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151
CHAPTER-6
GROWTH AND CHARACTERIZATION OF STOICHIOMETRIC PbS
THIN FILMS
This chapter presents the details of sample preparation and characterization of
stoichiometric thin films lead sulphide (PbS). In this chapter, we present the detailed
investigations on the characterization of PbS thin films during the course of research
work. X-ray differection (XRD), UV-Visible spectroscopy, scanning electron microscopy
(SEM) with EDAX, Atomic force microscopy (AFM) and I-V measurements have been
used to investigate the optical, structural, surface morphology and electrical studies of
vacuum evaporated PbS thin films deposited without and with H2S atmosphere.
152
CHAPTER-6 GROWTH AND CHARACTERIZATION OF STOICHIOMETRIC PbS THIN FILMS
6.1 Introduction:
The binary IV-VI compounds formed from Pb and group VI elements like S, Se
and Te are among the oldest known semiconducting materials and have been used for
many years for electronic & optoelectronic device applications. The first ever reported
solid state diode was made from single crystalline PbS and its rectifying properties were
exploited in early radio receivers. Later on, interest in IV-VI semiconductors shifted to
mid-infrared optoelectronic device applications such as photon detectors operating in the
3-14 µm wavelength range, taking advantage of the narrow band gap between valance
and conduction band of the IV-VI compounds. Recently IV-VI multi-quantum well
structures have also attracted a lot of attention for their potential as efficient thermo-
detective devices [1].
Lead sulphide (PbS) is unique interesting semiconductor due to its technological
importance in I-R field sensitive devices widely used as infrared sensor due to its 0.4eV
direct band gap [2-4]. The large excitation Bohr radius of PbS (i.e.18nm) [5], results in
strong quantum confinement of both electrons and holes in nano-sized structure, so that
the value of the band gap can be controlled by modifying particle size according to the
effective mass model [6]. This material has also been used in many fields such as
photography [7]. This property of PbS makes it desirable and attractive for new
applications such as solar cells [8,9] In addition, PbS has been utilized as photoresistance,
diode lasers, humidity and temperature sensors, decorative and solar control coatings
[10,11] fabricated near infrared (NIR) active solar cells based on PbS quantum dots and a
conventional conjugated polymer [12]. Thin films of PbS have been prepared by different
153
workers using various techniques such as spray pyrolsis [13], chemical vapor deposition
[14], chemical bath deposition [15-18], successive ionic layer and reaction (SILAR) [19,
20], atomic layer deposition [21], and vacuum evaporation [22]. The structural, electrical
and optical properties of vacuum deposited thin films of sulphide semiconductors are very
sensitive to the deposition conditions [23]. Stoichiometry can be restored by several
techniques as by, co-deposition of sulphur together with PbS [24] and depositing the film
in a controlled hydrogen sulphide atmosphere [25]. The deposition of sulphide
semiconductors in a controlled H2S ambient atmosphere yields better crystallinity,
orientations and pin-hole free films, which would be of great importance in device
fabrication.
The present chapter deals with the deposition of PbS thin films by evaporation of
powdered PSb material without and with H2S atmosphere under vacuum. The
compensation the sulphur deficiency in vacuum evaporated PbS thin films has been done
by exposing the film to a H2S ambient atmosphere during deposition in same manner as
in case of vacuum evaporated CdS and ZnS thin films as discussed in chapter 4 and
chapter 5 respectively. The higher reactivity of hydrogen sulphide will ensure a better
conversion of the dissociated cations (Pb ions) into compound sulphide semiconductors
(PbS) and also will not produce any excess of sulphure at the substrate.
6.2 Sample preparation:
In the present work the thin films of lead sulphide have been deposited by thermal
vacuum deposited technique onto highly cleaned glass and quartz substrates. For
comparison, films of PbS are deposited without and with H2S atmosphere. For sample
preparation lead sulphide powder (99.9%) of sigma Aldrich Company was used to
evaporate in deep mouthed molybdenum boat. The deposition takes place in a vacuum of
the order of 10-5 torr. H2S atmosphere was obtained by controlled thermal decomposition
154
of thiourea inside the vacuum chamber. For ambient atmosphere of hydrogen sulphide a
borosil test tube was used for the thermal decomposition of thiourea at 150ºC, it was
separated from the PbS molybdenum boat by a stainless steel heat shield. Before
deposition keeping the substrates at an elevated temperature of about 200ºC helps to eject
any sulphur atoms deposited due to thermal decomposition of PbS during evaporation.
The lead ions promptly recombined with the H2S it give better stoichiometry of the
deposited films. The general detailed summarized flow diagram of above used modified
thermal vacuum deposition technique for deposition of stoichiometric sulphide
semiconductors films has been discussed and illustrated in figure 2.5 of chapter 2 of the
thesis.
6.3 Sample Characterization:
The optical properties of as-deposited films especially absorption and transmission
have been evaluated at room temperature by spectro-photometric examination. Structural
analysis has been done by X-ray diffraction (XRD) patterns. The surface morphology was
examined by scanning electron microscope (SEM) with EDAX and atomic force
microscope (AFM). Electrical properties have been studied by measurement of I-V
characteristics of the films using electrometer.
6.4 Optical Properties:
The optical properties of vacuum evaporated PbS thin films deposited on quartz
substrates both without and with H2S atmosphere were studied by the absorption and
transmission spectra taken at room temperature with the help UV-vis-NIR
spectrophotometer (Varian cary 5000) as shown in figure 6.1 and figure 6.2 respectively.
The vis-NIR optical absorption spectra of both films deposited without and with H2S
atmosphere have not shown any appreciable change in the spectral dependence
absorbance in the spectral range 400 – 1500 nm.
155
400 600 800 1000 1200 14000.0
1.5
3.0
4.5
Abso
rptio
n
W avelength (nm )
b a
a. PbS without H 2S atm osphereb. PbS w ith H 2S atmosphere
Figure 6.1: Absorption spectra of vacuum evaporated PbS thin films deposited without and with H2S atmosphere on quartz substrate.
300 600 900 1200 15000
10
20
30
40
50
60
70
% T
W avelength (nm )
a. PbS w ithout H2S atmosphere
b. PbS w ith H 2S atm osphere
b
a
Figure 6.2: Transmission spectra of vacuum evaporated PbS thin films deposited
without and with H2S atmosphere on quartz substrate.
156
Both samples have low transmittance in the ultraviolet (UV) region below 400 nm
due to the strong absorbance in this region of wavelength. Thereafter, transmission
increased with increasing wavelength towards near-infrared (NIR) region. For low
wavelengths, there is no transmission because all the light is absorbed, while the steady
state in NIR regions resembles the absorption spectra [26].
6.4.1 Energy band gap:
To energy band gap of vacuum evaporated PbS films (for both without and with
H2S atmosphere) has been determined by Tauc’s plot for allowed direct transition. The
direct energy gap value of 1.59 eV for the vacuum evaporated PbS thin film (for both
without and with H2S atmosphere) was obtained by extrapolating the linear portion of the
(αhυ)2 versus hυ plot in figure 6.3. The large excitation Bohr radius of PbS (18 nm) [27],
results in strong quantum confinement of both electrons and holes in nano-sized structure.
The increased band gap of vacuum evaporated PbS thin film from 0.41 eV to 1.59 eV is
due to strong quantum confinement effect in nanocrystalline PbS thin films [15,28,29].
The value of Eg of PbS thin films deposited with H2S atmosphere can then be used to
calculate the value of n from the slope of plot of ln((αhν) vs ln(hν-Eg) as shown in figure
6.4. The slope of linear region of this plot comes out n value ~ ½, which indicated
allowed direct transition in as-deposited PbS film material.
157
0 1 2 30
2
4
6
8
10
(h
h
PbS thin film s w ith H 2S atm osphere
Eg =1.59 eV
Figure 6.3: Plot of (αhν)2 Vs hν for vacuum evaporated PbS film deposited with H2S atmosphere.
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
0.5
1.0
1.5
2.0
ln(hv-Eg)
ln(
h
Figure 6.4: Graph between ln(hν-Eg) and ln(αhν) of vacuum evaporated PbS thin
films deposited with H2S atmosphere.
158
6.5 Structural Properties:
The crystallinity and phase of the vacuum evaporated PbS thin films deposited
without and with H2S atmosphere on glass substrates were characterized by X-ray
diffraction (XRD) measurement using an x-ray diffractometer with CuKα radiation.
6.5.1 XRD:
X-ray diffraction patterns of the PbS thin films deposited on glass substrates
without and with H2S atmosphere with 2θ ranging from 20° to 80° are shown in figure
6.5. The XRD pattern of vacuum evaporated PbS/glass thin film deposited without H2S
atmosphere as shown in figure 6.5 (marked ‘a’) had a strong and high intensity peak at 2θ
value about 30° corresponds to the cubic (200) crystalline plane with low intensity cubic
(311) peak at 2θ value about 51° and cubic (400) peak at 2θ value about 62.5°. It is
observed that the preferred orientation growth along the (200) direction for PbS films
deposited without H2S atmosphere. While XRD pattern of vacuum evaporated PbS thin
film deposited with H2S atmosphere as shown in figure 6.5 (marked ‘b’) revealed that the
crystallography of the film is good and characterized by five principle peaks at 2θ values
of about 25.98°, 30°, 43.10°, 51°, 53.49° corresponding to cubic (111), (200), (220),
(311) and (222) orientations respectively. Along with these peaks some small (low
intensity) peaks at 2θ values of about 69°, 71° and 79° corresponding to cubic (331),
(420) and (422) orientations respectively were also observed. It is clear that PbS films
deposited with H2S atmosphere has the preferential orientation along cubic (111) plane.
By comparison with the standard data from JCPDF card No. 78-1901, all diffraction
peaks in both samples can be indexed as a cubic structure of PbS. The crystallite size (d)
using (111) peak was calculated by using Scherrer formula [30] for PbS films deposited
with H2S atmosphere and comes out as 29.79 nm.
159
20 30 40 50 60 70 80
2
Inte
nsity
(a.u
.)
a
b
b. PbS with H2S atmospherea. PbS without H
2S atmosphere
(400
)
(111
)
(200
)
(220
)
(311
)(2
22)
(331
)(4
20)
(422
)
(311
)
Figure 6.5: XRD patterns of vacuum evaporated PbS thin films deposited without and with H2S atmosphere.
6.6 Surface Morphology:
The Surface Morphology of vacuum evaporated PbS thin films deposited without
and with H2S atmosphere was examined by scanning electron microscopy (SEM) with
EDAX and atomic force microscopy (AFM).
6.6.1 Scanning Electron Microscopy (SEM) with EDAX:
Scanning electron microscopy (SEM) with Energy dispersive X- ray analysis
(EDAX) is a suitable technique to study the microstructure and composition of the as-
deposited thin films. The SEM micrographs (at different magnification 50,000X and
1,00,000X) of vacuum evaporated PbS thin films deposited without and with H2S
atmosphere are shown in figure 6.6 and 6.7 respectively. The SEM micrographs show
typical tightly adherent PbS films on highly cleaned glass substrates. It is cleared that the
160
surface of PbS films deposited without H2S atmosphere is rough and grains are not
distributed uniformly.
Figure 6.6: SEM micrograph of vacuum evaporated PbS thin film deposited without H2S atmosphere at different magnifications (i) at 50,000X and (ii) at 1,00,000X.
Figure 6.7: SEM micrograph of vacuum evaporated PbS thin film deposited with H2S atmosphere at different magnifications (i) at 50,000X and (ii) at 1,00,000X.
(i) (ii)
(ii) (i)
161
On the other hand the surface of PbS film deposited with H2S atmosphere has fine
grains which are more uniformly distributed. So, the films deposited in H2S atmosphere
are homogeneous, without any crack, rather dense and exhibit almost complete coverage
of the substrate. This shows that the better surface morphology is achieved in the films
grown in controlled H2S atmosphere.
EDAX analysis was performed for the elemental compositional analysis of the
film. Figure 6.8 and figure 6.9 shows the EDAX spectrum of PbS films deposited without
and with H2S atmosphere on glass substrates. Electron beam induced inner-shell
ionization and subsequent emission of characteristic fluorescence are analyzed in order to
obtain the composition. The peaks for Pb and S were found in the spectrums (figure 6.8
and figure 6.9) confirm the presence of the Pb and S as the components of the as-
deposited thin films of PbS. The Si peak appears in EDAX is due to the glass substrates
used in the deposition of film material. The element weight% and atomic% of Pb and S in
EDAX spectroscopy of vacuum evaporated PbS thin films deposited without and with
H2S atmosphere are presented in Table 6.1 and Table 6.2 respectively. The atomic
percentage of Pb and S shows excess of Pb in the vacuum evaporated PbS thin films. The
average atomic percentage of Pb:S was found to be 52.11:47.89 in PbS thin film
deposited without H2S atmosphere whereas it is found 50.87:49.13 in PbS thin film
deposited with H2S atmosphere. These results shows that S is significantly increasing in
vacuum evaporated PbS thin film deposited in H2S atmosphere which indicates that the
PbS films deposited with H2S atmosphere have better stoichiometry in comparison to PbS
films deposited without H2S atmosphere.
162
Figure 6.8: EDAX analysis of vacuum evaporated PbS thin films deposited without H2S atmosphere.
Table 6.1: EDAX analysis of vacuum evaporated PbS thin films deposited without H2S atmosphere.
Element Wt% At%
SK 12.44 47.89
Pb M 87.56 52.11
Matrix 100.00
163
Figure 6.9: EDAX analysis of vacuum evaporated PbS thin films deposited with H2S atmosphere.
Table 6.2: EDAX analysis of vacuum evaporated PbS thin films deposited with H2S atmosphere.
Element Weight% Atomic%
S K 12.95 49.13
Pb M 87.05 50.87
Totals 100.00
164
6.6.2 Atomic Force Microscopy (AFM):
The 2D and 3D atomic force microscopy (AFM) images and histogram
distribution of vacuum evaporated PbS thin films deposited on glass substrate without and
with H2S atmosphere are show in figure 6.10 (i), (ii), (iii) and figure 6.11 (i), (ii), (iii)
respectively. All the AFM images were taken for an area of 1000 x1000 nm orders show
that the particles are closely bonded. The AFM images of the vacuum evaporated PbS
film deposited on glass substrate with H2S atmosphere revealed that the grains are more
spherical in shape and are homogeneously distributed over the whole surface in
comparison to the PbS film deposited on glass substrate without H2S atmosphere. The
average grain sizes are observed to be increased in PbS films deposited with H2S
atmosphere. The average roughness also increases in vacuum evaporated PbS films
deposited with H2S atmosphere. The average grain size & roughness of PbS films
deposited without and with H2S atmosphere comes out 12 nm (grain size) & 2.6602
(roughness) nm and 14 nm (grain size) & 3.39607 nm (roughness) respectively. PbS film
with H2S atmosphere show cluster of particles with highly dense structure with high
packing density and have advanced surface and typical columnar structure with highly
dense grains.
165
Figure 6.10: (i) 2D and (ii) 3D AFM images of vacuum evaporated PbS thin film deposited
without H2S atmosphere of an area 1000 x 1000 nm orders.
Figure 6.10: (iii) Histogram distribution of AFM image of vacuum evaporated PbS thin film
deposited without H2S atmosphere of an area of 1000 x 1000 nm orders.
(i) (ii)
(iii)
166
Figure 6.11: (i) 2D and (ii) 3D AFM image of vacuum evaporated PbS thin film deposited with H2S atmosphere of an area 1000 x 1000 nm orders.
Figure 6.11: (iii) Histogram distribution of AFM image of vacuum evaporated PbS thin films deposited with H2S atmosphere of an area 1000 x 1000 nm orders.
(i) (ii)
(iii)
167
6.7 I-V measurements:
Schottky barrier junction:
Rectifying metal–semiconductor contacts, also known as Schottky barrier, are the
basic devices in the technology of semiconductors. When a metal is brought into contact
with a semiconductor, there is usually a redistribution of charges, which results in the
formation of depletion layer in the semiconductor. This deformation of the band edge at
the interface is called a Schottky barrier. The potential barrier, which forms when a metal
is contacted with a semiconductor, arises from the separation of charges at the metal–
semiconductor interface such that a high resistance region devoid of mobile carriers is
created in the semiconductor. The barrier results from the difference in the work functions
of the two substances. The current flows in a Schottky barrier diode because of charge
transport from the semiconductor to the metal or in the reverse direction. There are four
different mechanisms by which the carrier transport can occur: (1) thermionic emission of
electrons over the barrier, (2) quantum mechanical tunneling of electrons through the
barrier,(3) carrier recombination (or generation) in the depletion region, and (4) carrier
recombination in the neutral region of the semiconductor which is equivalent to the
minority carrier injection. Process (1) is usually the dominant mechanism in Schottky
barrier junctions and leads to the ideal diode characteristics. The standard Schottky
barrier theory should explain the saturation current JS of the general current–voltage (I–V)
characteristics as a function of temperature T [31,32].
퐽 = 퐽 [exp (푞푉 푘푇) − 1]⁄ … … … … … … … … … . . (6.1)
where J is the current density, V is the voltage, n is the ideality factor, q is the
electronic charge, and k is the Boltzman constant.
In the Schottky theory, JS (T) is mainly determined by the barrier height ФB and
the effective Richardson constant A*
168
퐽 퐴∗푇 exp (−푞∅ 푘푇) … … … … . . … … … … … … … (6.2)⁄
A*=4πm*qk2h-3 is the effective Richardson constant [33] for thermionic emission
corresponding to the electron effective mass m* in the semiconductor and h is the Planck
constant. The barrier height may be written as
∅ = (푘푇 푞)⁄ ln (퐴∗ 푇 퐽 ) … … … … … … … … … … … … . (6.3)⁄
The diode ideality factor (n) is given as
푛 = (푞 푘푇)⁄ [푑(ln 퐽) 푑푉⁄ ] … … … … … … … … … … … . . (6.4)
The ideality factor, n; is very close to unity at low doping and high temperature.
However, it can depart (increases) substantially from unity when the doping is increased
and temperature is lowered. For an ideal Schottky barrier, where the barrier height is
independent of the bias and current flows only due to thermionic emission, n= 1. Factors
which make n larger than unity are the field (bias) dependence of barrier height, electron
tunneling through the barrier, and the carrier recombination within the depletion region. A
plot of ln J versus V gives a straight line. The saturation current density JS can be
obtained by extrapolating the straight line to V=0. Knowing saturation current density JS,
Richardson constant A* and the temperature T, the barrier height ФB can be determined.
The ideality factor n is determined by using the slope of the plot of ln J versus V.
The barrier height and ideality factor of the junction were investigated by means
of current–voltage measurements. The I–V characteristics of the junction formed are
shown in figure 6.12 (i) & (ii) for vacuum evaporated PbS thin films deposited without
and with H2S atmosphere respectively and can be explained by diode rectifier theory.
Both contacts were of vacuum deposited aluminum on the same side of the sulphide films
with the gap of same side about 1mm between them. The I–V characteristics of the
junction can be analyzed in terms of the thermionic emission model of Schottky barrier
current transport as given by equation (6.1) and (6.2).
169
-20 -10 0 10 20
-0.0003
-0.0002
-0.0001
0.0000
0.0001
0.0002
0.0003
Cur
rent
(
V in (Volts)
PbS without H 2S atmosphere
-20 -10 0 10 20
-0.0000005
0.0000000
0.0000005
Cur
rent
(
V in Volts
PbS with H 2S atmosphere
Figure 6.12: Current–voltage characteristics of vacuum evaporated PbS Schottky junction without and with H2S atmosphere.
(i)
(ii)
170
0 5 10 15 20 25
-8
-12
-16
-20
lnJ F
VF(in Volts)
PbS without H 2S atmosphere
Figure 6.13: (i) Plot of ln JF versus VF for vacuum evaporated PbS film depossited without H2S atmosphere.
0 5 10 15-12
-14
-16
-18
-20
-22
-24
lnJ F
V F(in Vo lts)
PbS w ith H 2S atm osphere
Figure 6.13: (ii) Plot of ln JF versus VF for vacuum evaporated PbS film deposited with H2S atmosphere.
(i)
(ii)
171
Figure 6.13 (i) & (ii) shows a plot between the forward current density ln JF and
forward voltage VF for metal–semiconductor contact PbS thin films without and with H2S
atmosphere respectively. The extrapolated value of current density JF to zero voltage
gives the saturation current density JS, (JS -15.46 and -20.63 without and with H2S
atmosphere respectively) The barrier height of the PbS Schottky junction is obtained by
using equation (6.3) and has the value 0.35 eV and 0.34 eV without and with H2S
atmosphere respectively.
172
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