Room-temperature oxygen sensitization in highly textured, nanocrystallinePbTe films: A mechanistic studyJianfei 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 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v110/i8 Published by the American Institute of Physics. Related ArticlesPhotovoltaic effect of CdS/Si nanoheterojunction array J. Appl. Phys. 110, 094316 (2011) Charge transfer dynamics of 3,4,9,10-perylene-tetracarboxylic-dianhydride molecules on Au(111) probed byresonant photoemission spectroscopy J. Chem. Phys. 135, 174701 (2011) Analysis of lattice site occupancy in kesterite structure of Cu2ZnSnS4 films using synchrotron radiation x-raydiffraction J. Appl. Phys. 110, 074511 (2011) Photovoltaic property of BiFeO3 thin films with 109° domains Appl. Phys. Lett. 99, 122902 (2011) Magnetic field influence on the transient photoresistivity of defect-induced magnetic ZnO films Appl. Phys. Lett. 99, 112503 (2011) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Room-temperature oxygen sensitization in highly textured, nanocrystallinePbTe films: A mechanistic study

Jianfei Wang,1,a) Juejun Hu,1,2 Piotr Becla,1 Anuradha M. Agarwal,1 and Lionel C. Kimerling1

1Microphotonics Center, Massachusetts Institute of Technology, 77 Massachusetts Avenue,Cambridge, Massachusetts 02139, USA2Department 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 cm�3 is identified to be

the consequence of Fermi level pinning induced by the diffused oxygen. The successful demonstra-

tion of IR-sensitive PbTe films without the need for high-temperature processing presents an impor-

tant step toward monolithic integration of mid-wave PbTe infrared detectors on Si read-out integrated

circuits (ROICs). VC 2011 American Institute of Physics. [doi:10.1063/1.3653832]

I. INTRODUCTION

Lead chalcogenides (PbS, PbSe, and PbTe) are promis-

ing 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 chalco-

genides can exhibit high infrared detection sensitivity com-

parable to their single-crystalline counterparts after a high-

temperature oxygen-sensitization process. This unique char-

acteristic enables monolithic detector integration, i.e., inte-

gration 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 photodetec-

tors 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 photo-

generated 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

conduction, increased film resistivity and very long photo-

generated carrier lifetime; (3) oxygen sensitization is a

diffusion-limited process;17,20–24 (4) grain boundaries assist

in oxygen diffusion, as oxygen sensitization in polycrystal-

line films occurs over a much shorter time scale compared to

their single-crystalline counterparts;15–17 and (5) solution-

deposited PbSe films used in commercial IR detectors often

exhibit high sheet resistance (105�106 X), but reports on the

actual carrier concentrations in these films are scarce.

Polycrystalline lead chalcogenide films with high photo-

response 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 (Br�1) 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 incor-

porated by oxygen through diffusion and show good

photoresponse.19,28–30 Furthermore, based on the available re-

sistance values for commercial PbSe IR detectors, we estimate

a room temperature carrier concentration of 1016�1017 cm�3

depending on the film thickness and carrier mobility, which

is consistent with our observation in this work. However, liter-

atures on PbTe typically report carrier concentrations of

�1018 cm�3 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 sensiti-

zation (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%a)Electronic mail: wangjf05@mit.edu.

0021-8979/2011/110(8)/083719/8/$30.00 VC 2011 American Institute of Physics110, 083719-1

JOURNAL OF APPLIED PHYSICS 110, 083719 (2011)

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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� 10�7 Torr. The sub-

strates 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 A=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 thick-

ness 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 elec-

trical properties of PbTe films, two types of samples are pre-

pared. 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 oxy-

gen 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 tempera-

ture range of 80–340 K. No significant PbTe electrical prop-

erty variation is observed for films with Ge23Sb7S70 capping

layer and of different thickness values. Therefore, we con-

clude that the electrical properties we measured are domi-

nated by the intrinsic properties of PbTe films and the PbTe-

Ge23Sb7S70 interface has little impact on the measured film

properties.

Secondary ion mass spectrometry (SIMS) is employed

to determine the oxygen concentration and “through-

thickness” distribution in the PbTe films. Oxygen atomic

detection limit of the SIMS technique is approximately

1� 1018 atoms=cm3. Film phase composition and micro-

structure are evaluated using X-ray diffraction on a PANalyt-

ical X’Pert Pro diffractometer. Grain size and surface

roughness of the films are measured using a Digital Instru-

ments 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 identi-

fied using X-ray photoelectron spectroscopy (XPS) on a Kra-

tos 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

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 measure-

ment. 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-cen-

tered-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 A, very close to the

reported value of 6.454 A in the standard powder diffraction

file (PDF) card. However, the diffraction peak intensity val-

ues 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.

083719-2 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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(200) texture in the films deposited on an amorphous sub-

strate. 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 depend-

ence 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.

B. Oxygen concentration depth profiling and chemicalstates

Oxygen concentration depth profiles of the “high oxy-

gen 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 oxy-

gen concentration” sample comes from oxygen in-diffusion

when the films are exposed to air. Residual oxygen in the

“low oxygen concentration” samples most probably origi-

nates from the bulk evaporation source materials.

SIMS measurements are made on films after deposition

and exposure to air for a known time period; the correspond-

ing 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 10�16 cm2=s at room temperature. This high dif-

fusion coefficient suggests that the major contribution is

from “short-cut” diffusion paths, in particular grain

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).

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.

083719-3 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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boundaries.31 This is consistent with our micro-structural

analysis, the abundance of vertically aligned, “through-

thickness” 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 nanocrystal-

line 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 nano-

crystalline 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

films becomes heavily oxidized forming both Pb-O and Te-

O 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 situdoping 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 bind-

ing 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 there-

fore formally confirms the long-standing postulate of the for-

mation 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 resistiv-

ity 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 deposi-

tion 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 cm�3, which is among the lowest reported

values for PbTe [single crystalline PbTe (Ref. 32) and poly-

crystalline PbTe (Ref. 33), the calculated intrinsic value is

0.8� 1016 cm�3] 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 mid-

wave IR photodetector applications (1–5 lm wavelength)

and most commercially available TEC can cool

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.

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.

TABLE I. Key properties of the PbTe films with low and high oxygen con-

centration (i.e., with and without the Ge23Sb7S70 capping layer).

Low oxygen

concentration

High oxygen

concentration

Oxygen concentration (cm�3) 2.3� 1019 1.3� 1021

Electrical conduction type p-type p-type

Hole concentration at 300 K (cm�3) 7.36� 1016 12.5� 1016

Activation energy (eV) 0.125 0.109

083719-4 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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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 resis-

tivity, 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 concentra-

tion and Hall mobility of a 200 nm thick film. Figure 8(a)

suggests thermally activated process exists in the polycrys-

talline 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.

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 mill-

ing. The change of Pb-O and Te-O peak intensity after surface removal sug-

gests 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 calcu-

lated to be about 8:1, indicating oxygen preferentially bonds with Pb.

FIG. 7. (Color online) (a) Carrier concentration and (b) Hall mobility vs

PbTe film thickness taken at room temperature at three different time inter-

vals 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 cm�3

and a standard deviation of 0.3� 1017 cm�3. Hall mobility has an average

value of 81 cm2V�1s�1 and a standard deviation of 13 cm2V�1s�1.

TABLE II. Activation energy and power law’s exponent fitted from temper-

ature dependence of carrier concentration and Hall mobility for polycrystal-

line PbTe films of different thicknesses.

Film thickness

(nm)

Activation energy

(eV) Exponent

100 0.108 1.50

200 0.111 1.25

300 0.112 0.85

500 0.106 0.65

1000 0.109 1.86

083719-5 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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In polycrystalline materials, due to the permittivity dif-

ference between the grains and grain boundaries, a quasicon-

tinuous 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 sur-

face 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 Ea¼ (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 independ-

ent on surface states density, and the Fermi level position

becomes stabilized.36 This is consistent with our experimen-

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 sur-

face 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 tempera-

ture. An approximation of the mobility function can be writ-

ten as a combination of influences from lattice vibrations

(phonons) and from impurities,

1

l¼ 1

llattice

þ 1

limpurities

; (1)

where llattice / T�ð3=2Þ and limpurities / T3=2. Fitting tempera-

ture dependence of mobility gives a power law with an expo-

nent 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

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.

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 depend-

ence of resistivity which is mainly due to the hole concentration dependence

on temperature.

083719-6 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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dominates the electrical conduction in PbTe films, which could

be expected given the polycrystalline nature of the films. Fol-

lowing the discussion on p-type inversion channel conduction

in our previous work,29 and the microstructure study of the pol-

ycrystalline 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 thick-

ness. 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 acti-

vation model with weaker temperature dependence. Based

on impedance spectroscopy measurement results, Komissar-

ova et al. inferred that tunneling transport through grain

boundary barriers dictates the electrical properties of In-

doped polycrystalline PbTe films at low temperature.28

Alternatively, hopping conduction between localized states

in the mobility gap also exhibit weak temperature depend-

ence characterized by the famous T�1=x law, where x is a

constant greater than one depending on the system dimen-

sionality 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 con-

duction mechanism in the films.

In the high temperature range as shown in Fig. 9(b),

we observe exponential dependence of resistivity on tem-

perature. 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 tem-

perature. This suggests that the exponential dependence of

resistivity on temperature is mainly due to the hole concen-

tration. Therefore the fitted activation energy should be

almost identical to the energy fitted from temperature de-

pendence of hole concentration, i.e., Ea¼ (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 intro-

duces 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 acti-

vation energy Ea. This is consistent with the fitted Ea val-

ues for high (0.109 eV) and low (0.125 eV) oxygen

concentration samples listed in Table I. Figure 10 sche-

matically illustrates the band structure near grain boundary

region of the two types of PbTe samples.

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 photocon-

ductivity measurement equipment, no slow relaxation or re-

sidual 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 begin-

ning 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 pho-

ton energy larger than the bandgap of PbTe generates elec-

trons and holes in the bulk of grains, and these photo-

generated 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. 10. Schematic band structure of PbTe films near grain boundary

region: (a) “low oxygen concentration” sample and (b) “high oxygen con-

centration” 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 concen-

tration” sample the Fermi level becomes pinned resulting in larger band

bending and smaller activation energy.

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.

083719-7 Wang et al. J. Appl. Phys. 110, 083719 (2011)

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motion of the nonequilibrium holes along inversion channels

on grain surfaces gives rise to the observed photoconductivity.

Finally, to further identify which contribution is domi-

nant 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 photocon-

ductivity.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 oxy-

gen sensitization. Studies on the microstructure and electronic

structure of oxygen sensitized nanocrystalline PbTe films fur-

ther reveal the photoconductivity mechanism. PbTe films pre-

pared 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 aver-

age grain size down to 25 nm for the 100 nm thick film. Fur-

ther, the vertically aligned grain boundaries can serve as

diffusion “short-cuts” for oxygen, which are essential for oxy-

gen diffusion and sensitization processes at room temperature.

The large amount of oxygen incorporated results in band bend-

ing 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, nanocrystal-

line PbTe films can be deposited on substrates without

lattice-matching constraints, and thus facilitates integration

of thin PbTe layers with photonic crystal structures for reso-

nant-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 chal-

cogenide films without traditional high-temperature process-

ing, 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 facili-

ties. The authors acknowledge the Center for Materials Sci-

ence and Engineering at MIT for characterization facilities.

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