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CHAPTER 4

CHEMICAL SYNTHESIS, DEPOSITION AND

CHARACTERIZATION OF LEAD SELENIDE (PbSe) THIN

FILMS BY CHEMICAL AND PHYSICAL METHODS

4.1 INTRODUCTION

Nanocrystalline based solar cells have been demonstrated the next

generation of low cost alternative to traditional silicon based solar cells

(Ilan et al 2010 and Serap Gunes et al 2007). In this II-VI, IV-VI and I-III-VI

groups are the leading candidates for future solar applications. Now-a-days,

lead chalcogenide nanocrystalline semiconducting materials have attracted the

attention of many researchers because of their potential applications in solar

energy conversion devices (Enue et al 2011 and Kurtis et al 2009),

photosensors (or) photodetectors, (Jaime et al 1996 and Muñoz et al 1998),

thermoelectric devices and nonlinear optical devices. Among the available

lead chalcogenides, lead selenide (PbSe) thin film has been widely used as

thermoelectric materials for cooling and power generation applications

(Ibrahim et al 2008 and Xiaofeng Qiu et al 2010).

PbSe belongs to IV-VI compound semiconductor, possessing

excellent optoelectronic properties and it exhibits cubic structure in face

centered phase with the space group of 3Fm m , the atomic arrangement of

PbSe is shown in Figure 4.1 and direct bandgap (Eg) of 0.27 eV (bulk value)

at Room Temperature (RT) for thin films with crystallite size above 40 nm

(Rowe 1995 and Khokhlov 2003). However, optical bandgap of PbSe can be

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tuned (~1.5 eV) by decreasing the crystallite size (~ 4 nm) upon changing the

preparation conditions (Sasha Gorer et al 1995). This will pave the way to

PbSe films for the potential application in solar cells. It is worth to mention

here that the semiconductors with Eg range of 1-1.5 eV is suitable to achieve

high energy conversion efficiency (30%) when used as absorber material in

optoelectronic devices. (William Shockley and Hans J. Queisser 1961). In

addition, PbSe nanocrystals have major industrial uses such as field effect

transistors infrared detectors, light emitting devices etc. due to their unique

electronic, optical and physical properties (Anders Hagfeldt and Michael

Graetzel 1995 and Conyers Herring and Galt 1952).

Figure 4.1 Atomic arrangement of PbSe

4.2 DEPOSITION OF LEAD SELENIDE THIN FILMS USING

CBD

The nanocrystalline PbSe was synthesized and deposited by

chemical bath deposition method on glass substrate using various

concentrations of EDTA and the deposited thin films were vacuum annealed

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at 450°C. The lead selenide thin films were deposited via simple chemical

bath deposition technique at room temperature using lead nitrate, sodium

selenosulphate and sodium hydroxide as the starting materials. For thin film

deposition starting materials of 0.06 mol of lead nitrate and 0.6 mol of sodium

hydroxide (NaOH) were dissolved in 50 ml of distilled water in the glass

vessel.

The vessel with reactive solution was kept in the room temperature.

Initially, the solution became milky turbid due to the formation of Pb(OH)2

after that it changed as a clear solution. The different concentrations of

Ethylene Diamine Tetra acetic Acid (EDTA) such as 0.05 and 0.1 mol were

added into the solution, which easily binds with metal ions. A 50 ml of freshly

prepared sodium selenosulphate solution as a selenium source was used for

deposition. The substrate was fixed vertically inside the deposition bath and

the solution was stirred well with the help of magnetic stirrer to maintain the

homogeneous mixture. Normally CBD covers two sides of the substrates but

for analysis one sided film is required. Hence, the film deposited on one side

of the substrate was cleaned using cotton swabs moistened in dilute nitric

acid.

4.3 RESULTS AND DISCUSSION–CHEMICAL METHOD

4.3.1 Structural Properties

The X-ray diffraction pattern of the deposited PbSe material is

shown in Figure 4.2. The X-ray diffraction pattern was recorded using

BRUKER D2 PHASER desktop diffractometer. The presence of a number of

reflections indicates that the materials are polycrystalline in nature. The XRD

patterns are matched with the peak positions of standard X-ray powder

diffraction data (JCPDS card number 78-1902) and also matched with the

help of powderX software package and the values are tabulated in Table 4.1.

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The XRD pattern of the as-deposited film shows the cubic

structure, while on increasing the concentration of the EDTA from 0.05 M, an

additional peak is observed. This may be due to the high concentration of

EDTA in depositing solution which largely captures lead ions and releasing of

lead ions for deposition is not proportionate, hence the additional peak

appears due to the formation of diselinide. The peaks at (200), (111) and

(220) (Figure 4.2) show that the polycrystalline thin film exhibits in the cubic

phase. The line broadening is due to the size variation in the crystallites,

dislocations and lattice strains.

Figure 4.2 X-ray diffraction patterns of as-deposited and 450°°°°C

annealed PbSe films

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Table 4.1 Structural parameters of PbSe thin films deposited by chemical bath deposition

Details

2θθθθ (deg.)

(hkl) Crystallite

size 10-9 m

d-spacing (Å) Lattice parameter

(Å) Dislocation

density

lines/ m2

Strain

Exp Cal Exp Cal

Exp Cal

a a

PbSe as-deposited

25.61 25.25 111 - 3.476 3.525 - - - -

29.71 29.23 200 6.71 3.005 3.053 6.010 6.105 2.22×1016 0.081

41.87 41.82 220 - 2.156 2.158 - - - -

PbSe as-deposited

0.05 M EDTA

29.24 29.23 200 11.15 3.051 3.525 6.102 6.105 8.03×1015 0.049

41.69 41.82 220 - 2.165 3.053 - - - -

PbSe as-deposited

0.1M EDTA

29.29 29.23 200 6.75 3.047 3.053 6.093 6.105 2.19×1016 0.081

41.81 41.82 220 - 2.159 2.158 - - - -

PbSe annealed at

450 °C

25.41 25.25 111 - 3.502 3.525 - - - -

29.30 29.23 200 9.30 3.045 3.053 6.091 6.105 1.15×1016 0.059

41.77 41.82 220 - 2.161 2.158 - - - -

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4.3.2 Optical Properties

The thickness of the as-deposited PbSe film is 0.864 µm. The

optical transmittance and absorbance spectra of the PbSe films were recorded

in the range of 350-1100 nm. Figures 4.3(a) and (b) show the optical

transmittance and absorption of the PbSe film. The bandgap of as-deposited

PbSe film is found to be 1.14 eV and it gets reduced to 0.84 eV for the film

annealed at 450ºC (Figure 4.4(a)). Kassim et al (2010) have reported the

bandgap of PbSe thin films varies from 1.3 to 1.1 eV on increasing the

deposition temperature by chemical bath method. The extinction

coefficient (k) is calculated from the optical absorption spectra using the

relation given in Equation 3.11 and it’s shown in Figure 4.4(b). When the

deposited film was annealed at 450°C, it is inferred that the wavelength shifts

towards red region owing to the increase in particle size.

Figure 4.3 Optical spectra of as-deposited and 450°°°°C annealed PbSe

films (a) transmittance and (b) absorbance

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Figure 4.4 (a) Optical bandgap graph of PbSe films

(b) variation of extinction coefficient (k) with wavelength

for as-deposited and 450°°°°C annealed PbSe films

4.3.3 Surface Analysis

Scanning electron microscopy was employed for the investigation

of morphological features of the deposited thin films, which suggests that the

products exhibit high uniformity. From the surface morphology the

uniformly distributed crystal grains show a rod shaped particles on the surface

of the as-deposited thin film as shown in Figure 4.5(a). When as-deposited

sample was annealed at 450ºC the surface morphology changed to flower like

structure and also the agglomeration of crystal grains was observed (Figure

4.5(b)). The recorded EDX pattern shows the composition of Pb and Se.

Figure 4.6 shows the EDX spectrum with the inset image showing a selected

area where EDX was carried out on the surface of the deposited films. From

the obtained result % of Se is found to be more when compared to % of Pb.

This may be due to presence of more Se ions in the reacting solution. The

composition is roughly 1:2 (Pb:Se) or PbSe2.

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Figure 4.5 HRSEM image of (a) as-deposited and (b) 450ºC annealed

PbSe films

Figure 4.6 EDX spectrum of as-deposited PbSe film

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Figure 4.7 AFM micrographs of PbSe films (a) as-deposited

(c) annealed at 450°°°°C and (b, d) histograms of as-deposited

and annealed films

The surface morphology of PbSe films was studied using atomic

force microscopy. Figure 4.7(a-d) shows 3D view of AFM images and

histogram of the PbSe films deposited via chemical bath deposition method.

It was found that the small spherical grains of different sizes were

distributed on the surface of the films. When the sample was annealed at

450°C the grain size was found to be increased, owing to coalesce of

smaller grains into larger grains or agglomeration of the particles. The

surface roughness of the films was measured from the histogram image and

inferred from the study that the roughness of the film gets reduced after

annealing at higher temperature.

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4.3.4 Electrical Properties

Hall measurements were made at room temperature and a constant

current (1 nA) with the magnetic field of 0.57 Tesla using Hall Effect

measurement system. The deposited PbSe films have positive Hall coefficient

which confirms the p-type charge carrier. The Hall mobility of the chemically

deposited PbSe film is found to be 2.062 cm2/Vs, resistivity

(2.409 × 105 Ωcm), carrier concentration (η = 1.257 × 1011 /cm3) and

conductivity (4.151× 10-6 /Ωcm).

4.4 DEPOSITION OF PbSe FILMS BY PHYSICAL METHOD

Thermal evaporation impart some feasible device based qualities

like optimum stoichiometry, morphology and crystalline alignment to the

films, which are the key factors deciding the performance of the films for

their suitability in developing special devices. However, limited reports

(Arivazhagan et al 2011, Prabahar et al 2009 and Ma and Cheng 2011) are

available for the preparation of PbSe films by thermal evaporation technique

using the commercially available PbSe powder. But, no attempt has been

made to grow PbSe films using chemically synthesized PbSe nanocrystals by

thermal evaporation method. Such a study will be useful to identify clearly

the nanoparticle size dependent physical properties of PbSe films for any

technological applications. As mentioned above, the size and shape of the

PbSe nanoparticles play an important role in the physical properties, such as

its structural, morphology, optical and nonlinear optical properties (Terra et al

2010).

In the present work, as the first step, nanocrystalline PbSe was

synthesized by simple chemical method (co-precipitation) at 80°C. The

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reacting solution temperature is one of the most important parameters to

enhance the chemical reaction. The EDTA was used as a complexing agent to

have effect on the structural, morphological and compositional properties.

Later on, the synthesized nanocrystalline PbSe was used to deposit PbSe thin

films by thermal evaporation technique at different substrate temperatures like

RT, 150, 250, 350 and 450°C. Substrate temperature induced changes in

physical properties of PbSe films were studied.

4.4.1 Synthesis of PbSe Nanoparticles

The PbSe nanocrystalline material was prepared by a simple

chemical (co-precipitation) method using lead nitrate (Pb(NO3)2) and sodium

selenosulphate (Na2SeSO3) (source of lead ions and selenium) in the aqueous

alkaline media at 80°C. The starting materials of lead nitrate (0.06 mol) and

sodium hydroxide (NaOH) (0.6 mol) were dissolved in 250 ml of distilled

water in the 500 ml glass vessel. The vessel with reactive solution was

immersed into oil bath to maintain its temperature. A thermometer was placed

in the vessel to measure the temperature of the bath solution and also

temperature sensor, dimmer with temperature controller was attached to

maintain the constant temperature. Initially, the solution became milky turbid

due to the formation of Pb(OH)2 and turned as a clear solution. Then, 0.1 M

of EDTA was added with the solution, as a complexing agent, which easily

binds with metal ions. 250 ml of freshly prepared sodium selenosulphate

solution was used as a selenium source in the synthesis of nanoparticle PbSe.

The schematic of experimental setup is shown in Figure 4.8. The preparation

methods of sodium selenosulphate are discussed in the third chapter.

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Figure 4.8 Schematic of the apparatus used for synthesis of

nanoparticle PbSe

The above mixed solution was incessantly stirred by motorized

magnetic stirrer for 24 hr. The dark brown nano particle PbSe suspended at

the bottom of beaker and the product was washed three times with distilled

water to remove by-products present in the material. The harvested fine

product of PbSe was dried at 60°C for 2 hr.

4.4.2 Mechanism for Formation of Lead Selenide

The mechanism for the formation of lead selenide through chemical

reaction depends on the experimental conditions. The rate of growth mostly

dependent on the rate of release of Pb2+ ions from the complex and the

decomposition of sodium selenosulphate in aqueous alkaline medium. PbSe is

formed when the ionic product of Pb2+ and Se2− ions exceeds the solubility

product. The synthesized selenosulphate gradually releases selenide ions upon

hydrolytic decomposition in alkaline media as follows,

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2 23 4SeO OH HSe SeO− − − −+ → + (4.1)

22HSe OH Se H O− − −+ → + (4.2)

The released selenide ions combine with the lead ions to form the

compound, the mechanism of film formation can be understood from the

following reaction (Xinjun Wang et al 2010).

( )

2 2

complexPb EDTA Pb EDTA+ + −+ → (4.3)

( )

2 2

complexPb EDTA Se PbSe EDTA+ − −+ → + (4.4)

The various preparative parameters such as ion concentration,

temperature, pH of the solution and EDTA concentration were altered and

optimized to get good stoichiometric nano particle PbSe.

4.4.3 Thin Film Preparation

The PbSe films were deposited on clean microscopic glass

substrates by thermal evaporation using pelletized PbSe nano particle

(synthesized by the above process) under a chamber pressure of 1×10-5 Torr at

RT, 150, 250, 350 and 450ºC. The thickness of deposited film was in the

range of 0.450 - 0.615 µm as measured by stylus profilometer. Phase analysis

of PbSe films was performed by X-ray diffraction (XRD) studies using CuKα

radiation (λ = 1.5418 Å) over a 2θ scan range of 10-80º. Optical properties of

the films were studied by UV-Visible spectrophotometer and

photoluminescence spectrometer. Substrate temperature induced changes in

surface morphology of PbSe films were studied by HRSEM and AFM.

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Electrical properties of films were studied by Hall effect measurement system

(van-der-Pauw method).

4.5 RESULTS AND DISCUSSION-PHYSICAL METHOD

4.5.1 Structural Analysis

Figure 4.9 shows the X-ray diffraction pattern of as-synthesized

PbSe nanoparticle by chemical method. It shows that the as-synthesized PbSe

product is polycrystalline in nature and the observed d-spacing matches

closely with cubic structure (JCPDS card No. 78-1902). The appearance of

most prominent diffraction peak at 29.2° corresponding to (200) indicating

the predominant growth of crystallites along [100] direction. The XRD

patterns of grown PbSe films by thermal evaporation at different substrate

temperatures are shown in Figure 4.10. The crystalline nature of the films is

clearly evident from the XRD patterns of thermally evaporated PbSe films

and the observed d spacing agrees well with cubic phase as like synthesized

product. There is no major structural phase change observed in the films due

to substrate temperature up to 450ºC. It is clearly seen from Figure 4.10 that

the intensity of (200) oriented peak increases with increase in substrate

temperature.

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Figure 4.9 X-ray diffraction of synthesized PbSe powder

Figure 4.10 X-ray diffraction patterns of PbSe films deposited at various

substrate temperatures

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Table 4.2 Structural parameters of as-synthesized and different substrate temperatures deposited PbSe thin films

Temperature

2θθθθ (deg.)

(hkl) Crystallite

size 10-9 m

d-spacing (Å) Lattice parameter

(Å) Dislocation density

1015 lines/ m2

Strain

Exp Cal Exp Cal

Exp Cal

a a

Synthesized at 80ºC

25.17 25.18 111 - 3.535 3.534 - - - -

29.15 29.16 200 27.48 3.061 3.061 6.122 6.105 1.32 0.020

41.70 41.70 220 - 2.164 2.164 - - - -

49.34 49.34 311 - 1.846 1.846 - - - -

51.69 51.69 222 - 1.767 1.767 - - - -

60.45 60.44 400 - 1.530 1.530 - - - -

68.49 68.49 420 - 1.368 1.369 - - - -

76.12 76.12 422 - 1.249 1.249 - - - -

Thin film deposited at

Room Temperature

25.29 25.24 111 - 3.518 3.525 - -

29.14 29.23 200 18.06 3.062 3.053 6.125 6.105 3.06 0.031

41.75 41.82 220 - 2.162 2.158 - - - -

49.29 49.48 311 - 1.847 1.841 - - - -

51.77 51.84 222 - 1.765 1.762 - - - -

150ºC 29.26 29.23 200 22.76 3.049 3.053 6.099 6.105 1.93 0.024

250ºC 29.12 29.23 200 20.38 3.064 3.053 6.128 6.105 2.41 0.027

350ºC 29.31 29.23 200 29.39 3.044 3.053 6.089 6.105 1.16 0.019

450ºC 29.30 29.23 200 31.14 3.046 3.053 6.091 6.105 1.03 0.018

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The increasing peak intensity and narrowing of the peaks could be

an indicative of substrate temperature induced grain growth. This is clearly

observed from Table 4.2. The decreasing in dislocation density (δ) with

increasing substrate temperature is observed for the predominant peak (200).

The evaluated lattice parameters of films agree well with JCPDS data as

shown in Table 4.2. In addition, one can observe from Table 4.2 that the strain

of PbSe films decreases with increasing substrate temperature indicating the

release of intrinsic film stress while increasing the substrate temperature

thereby reducing the imperfections within the crystalline lattice.

4.5.2 Optical Properties

4.5.2.1 UV -Visible Spectroscopy

Optical (transmittance and absorbance) spectra recorded in the

visible and near infrared regions are related to electronic transitions and also

useful in understanding the electronic band structure of the semiconducting

films (El-Ocker et al 1990). The optical transmittance and absorbance spectra

of PbSe thin films deposited at different substrate temperatures were recorded

in the wavelength range of 400-1100 nm are shown in Figures 4.11(a)

and (b). It is observed from Figure 4.11(a) that the optical transmittance of

films decreases with increasing the substrate temperature. This may be

attributed to the grain growth of the films at higher substrate temperatures. On

the other hand, the samples exhibit a strong absorption in the visible region as

shown in Figure 4.11(b). In addition, from the optical absorption spectra it is

observed that the absorption edge shift towards higher wavelength region with

an increase in substrate temperature, indicating bandgap narrowing. The

optical bandgap of the film was evaluated using the relation 3.7.

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Figure 4.11 Optical spectra of PbSe thin films deposited at different

substrate temperatures (a) transmittance and

(b) absorbance

Figure 4.12(a) Optical bandgap graph of PbSe thin films (b) variation

of extinction coefficient (k) with wavelength for PbSe

thin films deposited at different substrate temperatures

PbSe is a direct bandgap system. Tauc’s plot, (αhν)2 with energy

(hν) shows a linear behaviour in the higher energy region which corresponds

to a strong absorption near the absorption edge. Figure 4.12(a) shows the

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Tauc’s plot of PbSe films prepared at different substrate temperatures. The

gradual reduction in Eg of the films with increasing substrate temperature is

clearly evident from Figure 4.12(a) which enumerates the crystallization of

the films. For instance, the bandgap of RT grown PbSe film is found to be

1.62 eV and it gets reduced to 1.46 eV for the film prepared at 450ºC. This

bandgap narrowing indicates that the top of the valence band and the bottom

of the conduction band are modified to various extents with increasing

substrate temperature.

The extinction coefficient (k) is calculated from the optical

absorption spectra using the relation given in Equation 3.11 and the variation

in k with respect to the substrate temperature is presented in Figure 4.12(b).

The extinction coefficient has high values near the absorption edge and it has

very small values at higher wavelength.

4.5.2.2 Photoluminescence properties

The photoluminescence (PL) spectra of RT and 450°C substrate

temperature deposited lead selenide thin films under various excitation

wavelength of 350, 400 and 450 nm are shown in Figure 4.13. The PL signal

depends on the density of photoexcited electrons, intensity of the incident

beam and also change with excitation position and wavelength (Timothy H.

Gfroerer 2000). It is observed that the emission peak shifts towards higher

wavelength region with an increase in the excitation wavelength and also with

increasing substrate temperature. This could be the indicative of variation in

the optical bandgap (narrowing) of the material. This inference is consistent

with our optical absorption study where the decrease in Eg is observed with

increasing substrate temperature. The compositional disorder present in this

material leads to the formation of broad emission peaks.

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Figure 4.13 Photoluminescence spectra of (a) RT and (b) 450°°°°C grown

PbSe thin films recorded at different excitation

wavelengths: (i) 350 nm, (ii) 400 nm, and (iii) 450 nm

The observed high intense peak in PL spectra is attributed to the

excitons (electron-hole pair) recombination of Pb and Se nanoparticles.

Soumyendu Guha et al (1998) have observed the peak shift in emission band

when the PbSe films were excited with different energy. In the present

investigation the PL excitation with different energies and the corresponding

emission shift are reported. The shift in these PL bands is attributed to the

difference in their excitons transition. This property reveals that the material

can be used for tunable IR detectors.

4.5.3 Surface Morphology

High Resolution Scanning Electron Microscope was used to study

the surface morphology of the synthesized material and grown PbSe thin

films. The HRSEM image (different magnifications) of synthesized PbSe

nanoparticle is shown in Figure 4.14(a)-(c). The morphology (Figure 4.14(b))

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clearly shows the accumulation of almost uniform cubical nanoparticles in the

synthesized product. The approximate sizes of the cubes are in between

350 - 610 nm. The compositional nature of the synthesized PbSe product was

studied by EDX and is shown in Figure 4.14(d). It is not observed any other

peaks except the strong identifications of Pb and Se that indicates the purity

of synthesized product. The atomic percentage of Pb and Se are found to be

50.47 and 49.53, respectively which revealed the stoichiometric nature of the

synthesized PbSe nanoparticles by the simple chemical approach.

Figure 4.14 HRSEM micrographs (different magnifications) of

synthesized PbSe powder: (a) ×14000, (b) ×25000, and

(c) ×40000. (d) EDX spectrum

The HRSEM micrograph of RT grown PbSe thin film is shown in

Figure 4.15(a). From the image it is inferred that the particles are arranged

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randomly and densely packed on the surface of the substrate. The appearance

of random crystals in the micrograph is also corroborated from the observed

multiple reflections in XRD pattern of room temperature deposited film.

Figures 4.15(b)-(e) show the HRSEM image of PbSe thin films prepared at

150, 250, 350 and 450ºC, respectively. The uniformly distributed particles are

observed in the morphology for the films grown at 150 and 250ºC

(Figure 4.15(b) and (c)). Whereas, when the substrate temperature is

increased to 350ºC the film morphology becomes smoother and a few

nanorods are also observed, as shown in inset of Figure 4.15(d). In the case of

450ºC sample, the image (Figure 4.15(e)) shows the agglomeration of grains

and also the block shaped particles. The EDX studies revealed the deposited

PbSe films at all the substrate temperatures are close to the stoichiometry.

Figure 4.15 HRSEM micrographs of PbSe thin films deposited at

different substrate temperatures: (a) RT, (b) 150, (c) 250,

(d) 350 and (e) 450ºC

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Atomic force microscopy (AFM) is one of the most effective ways

for the surface analysis due to its high resolution. This technique offers digital

images which allow qualitative measurements of surface features, such as root

mean square roughness and the analysis of images from different point of

view, including three dimensional simulations.

Figure 4.16 AFM images of (a) Room temperature and (c) 450°°°°C

deposited PbSe thin films, (b) and (d) corresponding RMS

roughness histograms

Figure 4.16 shows the AFM images of PbSe films deposited at RT

and 450ºC. The AFM image of RT deposited sample (Figure 4.16(a))

indicates the growth of a granular film. When the substrate temperature was

increased to 450ºC, the AFM image shows (Figure 4.16(c)) that the films

deposited are very dense and well crystallized in the nano form on the surface

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and uniformly covered by spheroid-like structures. It is noted that film

composed of isolated nanoparticles throughout the surface with an average

height of 134, 104 nm for RT and 450°C deposited samples, respectively. In

fact, the AFM images (not shown) for the other three substrate temperatures

also show similar morphological changes. The average route mean square

(RMS) surface roughness of the films does not show much variation in RT

and 450°C deposited PbSe samples, this can be easily viewed from the

histograms in Figure 4.16(b) and (d).

4.5.4 Electrical Properties

The electrical properties of PbSe film deposited at 450ºC were

studied by the Hall effect measurement using the van-der-Pauw method. Hall

measurements were made at room temperature with a constant current (1 µA)

and the magnetic field of 0.57 Tesla. The deposited PbSe thin films have

positive Hall coefficient confirming the p-type charge carrier. The Hall

mobility of the deposited PbSe film is found to be 5.5 cm2/Vs, resistivity

8.15 × 103 Ωcm, carrier concentration η = 1.35 × 1014 /cm3 and conductivity

1.22× 10-4 /Ωcm.

4.6 CONCLUSION

The PbSe thin films deposited via chemical method using 0.05 and

0.1 M concentration of EDTA. X-ray diffraction shows the deposited film

exhibits cubic phase. When the EDTA of 0.1 M was added to the reacting

solution, the deposited film shows an additional peak in the XRD pattern. In

Physical methods the substrate temperature induced changes in physical

properties of thermal evaporated PbSe thin films. The chelating agent EDTA

enhances the growth and controls the shape of the crystalline powder.

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Nanoparticle of lead selenide semiconducting material with stoichiometry was

synthesised using simple chemical method which exhibits cubic phase. XRD

pattern reveals the substrate temperature induced grain growth and decrease in

the dislocation density. The optical study reveals that the absorbance band

shifts towards the red region after annealing at 450°C. A gradual reduction in

optical bandgap is observed with increasing substrate temperature, which is

associated with the crystallization of the films has been observed. The

variation of optical bandgap is due to the change in the particle size.

Surface properties were studied by HRSEM and AFM. From the

HRSEM micrograph, it is found that rod shaped particle after annealing it

gives rise to a flower like structure which agglomerates and scatter over the

surface of the film. EDX spectrum reveals that the percentage composition of

Pb:Se∼1:2, due to captures of Pb ions by the Chelating agent creating excess

of Se ions. The scanning electron microscopic analysis of the synthesized

nano particle shows cubic shape with regular morphology. 3D view of AFM

shows that spherical grains get agglomerated and the surface roughness of the

deposited film reduces and it entirely spreads over the surface of the substrate,

similar morphology has been observed for thermal evaporated thin films.

From the observation, it is concluded that the 450°C substrate temperature is

the best condition to prepare good quality lead selenide thin films. The

positive Hall coefficient obtained for the deposited film by physical and

chemical methods confirm the p-type conductivity.