, 15, 5]. However, a fine contrast modulation (FCM) noted by A. G. Norman
and G. R. Booker and others appears both inside and outside the miscibility gap
Different authors have various explanations for this. This modulation may
[16].
not be described by the phase diagrams calculated previously because it occurs by
a different mechanism.
Instead of being described by the bulk diagram, perhaps
another model of phase separation in epitaxial layers is needed. Another possibility
is that the modulation does not indicate spinodal decomposition. Glas argues that
FCM can be consistent with homogeneity [12]. Other effects, such as strain, may
cause the contrast change appearance. The dimensions of this FCM will be described
below.
The issue of growth technique has often surfaced in relation to growing inside or
outside the miscibility gap. Equilibrium techniques, such as liquid phase epitaxy,
LPE, expectedly adhere to the convention of showing contrast modulation inside the
miscibility gap (except for FCM) and many experiments have been done on LPE
specimen to show this [16, 4]. Researchers questioned whether this would also prove
true for non-equilibrium techniques such as Molecular Beam Epitaxy, MBE, and
Organo-Metallic Vapor Phase Epitaxy, OMVPE (or Metallo-Organic Chemical Vapor Deposition, MOCVD). Another form of epitaxy is Gas Source Molecular Beam
Epitaxy (GSMBE). Samples observed so far have generally followed the trend of
showing contrast inside the spinodal [16].
As indicated by the name given to fine contrast modulation, coarser modulations
have been noted. The range in sizes is indicative of the number of variables that
bring about spinodal decomposition, including composition and growth conditions.
The modulation variation is also not only between samples and studies, but also
between directions in the same specimen. The phases have different lengths in the
different directions.
The coarser structure in Norman's work [16] is thought to arise during growth at
the liquid solid interface present in LPE. The finer structure (FCM) is contributed
to decomposition during cool down from the growth temperature, hence it is present
even in stable samples as all samples must cool down from growth temperature. As
Author
Growers
Norman
Norman
Glas
Glas
Seong
Seong
Wang
Peiro
Peiro
LaPierre
LaPierre
LaPierre
Volkov
Bulsara
Bulsara
Dim.(nm)
30
150
15
100-200
10-15
15
5
50-100
100-200
10-20
10
100
70
20
800 (waviness)
20 (phase sep)
Direction
none
100 010
100 010
100 010
100 010
none
two 110 type
010 type
010 type
011
011il
100
110
110
110
Technique
LPE
LPE
LPE
LPE,MBE
LPE,MBE
MOCVD
MOCVD
MBE
MBE
MBE
GSMBE
GSMBE
GSMBE
LPE
OMVPE
OMVPE
Type
pv
pv
pv
pv
pv, xsec
pv
pv
xsec
pv
pv
xsec
xsec
xsec
xsec
pv
pv
Ref.
[4]
[16]
[16]
[12]
[12]
[14]
[14]
[11]
[13]
[13]
[5]
[5]
[5]
[17]
[6]
[6]
Table 3.1: Varied Decomposition Results
the sample cools, the composition may pass through spinodal regions of the phase
diagram.
In Seong's work [14], the "coarse" modulation noticed is on a similar scale of the
"fine" modulation of other works. The coarse modulation was identified as spinodal
decomposition whereas in [12], similar modulation was declared not to be spinodal
decomposition. The difference may lie in the difference of materials (InGaAs vs.
InGaAsP) or any differences in growth conditions. The 5nm variation in Seong was
attributed to random displacement of atoms and did not occur in any particular
direction [14].
The experiments are listed in the order that they were published. For the earlier
experiments, modulation is usually in the [100] or [010] directions. This would be
expected from the bulk model (explained in Chapter 2). Newer publications have
noted modulation in the [110] directions instead [5, 17, 6]. This is still a point of
confusion within the field. R. R. LaPierre explains that a mechanism different from
that of bulk decomposition may be occurring [18]. He calls for a new model that de-
scribes surface decomposition that may be happening on the two-dimensional growth
plane. As epilayers grow, the material may be strained and distorted in the growth
direction differently from directions parallel to the substrate. This is the nature of
lattice matching in epitaxy and it gives some description of why a different model
that incorporates this epi-specific concern may be needed.
Another model popular in the earlier articles is characterization of the decomposition as a "columnar" growth of phases. This argument is compatible with the 2D
surface decomposition occurring at the growth surface. Phases may grow in columns
parallel to the growth direction as the epilayers grow. This is supported by the absence
of modulation in the growth direction, typically [001]. Even works that noted some
modulation in the [001] direction still characterize the decomposition as columnar
[11]. However, LaPierre again notices a different structure. For the phase separation
he observes, the growth direction dimension is smaller than one of the dimensions
parallel to the substrate. The phase regions which the dimensions describe represent
GaP and InAs rich regions alternating with nominal composition regions. This creates a changing band structure in which carriers can become trapped in the smaller
InAs-rich bandgap. This causes a shift in the PL spectra and helps explain why
performance may degrade for a device composed of spinodally decomposed material.
The diagram in Figure 3-3 represents the phases and their orientations visually.
Electron diffraction patterns (DP) were also investigated for evidence of spinodal
decomposition. Some authors noted satellite spots in their patterns [16]. The spacing
of the satellite spot from its main spot is theoretically related to the spacing of the
modulation. Since their relation is through reciprocal space, a fine modulation has a
better chance of producing a visible spot because the spacing of the modulation in
reciprocal space is larger. If the spots are too close, as they might be for a larger
real space modulation, they are not distinct from the reciprocal lattice points. They
are either enveloped in the spots or producing an elongated appearance of the spots.
Therefore, the direction of elongation may be indicative of modulation direction.
Electron DP of pv samples in [13] and [14] show elongation of spots in the [010]
and [001] directions that accompany the noted composition modulation. In another
1100]
(a)
S 100 A
InGaAsP
layer
[07
InP substrate
a1000 A
=70
N GAP-rich
tic
(b)
'
• InAs-rich
,ooA
Ldistance
ong [01]
VB
noriin
GaP-rich
CK
T> 100
,
;nAs-rich
T<100 K
Figure 3-3: Phases interpreted by LaPierre. Note dimension orientation (a) and
corresponding bandgap diagram (b) [5].
study, authors tried annealing epilayers to determine if the microstructure would be
affected and therefore give information about the kinetics of the decomposition [14].
They used electron diffraction to characterize the material before and after annealing. For InGaAs, grown by MOCVD at 717 0 C, 15nm and 5nm contrast modulations
were observed during imaging. After annealing at 6000 C for 744 hours, the larger
modulation disappeared while the smaller remained. (Recall, the 15nm contrast was
identified as spinodal decomposition.) Although the microstructure changed after annealing, the electron diffraction pattern remained the same. This indicates that the
elongation of the diffraction spots in the [010] and [100] directions may correspond
to the finer 5nm contrast modulation that remains after annealing. The disappearance of the 15nm contrast is explained by atomic diffusion. Given heat and time,
the atoms reordered themselves and the phase separation diffused out. The fact that
the 5nm contrast remains suggests thermodynamics is not responsible for that phase
separation (if phase separation is really occuring). Another study of annealing used
0
an opposite argument [5]. A short anneal at higher temperature (10 min at 700 C)
was done on decomposed samples. PL and X-ray done on samples before and after annealing showed no change. The authors explain that annealing would remove
bulk decomposition but possibly not decomposition of another origin (i.e. surface
decomposition during growth). Therefore, the remaining contrast may be explained
by surface growth decomposition mechanisms and any disappearing contrast may be
attributed to bulk phase separation. Another possible explanation is that the finer
modulation is not actual composition modulation and the DP elongation does not
indicate decomposition.
Another feature of DP's that have received attention is the streaking between
reciprocal space spots. V. V. Volkov notes that InGaAsP grown by LPE produces
streaking in the [110] directions (for a (100) xsec sample) [17].
This streaking is
seen in over-exposed negatives of the DP and is believed to be related to short range
ordering in the InGaAs layer of the structure. In F. Glas' discussion of streaking,
he notes that citing of streaking in the [110] type direction is not necessarily due to
composition modulation. It may simply be the result of strain fields in the material,
a condition that may be consistent with homogeneity.
An article on InGaAs grown by OMVPE notes phase separation in [110] directions
and an unusual contrast modulation which the author attributes to strain fields from
misfit dislocations [6].
The wavy contrast in the [110] direction was noted on a
pv sample and related to compositional modulation. The contrast only appears in
one [220] reflection. In a xsec sample, striations layered in the [001] direction were
reported and attributed to phase separation. The features attributed to strain fields
also appear in pv micrographs of another study, but are not explained [17]. A picture
of this modulation is shown in Figure 3-4.
A few authors have looked into strain issues. The strain resulting from lattice
mismatch is hypothesized to impact the phase stability [17].
Strain causes a shift
in the phase diagram location, with tensile strains causing a movement towards the
unstable region and compressive strains causing a movement away from the unstable
region. In comparing 0.5 % strained samples strained compressively (+) or tensily
(-), the tensile samples showed a sharper contrast modulation, indicating a position
Figure 3-4: PV sample contrast modulation from Bulsara [6]. The contrast lines are
oriented in the [110] direction.
further in the miscibility gap [18]. Any new model describing epilayer decomposition
would likely need to include this factor also.
Some final factors that influence modulation are growth temperature and film
thickness. As these factors increase, the wavelength of modulation decreases (in [13]).
The decrease in modulation as the film thickens is probably related to the different
strain in epilayers of different thicknesses. Increased growth temperature would cause
a shift away from the spinodal if the composition is constant, leading to a decrease
in the modulation. This has been shown to be likely through the general correlation
of composition relative to the miscibility gap and contrast definition.
The issues and variables involved with spinodal decomposition investigation are
numerous. Most of the works cited here have made efforts to understand the process
by starting with an understanding of bulk thermodynamics and extending it. Suggestions for surface decomposition models indicate the current direction of research.
This work extends from the same basis. The results will be discussed in comparison with these previous experiments to contribute to the understanding of spinodal
decomposition in epitaxial semiconductors.
Chapter 4
Experiment and Procedures
TEM sample preparation for this project required significant time and development.
Because GaSb is a soft material, great care must be taken in preparing and handling the samples. In general, TEM samples are mechanically polished and then ion
milled to electron-transparent thickness (- 100mm). Conventional techniques involve
a planar polish and then milling until a small puncture in the sample is made. The
area around this hole is then the thinnest area since the ion beam is directed toward
the sample at an angle during milling. Motivated by some of its advantages, this
project used a wedge polishing method as well as the conventional method to prepare
samples.
4.1
Wedge Polishing
Wedge polishing involves making the sample into the shape of a wedge where a very
thin edge is polished on one side of the sample. This thin edge then requires careful
handling but less milling to further thin it. Since milling can introduce damage in
soft materials such as GaSb, obscuring the microstructure, wedge polishing methods
offer potential advantages for this project by reducing the milling time. The faster
preparation technique also makes this a more efficient way to make samples. Another
motivation for wedge polishing is the theoretical ability to thin cross-section samples
deeper into the structure during milling. Therefore, thicker epitaxial layers are easier
to observe. Some have also used the wedge method for selected-area sample preparation [19]. By positioning the wedge in a specific area, known features can be analyzed
via TEM.
The wedge sample polishing method can be used with both types of samples used
for this project, cross-section (xsec) and planview (pv). The method for xsec will
be described since pv is an easier method requiring only slight modification of the
procedure.
Two small pieces of the wafer are cleaved and glued together, epilayer to epilayer,
using M-bond 610 adhesive. Two supporting substrate pieces of approximately the
same size are then glued to the backsides of the sample wafer pieces to make a fourlayer sandwich. The sandwich is then cut into cross-sections about 400 ,/m thick.
The cut surface is then planarly polished with diamond-impregnated films so that
the surface is reflective and with as few scratches as possible. (Scratches are locations
where samples could break when they become thinner.) Then, the sample is turned
over and remounted with the unpolished cross-section surface face up. At this point,
an approximate one degree angle wedge is polished into the sample so that the thinnest
edge is along the cross-section (perpendicular to the epilayer surfaces that are flush
together in the middle of the sandwich piece). Once the wedge is polished into the
sample, a Mo support ring is mounted on the sample with M-bond to allow handling
once the sample is removed from the mount. The ring is positioned so that the thin
edge is along the diameter of the hole. The sample can then be ion milled.
A diagram of this procedure showing the sample geometry is shown in Figure 4-1.
The shaded regions are the epilayers sandwiched together. The figure does not show
the substrate support pieces as they are not crucial for understanding the method.
They would simply add an extra layer on either side of the structure. The diagram
also shows the conventional method so that the differences between the methods are
obvious. The initial gluing and slicing steps are the same and the different procedures
after that are depicted.
A Gatan Dual Ion Mill with Ar+ ion flow and a liquid Nitrogen cooled stage was
used to thin the samples further to electron-transparent thickness. The conditions for
Slice
Conventional
Wedge
Figure 4-1: Comparison of conventional and wedge preparation methods.
proper sample thinning have been somewhat elusive. The main parameters are gun
voltage, current, angle, and milling time. Thinning samples at voltages greater than
5kV and angles greater than 120 has generally resulted in damaging the samples.
Typical conditions are 3kV and 0.5mA current at an angle of 100. Depending on
sample thickness, the milling time has been 1-5 hours. Once the sample is milled
correctly, it is ready for TEM viewing. If samples are damaged, they may have a
spotted appearance or a clustered, rough appearance. This is the result of bombarding
the sample with too much energy or redepositing material (may occur if milling is
done for long periods at low angles). Both prevent viewing of sample microstructure.
For pv sample preparation, only one piece is needed (epilayers are not glued together) and the initial planar polishing step is not needed. A single piece of the wafer
is mounted epilayer side down. The orientation of the wedge is also not critical. Polishing the wedge along any edge is acceptable. Support ring attachment and milling
proceeds as above.
4.2
Sample Types
In order to fully characterize spinodal decomposition, viewing a layer from three different structural directions is recommended. Two different cross-section samples are
required to view the (110) directions 900 to one another in the layer. Decomposition
may be visible in one direction and not another, requiring both for any conclusive
results based on xsec samples. The third sample is the pv sample that allows viewing
of the sample down the (001) growth direction.
4.3
Sample Set
The samples analyzed in this study were GaInAsSb layers grown on GaSb substrates
by OMVPE. The samples were chosen to represent extremes in decomposition. Their
information is shown in Table 4.1. The substrate offcut information is interpreted as
20 towards the nearest (110) plane and 60 towards the (111)B plane. The temperature
listed is growth temperature and RT PL abreviates Room Temperature Photoluminescence results (the material wavelength). The variation of conditions indicates that
the intent of this project is not to provide a systematic experiment from which specific
effects can by quantifiably determined. The intent is to image decomposition in this
alloy for the first time and discover how to study the phase instability effectively. Suggestions of how the conditions affect stability and to what extent will also be made.
The wide range in temperature and composition in these samples will give an indication of the parameters' impacts. Since 269 and 392A have similar growth conditions
except for the temperature and composition, their comparison should be helpful. The
value of doing both 392A and 760 when their compositions are similar is to notice
the effect of growth temperature and substrate misorientation on decomposition.
The compositions of samples 760 and 392A place them in or near the miscibility
gap. Sample 269 is outside the spinodal isotherm. This can be seen by eye in relation
to the phase diagram shown in Chapter 2. Although the closest isotherm is 6000 C,
an intuitive feel can be gained for their relative stabilities. Since sample 269 appears
Sample
269
Temp oC
575
Offcut
2°(110)
% In Comp
6.4
RT PL(pm)
1.9
392A
550
20(110)
18
2.4
525
60 (111)B
16
2.3
760
Table 4.1: Sample Set Information
to be outside the miscibility gap, we hypothesize that it is less likely to decompose.
Similarly, since 392A and 760 are inside or close to the gap, they are more likely to
decompose.
For a slightly more quantitative approach, the equations of Chapter 2 were used
to calculate the stability of the samples in this study. The In compositions (x)
were estimated for the sample set and are indicated in the table. From this, the
As composition (y) can be calculated assuming a lattice-matched situation, which is
appropriate here.
Y=
0.867x
0.867x
1 - 0.048x
(4.1)
The composition values can be inserted into the equations, noting that x in
Stringfellow's equations refers to Ga composition, while x refers to In for most of
this project [1]. The stability relative to the spinodal is then known.
For sample 269, % In = 6.4 and %As = 5.6, giving a positive, and thus stable
result to the stability criteria. The exact number could vary due to rounding and
constant values and was calculated as 583,432 for 269. For sample 760, %In = 16
and %As = 14, giving 17,160. Sample 392A was barely stable with a value of 6,454
for %In = 18 and %As = 16. The magnitude of the numbers indicates their relative
stability. The sample compositions are plotted on the calculated isotherm plot similar to that in Chapter 2 in Figure 4-2, only including the isotherms of the sample
growth temperatures for a more accurrate correlation between sample composition
and isotherm. The placement relative to the spinodal is clearer in the figure. Again,
recall that the spinodal isotherms are calculated for bulk material and are thus still
an estimate of probable stability.
SpinodalIsotherms_for_GaInAsSb
As
0.2
o0
\
0'.2
0:4
0:6
08
Ga
Figure 4-2: Spinodal isotherms calculated from Stringfellow [1]. The innermost dotted
contour is for 100000C, the outter solid contour is for 5250C, and the third contour is
for 575°C. The points plotted are: square - 392A, circle - 760, diamond - 269.
Chapter 5
Results and Discussion
5.1
Sample Preparation
One xsec and one pv of sample 269 provided good micrographs. The epilayer was
mostly milled away in a second perpendicular xsec. One pv of sample 760 was prepared due to time constraints. This sample was largely damaged during ion milling.
One xsec and two pv of sample 392A were prepared. The second xsec sample was
damaged during ion milling. One pv sample was prepared with the conventional
method and one with the wedge method. This brings forward two points of discussion: effectiveness of the wedge method and the conditions that produce ion milling
damage.
5.1.1
Wedge Method
Using the wedge polishing method involves trade-offs. This technique requires great
care because the wedge sample is very fragile. A number of samples were broken or
unsalvageably damaged during preparation. However, the milling time of surviving
samples was greatly decreased (1-2 hrs or less vs 2 days). This decreases the time the
soft GaSb material was bombarded by the ion beam and made the sample-making
process more efficient. A pv sample of 392A prepared by the conventional method
and the same material prepared with the wedge technique show that the sample
Figure 5-1: PV sample of 392A prepared by conventional technique.
appearances and microstructures viewed are comparable. The pictures in Figures 5-1
and 5-2 show the similarities between a conventionally polished sample and a wedge
polished sample, respectively.
The amount of thin area available for viewing on the wedge sample appears to be
less than the area visible on the conventional sample. This is probably due to the
wedge angle. If the sample is optimally 100 to 200nm thick for electron transparency,
with a one degree angle wedge, 5.7-11.5 pum into the wedge from the edge should be
visible. Depending on the thickness of the sample and if any of the thinnest area was
removed or broken off (very likely), the area could be barely sufficient or abundant
for viewing. The importance of this dependency is related to the magnification at
which the samples need to be viewed. It appears that a magnification of 39kX shows
features of interest nicely. This magnification is low enough to require a good amount
of area of sample to be thin. If the angle were decreased, the area of thin viewing
might increase. However, the extent of the effect on the fragility of the sample is
unknown.
Figure 5-2: PV sample of 392A prepared by wedge technique. Features are similar to
those seen in conventionally prepared sample.
5.1.2
Ion Milling Conditions
The quality of a sample is dependent on the conditions used during milling. The
optimal conditions seem to vary for each sample, however, some general trends were
noticed as well as the degree of sensitivity of the conditions. Milling at higher voltages,
higher gun angle, and long times produced damaging conditions as shown in Figure
5-3.
This sample was ion milled at 5kV and 12 - 130 for 8.4 hours and then ion milled
for one hour at 3kV and 100. The speckled appearance is attributable to ion milling
damage. This sample was prepared by the conventional method, which tends to
produce thicker samples for milling. The thicker the sample, the longer the milling
time required. For this material, attempting to speed up the ion milling process by
using higher voltages (5kV) creates damage. A lower voltage (3kV) should be used,
even though longer times are required.
Figure 5-4 shows a different kind of damage. The "bubbly" appearance near the
edge may be redeposited material removed by the ion beams. This sample was milled
Figure 5-3: Speckle ion mill damage visible. Sample milled at 5kV for over 8 hours
Figure 5-4: Ion mill damage in wedge sample milled for 3hrs at 3kV.
Figure 5-5: Although dark, this picture shows a wedge sample with a fairly clean
edge.
at 3kV and 100 for 3 hours. A possible explanation for the damage observed at this
shorter time is that the ion guns were unbalanced. One gun may have been operating
at more than 3kV, resulting in sample damage. Another explanation is that since
this is a wedge sample, even less time is needed to mill quality samples. Indeed, a
good sample was milled in one hour and is shown in Figure 5-5 for comparison. This
sample is also a wedge sample and shows a fairly clean edge, although the picture
contrast is a little dark.
The conditions for milling vary between samples and methods. Both methods can
produce good samples as long as the conditions are adjusted appropriately.
5.2
Microstructure
Since many previous works identify fine contrast modulation as an indication of decomposition, initial work concentrated on characterizing these features. A fine contrast modulation (FCM) is observable in the epilayer of most samples. This modulation has a "textured" or "speckled" appearance. An example of the FCM is shown
Figure 5-6: Sample 269 (0il) xsec. Note epi-substrate boundary crossing the picture
diagonally. FCM is seen in the epilayer and the substrate does not appear completely
uniform.
in Figure 5-6. It shows a cross-section sample with the GaSb substrate to the left
of the faint line diagonally traversing the picture (from top left to lower right) and
the GaInAsSb quaternary to the right of the boundary. This is the highest contrast
picture achieved for this sample, taken with g = 220-type two-beam condition. Notice
that in this picture the substrate does not have a totally uniform contrast appearance.
It appears to have some contrast change also. Regardless of cause (possible sample
preparation artifact), this indicates that FCM may not be a unique and distinct way
to characterize phase instability.
Figure 5-7 also shows a picture of FCM. This is the typical appearance of FCM
in sample 392A, the sample bordering the bulk thermodynamic miscibility gap. The
contrast modulation is more distinct and even seems to have a particular orientation
tendency. The figures 5-6 and 5-7 are both at 200kX magnification, are (0il) xsec
samples, and use a g=220-type 2-beam reflection.
The hint of a larger modulation is also present in Figure 5-7. The dimension seems
to be 125nm. However, this is not necessarily a periodic modulation. Conclusions are
Figure 5-7: Sample 392A, (011) xsec. Note fine and coarse modulation.
difficult to draw from a small area in one picture. Micrographs of lower magnification
show that periodicity is not strictly regular and modulation may be occurring on a
number of scales.
The pv sample of 760 also contains fine contrast modulation like that seen in 269.
The larger modulation does not seem to be present although more samples would
be needed to verify this. If the coarser contrast is indeed missing even though the
composition is somewhat close to the phase boundary, the difference between 760
and 392A may be due to their different substrate orientations or growth temperature.
The onset of decomposition for epilayers on 6°(111)B offcut substrates may differ
from that of 2'(110). With so few samples, only suggestions can be made at this
point.
Very distinct coarser-scale contrast modulation is seen in a pv sample of 392A in
Figure 5-8. Again, the periodicity is irregular but seems to average around 300nm.
This contrast modulation on a coarser scale is much more distinct than any of the
coarse contrast noted in previously published works. The FCM is also visible between
the distinct lines. The absence of these bands in sample 269, and thus the ability
to distinguish between the two microstructures, indicates that they may be better
features for comparing phase stability.
Figure 5-8: Coarse and fine contrast noted in sample 392A pv sample.
A closer look at sample 269 reveals a hint of a distinct coarser modulation. Figure 5-9, which is at the same magnification as Figure 5-8, shows faint contrast lines
extending in the growth direction of this cross-section sample. The lighter region
below the horizontal line is the substrate and the gray area with clearer fine contrast
modulation is the epilayer. The blurred dark lines and region extending in the vertical direction are sample bending contours. The faint distinct lines of interest and
especially the distinct contrast lines in 392A may indicate boundaries between phases
of different composition. If the features can be correlated to decomposition, this suggests that 269 is in an earlier stage of decomposition. This correlates to the trend
suggested by the bulk thermodynamic phase diagrams. 392A is near the miscibility
gap and is therefore more likely to decompose.
The important question now is how well do the features correlate to phase decomposition? Difference in composition can be indicated by these contrast changes in
TEM images because of the differing structure factor and/or strain associated with
the different phase structures. However, only a compositional analysis can verify this
connection. STEM (Scanning Transmission Electron Microscope) with EDX (Energy
Figure 5-9: Sample 269 xsec sample. Faint vertical line hints at features like those
seen in 392A related to noted coarse contrast modulation.
Dispersive X-ray) capabilities was used to determine the relative composition changes
across the 392A sample depicted in Figure 5-8. This instrument has the capability of
distinguishing 1% compositional changes for a good sample.
Figure 5-10 shows the appearance of the 392A sample in the STEM. The contrast
lines visible in the TEM micrograph look more like bands with some depth to them
in this image. Sampling the composition across these features with the EDX shows
that there is some compositional modulation. The narrow-band areas appear to have
a higher As/Ga ratio than the larger in-between areas. The In/Ga and Sb/Ga ratios
decrease in the narrow-band area. Since EDX is very accurate in indicating relative
change, the most helpful calculation is the comparison of the integrated peaks of x-ray
energy for the elements in these two areas.
Table 5.1 shows the average ratios of integrated peak energy (denoted by Nezement)
in the different types of regions. Only one data point exists for each ratio for the Average area and Forming band areas. The in-between area and narrow-band area statistics summarize four data points each. The standard deviations are small enough that
the composition change between the two areas is significant. The larger in-between
Figure 5-10: STEM micrograph of 392A. Note striations (narrow bands) crossing
picture diagonally.
Region
Average area
Larger in-between area
Narrow-band area
Forming band
NIn/NGa
NSb NGa
NAsINGa
0.383
0.390 ± 0.006
0.341 ± 0.018
0.38
1.32
1.343 ± 0.014
1.212 ± 0.040
1.27
0.22
0.209 ± 0.004
0.274 + 0.022
0.24
Table 5.1: EDX integrated intensity ratios.
band area ratios are very close to the average area ratios. This is understandable
considering that this region is present in a much higher percentage than the narrowband area. The relative changes suggest that the structure may be forming GaAs-rich
(InSb-poor) regions in the narrow-band features.
There also appears to be a finely striated region in the middle of one of the
inbetween phase areas of Figure 5-10. This region had a slightly larger As/Ga ratio
than the surrounding large phase, although the difference was not as large as observed
in the small band phase. Therefore, this may be an early stage of phase formation.
It is called the forming band region in Table 5.1.
Unfortunately, the fine contrast modulation present was too fine for accurate compositional analysis. Therefore, the origins of this modulation are inconclusive at this
stage. The modulation may indicate some phase separation, if not spinodal decomposition. Modulation has also been consistent with homogeneity [12].
In fact, F.
Glas argues that homogeneous material must exhibit FCM. Statistical fluctuations
in atomic placement on the lattices, and thus slight fluctuations in composition, will
occur.
The distinction that Figure 5-8 is a pv sample and Figure 5-9 is a cross-section is
an important one. The faint contrast lines in 5-9 are oriented in the growth direction.
The contrast lines in the pv sample tend to orient in a 100 type direction. This gives
indications of a 3D structure and of the importance of sample viewing direction.
Depending on the orientation of phases in the microstructure, xsec samples sliced 900
from each other could produce different contrast appearances. Therefore, both xsec
are needed to fully describe the structure. The pv sample should show some effects
from both xsec, but might miss features oriented in the growth direction. As both
xsec and pv samples indicate, all three sample types provide valuable information for
this study.
As mentioned before, the [100] directions are the "easy" axis for the zinc-blend
structure [2]. This means the atoms may diffuse easily along these directions. This
does not exactly correlate to the structure seen in the STEM and TEM. The bands
viewed would be expected along with a similar pattern perpendicular to the first.
Figure 5-11: Surface of 269 smooth except for a few defects.
Either one direction is preferential or this mechanism does not explain the origin.
Another possible explanation is the offcut of the wafer. 392A was grown on a (100)
substrate miscut 20 towards the nearest (110) plane. The direction of the nearest
(110) plane is actually the [100] direction. If the offcut direction created a tendency
to diffuse in that direction, this is a possible explanation for the [100] microstructural
orientation. It is also consistent with the lack of perpendicular features.
5.3
Surface Structure
Analyzing the surface roughness of samples 269, 760 and 392A also provides interesting results. A Nomarski optical photograph of sample 269 is shown in Figure 5-11.
Besides a few defects, the surface is smooth and shows no visible texturing. Sample
760 also has a fairly smooth surface. In contrast, sample 392A has a rough surface
with much texturing, as shown in Figure 5-12. The rms roughness values, obtained
via Atomic Force Microscopy, AFM, are shown in Table 5.2.
Figure 5-13 shows a 3D depiction of the surface roughness of sample 269 and Figure
Figure 5-12: Surface of 392A rough and textured in [110] direction.
Sample
269
760
392A
392A
Rms roughness(nm)
1.078
1.256
5.948
6.660
Max peak
6.428
8.429
37.66
44.252
Table 5.2: AFM Results
Scan size
1pm X 1pm
1pm X 1pm
5pm X 5pm
10pm X 10pm
Figure 5-13: AFM plot of 269 with z scale = 10nm/div
5-14 shows a 3D depiction of the surface roughness of 392A obtained via AFM. Note
that the z-scales are different and that 269 is much smoother than 392A. The amount
of area captured in the scans are also different. Enough area to suitably show the
features was captured (scale shown in figures).
The slight roughness in 269 is oriented in the [010] type direction of wafer offcut.
The surface roughness and offcut direction seem to correlate for this sample showing
little or no decomposition on a 20(110) substrate. The orientation of the roughness
of sample 392A differs from that of 269. The surface features are oriented in the
[011] direction and do not seem to correlate with the coarse microstructural features
oriented in the [100] direction.
Similar observations have been noted for other 20(110) samples viewed with Nomarski microscopy. Higher Indium content layers seem to produce a surface roughness
orientation in the [110] direction while lower In content sample texture follows the
[010] offcut direction (as in 269). The surface roughness orientation for higher In samples may be related to the adatom diffusion direction in the [Oil]. Furthermore, the
greater diffusion in the [0il] direction as opposed to the [011] may explain why the
Figure 5-14: AFM plot of 392A with z scale = 50nm/div
roughness is oriented in ridges all parallel to each other (instead of in a perpendicular
pattern or in any other direction). Since the differentiating factor is In content and
higher In contents are more likely to decompose, there seems to be some correlation
that relates decomposition and surface orientation. Therefore, although a correlation
between the extent of roughness and the microstructure seems clear, other conclusions
about feature orientation are unclear.
Figure 5-15 shows the AFM plot of sample 760. The slight roughness in this
sample is oriented in the [li0] direction. However, since this sample was grown on
a 60 (111)B substrate, the orientation tendencies may be entirely different.
If the
trend is that faint roughness alligns with offcut direction, as it does in sample 269,
760 follows this tendency. An offset towards a (111) plane would be in the [li0]
direction. Therefore, it makes sense that surface features would allign this way. Other
layers grown on 60 (111)B substrates all show surface texture orientation in the [110]
direction, regardless of In content. This shows that different tendencies exist in the
different misoriented substrates. Again, surface orientation correlations are unclear
but potentially informative.
Figure 5-15: AFM plot of 760 with z scale = 10nm/div
5.4
Comparisons
The reader may have noticed that the distinct contrast lines of 392A resemble the
contrast lines of Bulsara's work, described in a previous chapter [6]. The decomposition in these two may take the same form and they do appear to be similar.
However, the contrast in Bulsara's samples is oriented in the (110) direction, while
the phases for the sample of this work are oriented in the (100)-type direction. The
nature of the samples differ so that the features probably arise from different effects.
Bulsara's samples are relaxed epilayers on graded buffers while this study focuses on
lattice-matched epilayers. The features are related to the underlying misfit dislocation
network (as the orientation directions match) in Bulsara's work. No such network
has been observed or is expected in 392A. A better understanding of the strain and
decomposition in both these sample types is needed to explain the similarity of the
decomposition appearances.
The surface growth and bulk decomposition mechanisms may both have an effect.
And, as shown by the AFM results, the effect may vary depending on composition or
phase separation present. The [100] preferential direction is associated with a bulk
effect. However, one work mentioned in Chapter 3 does note modulation in the [011]
Figure 5-16: DP taken from the quaternary epilayer. Faint streaking is noticible.
The explanation for the different results obtained is still unknown.
Neither a simple bulk or surface model seem to be adequate for describing these
complicated systems. It must also not be taken for granted that all phase separation
directions [5].
is spinodal decomposition. Phase separation may occur by nucleation and growth or
perhaps by other methods [2]. There is much to learn about this III-V material phase
stability issue.
5.5
Diffraction Patterns
Many have looked to diffraction patterns (DP) for indication of decomposition. In
this study, no satellite spots were noted in the sample DP's. However, light streaks
did appear between reciprocal space spots and were oriented in the [111] directions
and a [100] direction for DP taken down a [110] type direction (xsec sample). These
diffraction patterns are shown in Figures 5-16 and 5-17.
Interestingly, this streaking was seen in both quaternary and substrate patterns.
In fact, the substrate streaking appeared to be a stronger effect than the epilayer
Figure 5-17: DP taken from GaSb substrate. Streaking is more noticible than in
quaternary.
streaking. Since the substrate is the binary GaSb which cannot exhibit ordering or
phase separation, we question the origin of this streaking. Glas states that streaking
is not necessarily indicative of ordering or a compositional variation [12]. It may arise
simply from strain fields in the material, which are compatible with homogeneity
[12]. This streaking phenomena in both substrate and quaternary is observed in both
sample 269 and 392A.
Chapter 6
Conclusions
The effort to understand spinodal decomposition and the results of this work direct
us back to thermodynamics and kinetics. A model appropriate for epitaxial phase
stability including possible bulk, surface, and strain factors is needed. Decomposition
may be thermodynamically possible in sample 269, but the low temperature of growth
(575 0 C) may limit the kinetics. The diffusion to decompose is prevented by the low
temperature. This is supported by the general trend that as growth temperature
decreases, epilayer quality increases (determined by surface appearance and electric
properties) [20]. Although 392A was grown at a lower temperature (550 0 C) than 269,
the higher Indium composition may provide a large enough driving force so as to
lower the activation barrier for diffusion, causing phase separation.
Although many questions remain, this project did increase understanding of spinodal decomposition and how to characterize it in the GaSb system of interest. The
wedge method is a time saving preparation method that can produce valuable results
comparable to the conventional preparation method. The sensitivity of the material
to ion milling conditions was demonstrated and appropriate conditions were characterized.
Fine contrast modulation in the TEM micrographs proves to be a difficult feature
to distinguish and quantify. However, a coarser modulation was found to correlate to
likelihood of decomposition and possibly surface roughness. The direction of surface
features may also be related to decomposition, depending on substrate misorientation.
A more thorough characterization of these features is needed to better understand
their origin. This will involve investigating the correlation between surface features
and microstructure more thoroughly. The different orientation of these features remains a puzzle.
Several systematic experiments are needed to verify and extend the suggestions of
this work. First, the sample sets for 269, 760 and 392A should be completed to include
a pv and two xsec samples for each. Compositional analysis could be performed on
each to verify the results thus far. Also, samples with varying degrees of strain could
be studied to understand this effect more. For all these samples, a more thorough
investigation of the DP's would also be needed. The streaks observed in the patterns
could be imaged to obtain different types of dark-field images (differing from the
bright-field imaging of this work).
Another dimension to add to the project could be a correlation with X-ray diffraction and PL data in a systematic study. These additional methods would help verify
the results seen in TEM. This project was really just the beginning of the GaSb
spinodal decomposition investigation.
Future work can continue with the current
understanding and consciousness of the issues raised in completing this investigation
thus far.
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