Room-temperature oxygen sensitization in highly
textured, nanocrystalline PbTe films: A mechanistic study
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Wang, Jianfei, Juejun Hu, Piotr Becla, Anuradha M. Agarwal,
and Lionel C. Kimerling. “Room-temperature oxygen
sensitization in highly textured, nanocrystalline PbTe films: A
mechanistic study.” Journal of Applied Physics 110, no. 8 (2011):
083719. © 2011 American Institute of Physics
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Room-temperature oxygen sensitization in highly textured, nanocrystalline
PbTe films: A mechanistic study
Jianfei Wang, Juejun Hu, Piotr Becla, Anuradha M. Agarwal, and Lionel C. Kimerling
Citation: J. Appl. Phys. 110, 083719 (2011); doi: 10.1063/1.3653832
View online: http://dx.doi.org/10.1063/1.3653832
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JOURNAL OF APPLIED PHYSICS 110, 083719 (2011)
Room-temperature oxygen sensitization in highly textured, nanocrystalline
PbTe films: A mechanistic study
Jianfei Wang,1,a) Juejun Hu,1,2 Piotr Becla,1 Anuradha M. Agarwal,1 and Lionel C. Kimerling1
1
Microphotonics Center, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139, USA
2
Department of Materials Science and Engineering, University of Delaware, DuPont Hall,
Newark, Delaware 19716, USA
(Received 6 May 2011; accepted 20 September 2011; published online 31 October 2011)
In this paper, we report large mid-wave infrared photoconductivity in highly textured, nanocrystalline
PbTe films thermally evaporated on Si at room temperature. Responsivity as high as 25 V=W is
measured at the 3.5 lm wavelength. The large photoconductivity is attributed to the oxygen
incorporation in the films by diffusion. Carrier concentration as low as 1017 cm3 is identified to be
the consequence of Fermi level pinning induced by the diffused oxygen. The successful demonstration of IR-sensitive PbTe films without the need for high-temperature processing presents an important step toward monolithic integration of mid-wave PbTe infrared detectors on Si read-out integrated
C 2011 American Institute of Physics. [doi:10.1063/1.3653832]
circuits (ROICs). V
I. INTRODUCTION
Lead chalcogenides (PbS, PbSe, and PbTe) are promising material candidates for mid-wave infrared (IR) detection
(2–5 lm wavelength) because of their superior chemical and
mechanical stability over HgCdTe alloys. Their detection
wavelength range can be further extended into the far-wave
infrared by capitalizing on their bandgap tunability through
alloying with tin chalcogenides.1–3 In addition, while the
HgCdTe system is highly sensitive to crystal defects and
thus only single-crystalline HgCdTe materials qualify for IR
detection, it has been found that polycrystalline lead chalcogenides can exhibit high infrared detection sensitivity comparable to their single-crystalline counterparts after a hightemperature oxygen-sensitization process. This unique characteristic enables monolithic detector integration, i.e., integration of lead chalcogenide detectors with silicon read-out
integrated circuits (ROIC) without resorting to complicated
flip-chip bonding as in the case of HgCdTe detectors. Such
integration has been demonstrated in a few reports to date,
which clearly illustrates the competitive advantage of lead
chalcogenide materials for mid-IR detection over HgCdTe.4–6
Highly sensitive mid-IR lead chalcogenide photodetectors have been demonstrated through annealing as-deposited
films in an oxidizing atmosphere.7,8 Oxidation annealing is
known to incorporate oxygen in the film and sensitize the
infrared photoresponse of lead chalcogenide.5,9–24 Oxygen
can introduce gap states which deplete electrons and result in
spatial separation of carriers, thus increasing the photogenerated carrier lifetime, and enhancing photoconductivity.
Some key findings are: (1) polycrystalline lead chalcogenide
films evaporated from stoichiometric bulk are usually n-type
due to the poor sticking coefficient of chalcogen atoms and
hence chalcogen deficiency. After sensitization, oxygen
introduces gap states which can deplete electrons from the
conduction band; (2) oxygen-sensitized films exhibit p-type
a)
Electronic mail: wangjf05@mit.edu.
0021-8979/2011/110(8)/083719/8/$30.00
conduction, increased film resistivity and very long photogenerated carrier lifetime; (3) oxygen sensitization is a
diffusion-limited process;17,20–24 (4) grain boundaries assist
in oxygen diffusion, as oxygen sensitization in polycrystalline films occurs over a much shorter time scale compared to
their single-crystalline counterparts;15–17 and (5) solutiondeposited PbSe films used in commercial IR detectors often
exhibit high sheet resistance (105106 X), but reports on the
actual carrier concentrations in these films are scarce.
Polycrystalline lead chalcogenide films with high photoresponse have been reported to have large grain sizes, i.e.,
0.4–0.7 lm for PbSe (Refs. 25 and 26) and 0.9 lm for PbS.27
The films achieve high photoresponse through oxidation
annealing process with iodine (I2) to facilitate the film
recrystallization process and incorporation of oxygen,25,26 or
through bromide ion (Br1) to control film morphology and
grain size.27 As a comparison, nanocrystalline PbTe films
with 50–75 nm grain size are found to be more easily incorporated by oxygen through diffusion and show good
photoresponse.19,28–30 Furthermore, based on the available resistance values for commercial PbSe IR detectors, we estimate
a room temperature carrier concentration of 10161017 cm3
depending on the film thickness and carrier mobility, which
is consistent with our observation in this work. However, literatures on PbTe typically report carrier concentrations of
1018 cm3 in polycrystalline films, 10 times higher than our
results, probably due to insufficient Fermi-level pinning (due
to formation of trap states near Fermi level) as we will explain
later. In this paper, we report significant photoconductivity in
nanocrystalline PbTe films without high temperature sensitization (grain size 25 nm). The scope of this paper further
includes study of the oxygen sensitization mechanism and the
electronic structure of oxygen sensitized PbTe films.
II. EXPERIMENT
PbTe films are prepared using single-source thermal
evaporation from stoichiometric PbTe bulk of 99.999%
110, 083719-1
C 2011 American Institute of Physics
V
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083719-2
Wang et al.
purity. Oxide coated Si wafers (6 in. Si wafers with 3 lm
thermal oxide from Silicon Quest International) are used as
starting substrates. The thermal evaporation runs are carried
out at a base pressure lower than 5 107 Torr. The substrates are held on a rotating substrate holder maintained at
room temperature throughout the deposition. Film deposition
rate is monitored in situ with a quartz crystal sensor and is
maintained at 8–10 Å=s. The thermal evaporation technique
allows deposition of films with high uniformity across an
entire large-area substrate, with improved throughput and
much lower cost compared to molecular beam epitaxy
(MBE) or metalorganic chemical vapor deposition
(MOCVD). Film thickness is measured using a KLA Tencor
P-16 surface profilometer, and we confirm excellent thickness uniformity across an entire substrate with thickness
variations < 3%. Films with different thickness values from
100 to 1000 nm are deposited under identical conditions to
study the microstructural evolution.
To quantitatively evaluate the impact of oxygen on electrical properties of PbTe films, two types of samples are prepared. In one case a thin layer of thermally evaporated
Ge23Sb7S70 glass film is evaporated onto as-deposited PbTe
in the same deposition chamber without breaking vacuum
(denoted as “low oxygen concentration”); and in the other
the as-deposited PbTe films are allowed to be directly
exposed to air after deposition to enable oxygen diffusion
into the films (denoted as “high oxygen concentration”). The
Ge23Sb7S70 glass has been proven to be impermeable to oxygen up to 200 C and is chemically stable. Tin metal contacts
are pre-deposited through a shadow mask onto the substrate
for electrical measurements. The Ohmic nature of electrical
contact is confirmed by I–V measurements in the temperature range of 80–340 K. No significant PbTe electrical property variation is observed for films with Ge23Sb7S70 capping
layer and of different thickness values. Therefore, we conclude that the electrical properties we measured are dominated by the intrinsic properties of PbTe films and the PbTeGe23Sb7S70 interface has little impact on the measured film
properties.
Secondary ion mass spectrometry (SIMS) is employed
to determine the oxygen concentration and “throughthickness” distribution in the PbTe films. Oxygen atomic
detection limit of the SIMS technique is approximately
1 1018 atoms=cm3. Film phase composition and microstructure are evaluated using X-ray diffraction on a PANalytical X’Pert Pro diffractometer. Grain size and surface
roughness of the films are measured using a Digital Instruments Nanoscope IIIa AFM. Cross-sectional TEM images
are taken using a JEOL 200CX General Purpose TEM. The
chemical states of the elements in the PbTe films are identified using X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS Ultra Imaging XPS system. During the XPS
analysis, we use the C 1s peak with a binding energy of
284.448 eV for energy calibration.
Hall experiments are performed using a van der Pauw
technique at a magnetic field of 4000 G. IR light from a
blackbody is monochromatized through a sodium chloride
prism and modulated at 10.3 Hz for the photoconductivity
measurement. An SP2402 thermoelectric cooler (TEC) from
J. Appl. Phys. 110, 083719 (2011)
Marlow Industries Inc. is used for cooling samples down to
60 C during the photoconductivity characterization. A
bias current of 0.1 mA is held constant during the measurement. The incident IR light power density on the samples is
calibrated using a thermopile reference.
III. RESULTS AND DISCUSSION
A. Microstructure analysis
XRD spectra in Fig. 1(a) confirm that PbTe films of all
thicknesses are polycrystalline and contain a single face-centered-cubic (fcc) crystalline phase with a rock salt structure.
Detailed peak-by-peak analysis indicates all the films have a
lattice parameter of 6.459 6 0.001 Å, very close to the
reported value of 6.454 Å in the standard powder diffraction
file (PDF) card. However, the diffraction peak intensity values in the measured spectra differ vastly from those quoted
from the standard PDF card in Fig. 1(a), indicating strong
FIG. 1. (Color online) (a) XRD spectra of thermally evaporated PbTe films
on oxide-coated Si substrates. The film is a polycrystalline single fcc phase
with strong (200) texture. Standard XRD data from PDF card # 03-065-0137
is also shown at the bottom for comparison. (b) Thickness dependence of
peak intensity ratio, i.e., I(200)=I(220), showing the degree of texture
increases sharply with decreasing film thickness. The ratio for the standard
randomly oriented PbTe sample of PDF card # 03-065-0137 is 1.45.
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Wang et al.
J. Appl. Phys. 110, 083719 (2011)
FIG. 2. Schematic illustration of two-step nucleation and growth model for
polycrystalline PbTe films on the SiO2=Si substrate. (a) Initial nucleation
and growth of PbTe grains on the SiO2=Si substrate. The arrows are pointing
to the [200] direction in PbTe. (b) Film growth stage. New grains with more
random orientations are nucleated and grow on nucleation sites created in
step (a).
(200) texture in the films deposited on an amorphous substrate. The degree of texture can be quantified using the peak
intensity ratio between the (200) peak and the (220) peak as
shown in Fig. 1(b). In randomly oriented polycrystalline
PbTe, the ratio is 1.45. In contrast, our films show strong
(200) texture and the degree of texture increases prominently
with decreasing film thickness. For the 100 nm thick film, an
intensity ratio up to 180 is observed. Notably, this degree of
texture is almost 50 times stronger compared to PbTe films
deposited on soda-lime glass substrates in our previous
study.29
The evolution of texture as a function of film thickness
and substrate type can be explained based on a two-step
nucleation and growth model shown in Fig. 2. Figure 2(a)
illustrates the kinetics of early stage nucleation and film
growth which are primarily determined by interface energy
between PbTe and SiO2=Si substrate. In this stage, [200]
becomes the preferred film growth direction due to the lower
interface energy between (200) planes and the substrate, and
thus strong (200) texture is expected. In thick films shown in
Fig. 2(b), the growth of (200) grains creates new nucleation
sites in the grain boundary regions, where nucleation of
grains with random orientation occurs. As a consequence,
the degree of texture decreases in thick films. We shall note
that the transition between the two growth phases during
film growth is a gradual process. This is evident in Figs.
3(a)–3(e), which show the progressive increase in density as
well as size of randomly oriented grains as film growth
proceeds.
This two-step growth model is further verified by AFM
surface morphology studies. As shown in Figs. 3(a)–3(e), the
average grain size monotonically increases with increasing
film thickness from 100 nm to 1000 nm. Indicated by arrows
in Figs. 3(c) and 3(d), non-equiaxial grains evolve in thick
films, consistent with our hypothesis of two-stage nucleation
and growth model. Figure 3(f) plots the thickness dependence of average grain size and rms surface roughness. As
film thickness decreases, average grain size decreases from
50 nm for 500 nm thick film to 25 nm for 100 nm thick film,
accompanied by an rms surface roughness decrease. The
cross-sectional TEM image in Fig. 4 shows the columnar
structure of a 500 nm thick film. The “through-thickness”
grain boundaries are clearly seen as indicated by the white
arrows.
FIG. 3. (Color online) AFM surface morphology images of thermally
evaporated PbTe films show the nanocrystalline structure (a) 100 nm, (b)
200 nm, (c) 300 nm, (d) 500 nm, and (e) 1000 nm. Arrows in (c) and (d)
indicate randomly oriented grains. (f) Thickness dependence of surface
roughness and average grain size. Both surface roughness and average grain
size decrease rapidly with decreasing film thickness.
B. Oxygen concentration depth profiling and chemical
states
Oxygen concentration depth profiles of the “high oxygen concentration” and “low oxygen concentration” films
are shown in Fig. 5 and the average oxygen concentration
values are listed in Table I. The two orders of magnitude
concentration difference in the samples with and without
capping layers suggests that most oxygen in the “high oxygen concentration” sample comes from oxygen in-diffusion
when the films are exposed to air. Residual oxygen in the
“low oxygen concentration” samples most probably originates from the bulk evaporation source materials.
SIMS measurements are made on films after deposition
and exposure to air for a known time period; the corresponding diffusion coefficient fitted from the concentration profile
thus may be calculated by fitting the concentration profile to
the diffusion equation. The fitted diffusion coefficient is in
the order of 1016 cm2=s at room temperature. This high diffusion coefficient suggests that the major contribution is
from “short-cut” diffusion paths, in particular grain
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TABLE I. Key properties of the PbTe films with low and high oxygen concentration (i.e., with and without the Ge23Sb7S70 capping layer).
Oxygen concentration (cm3)
Electrical conduction type
Hole concentration at 300 K (cm3)
Activation energy (eV)
FIG. 4. Cross-sectional TEM image shows the columnar structure of a 500
nm thick film. “Through-thickness” grain boundaries are indicated by the
white arrows.
boundaries.31 This is consistent with our micro-structural
analysis, the abundance of vertically aligned, “throughthickness” grain boundaries provides numerous diffusion
short-cuts, which enables efficient oxygen diffusion without
the need for high-temperature processing. The fine grain
structure and the columnar grain structure of the nanocrystalline films are thus ideal for enhanced oxygen diffusion and
low-temperature sensitization process. As we will discuss in
greater detail in the subsequent sections, this unique nanocrystalline columnar microstructure is critical in shaping
electrical properties of the films and achieving high
photoconductivity.
Using XPS, we have identified two chemical states for
both Pb and Te, one is due to normal Pb-Te bonds in PbTe,
and the other resulting from oxidation of Pb and Te. Figures
6(a) and 6(b) show the XPS spectra of Pb 4f and Te 3d lines
in the “high oxygen concentration” sample, respectively.
The black solid curves in Figs. 6(a) and 6(b) correspond to
XPS spectra collected on the surface of the films, and the red
dotted curves are spectra taken on films after removal of the
surface layer using Ar ion milling. Clearly, the surface of the
FIG. 5. (Color online) Oxygen concentration depth profiles obtained by
SIMS for PbTe films with low and high oxygen concentrations (i.e., with
and without the Ge23Sb7S70 capping layer). The two orders of magnitude
concentration difference in the samples suggests that most oxygen in the
“high oxygen concentration” sample comes from oxygen in-diffusion when
the films are exposed to air.
Low oxygen
concentration
High oxygen
concentration
2.3 1019
p-type
7.36 1016
0.125
1.3 1021
p-type
12.5 1016
0.109
films becomes heavily oxidized forming both Pb-O and TeO bonds. Oxygen concentration is significantly reduced
beneath the surface layer, which is consistent with the SIMS
measurement and confirms that the oxygen incorporation
occurs through diffusion from the ambient rather than in situ
doping during deposition. To identify the chemical binding
states of diffused-in oxygen, Fig. 6(c) shows the XPS spectra
of oxygen in the sample after removal of the surface layer. A
sputtered TeO2 film is used as the reference sample to
unequivocally identify the peak corresponding to Te-O binding in Fig. 6(c). From the measurement we calculate the ratio
of oxygen bonded to Pb and Te to be about 8:1. Thus even
though both Pb-O and Te-O bonds are formed in the PbTe
films, oxygen preferentially bonds with Pb. This result therefore formally confirms the long-standing postulate of the formation of (PbO)2þ complex in lead chalcogenides, which
serves as electron traps and leads to p-type conduction.11
C. Hall experiment
Hall experiment and temperature dependence of resistivity are studied to further ascertain the electronic structure of
oxygen sensitized PbTe films. Room temperature carrier
concentration values of the PbTe samples with low and high
oxygen concentrations are listed in Table I. Both types of
samples exhibit p-type conductivity. Furthermore, for films
of different thicknesses exposed to air directly after deposition and thus with high oxygen concentrations, we confirm
that they are all p-type. As shown in Figs. 7(a)–7(b),
repeated Hall experiments performed over a period of 4
months after film deposition do not show any change within
the accuracy of the experiment, and no significant thickness
dependence of either carrier concentration or Hall mobility
has been observed. These findings provide further evidence
that oxygen diffusion in the nanocrystalline PbTe films is a
very rapid process which quickly reaches saturation over
time. The room temperature carrier concentration is
(1.2 6 0.3) 1017 cm3, which is among the lowest reported
values for PbTe [single crystalline PbTe (Ref. 32) and polycrystalline PbTe (Ref. 33), the calculated intrinsic value is
0.8 1016 cm3] and is more than an order of magnitude
lower than previously reported values in polycrystalline
PbTe.33 Hall mobility of the films is (81 6 13) cm2=V=s,
which is about one order of magnitude lower than the values
in single crystals34 and epitaxial layers.35
Since we target at TEC-cooled short-wave and midwave IR photodetector applications (1–5 lm wavelength)
and most commercially available TEC can cool
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083719-5
Wang et al.
J. Appl. Phys. 110, 083719 (2011)
FIG. 7. (Color online) (a) Carrier concentration and (b) Hall mobility vs
PbTe film thickness taken at room temperature at three different time intervals after deposition using a van der Pauw technique. No thickness or time
dependence of either carrier concentration or Hall mobility has been
observed. The carrier concentration has an average value of 1.2 1017 cm3
and a standard deviation of 0.3 1017 cm3. Hall mobility has an average
value of 81 cm2V1s1 and a standard deviation of 13 cm2V1s1.
FIG. 6. (Color online) (a) XPS spectra of lead 4f peaks; (b) XPS spectra of
tellurium 3d peaks. The black solid curves in (a) and (b) correspond to XPS
spectra collected on the surface of the films, and the red dotted curves are
spectra taken on films after removal of the surface layer using Ar ion milling. The change of Pb-O and Te-O peak intensity after surface removal suggests the presence of a surface oxide layer in the sample. (c) XPS spectrum
of oxygen 1s peaks for the PbTe film after removal of a surface oxidation
layer by Ar ion milling. Two chemical binding states due to oxidation of Pb
and Te are identified and the ratio of oxygen bonded to Pb and Te is calculated to be about 8:1, indicating oxygen preferentially bonds with Pb.
photodetectors down to 60 C (213 K), we perform Hall
experiment in the temperature range of 213–300 K in order
to study two separate components’ contribution in film resistivity, i.e., carrier concentration and mobility. The former
could help understanding the band structure, and the latter
could facilitate identifying the scattering process in electrical
conduction. All the films in the studied temperature range
are p-type, and films of all thicknesses yield similar results
and key parameters are summarized in Table II. Figure 8
shows typical temperature dependence of carrier concentration and Hall mobility of a 200 nm thick film. Figure 8(a)
suggests thermally activated process exists in the polycrystalline PbTe films and the activation energy is fitted to be
0.111 eV. The activation energy fitted for films of other
thicknesses are listed in Table II, and all values are around
0.11 eV.
TABLE II. Activation energy and power law’s exponent fitted from temperature dependence of carrier concentration and Hall mobility for polycrystalline PbTe films of different thicknesses.
Film thickness
(nm)
100
200
300
500
1000
Activation energy
(eV)
Exponent
0.108
0.111
0.112
0.106
0.109
1.50
1.25
0.85
0.65
1.86
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083719-6
Wang et al.
FIG. 8. (Color online) (a) Carrier concentration and (b) Hall mobility as a
function of temperature. Temperature dependence of carrier concentration
suggests thermally activated process in the PbTe film and Fermi level is
0.111 eV above the valence band. Temperature dependence of mobility
shows ionized defect scattering dominates the electrical conduction in the
PbTe film.
In polycrystalline materials, due to the permittivity difference between the grains and grain boundaries, a quasicontinuous energy spectrum of grain surface states should be
expected. However, according to our XPS results the grain
boundary areas could include high permittivity compounds
such as PbO, thus the width of the energy spectrum of the
grain surface states may be found to be less than kT and we
can assume the existence of a monoenergetic level of the surface states on the grain surfaces.36,37 Therefore, a schematic
of the band diagram on near a grain boundary could be
employed as in Fig. 4 in Ref. 29. According to this model,
Hall carrier concentration is thermally activated and can be
described by an activation energy Ea ¼ (EF EV) ES,
where (EF EV) is the energy separation between valence
band edge and Fermi level in crystalline PbTe grains and ES
corresponds to the band bending. As demonstrated before,
diffused oxygen leads to formation of acceptor states in grain
boundaries, and these acceptor states induce band bending
on the surface of PbTe crystalline grains, forming p-type
conduction channels on the grain surfaces. Neustroev and
Osipov have analyzed the case of high grain surface states
densities. The Fermi level on grain surfaces will reach the
minimum energy of the surface states, ES becomes independent on surface states density, and the Fermi level position
becomes stabilized.36 This is consistent with our experimen-
J. Appl. Phys. 110, 083719 (2011)
FIG. 9. (Color online) (a) Temperature dependence of resistivity of PbTe
films with high and low oxygen concentrations from 80 K to 340 K. Both
samples are 1000 nm in thickness. (b) Enlarged portion of interest in (a)
from 240 K to 340 K with fitted data shows exponential temperature dependence of resistivity which is mainly due to the hole concentration dependence
on temperature.
tal result, i.e., almost identical activation energy Ea for films
of different thicknesses. For these films with high oxygen
concentration, the grain surface states density is high enough
to “pin” the Fermi level. Therefore (EF EV) and ES, and
thus Ea should be independent on film thickness even though
the microstructure of these films changes gradually with
thickness.
Another consequence of high density of the grain surface states and Fermi level pinning is that the temperature
dependence of Hall mobility should be described by a power
law instead of a thermally activated behavior.36 Mobility is
usually a strong function of material impurities and temperature. An approximation of the mobility function can be written as a combination of influences from lattice vibrations
(phonons) and from impurities,
1
1
1
¼
þ
;
l llattice limpurities
(1)
where llattice / T ð3=2Þ and limpurities / T 3=2 . Fitting temperature dependence of mobility gives a power law with an exponent of 1.25 as shown in Fig. 8(b) for 200 nm thick film. The
fitted exponent numbers for other thicknesses are summarized
in Table II. The result indicates that ionized defect scattering
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083719-7
Wang et al.
J. Appl. Phys. 110, 083719 (2011)
dominates the electrical conduction in PbTe films, which could
be expected given the polycrystalline nature of the films. Following the discussion on p-type inversion channel conduction
in our previous work,29 and the microstructure study of the polycrystalline PbTe films in this paper, we conclude the defects
mainly exist in the grain boundary regions and they result from
the diffused oxygen induced acceptors.
D. Electrical property and photoconductivity
Temperature dependent resistivity of the two types of
PbTe samples with low and high oxygen concentrations are
shown in Figs. 9(a) and 9(b) and both are 1000 nm in thickness. Figure 9(a) shows an experimental result from 80 K to
340 K, and Fig. 9(b) is the enlarged portion of interest from
240 K to 340 K with fitted data. In the low temperature range
(< 240 K) we can see clear deviation from the thermal activation model with weaker temperature dependence. Based
on impedance spectroscopy measurement results, Komissarova et al. inferred that tunneling transport through grain
boundary barriers dictates the electrical properties of Indoped polycrystalline PbTe films at low temperature.28
Alternatively, hopping conduction between localized states
in the mobility gap also exhibit weak temperature dependence characterized by the famous T1=x law, where x is a
constant greater than one depending on the system dimensionality and the form of density of states function near
Fermi level.38 Lastly, it is well known that bandgap energy
of PbTe changes significantly from room temperature
(0.31 eV) to 77 K (0.20 eV),39 and thus large deviation of
band structure and electronic transport behavior could be
expected in the low temperature range. More experimental
efforts will be necessary to confirm the low temperature conduction mechanism in the films.
In the high temperature range as shown in Fig. 9(b),
we observe exponential dependence of resistivity on temperature. According to Figs. 8(a) and 8(b) and Table II, the
Hall experiment performed at different temperatures shows
that the hole concentration obeys exponential dependence
while mobility follows the power law dependence on temperature. This suggests that the exponential dependence of
resistivity on temperature is mainly due to the hole concentration. Therefore the fitted activation energy should be
almost identical to the energy fitted from temperature dependence of hole concentration, i.e., Ea ¼ (EF EV) ES.
Since it has been established that oxygen is concentrated
on the surfaces of films and in the grain boundary regions
instead of penetrating into the interior of the grains,36 we
assume (EF EV) is the same for high and low oxygen
concentration films. Additionally, we note that oxygen
forms acceptor states in grain boundary regions and introduces band bending, thus in the high oxygen concentration
samples the band bending ES should be larger than in the
low oxygen concentration sample, resulting in smaller activation energy Ea. This is consistent with the fitted Ea values for high (0.109 eV) and low (0.125 eV) oxygen
concentration samples listed in Table I. Figure 10 schematically illustrates the band structure near grain boundary
region of the two types of PbTe samples.
FIG. 10. Schematic band structure of PbTe films near grain boundary
region: (a) “low oxygen concentration” sample and (b) “high oxygen concentration” sample. The relevant energy points are Ec conduction band edge,
Ev valence band edge, EF Fermi level, ES bend banding on the surface of a
grain, and Ea activation energy of electrical conduction according to the
grain boundary conduction channel model. In the “high oxygen concentration” sample the Fermi level becomes pinned resulting in larger band
bending and smaller activation energy.
We measure excellent photoconductive signal in the
wavelength range of 0.8–5 lm in the nanocrystalline PbTe
films of all thicknesses. The responsitivity spectrum of a 100
nm thick film is shown in Fig. 11 as an example. Up to 300
Hz, the maximum modulation frequency of our photoconductivity measurement equipment, no slow relaxation or residual conductivity has been observed which is evidenced by
the anomalously long relaxation times (on the order of
minutes, hours, or longer) of the photoresponse at the beginning or end of illumination.40 To explain the mechanism
underlying photoconductivity in PbTe, here we refer to the
theory described by Neustroev and Osipov for the other two
similar lead chalcogenides, i.e., PbS (Ref. 37) and PbSe.41
In our polycrystalline PbTe films, the existence of grain
surface states due to the diffused oxygen leads to p-type
inversion channels between grains. Incident light with photon energy larger than the bandgap of PbTe generates electrons and holes in the bulk of grains, and these photogenerated carriers could diffuse to the grain surfaces and get
spatially separated. Electrons are collected at the interface of
the quasineutral bulk of grains and the surface space charge
region, while holes are collected on grain surfaces (Fig. 10).
Thus the band bending on the grain surfaces and the width of
the surface space charge region could also be reduced. The
FIG. 11. Responsivity spectrum of 100 nm thick nanocrystalline PbTe film
under 0.1 mA bias current and at 60 C cooled by a TEC. Film shows
excellent photoconductive signal in the wavelength range of 0.8–5 lm, and
responsivity as high as 25 V=W is measured at 3.5 lm wavelength.
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083719-8
Wang et al.
motion of the nonequilibrium holes along inversion channels
on grain surfaces gives rise to the observed photoconductivity.
Finally, to further identify which contribution is dominant in the intrinsic physical process of photoconductivity in
our PbTe films, i.e., illumination-induced change in the
effective Hall mobility or hole concentration, the activation
energies of four parameters should be determined at the
same time: hole concentration, the electrical conductivity,
the photoconductivity, and the decay time of the photoconductivity.37,41 In this paper the first two activation energies
are mainly discussed and more work is underway focusing
on the last two activation energies.
IV. CONCLUSIONS
To sum up, we demonstrate high photoconductivity in
thermally evaporated PbTe films without high temperature oxygen sensitization. Studies on the microstructure and electronic
structure of oxygen sensitized nanocrystalline PbTe films further reveal the photoconductivity mechanism. PbTe films prepared by single source thermal evaporation exhibit strong (200)
texture with columnar grain microstructures, which is dictated
by preferential nucleation orientations. The film grain size
decreases with decreasing film thickness and reaches an average grain size down to 25 nm for the 100 nm thick film. Further, the vertically aligned grain boundaries can serve as
diffusion “short-cuts” for oxygen, which are essential for oxygen diffusion and sensitization processes at room temperature.
The large amount of oxygen incorporated results in band bending on grain surfaces, Fermi level pinning, and p-type inversion
channels, leading to high infrared photoconductivity in the
PbTe films. Peak photo-responsivity up to 25 V=W is attained.
Unlike their single crystalline counterpart, nanocrystalline PbTe films can be deposited on substrates without
lattice-matching constraints, and thus facilitates integration
of thin PbTe layers with photonic crystal structures for resonant-cavity-enhanced (RCE) infrared photodetectors,42 and
multi-color detection.43 Our results also provide a method to
achieve sensitization and high infrared response in lead chalcogenide films without traditional high-temperature processing, and open up an avenue toward the Holy Grail: RCE
multispectral infrared photodetectors and focal plane arrays
monolithically integrated on silicon ROICs.
ACKNOWLEDGMENTS
This work is supported by DSO National Laboratories,
Singapore. The authors would like to thank Microsystems
Technology Laboratories at MIT for metal deposition facilities. The authors acknowledge the Center for Materials Science and Engineering at MIT for characterization facilities.
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