Preparation of Nanocrystalline PbS Thin Films by Chemical Bath Deposition M. Abass

1, E..Al-Fawadi1 and A-K. Al-Samuraee2 1 Department of Physics, Baghdad Univ., Baghdad, Iraq 2Ministry of science & Technology, Baghdad, Iraq Keywords: PbS film, Chemical bath deposition, AFM

Abstract

A Polycrystalline PbS thin films of a thicknesses (0.2-0.85m) were deposited on glass substrates using chemical bath deposition technique. The details of the preparation method are well described. The method is relatively simple and is easily controlled and sufficient to produce large area and good adherence to the substrate films.

The structure and crystallite size of these films were established using atomic force microscope (AFM) and X-ray diffraction (XRD), it shows that the structure is polycrystalline and the average crystallite size is around 20 nm.

The optical properties; absorption, transmission, and reflection, as a function of thickness were measured. These measurements showed that increasing of film thickness always led to; decreasing the optical energy gap (Eg), increasing the absorption coefficient, shifting the absorption edge slightly towards higher wavelength, and also increasing the density of state .The extinction coefficient, refractive index and dielectric constant have been studied as well as.

Introduction

Nanotechnology deals with nanometer size objects. Size dependence new physical aspect

have been generating a continuous thrust for new practical applications[1]. In recent years, there has been increasing interest in the size dependent electrical and optical properties of semiconductors. In nanocrystallites, the electrons confined in a narrow potential well, exhibit extraordinary characteristics in the optical and electrical properties [2]. When the width of such confining potential is very small, i.e., of the order of few hundred angstroms, a quantum size effect arises [3]. During the last few years, the research on the growth of nanoparticle PbS films has increased, for its technological applications in photo resistance, laser diodes, decorative coatings and optoelectronic devices [2]. Many researchers [4-11] have reported properties of chemically bath deposited thin films. The method of chemical bath deposition (CBD) is based on successive adsorption and reaction of species on the substrate surface from aqueous solutions, it does not require sophisticated and expensive instruments. By CBD method thin films have been grown easily. Growth of nano particle films for device application is still an emerging field of research. The CBD technique has been used extensively for the deposition of thin chalcogenide films. This is because the technique processes a number of advantages such as low cost, low working temperatures, and easy coating of large surfaces over other thin films deposition method. A CBD technique also offers the advantage of being able to deposit films on different kinds, shapes and size of substrates [11]. CBD technique has also been found to be a very suitable method for deposition of polycrystalline PbS thin films with good photoconductive properties. By CBD the dimensions of the crystallites can be varied controlling deposition parameters, reaction time,

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temperatures, PH, presence of impurities in the solution, Most of the works on nano semiconductor particle reported so far have been restricted to optical absorption. The absorption edge has been found blue shifted as the particles size reduces. In this paper we present the synthesis of nanocrystalline PbS thin films using CBD method. Preparative parameters such as concentration Pb+2 and S-2 ions, temperatures and PH were optimized to obtain good quality of PbS thin films on glass. The size dependent particles of films were studied by XRD, optical absorption as a function of thickness.

Experimental

The important thing before the deposition of films is the careful cleaning of the substrates.

As the cleanliness of the substrate has a direct bearing on the adherence of the film, the following procedure was adopted for cleaning the substrates. The glass slides were cleaned by water and soap then washed with double distilled water subsequently, the substrates cleaned with alcohol using ultrasonic for 30 min and dried, followed by chemical polishing in a mixture of( K2Cr2O7 :H2SO4 ) , HNO3 to introduce functional groups called nucleation and or epitaxial centers which formed the basis for thin films growth. Finally, the samples were washed repeatedly in bi-distilled water.

PbS films were grown on ordinary glass slide substrates by CBD method .The reactive solution was done in a 100 ml chemical cell. It consisted of a glass cell surrounded by a glass jacket in which water is circulated in order to allow temperature control by means of a thermostatic bath.

The deposition bath contained an aqueous solution, prepared by the sequential addition of 5ml of 5ml of 0.5M lead acetate, 5ml of 2M sodium hydroxide, 6ml of 1M thiourea and 2ml of 1M triethanolamine. The solution was diluted with deionized water to complete a total volume of 100ml.

Cleansed glass substrates were immersed into the solution, supported on the wall of chemical cell. The solution was stirred continuously during growth with a constant stirring. The temperatures of the solution for growth were 10 0C and kept constant for all deposition time.

The PH of the solution was kept at 10 and was measured by PH meter (HANNA HI-8314).Microstructure surface topography was estimated using AFM by AA3000 Angstrom advanced lmtd. . The result of films were uniform, homogeneous, well adhered to the substrates and brown black in color.

The structure of the films of thicknesses (0.2, 0.37, 0.65, 0.85m) was obtained using X-ray diffractometer (XRD) (Shimadzo 6-2006, with Cu-K radiation having wavelength = 1.5406 A). Optical absorption studies of the thin films in the wavelength range 200800 nm were conducted using spectrophotometry (Model UV-160, (Shimadzu, Kyoto). The thicknesses were measured using digital coating thickness gauge TT260 instruction manual.

Results and Discussion

The CBD is based on sequential reaction at the substrate surface. The formation of PbS

may involve the following steps. In an aqueous solution lead acetate releases Pb2+ ions as

Pb(CH3COO)2 Pb2+ + 2CH3COO (1)

In alkaline medium, dissociation of thiourea (NH2)2CS takes place:

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S ||

NH2 C = NH2 + OH CH2N2 + H2O + SH (2)

SH + OH S2 + H2O (3)

Pb2+ + S2 PbS (4)

The reaction solution color changed to brownish black after being heated for ten minutes, indicating initiation of chemical reaction. At the end of different duration of depositions, the substrates coated on both sides with blackish films, were removed from the bath, washed well with deionized water and dried. The PbS thin film deposited on the side of the substrate facing the beaker wall was retained for all analysis. The coating on the outer side was removed with dilute acid and preserved in airtight chamber. Using optimized conditions and by changing deposition time period the films having thickness (0.2, 0.37, 0.65 & 0.85) m was prepared Fig.(1).

Fig.(1) Variation of PbS film thickness with deposition time.

AFM Characterization

AFM images (2D & 3D) using tapping tip mode were prepared of the PbS thin films on glass substrates deposited at various thicknesses as shown in (Fig. 3a,b,c).

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Fig (2) AFM images (2D & 3D) ;a-0.25 , b-0.65,and c-0.85 m.

A dense surface of the thin films was obtained at all cases. The thin films were composed of multilayered grains. When the deposition thickness was increased, the crystallization of PbS thin films was improved, when the deposition thickness was around .25mm, the grain size of the thin films was relatively uniform, and the crystalline growth was perfect. In addition, preferential film growth improved with increasing thickness, which was in accordance with XRD analysis.

Therefore, deposition rate was important during the deposition process. It significantly influenced the crystallizations and morphologies of the PbS thin films. Fig.(3) shows the best grain analysis report which estimated for the more thicker film (around 48 nm) .

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Fig (3) AFM grain analysis report for the PbS film of 0.85 m thickness.

X-ray Diffraction

Fig.(4) shows the XRD patterns of the PbS films deposited at several thickness, the patterns display four diffraction peaks at 2 values of approximately 25.8, 29.9, 42.9 and 50.7o which correspond to the diffraction lines produced by the (111), (200), 220) and (311) crystalline planes of the PbS cubic phase (galena), respectively.

It is observed a preferred orientation growth along the (200) direction. It is also observed that the intensity of the peaks increases with increasing thickness of the films.

Fig.(4) X-ray diffraction of PbS films at different chemical bath thickness.

Optical Properties

The transmittance spectra for PbS films at different thicknesses were measured and the results showed a decrease in transmission wavelength with increasing the thickness as shown in fig (5), and this is in agreement with the results obtained by Ileana et. al [12], and this explained

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through the film morphology , as the film thickness increases the grain size increases too, as has been mentioned previously, the surface roughness increases with increasing grain size which led to decrease in transmission. The absorption edge was shifting to higher wavelength with increasing of thickness and this is may be attributed to the creating levels at the energy band by increasing of thickness, which leading to shifts to lower energies and this may be indicate that increasing of thickness leads to increasing new state in the energy gap. Visualized as an increase in the width of concentration tail leading to a shift of Fermi level towards the valence or conduction band.

0

0.5

1

1.5

2

2.5

3

0.2 0.4 0.6 0.8 1 1.2

(m)

T

t=0.2umt=0.37umt=0.65umt=0.85um

Fig.(5) The transmission spectra for PbS films at different thicknesses (0.2, 0.37, 0.65, 0.85 m).

The spectrum of absorptions were also estimated as shown in Fig.(6). It is opposite to that of transmittance spectrum.

0

0.2

0.4

0.6

0.8

1

1.2

0.2 0.4 0.6 0.8 1 1.2

(m)

A

t=0.2umt=0.37umt=0.65umt=0.85um

Fig. (6) The absorptions spectra for PbS films at different thicknesses (0.2, 0.37, 0.65, 0.85 m). .

Fig.(7) shows the optical absorption coefficient () which was calculated in the wavelength

range (300-1100) nm. It shows that the value of increases with increasing film thicknesse .This is due to the increase the density of localized state by increasing thicknesses which leads to increase the absorption coefficient, and the estimated values of seemed to be greater than the values of which obtained by Joshe et al. [4] and Oriaku et al. [11] .

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0

1

2

3

4

5

6

7

0.2 0.4 0.6 0.8 1 1.2

(m)

(cm

-1)x

10-4

t=0.2umt=0.37umt=0.65umt=0.85um

Fig (7) The absorption coefficient spectra for PbS films at different thicknesses

The optical energy gap(Eg) of PbS thin films are calculated using the following equation

h =B(h-Eg)r (5)

where the h is the photon energy, B and r are constants.

Fig.(8) presents the (h)2 as a function of h in accordance with eq.(5)

0

50

100

150

200

250

300

1 2 3 4 5

h(eV)

( h

)2 x10

8 (cm

-1.e

V)2

6

t=0.2umt-0.37umt=0.65umt=0.85um

Fig.(8) the variation of (h)2 as a function of h for PbS films.

The extrapolation of the linear region gave values of Eg as listed in Table(1)

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Table (1) Values of the optical constants of PbS films.

th m x104 (cm-1)

Eg(eV) Eu(eV) n k 1 2

0.20 1.94 2.2 0.94 2.05 0.093 4.2 0.38

0.37 2.33 2.15 1.07 2.13 0.111 4.5 0.47

0.65 2.61 2.13 0.905 2.14 0.124 4.57 0.53

0.85 3.18 2.1 1.108 2.03 0.152 4.119 0.618

The value of Eg values decreases with increasing thicknesses and this is attributed to the

increases of density of localized state in the energy gaps These values of Eg are agreed with those ones published [9-12]. Elsewhere of tails width (Eu) in the energy gap were obtained by plotting Ln as a function of h, and the value of Eu extracted from the reciprocal slope of the linear part. As shown in Fig.(9) .

Fig.(9) The variation of Ln as a function of h for PbS films.

Increasing of Eu values with increasing thicknesses is attributed to the increases of the defects and localized state in the energy gap. The value of extinction coefficient (k) is shown in Fig.(10).The behavior of k is similar to the corresponding and the value of k increases with increasing thicknesses and this is also due to the same reason mentioned previously.

Fig.(10) The extinction coefficient k as a function of wavelength for PbS films.

0

0.05

0.1

0.15

0.2

0.25

0.2 0.4 0.6 0.8 1 1.2

(m)

K

t=0.2umt=0.37umt=0.65umt=0.85um

k

9.5

10

10.5

11 th=0.2um th=0.37um th=0.65um

2 2.2 2.4 (eV)h

th=0.85um

2.6 2.8 3

Ln

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The value of refractive index (n) is shown in Fig.(11) and results show good agreement with that of Oriaku etsl[11].

0

0.5

1

1.5

2

2.5

0.38 0.58 0.78 0.98 1.18

(m)

n

th=0.2um

th=0.37um

th=0.65umth=0.85um

Fig.(11) The refractive index n as a function of wavelength for PbS films. The real imaginary part of the dielectric constant (1 and 2) are shown in Fig.(12) and(13)

respectively , the behavior of 1 is similar to refractive index because of the smaller value of k2 comparison of n, while 2 is mainly depends on the k values, which are related to the variation of absorption coefficient. These measurements are agreed with the data of Oriaku etsl[11}.

00.5

11.5

22.5

33.5

44.5

5

0.2 0.4 0.6 0.8 1 1

(m)

1

.2

0.2um0.37um0.65um0.85um

Fig.(12) The real part of dielectric constant 1 at different thicknesses .

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.2 0.4 0.6 0.8 1 1.2

(m)

2

0.2um

0.37um

0.65um0.85um

Fig.(13) The imaginary part of dielectric constant 2 at different thicknesses.

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Conclusion Nanostructure PbS thin films at low temperatures were prepared by a simple bath

deposition method. The results of AFM & XRD tests showed that the deposited PbS thin films consist of nanosize grains and the grain size increases with increasing film thickness. XRD shows nanocrystalline structure with cubic phase (galena). The optical properties of these films have been studied and show that PbS thin films have allowed direct transition and the values of energy gap varied between (2.2-2.1) eV with increasing film thickness. Other the optical parameters have also been calculated and presented. References [1] S.B.Qadri, J.Yang, B.R.Ratna, F.E.Skelton,and J.H. Hu, Appl.Phys. Lett, Vol 69, 1969, p 2205. [2] A.U.Ubale, A.R. Junchara, N.H. Wadibhasme, A.S.Darya Purkab, R.B. Mankar and V.S.Sangawar, Thickness dependent structural, electrical and optical properties of chemically deposited nano particle PbS thin film, Turk. J. Phys, Vol 31,2007, p 279-286. [3] C.D.Lokhande, A.U. Ubale and P.S. Patil, Thin Solid Films, Vol 302, 1997, p 1. [4] K.R. K.Joshi, A. Abke. Kanjilal, H.K. Sehgal, Solution grown PbS nano particle PbS film, Applied Surface Science, Vol 221(No.1),2004, p 43-47. [5] E.Pantia, L.Pintilie, I.Matei, T. Botila, E.Ozbay, Chemically prepared nano crystalline PbS thin film, Journals of Optoelctronics and Advanced materials, Vol 3 (No.1),2001,p525-530. [6] S. Mahmoud and O. hamid, Growth and characterization of Lead Sulfide films deposited on glass substrate, Fizika A(Zagreb), Vol 10 ,2001, p 21-30. [7] J.J.Valen Zula. Jaurgue, R.Ramire Bna, A. Mednza-Kalvan and M. Soltelo Lerma, Optical properties of PbS thin film chemically deposited at different temperatures, Thin Solid Films, Vol 441,2003, p 104-110. [8] V. Stancu, E.Pentia, A. Goldenblum, M.Buda, G. Iardache and T. Botila, A comparative study of microcrystalline and nanocrystalline lead sulfide based PbS/SiO2/Si Heterostructure, Romanian Journal of information science and technology, Vol 10 (No.1)2005,53-66 . [9] A. Popa, M.Lisca, V. Stancu, M.Buda, E. Pentia and T. Botila, Journals of optoelectronics and advanced materials, Vol 8 (No.1),2006,p 43-45. [10] J.A. Amusa, G.R. Fajinmi and Y.K.Sanusi, Deposition time dependence on Absorpivity of chemically deposited lead sulfide thin films, Research Journals of Applied Science,Vol 2 (No.9),2007,p 931-943. [11] C.I. Oriaku and J.C. Osuwa, Analysis of thin chalcogenide PbS films prepared from chemical path, The Specific Journals of science and technology, Vol 9 (No.1),2008,p 461-467. [12] I. Leanapop, C. Nascu, V. Ionesch, E. Indrea and Bratu, Structural and optical properties of PbS thin films obtained by chemical deposition, Thin Solid Films, Vol 307,1997, p 240-244.

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