ARCHIVES
MASSACHUSETTS INSTITUTE
OF TECHNOLOLGY
Infrared Photoconductive PbTe Film
Processing and Oxygen Sensitization
JUN 08 2015
by
LIBRARIES
Christopher J. Klingshim
Submitted to the Department of Materials Science and Engineering as partial fulfillment
for an S.B. degree in the Department of Physics at the
Massachusetts Institute of Technology
June 2015
C 2015 Christopher J. Klingshim. All rights reserved.
The author hereby grants to MIT permission to reproduce and to
distribute publicly paper and electronic copies of this thesis document in whole or in part
in any medium now known or hereafter created.
Signature of Author:
Signature redacted
Department of Physics
May 8, 2015
Certified by:
Signature redacted
Anuradha M. Agarwal
Principal Research Scientist, Materials Processing Center
Thesis Advisor
Certified by:
Signature redacted
Professor 9aterals
1,1,,,
Accepted by:
Lionel C. Kimerling
nce and Engineering
Thesis Reader
/
Signature redacted
_
Geoffrey Beach
Professor of Materials Science and Engineering
Chairman, Undergraduate Committee
Infrared Photoconductive PbTe Film
Processing and Oxygen Sensitization
by
Christopher J. Klingshirn
Submitted to the Department of Materials Science and Engineering on May 8, 2015 as partial
fulfillment for an S.B. degree in the Department of Physics
Abstract
Infrared (IR) thermal detectors and photodetectors have significant applications including
thermal imaging, infrared spectroscopy and chemical and biological sensing. In this work we
focus on photodetectors, which typically use narrow gap semiconductor materials requiring
cryogenic cooling to provide measurable signals above thermally generated noise. Our study
investigates one class of photodetectors, namely photoconductive semiconductor films. When
embedded within resonant cavities, these films are additionally capable of precise detection at
narrow, selectable bands and enable the development of monolithically-integrated detectors that
are physically small, highly responsive and able to record data autonomously. Lead
chalcogenides such as PbTe are ideal photoconductive material candidates because (i) low-cost
thermal deposition produces polycrystalline films that exhibit good mid-IR responsivity without
being subject to lattice-matching constraints, and (ii) they do not require cryogenic cooling. We
show that the responsivity of polycrystalline PbTe is enhanced by oxidation annealing.
This investigation sought to determine a viable set of processing conditions for thermally
depositing oxygen-sensitized PbTe photoconductors on Si substrates. Depositions were
performed under high vacuum on the order of 1 0-6 Torr. Physical shadow-mask and
photolithographic techniques were used to pattern the films in order to produce photoconductive
samples with varied film and electrical contact geometries. The introduction of non-functional
"dummy layers" within 100-300 pm of the usable samples prevented undesired film peeling
during the lift-off process.
PbTe films displayed an FCC rocksalt structure and slight preference for (200) texture when
thermally deposited on a Si substrate. A 250-nm thick sample exhibited large photoconductivity,
with responsivity higher than 100 V/W between 2-3 jim wavelengths, a factor of 4 higher than
literature values for similar films. Sn metal formed highly ohmic contacts with the PbTe layer,
permitting Hall experiments that showed the film to be p-type with a carrier concentration of
1.49 x 1017 cm-3 and Hall mobility of 21 cm 2 V- 1 s-. The carrier concentration was thermally
activated with activation energy of 0.137 eV. These values are comparable to past experiments in
which the film was sensitized by exposure to oxygen at ambient conditions. Further research is
needed to establish the exact origin of the enhanced photoconductivity observed.
3
Thesis Advisor: Anuradha M. Agarwal
Title: Principal Research Scientist, Materials Processing Center
Thesis Reader: Lionel C. Kimerling
Title: Professor of Materials Science and Engineering
Acknowledgements
I would like to thank Dr. Agarwal for her advice, guidance and feedback over the course of this
research.
I would also like to thank Prof. Kimerling for his advice and support, not only during this project
but throughout the past three semesters.
I am grateful to Vivek Singh for all of his help with sample preparation, measurements, and
interpretation of the results, even while working on his own doctoral thesis.
4
Contents
Abstract
3
Acknowledgements
4
1 Introduction
7
1.1 Motivation
7
1.2 Photoconductivity mechanism
8
1.3 Structural, electrical, and optical properties of PbTe films
10
1.4 Photoconductivity and oxygen sensitization
12
13
2 Materials and methods
2.1 Film deposition using thermal evaporation
13
2.2 Patterning techniques
15
2.3 Photolithographic pattern designs for 2- and 4-contact films
19
2.4 Structural characterization
21
2.5 Electrical characterization
21
2.6 Photoconductivity measurement
23
24
3 Results and discussion
3.1 Structural characterization
24
3.2 Electrical characterization
25
5
28
3.3 Photoconductivity
4 Conclusions
30
4.1 Summary
30
4.2 Future work
31
33
References
6
1 Introduction
1.1 Motivation
Infrared (IR) detection devices have numerous scientific and industrial applications,
including thermal imaging and IR spectroscopy, particularly for identifying chemical and
biological species that absorb characteristic wavelengths between 3-12 jim [1]. In principle, any
physical phenomenon requiring an energy transfer on the order of 0.1-1 eV can serve as a
mechanism for IR detection [2]. IR wavelengths on the order of 10 pim correspond to
temperatures on the order of 100 K and can be detected using thermal phenomena such as the
Seebeck effect, as seen in thermocouples, or a temperature-dependent resistivity as in
bolometers. For more precise detection of narrower bands, or of shorter mid-IR wavelengths,
photodetectors which use semiconductor devices exhibiting photovoltaic properties (e.g., pn
diodes) or photoconductivity are commonly employed in conjunction with resonant cavities
[2,3]. Such devices enable the development of monolithically integrated detectors that offer
advantages including small device footprint, high sensitivity at specified wavelengths, and the
ability to record data without human intervention [1].
A photodetector can be made responsive to desired wavelengths by selecting an
appropriate semiconductor material and doping parameters. Common photoresponsive materials
include IV-VI semiconductors such as the lead chalcogenides PbS, PbSe and PbTe, JJ-VI
semiconductors such as HgCdTe, and numerous III-V configurations [2]. In addition to the
performance metrics, the conditions required to fabricate photoresponsive films can vary
significantly depending on the materials used. For example, single-crystalline HgCdTe is often
7
used for the photoresponsive layer, but the molecular beam epitaxy technique required to
produce an appropriate high-purity single-crystalline film is difficult and expensive [1].
Polycrystalline lead chalcogenide films such as PbTe can be deposited easily using thermal
evaporation and still exhibit high photoresponsivity in the mid-IR regime [1]. The polycrystalline
nature of thermally-grown PbTe means it is not subject to lattice-matching constraints and is
more easily integrated with any desired substrate. Additionally photonic structures such as
resonant cavities and waveguides can be fabricated on multiple levels, hence enabling vertical
integration when necessary [7]. Lower processing costs and the potential for monolithic
integration make PbTe detector films worthy of further investigation for mid-IR sensing
applications.
1.2 Photoconductivity mechanism
Photoconductivity is the increase in a material's electrical conductivity due to an incident
radiation flux. Photons with energies greater than the bandgap energy Eg of the material can
generate electron-hole pairs that increase its carrier concentration, and thus its conductivity [3].
The conductivity u of a material is related to its carrier concentrations and mobilities by
a= nep1n + peup,
(1)
where n and p represent the electron and hole concentrations, pi and p, are their respective
mobilities, and e is the elementary charge [1]. The change in conductivity AU is therefore
proportional to the photo-induced changes in carrier concentrations An and Ap,
Au = Anepn + Apep ,
provided that illumination does not significantly affect the mobilities. Petritz and Slater have
8
(2)
found this assumption to be reasonable for PbTe [4-6].
Change in conductivity is measured by subjecting the photoconductive sample to a fixed
bias current or voltage and measuring the change in the circuit voltage or current in response to
illumination of the photoconductor [3]. Fig. 1 illustrates a simple circuit for measuring
photoconductivity under a bias current. When the sample resistance is small, a large load
resistance RL >> Rsa,nepi is placed in series in order to maintain a constant bias current. The change
in voltage across the sample due to incident light is proportional to AU when AU << a [1].
Therefore a convenient metric for photoconductivity is the responsivity R, defined as the change
in voltage A Vper watt of incident optical power,
AV
R =
opt
A'(3 ,
where Iop, is the incident optical intensity per unit area and A is the detector area. When the
sample resistance is large, as in a photodiode, a bias voltage is applied and the current flowing
through the sample is measured instead; responsivity is then expressed in A/W.
9
(3)
Incident
radiation
Ohmic
metal contact
Photoconductor
RL
Voltage
signal
Fig. 1 A simple circuit for measuring the response of a photoconductive material to incident radiation.
When RL >>
Rsampie,
there is a constant bias current in the circuit and the voltage signal across the
photoconductor may be used to calculate Au, as well as the responsivity R in volts per watt of incident
optical power.
The photoconductor is connected to the circuit by metal contacts known to exhibit ohmic
behavior, such as Sn on PbTe [3,7]. Ohmic contacts are needed to prevent non-linearities from
disrupting the relationship between the photo-induced change in conductivity and the measured
quantity (current or voltage) used to represent it.
1.3 Structural and electrical properties of PbTe films
Numerous techniques exist for depositing films of atomistic or molecular species,
including thermal evaporation, ion beam deposition, molecular beam epitaxy (MBE), sputtering,
and chemical vapor deposition [8]. Thermal evaporation is often used for the deposition of
10
polycrystalline lead chalcogenides because it can produce a photoresponsive film at low cost and
with relative ease of operation [6,9]. However, it is difficult to control the microstructure of
thermally deposited films. In crystalline form PbTe exhibits a face-centered cubic (FCC) rocksalt
structure with a lattice parameter of 6.454 A [7]. PbTe films thermally deposited on oxide-coated
Si substrates are polycrystalline, with the average surface roughness and grain size increasing
with film thickness [7]. Wang et al. found nanocrystalline PbTe films to favor the (200)
orientation, with the degree of (200) texture increasing with decreasing film thickness [7]. This
relationship suggests that the grain orientation is initially determined by the surface energy at the
PbTe-SiO 2 interface [7]. The degree of texturing and average grain size have been found to
diminish at lower substrate temperatures, presumably due to a deficiency in thermal energy
required for ordering [10].
PbTe belongs to the IV-VI family of narrow-bandgap semiconductors that have
applications in IR laser diodes as well as IR detectors [11]. Bulk PbTe has a bandgap of 0.31 eV
at 300 K; however an optical bandgap of 0.39 eV has been measured in nanocrystalline PbTe
films, likely due to the small crystallite size (~50-100 nm) contributing to a quantum
confinement effect [9]. The formation of ternary compounds with elements such as Sn or Cd in
addition to S, Se or Te allows bandgap tunability between 0.05-0.5 eV, corresponding roughly to
wavelengths of 3-30 pm [11], a desirable feature for narrow-band or multi-spectral IR sensing
applications.
11
1.4 Photoconductivity and oxygen sensitization
The photoconductive properties of PbTe have been known since the 1950s, when
polycrystalline lead-salt films were used in missile defense systems for detecting hot exhaust
gases [2]. However, the photoconductive response is known to be weak in films without some
degree of oxygen incorporation [12]. Oxygen diffusion into the film introduces (PbO)" acceptor
states in grain boundaries that reduce the concentration of electrons in the conduction band,
converting the film from n- to p-type, and increasing the carrier lifetime and thus the
photoconductivity [7,10]. Due to the importance of grain boundaries in the oxygen-incorporation
process, smaller average grain size tends to correspond to higher photoconductivity [7,10]. Bode
and Levinstein report that substrates held at lower temperatures during PbTe deposition are
significantly more susceptible to oxygen sensitization [12]. This trend is consistent with more
recent observations linking lower substrate temperatures to smaller average grain size and
minimal (200) texturing, which increase the grain boundary density and facilitate the
incorporation of oxygen [7,9,10]. In early investigations oxidation annealing was performed at
high temperatures (500-600 'C) [10], but Wang et al. have demonstrated successful oxygen
sensitization at ambient temperature and pressure [7]. Peak mid-IR responsivities between 10-25
V/W have been observed in nanocrystalline PbTe films treated in this manner [7].
12
2 Materials and methods
2.1 Film deposition using thermal evaporation
The photoconductive films used in this investigation were deposited using a dual-source
thermal evaporation apparatus assembled by PVD Systems. Sn metal contact films were created
using a similar thermal evaporation apparatus from the Kurt J. Lesker company. A schematic of a
thermal evaporation chamber is presented in Fig. 1. To deposit a film, the substrate is placed in
the chamber above a heated Ta boat containing source material in the liquid state. Because the
source material vapor pressure is higher at the surface of the melt than at the substrate, there is a
net flux of source material from the boat towards the substrate. The source boat aperture is small,
on the order of 1 cm, and the boat may be modeled as a point source that generates spherical
surfaces of constant vapor density. The thickness variation associated with projecting the vapor
flux on a flat substrate was minimized by using a source-substrate distance of-50 cm, an order
of magnitude larger than the typical substrate width of roughly 5 cm.
13
holder
thickness
monitor
substrate
shutter
DIFF.
PUMP
souroe
boats
ROUGHING
PUMP
VACUUM
:CHAMBER
|o
...
POWER
SUPPLIES
GND
Fig. 2 A thermal evaporation chamber similar to the apparatus used in the investigation. Molten source
material evaporates outwards from a heated boat and travels towards the comparatively cool substrate
where it condenses. Changes in the natural frequency of a quartz crystal monitor enables real time
measurement of the rate of deposition on the substrate, and thus the film thickness. Adapted from [13].
The deposition rate is controlled by varying the current used to heat the source boat;
deposition rates between 6-8 A/s were used in this investigation. The rate is measured throughout
the process via the changing natural frequency of a quartz crystal monitor, which is offset from
the source vapor path to avoid shadowing the substrate. The deposition rate observed by the
crystal monitor is related to the rate of deposition on the substrate by an empirically-determined
constant known as the tooling factor, which is the ratio of the measured film thickness following
the deposition (e.g., via profilometry) to the thickness indicated by the crystal monitor. The
14
tooling factor depends on the relative positions of the source boat, crystal monitor and substrate
within the apparatus as well as the material being deposited; a value of 37 was used in this
investigation for PbTe deposition on silicon dioxide on a silicon wafer.
A mechanical shutter allowed the deposition rate to be stabilized at the desired value
before the sample was exposed, and kept contaminants that may have been present on the solid
source material from being deposited on the substrate. To prevent scattering of the source vapor,
depositions were performed under high vacuum, at pressures on the order of 10-6 Torr (about 10-'
Pa) or lower, at which the mean free path of the source vapor atoms is significantly longer than
the length scale of the deposition chamber. For an ideal gas, the mean free path A is described by
the expression
kT
(4)
\-27rdp
where d is the atomic diameter, p is the gas pressure, T is the absolute temperature, and k is the
Boltzmann constant. Using the approximate values T = 1200 K, p = 10- Pa and d = 4 A, A for
PbTe is about 230 m. The vacuum in the PbTe deposition chamber was created by an oil
diffusion pump in series with a mechanical roughing pump, while the metal deposition apparatus
employed a faster turbomolecular pump and roughing pump. In all cases the chamber was
pumped for several hours prior to deposition to eliminate residual water vapor or other adsorbed
gases from the chamber walls.
2.2 Patterning techniques
Patterning allowed several small samples to be created on a single substrate by keeping
the film from adhering to the substrate in certain regions. In all cases the substrate was one
15
quarter of a 4-inch Si wafer with a 3-pm layer of thermally-grown oxide. Both physical shadowmask and photolithographic techniques were used to implement patterns for films and metal
contacts of various sizes and geometries. Fig. 3 demonstrates the creation of a set of films and
overlaid metal contacts using metallic shadow masks (the "PbTe-first" configuration).
(a)
(b)
Substrate
PbTe
(c)
(d)
Sn
mom
Fig. 3 Shadow-mask fabrication technique for PbTe films and metal contacts. (a) A shadow mask for
patterning the film was held in front of the substrate during the PbTe deposition, creating the samples
shown in (b). The mask illustrated in (c) determined the position of the metal contacts as depicted in (d).
16
Separate depositions were required to make the films and contacts. Prior to each
deposition the appropriate mask was affixed in front of the substrate using double-sided Kapton
tape. Shadow masks were also used to deposit metal contacts (-500 nm thermally-deposited Sn)
underneath the PbTe layer. The shadow-mask technique was used to create a set of thin PbTe
films with a target thickness of 100 nm at a pressure of 4
films during a deposition at a pressure of 2
x
x
106 Torr as well as thicker 250 nm
106 Torr.
A photolithography and lift-off technique was also used for patterning in order to achieve
higher resolution, ptm-scale pattern features [14]. This process is depicted in Fig. 4. The
substrate was first prepared by uniformly coating it with a negative photoresist (NR9- 1 OOOPY
from Futurrex Inc.), an organic material which becomes insoluble to a developing agent when
exposed to ultraviolet (UV) light. Spin-coating the substrate at 3500 rpm for 30 s yielded a
photoresist film about 1.2 im thick. The substrate was soft-baked on a 150 'C hot plate for 65 s
to drive off excess solvent. A polyester-based photomask (Fig. 4a) was then used to selectively
block UV light (80 s exposure to 365 nm light with an intensity of 9-10 W cm-2 ) from regions of
the substrate that were to be deposited with PbTe. A 65 s post-exposure bake at 110 'C crosslinked the photoresist and made it insoluble in the developer (Futurrex RD6). In the un-exposed
regions, the photoresist was removed by the developer, leaving the substrate exposed during the
deposition process (Fig. 4b). Acetone was used after film deposition (Fig. 4c) to dissolve the
remaining photoresist and "lift off' the deposited film above it (Fig. 4d-e).
17
4--
()
Substrate
Photomask
Unexposed photoresist
removed by developer
Exposed photoresist
(b)
remained
Deposited film
(C)
(d)
Acetone used to dissolve
remaining photoresist, lifting
off film from undesired
regions
LTLATL
Final deposited film
and substrate
(e)
Fig. 4 Photolithography and lift-off patterning process [15]. (a) Dark regions of the photomask blocked
UV light to the regions where the film was to be deposited. (b) The unexposed photoresist was removed
by developer. (c) The system after film deposition. (d) Acetone was used to dissolve the remaining
photoresist, lifting away the undesired regions of deposited film. (e) The final film and substrate.
As with the shadow mask, photolithography and lift-off could also be used to deposit the
metal contacts first, followed by the PbTe layer. This option provided the flexibility to
investigate both configurations for potential performance differences and is particularly
important when additional layers are to be added beyond a photoconductor and metal contact,
such as the GeSbS glass capping layer used by Wang et al. to control the extent of atmospheric
oxygen diffusion in PbTe photoconductors [7].
18
2.3 Photolithographic pattern designs for 2- and 4-contact films
What constitutes an optimal photoconductor design depends on the intended use. For
example, the structural characterization techniques such as profilometry and X-Ray Diffraction
(XRD) described in Section 2.4 require uniform thickness and composition across a suitable
distance, but do not involve electrical contacts. Simple electrical measurements such as
responsivity and I-V characterization require 2-contact samples, often with a simple rectangular
and
geometry. The more sensitive Hall experiments used to determine carrier type, concentration
mobility must be conducted on 4-contact continuous, symmetrical films of uniform thickness as
illustrated in Fig. 6. [16]. The Hall technique is discussed further in Section 2.5.
An even more basic requirement than high measurement accuracy is that the films and
contacts not be damaged during the lift-off process, which can be highly sensitive to processing
conditions such as the overall fraction of substrate coverage [14]. During initial attempts to
perform lift-off when only a small fraction of the substrate area was covered with PbTe (similar
to the degree of coverage in Fig. 3b), small- and medium-scale features did not remain properly
adhered to the substrate, lifting off along with the exposed photoresist as depicted in Figure 5.
Damaged PbTe films
was
Fig. 5 PbTe features were damaged when the overall PbTe coverage fraction was low. This problem
resolved by surrounding functional devices with "dummy layers" to increase the uniformity of coverage.
19
This problem was resolved by surrounding the PbTe and Sn mask features with a
"dummy layer" that extended to within 100-300 ptm of the functional device features. Although
dummy regions did not serve a functional purpose, they played an important role during the liftoff process by increasing the film coverage fraction near the devices. Covering a higher fraction
of the substrate increased the film uniformity, preventing it from peeling. Two sets of 4-contact
devices and surrounding dummy regions are depicted in Fig. 6. Pairs of alignment marks allowed
the second pattern to be properly placed above the first, ensuring that the dummy regions did not
interfere with the functional devices.
5 mm
Substrate
Dummy
regions
Alignment
marks
Fig. 6 Two sets of devices (PbTe shown in black, Sn in purple) and dummy regions (green), which
surrounded the functional films and contacts to within 100-300 lam. The dummy layers prevented
unintended peeling of films during the lift-off process. Cross-shaped alignment marks at the mask edges
prevented the dummy regions from becoming skewed and interfering with functional devices.
A complete set of devices with Sn contacts is depicted in Fig. 7.
20
L
Fig. 7 Fabricated devices illustrated in Fig. 6, which were surrounded within 100-300 pm by dummy
layers. None of the devices displayed observable peeling.
2.4 Structural characterization
Film thickness was measured using a Veeco Dektak 150 surface profilometer. The 12.5
gm tip was too large to render small-scale features and surface roughness, but the resolution was
sufficient to confirm that the film thickness was broadly uniform across the substrate. X-Ray
Diffraction (XRD) was performed on some samples to determine the crystal structure and
preferred grain orientation. The scan rate was 3 degrees per minute with a step size of 0.02
degrees.
2.5 Electrical characterization
I-V curves were recorded for all viable films with 2 or 4 contacts in order to determine
whether they exhibited ohmic behavior indicated by a linear I-V characteristic). Voltage was
varied between -5V and +5V in 0.2 V increments. For 4-contact samples, separate I-V curves
were measured for each diagonal. For some samples, additional measurements were taken while
the sample was shielded from ambient light in order to determine any effect it may have on I-V
behavior.
21
Hall experiments were conducted at room temperature on 4-contact samples in order to
determine the dominant carrier type, concentration and Hall mobility. Hall measurements require
specific geometries - the sample thickness must be significantly smaller than its length or width,
it must not have any holes, and the 4 electrical contacts must be positioned on the sample corners
as in Fig. 6 [16]. The carrier type is indicative of the degree of oxygen sensitization; pure,
thermally-deposited PbTe films tend to be n-type due to poor adhesion of chalcogen atoms on the
substrate, but transition to p-type due to the presence of oxygen-induced (PbO)" acceptor states
in grain boundaries [7,9]. The activation energy extracted from the variation of resistance with
temperature (set by a thermoelectric cooler between -65 'C and room temperature) indicated the
extent of band bending caused by oxygen incorporation (Fig. 8).
Grain boundary
Grain 1
Grain 2
Electrons
Ec
- - - - ---a
EF
E
E Es
~-2
Holes
0
p-type channel
Fig. 8 Energy band diagram in the vicinity of a PbTe grain boundary [9]. E, is the valence band edge, E,
is the conduction band edge, Ef is the Fermi level, E, is the degree of band bending caused by oxygen
incorporation, and E, is the activation energy of conduction, which was measured via the variation of
resistance with temperature.
22
2.6 Photoconductivity measurement
Photoconductivity measurements were conducted using IR light from a blackbody source
passed through a Perkin-Elmer NaCl monochromator, with a chopping frequency of 10 Hz. The
signal was amplified with a lock-in amplifier using a time constant of 2. Voltage response
measurements were made in 1 00-nm steps from 800-5000 nm under a bias current of 0.1 mA.
The sample responsivity
RPbTe
was determined by comparison to a reference thermocouple
detector with a known responsivity R,,f of 0.3 V/W and area of 1 mm 2 via the expression
VPbTe Aref
RpbTe
= Rree
,
(5)
VrefArbe
which is derived from equation 3 and relates the responsivity of the PbTe sample to that of the
reference thermocouple by their areas A and voltage-response ratios V. Samples were placed in
an evacuated chamber and cooled by a thermoelectric cooler (TEC) operating at -62 'C in order
to minimize the influence of thermal noise, as well as to simulate the typical operating
temperature of thermoelectrically-cooled IR detectors [7].
23
3 Results and discussion
3.1 Structural characterization
Surface profilometry indicated that sample thicknesses were all within 10% of their target
values. The 100-nm film was shown to be 91 nm thick and the 250-nm film was shown to be 246
nm thick, indicating good calibration of the deposition apparatus and selection of tooling factor.
XRD analysis confirmed the presence of the expected FCC rocksalt structure and
revealed preferential (200) texture in the films. Peak magnitudes for the 250-nm thick sample
were compared with reference data from powder diffraction file (PDF) card #04-004-4327,
presented in Fig. 9. The I(200)/I(220) intensity ratio for the sample is 4.2, higher than the value
of 1.45 given by the reference data for randomly-oriented crystallites. This relationship is
consistent with the literature, although Wang et al. observed a much higher (200)/(220) intensity
ratio of about 60 for a similar 300-nm thick film [7]. The smaller degree of ordering could have
been the consequence of a lower substrate temperature during deposition, in accordance with the
observed trend from previous research [10], though the substrate temperature was not controlled
and a causal relationship can therefore not be established.
24
(a)
Evaporated 250 nm PbTe film
(200)
-
12000
10000
t8000
~6000
CD 4000
(220)
2000
(222)
(
J
L
0
20
30
(420)
(311)~ (400)
40
A
60
50
(422
70
4
(440) (600)80
90
100
20 (Deg)
100
80
Standard XRD Data of PbTe Powder
(b)
~E 60
.
40
-
20
0
'
-' 1.' I ' .-'I'
20
30
40
60
50
70
80
90
100
20(Deg)
Fig. 9 (a) XRD spectrum of thermally-deposited 250-nm PbTe film on Si. The film exhibited the
expected FCC rocksalt structure and (200) texture, indicated by an 1(200)/I(220) intensity ratio of 4.2.
(b) Normalized reference XRD spectrum for PbTe powder (PDF card #04-004-4327), with an
1(200)/1(220) ratio of 1.45.
3.2 Electrical characterization
Strong linearity in the I-V measurements confirmed that thermally-deposited Sn formed
Ohmic contacts with the PbTe films. Portions of the I-V characteristics for a set of six 2- and 4contact films deposited using the shadow mask (Fig. 3) are plotted in Fig. 10.
25
20
-1a
+A-b
30
2an
--'
2b
-3a
19
~ -
20-
3bi
18
104-contact samples
(b)
(a)
0
17
C
3
2
1
2.5
v (V)
2.6
2.8
2.7
2.9
3
V (V)
(C)
4
53
L6u
U
3
2
1
Diag. b \/
.Diag. a
Fig. 10 Linear I-V characteristics of the 2- and 4-contact photoconductors indicate the ohmic nature of
the PbTe-Sn contact. (a) The 2-contact samples #4, 5, and 6 compared with the 4-contact diagonal
measurements. (b) I-V slopes measured along the diagonals a and b of the 4-contact samples #1, 2, and 3
were highly consistent. (c) Arrangement of samples on the substrate and their approximate relative sizes.
The constant slope of each I-V curve yields the sample resistance Rsampie via Ohm's law,
I
1
V
Rsample
(6)
Sample resistances were in the range of 104
-
10' Q and are tabulated in Table 1. Close
agreement between the resistances of the identical 2-contact samples #4 and 5 and all of the 4contact samples indicate that the film thickness and composition were uniform across the film.
Exposure to ambient light did not cause any noticeable deviation from the dark measurements.
26
-
4-contact
2-contact
Sample #
la
1b
2a
2b
3a
3b
4
5
6
Resistance (kQ)
153
144
153
153
152
155
84
82
31
Table 1 PbTe sample resistances were consistent among all 4-contact samples as well as among the 2contact samples #4 and 5, which were the same size, indicating uniformity of film thickness and
composition.
Hall measurements on the 100 nm film were inconclusive; however, the data taken on the
250 nm sample are generally consistent with expectations. The film was shown to be p-type with
a majority carrier concentration was 1.49 x 1017 cm-3 , indicative of some degree of oxygen
incorporation, barring a significant presence of other p-type impurities. This carrier concentration
is significantly greater than the calculated intrinsic value of 8 x 1015 cm-3 , but only 20% above
the 300 K hole concentration reported by Wang et al. for thermally-deposited PbTe films exposed
to atmospheric oxygen at room temperature [7]. The observed Hall mobility was 21 cm 2 V-1 s-1, a
factor of 4 below the value measured by Wang et al.
Analysis of resistance variation with temperature yielded the carrier concentration
activation energy, plotted in Fig. 11. Resistance exhibited a strong exponential dependence on
temperature, suggesting that carrier concentration is determined by a thermally activated process.
The associated activation energy of 0.137 eV is consistent with literature results for PbTe films at
temperatures greater than 200 K, which found activation energies of 0.109 eV [7] and 0.144 eV
[9]. It is also lower than the conduction activation energy expected for intrinsic PbTe, indicating
the presence of band bending, which in conjunction with the observed p-type conduction
supports the model of oxygen-induced (PbO) 2 ' acceptor states in the grain boundaries.
27
3
13
U
(R,T) data
Linear fit
10
R 2 = 0.99
Ea = 0.137 eV
0.003
0.005
0.004
)
l/T (K-
Fig. 11 PbTe film resistance varied exponentially with temperature between 210-300 K, suggesting a
thermally activated mechanism governing carrier concentration with activation energy Ea of 0.137 eV.
This observation is consistent with the literature for PbTe films in the same temperature range [7,9].
3.3 Photoconductivity
Photoconductivity was not observed in the 100 nm film because it exhibited bolometric
behavior, as indicated by an exponentially decreasing variation of voltage signal with incident
wavelength, and rapidly decreasing resistance with temperature (from 445 kQ at -45 'C to 66 kQ
at room temperature). However, the 250 nm film was highly photoconductive. The PbTereference voltage ratio is plotted in Fig. 12. The corresponding responsivity was greater than the
values observed by Wang et al. by a factor of 4, with a responsivity of over 100 V/W between 23 pm as illustrated in Fig. 13 [7]. A plateau is seen beyond 4 pm in contrast to the expected drop,
which may indicate a lower bulk PbTe bandgap at low temperature, or simply noise in the signal.
Higher responsivity could be a result of the comparatively low degree of (200) texturing, which
may have increased the surface roughness and led to greater absorption of the incident light.
28
800 i '
I
'
I
I
i
i
i
1
2
3
4
5
'
i
-
600
-
400
low
r-
'
-
200
U
Wavelength (pmir)
Fig. 12 Sample-reference voltage ratio VpbTe ref of the 250 nm PbTe film as a function of incident
wavelength. The voltage ratio is directly related to the responsivity (plotted in Fig. 13) by equation 5.
140
v
250 nm
200 nm*
--- 300 nm*1
-
120
100
80
0
0
60
40
*Wang et al.
20
0
1
3
2
4
5
Wavelength (pin)
Fig. 13 Responsivity as a function of wavelength for the 250-nm sample tested in this investigation. Peak
responsivity between 1-4 pm was up to a factor of 4 greater than in comparable nanocrystalline PbTe
films deposited thermally on Si/SiO 2 substrates, such as those tested by Wang et al. [7].
29
4 Conclusions
4.1 Summary
Even prior to the collection of quantitative data, this investigation demonstrated
processing techniques for fabricating viable photoconductive PbTe films for use in research.
Thermal evaporation was an effective technique for depositing sufficiently uniform films of the
desired thickness. A physical shadow mask was successfully used to place several
photoconductive films and electrical contacts on a single Si substrate, including many of the
devices analyzed in Section 3 of this thesis. With photolithography and lift-off patterning,
dummy layers were found to be effective in preventing undesirable film peeling by increasing
the fraction of substrate coverage in the vicinity of functional devices.
The quantitative results largely confirm the results of previous research on PbTe
photoconductors. PbTe exhibited an FCC rocksalt structure with a slight preference for (200)
texture when thermally deposited on an Si substrate. Sn metal was found to form an ohmic
contact with the PbTe film, enabling Hall effect and photoconductivity measurements to be
performed. The carrier type, concentration and conduction activation energy were comparable to
a similar film that had been sensitized with oxygen under ambient temperature and pressure,
suggesting the presence of oxygen-induced band bending, and the Hall mobility was smaller than
the literature value by a factor of 4 [7].
Some inconsistencies with past investigations also arose. The 250-nm sample exhibited a
significantly larger responsivity than similar films in past investigations, as well as a lower
degree of (200) texturing. In contrast, the 1 00-nm film, which was similar to one that was
30
previously found to exhibit high photoconductivity, was instead bolometric. No photoconductive
response could be observed, and furthermore both the I-V and Hall measurements were
inconclusive. The failure of the 1 00-nm film in multiple ways and against expectations from the
published literature can likely be attributed to a processing error, though its exact nature could
not be determined.
4.2 Future work
The 250 nm samples studied in this investigation exhibited remarkable photoresponsivity
in excess of 100 V/W for certain wavelengths in the mid-IR band, a factor of 4 greater than the
best performing films discussed in the literature [1,7]. It would be a significant advancement to
identify and reproduce the mechanism responsible for its high responsivity. A possible link
between the lower degree of (200) texturing, surface roughness and photoconductivity should be
investigated further. Other factors worthy of further investigation include processing conditions
such as deposition background pressure, deposition rate and substrate temperature. Additional
measurements should also be performed. Measurement of the grain size could be performed
using XRD or atomic force microscopy (AFM), and the degree of oxygen incorporation could be
quantified using X-ray Photoelectron Spectroscopy or Secondary-Ion Mass Spectrometry, and
the surface roughness could be quantified using surface profilometry or AFM. Further research
on highly responsive PbTe photodetectors will be essential to the development of monolithically
integrated IR detectors for remote, small-scale chemical and biological sensing and thermal
imaging applications.
31
32
References
[1] V. Singh et al, "Evanescently coupled mid-infrared photodetector for integrated sensing
applications: Theory and design," Sensors and Actuators B 185 (2013) 195-200.
[2] Antoni Rogalski, "Infrared detectors: status and trends," Progress in Quantum Electronics 27
(2003) 59-210.
[3] Antoni Rogalski, InfraredDetectors, Second Edition, CRC Press, Boca Raton, FL, 2011.
[4] R.L. Petritz, Theory of photoconductivity in semiconductor films, Physical Review 104
(1956) 1508-1516.
[5] J.C. Slater, Barrier theory of the photoconductivity of lead sulfide, Physical
Review 103 (1956) 1631-1644.
[6] Wang, Jianfei, Resonant-cavity-enhancedmultispectral infraredphotodetectorsfor
monolithic integrationon silicon, Thesis (Ph. D.) - Massachusetts Institute of Technology, Dept.
of Materials Science and Engineering, 2010.
[7] Wang et al., "Room-temperature oxygen sensitization in highly textured, nanocrystalline
PbTe films: A mechanistic study," Appl. Phys. Lett. 110, 083719 (2011).
[8] C. G. Granqvist, "Preparation of Thin Films and Nanostructured Coatings for Clean Tech
Applications: A Primer," Solar Energy Materials & Solar Cells 99 (2012) 166-175.
[9] Wang et al., "Structural, electrical, and optical properties of thermally evaporated
nanocrystalline PbTe films," J. Appl. Phys. 104, 053707 (2008).
[10] Z. Dashevsky, R. Kreizman, and M. P. Dariel, "Physical properties and inversion of
conductivity type in nanocrystalline PbTe films," J. Appl. Phys. 98, 094309 (2005).
[11] M. Tacke, A. Ishida; C. Klingshim; SpringerMaterials; smlbs_978-3-540-69990-3_40
(Springer-Verlag GmbH, Heidelberg, 2001), http://materials.springer.com/lb/docs/smlbs_978-3540-69990-3_40; accessed: 20-04-2015, 12:37:20 GMT-0400 (EDT).
[12] D. E. Bode and H. Levinstein, "Effect of oxygen on the electrical properties of lead telluride
films," Phys. Rev., 96, 259 (1954).
[13] Retrieved from
<http://stuff.mit.edu/afs/athena.mit.edu/course/3/3.082/www/team2_ft2/Pages/processing.html>
on 20 April 2015.
33
[14] MicroChemicals GmbH, "Lift-off Processes with Photoresists" (2009). Retrieved from
<http://www.microchemicals.com/technicalinformation/lift-off photoresist.pdf> on 08 January
2015.
[15] "Lift-off process for thin films production," Collaborative Open Resource Environment for
Materials, University of Liverpool. Retrieved from
<http://core.materials.ac.uk/search/detail.php?id=3335> on 22 April 2015.
[16] M. Grundmann, The Physics ofSemiconductors, Springer-Verlag, Berlin, 2006.
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