Effect of oxidation conditions on the phase composition, structure, and properties of photosensitive lead sulfide layers

Article abstract of DOI:10.1023/A:1011645730621

The oxidation of single-crystal and polycrystalline lead sulfide was studied as a function of process duration and temperature. The phase changes during oxidation were followed by thermal analysis, x-ray diffraction, and x-ray photoelectron spectroscopy. The transport properties of as-grown and oxidized PbS layers were measured. A mechanism was proposed for the bulk and surface processes resulting in the formation of photosensitive PbS-based structures.

Full text of DOI:10.1023/A:1011645730621

Inorganic Materials, Vol. 37, No. 9, 2001, pp. 910–914. Translated from Neorganicheskie Materialy, Vol. 37, No. 9, 2001, pp. 1074–1079.
Original Russian Text Copyright © 2001 by Kamchatka, Chashchinov, Chesnokova.
Effect of Oxidation Conditions on the Phase Composition,
Structure, and Properties of Photosensitive Lead Sulfide Layers
M. I. Kamchatka, Yu. M. Chashchinov, and D. B. Chesnokova
St. Petersburg State Electrotechnical University, ul. professora Popova 5, St. Petersburg, 197376 Russia
Received August 11, 2000
Abstract—The oxidation of single-crystal and polycrystalline lead sulfide was studied as a function of process
duration and temperature. The phase changes during oxidation were followed by thermal analysis, x-ray dif-
fraction, and x-ray photoelectron spectroscopy. The transport properties of as-grown and oxidized PbS layers
were measured. A mechanism was proposed for the bulk and surface processes resulting in the formation of
photosensitive PbS-based structures.
INTRODUCTION
x-ray diffraction (XRD) and x-ray photoelectron spec-
troscopy (XPS). The Pb 4f, S 2p, and O 1s XPS spectra
of both as-grown and oxidized layers were analyzed
using the Curve Fit routine.
Annealing in air or oxygen atmosphere is known to
increase the photoconductivity of polycrystalline lead
sulfide layers. However, the effect of oxidation condi-
tions on the processes involved, phase composition of
the layers, and their properties has not yet been fully
investigated. Various and often contradictory data have
been reported in the literature as to the phases resulting
from the oxidation of lead sulfide. The phases men-
tioned most frequently are PbSO , PbO · PbSO , PbO,
The lattice parameter of lead sulfide was determined
by precision XRD measurements using fine-particle Ge
powder as an internal standard. The powder was prean-
nealed to relieve internal stress. The amount of added
Ge was adjusted so that the neighboring high-angle
reflections from germanium and lead sulfide were close
in intensity. We also measured the dark resistance Rd,
thermoelectric power α, and spectral response of the
layers. Carrier concentration (n, p) was determined
from 77-K Hall effect measurements; α was measured
by a hot-probe technique.
4
4
and PbO [1–10]. Detailed analysis of the surface pro-
2
cesses underlying the oxidation of lead sulfide and
accurate identification of the forming phases are crucial
for understanding the photoconductivity mechanisms.
In this work, we analyze the effect of preparation
conditions on the composition, properties, and structure
of photosensitive oxidized lead sulfide layers.
RESULTS AND DISCUSSION
Structural and transport properties of as-grown
layers. The layers grown from preannealed lead sulfide
with the use of a sulfur vapor source were single-phase
EXPERIMENTAL
Single-crystal and polycrystalline lead sulfide layers and uniform in composition and electrical properties.
were produced by the hot-wall method in a semi-closed The high uniformity of the layers was ensured by using
system with or without an additional sulfur vapor nearly equilibrium deposition conditions. The layers
source. As the starting charge, we used polycrystalline grown without a sulfur vapor source contained high
lead sulfide preannealed under dynamic vacuum at tem- concentrations of sulfur vacancies and, occasionally,
peratures corresponding to the conditions of subse-
quent sublimation. During growth, we varied the sulfur
Pb metal and were n-type. Increasing pS2 (sulfur vapor
source) reduced the electron concentration to the point
of type conversion. The dependences of the carrier con-
vapor pressure pS2 and deposition temperature T to
d
control the type and concentration of dominant point
defects. The grown layers were annealed in an oxygen
atmosphere at widely varied temperatures and pres-
sures. The annealing duration was varied from several
seconds to several hours. After annealing, the layers
were air-quenched to room temperature.
centration and thermoelectric power on T and p
d
S2
were similar to those for lead sulfide crystals. In view
of this, we used the thermodynamic parameters of
defect formation in lead sulfide crystals to evaluate the
concentration of dominant native point defects in the
layers as a function of pS2 and Td.
The phase changes during oxygen annealing were
followed by thermal analysis (DTA + TG + DTG) at
Phase composition of oxidation products. The
different heating rates and sensitivities. The phase com- surface phase composition of oxygen-annealed lead
position of the oxidation products was determined by sulfide was determined by XRD and XPS. Since refer-
0
020-1685/01/3709-0910$25.00 © 2001 MAIK “Nauka/Interperiodica”

EFFECT OF OXIDATION CONDITIONS ON THE PHASE COMPOSITION, STRUCTURE
911
ence XPS data for PbO · PbSO are not available in the
T, K
1400
4
literature, we prepared high-purity PbO · PbSO and
4
measured its XPS spectra. A chemical shift relative to
lead sulfate was revealed only for the Pb 4f emission.
The Pb 4f binding energy was found to coincide with
that in PbO [11], which can be related to some struc-
1200
1000
tural features of PbO · PbSO (sp. gr. C2/m). As follows
4
from structure refinement (Weissenberg photographs,
Fourier synthesis) [12], the oxygen atoms in PbO ·
PbSO are tetrahedrally coordinated by Pb, and the Pb–
4
O bond distance is the same as in PbO.
Figure 1 displays typical thermoanalytical curves of
lead sulfide oxidation in an oxygen atmosphere. The
T(τ) curve indicates that the processes involved are not
monovariant. The curve, however, provides little infor-
mation, especially at high temperatures, where many
processes occur in the Pb–S–O system. In contrast, the
DTA, DTG, and TG curves show well-defined, per-
fectly correlated extrema. As follows from the TG data,
the sample weight decreases in the initial stage of oxi-
dation (355–420 K). The weight loss is due to water
desorption, as evidenced by O 1s XPS data, indicating
the presence of chemisorbed (and, undoubtedly, phys-
isorbed) water molecules on the surface of unoxidized
lead sulfide. Above 420 K, we observe a large exo-
therm, accompanied by a sharp increase in sample
weight. In addition, the DTA, DTG, and TG curves
show a kink around 470 K, which is probably related to
rapid lead sulfide oxidation in the oxygen-saturated
surface region. The signal rises steadily with increasing
temperature, up to 850 K, indicating an increase in the
oxygen-indiffusion and reaction rates. The oxidation
products obtained below 850 K were found to consist of
lead sulfate only:
8
6
00
00
TG
DTA
400
200
í
DTG
0
10
20
30
40
50
60
70
τ, min
Fig. 1. Thermal analysis of PbS oxidation.
from our TG data that reaction (2) prevails in the temper-
ature range in question. At the same time, reaction (3)
also cannot be ruled out, because the observed weight
PbS + 2O = PbSO .
(1) loss does not correspond to reaction (2), even with
2
4
allowance made for lead sulfide sublimation at elevated
temperatures.
Between 850 and 960 K, we observe endothermic oxi-
dation, accompanied by a sharp weight loss and result-
ing in PbO · PbSO , instead of PbSO . The formation
According to our TG data, only Ӎ0.7 wt % of the
PbS was oxidized, and the thickness of the oxide layer
was no greater than 30 nm because the oxidation pro-
cess was diffusion-limited. The oxidation rate was
slower for compositions near the Pb-rich phase bound-
ary of lead sulfide.
4
4
of PbO · PbSO may be due to a number of processes:
4
(
1) Reaction between PbS and the PbSO formed in
4
the initial stages of oxidation, which is thermodynami-
cally unstable in contact with PbS in the temperature
and oxygen-pressure ranges under consideration,
The features observed in the TG, DTA, and DTG
PbS + 7PbSO = 4(PbO ⋅ PbSO ) + 4SO .
(2) curves above 900 K are due to the formation of 2PbO ·
4
4
2
PbSO , 4PbO · PbSO , PbO, and Pb and the eutectic
4
4
(
2) Direct oxidation of PbS via oxygen indiffusion
transformations and melting of the oxidation products.
Since photosensitive PbS layers are commonly pre-
pared at lower temperatures, these processes are not
considered here. Note that the formation of Pb metal
(3) was observed above 1170 K.
or lead and sulfur outdiffusion through the PbSO layer
4
produced in the initial stage of oxidation,
7
4
1
2
1
2
PbS + --O = --(PbO ⋅ PbSO ) + --SO .
2
4
2
Structural and transport properties of oxidized
(3) PbS decomposition. The extent of this reaction is
layers. The effect of oxidation conditions on the trans-
port properties and lattice parameter of lead sulfide lay-
ers is illustrated in Figs. 2–5. Figure 2 shows the varia-
insignificant even above 1100 K [5], and it was left out
of consideration.
Reaction (2) is accompanied by weight loss, while tion in the effective thermoelectric power of initially
reaction (3) involves weight gain. Therefore, it follows p-type (curve 1) and n-type (curve 2) layers during oxy-
INORGANIC MATERIALS Vol. 37 No. 9 2001

9
12
KAMCHATKA et al.
α, µV/K
gen annealing at 870 K. Figure 3 displays the dark
resistance vs. oxidation time data for the same layers.
At oxidation temperatures in the range 700–900 K, the
thermoelectric power and dark resistance vary in a sim-
ilar manner. Figure 4 shows the lattice parameter of
n-type PbS as a function of oxidation time. The as-
grown lead sulfide layers showed no photoresponse.
Significant photoconductivity was only observed after
oxygen annealing (Fig. 5).
According to the variation in thermoelectric power,
the process can be divided into two stages: α increases
in the first stage and decreases in the second. Consider
the properties and phase composition of the layers in
either stage.
4
00
00
0
*
*
*
*
1
2
*
*
*
*
2
*
*
*
*
*
*
*
*
*
*
*
*
*
*
–
200
In the first stage, the hole concentration in the p-type
1
00
101
102
103
layers increases, as evidenced by α and R data (Figs. 2,
d
Oxidation time, s 3, curves 1). Oxidation of n-type layers results in a con-
version to p-type (Fig. 2, curve 1). The dark resistance
Fig. 2. Thermoelectric power as a function of oxidation time
at 870 K for (1) p- and (2) n-type PbS layers.
rises until the type conversion and then decreases
Fig. 3, curve 2). At the end of the first stage, thin
(
islands of new phases appear on the layer surface:
–
4
R × 10 , Ω
d
PbSO at low oxidation temperatures and PbO · PbSO
4
4
1
0
5
0
above 853 K. These phases are missing, however, in the
interior of PbS grains. Both phases have a larger band
gap compared to PbS. Since they are present in small
amounts and are distributed nonuniformly, the mea-
sured electrical properties characterize the grain bulk.
In the presence of PbSO , the photosensitivity
4
remains low.
These findings lead us to conclude that the main
process in the first stage of oxygen annealing is oxygen
diffusion from the gas phase into the interior of PbS
grains. In the near-surface region, oxygen occupies
vacant sulfur sites and, possibly, interstitial positions,
thereby compensating the donor effect of sulfur vacan-
cies. The decrease in vacancy concentration correlates
with the increase in lattice parameter (Fig. 4). The con-
centration of sulfur vacancies in n-type PbS layers is
2
1
00
101
102
1
Oxidation time, s
Fig. 3. Dark resistance as a function of oxidation time at
70 K for (1) p- and (2) n-type PbS layers.
1
9
–3
8
~10 cm , and that in p-type layers is about two orders
of magnitude lower [13]. Therefore, in initially n-type
layers, the filling of sulfur vacancies with oxygen takes
much longer times and the oxygen concentration is
notably higher as compared to p-type layers with the
same final value of α. Thus, in the first stage of anneal-
ing, oxygen acts as an electrically active point defect
whose diffusion gives rise to a nonuniform distribution
of carrier concentration in PbS. The direct effect of
chemisorbed oxygen on the carrier concentration in the
surface region can be neglected owing to the suffi-
ciently high annealing temperature and the formation
of oxygen-containing phases in the surface region.
a, Å
5
5
5
5
.9360
*
*
*
*
.9358
.9356
.9354
**
*
*
*
*
*
*
*
*
In the second stage of oxidation, α decreases and
changes sign (Fig. 2) and R increases, independent of
d
the initial conductivity type. The growing islands of
new phases merge into a continuous layer whose thick-
ness attains 20–30 nm at long oxidation times. The
0
500
1000
1500
Oxidation time, s
rapid formation of PbO · PbSO is due not only to oxy-
Fig. 4. Lattice parameter as a function of oxidation time at
70 K for n-type PbS.
4
8
gen indiffusion from the gas phase but also to oxygen
INORGANIC MATERIALS Vol. 37 No. 9 2001

EFFECT OF OXIDATION CONDITIONS ON THE PHASE COMPOSITION, STRUCTURE
913
CONCLUSION
2
1
.0
.5
The observed phase changes in lead sulfide layers
during oxygen annealing and the associated variations
in their transport properties indicate that the resulting
photosensitive layers are highly nonuniform. The inte-
rior region of the grains consists of PbS, with a nonuni-
form distribution of electrically active point defects
3
0
(sulfur and lead vacancies and oxygen) and, accord-
ingly, carrier concentration, especially in the layers
which were n-type before annealing. The near-surface
region has a high resistance. The surface of the grains is
1
covered with a PbO · PbSO dielectric layer. This model
4
is consistent with the observed spectral response of oxi-
dized lead sulfide layers.
0
2
4
Wavelength, µm
Fig. 5. Spectral response of PbS-based layers after oxygen
annealing at 870 K for (1) 100, (2) 800, and (3) 1200 s.
ACKNOWLEDGMENTS
We are grateful to S.I. Andreev for his assistance
with the x-ray photoelectron spectroscopic measure-
ments.
and sulfur outdiffusion from the grain bulk. XPS anal-
ysis combined with depth profiling by Ar ion milling
demonstrates that, after the removal of PbO · PbSO4,
+
This work was supported by the RF Ministry of
the Pb 4f and S 2p binding energies are the same as in Education (Basic Research in the Field of Electronics
PbS, but there is a pronounced Pb enrichment in the top and Radio Engineering).
surface layer. The oxygen and sulfur deficiency leads to
a low hole concentration and high resistance (R ≥
d
1
0 MΩ). Thus, a sharp modulation of carrier concentra-
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2
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4
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