INTRODUCTION:

Synthesis and characterization of IV-VI semiconductors are of great importance because of these materials can be used in a variety of applications such as solar cells, photoconductors, sensors, optical mass memories and solar selective coatings. Various techniques so far adopted for the preparation of SnSe thin films include electrodeposition¹, atmosphere pressure chemical vapour deposition², chemical bath deposition method^3,4, thermal evaporation⁵, spray pyrolysis technique⁶ and reactive evaporation⁷. Chemical bath deposition technique is an attractive technique for preparation of thin films on surface of substrates, the main advantage being the simple, low cost and convenient for large area deposition of films. This technique has been actively studied for growth of binary^8-13 and ternary^14-16thin films.

In the present paper, we have employed chemical bath deposition method for the growth of SnSe onto substrates. We have investigated the surface topography of SnSe films deposited onto microscope glass slides at various bath temperatures using atomic force microscopy.

MATERIAL AND METHODS:

0.94g of tin chloride (SnCl₂) in 10 mL water was used to react with the medium containing 0.16M of sodium selenosulphate (10 mL). The sodium hydroxide is added dropwise to obtain an alkaline media of pH 11. All these chemicals used for the deposition were analytical grade. All the solutions were prepared using deionized water using a Millipore alpha Q deionised (18.2 MWcm). The substrates (microscope glass slide) were mounted vertically in the bath. Prior to deposition, the substrate was degreased in ethanol for 10 min, followed by ultrasonically cleaned with distilled water for another 15 min and finally dried in desiccators. The deposition was carried out at different bath temperatures (65, 70, 75 and 80 °C) in order to investigate the influence of temperature on surface topography of thin films. After the completion of deposition (45 min), the glass slide was taken out of the beaker, washed with distilled water and dried in desiccators for further characterization.

Surface topography was investigated using an atomic force microscopy (Quesant Instrument Corporation, Q-Scope 250) technique. The AFM was operated in contact mode, with a commercial Si₃N₄ cantilever. The elemental composition of the films was studied by scanning electron microscope (JEOL, JSM-6400) attached with energy dispersive analysis of X-ray (EDAX) analyzer. Optical absorption study was carried out using the Perkin Elmer UV/Vis Lambda 20 Spectrophotometer. The film-coated microscope glass slide was placed across the sample radiation pathway while the uncoated microscope glass slide was put across the reference path. The data obtained from the films was used to investigate the band gap energy and transition type of the thin films.

RESULTS AND DISCUSSION:

Figure 1 shows the three-dimensional atomic force microscopy images for an area of 10 x 10 μm for SnSe films deposited at different bath temperatures. Atomic force microscopy is a very useful method to investigate the surface topography of thin films. The structure, grain size, thickness and surface roughness of films could be studied using this technique. Our observations have shown that preparation of thin films at lower bath temperature (65 °C) leads to a smooth, homogeneous and small grained surface. Meanwhile, increasing the bath temperature causes in the appearance of randomly distributed large grains that form an irregular grain shapes.

(a)

(b)

(c)

(d)

Figure 1: Atomic force microscopy images of SnSe thin films deposited at different bath temperatures (a) 65 °C (b) 70°C (c) 75°C (d) 80 °C

Figure 2 shows the thickness versus bath temperature variation for the SnSe thin films deposited at various bath temperatures. From the data obtained, it is clear that the thickness of the films increased from 652, 821, 952 to 1052 nm as the bath temperature was increased from 65, 70, 75 to 80 °C, respectively. Thicker films could be observed for the films deposited at 80 °C compared with other bath temperatures. This phenomenon proved that films grown onto substrates are dependent on bath temperature.

Figure 2: The thickness of SnSe thin films with variation bath temperature.

Figure 3: The surface roughness of SnSe thin films with variation bath temperature.

The surface roughnesses of samples are shown in Figure 3. The roughness measurements of the samples follow a similar trend. It can be observed that the roughness increases (from 64, 107, 137 to 141 nm) with increasing bath temperature in this experiment. The lower roughness in samples, the smaller grains grown onto substrate we had. It is evident that the grain size increases from 0.4-0.6μm (65 °C), 1-1.5μm (70 °C), 1.6-1.8μm (75 °C) and 2-2.5mm (80 °C) with the increase of bath temperature.

Figure 4: Absorption spectrum of SnSe thin films deposited at 65 °C

The optical properties of the films were studied from absorption measurements in the wavelength range 300-900 nm. Figure 4 shows the absorption spectrum of SnSe films deposited at 65°C. An increase in the absorbance can be seen in the visible region representing that the films absorbs energy in visible region.

In order to decide the band gap energy, the equation of Stern was used.

(1)

Where, v is the frequency, h is the Planck’s constant and k equals a constant. The n value for direct band gap and indirect band gap is 1 and 4, respectively. Using this relation, a graph is plotted between the square of Ahv and hv to get a straight line (Figure 5). The extrapolation of straight line to (Ahv)²=0 axis gives the value of the band gap. The band gap energy of the SnSe films deposited at 65 °C is 1.5 eV. It is in close approximation to the value reported by Kumar et al⁵, Mariappan et al⁶ and John et al⁷ for the SnSe films deposited using thermal evaporation, spray pyrolysis and reactive evaporation method, respectively.

Figure 5: Plot of (Ahv)² against the photon energy of SnSe thin films deposited at 65 °C.

Figure 6: EDAX spectrum of SnSe thin films deposited at 65 °C.

Chemical composition of SnSe thin films deposited at 65 °C was analyzed by energy dispersive X-ray analyzer (EDAX). EDAX analysis showed the presence of tin and selenium in this sample. The atomic percentage of Sn:Se was 49.31:50.69 as shown in Figure 6.

CONCLUSIONS:

SnSe thin films have been deposited by chemical bath deposition method from an aqueous solution. We have observed a very strong dependence of the surface roughness, film thickness and grain size on the growth temperature. Larger grain size, thicker and rougher films could be observed for the films deposited at higher bath temperature.

ACKNOWLEDGEMENTS:

The authors would like to thank the Department of Chemistry, Universiti Putra Malaysia for the provision of laboratory facilities and Ministry of Science, Technology & Innovations for the National Science Fellowship.

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Received on 11.02.2012 Modified on 15.02.2012

Asian J. Research Chem. 5(2): February 2012; Page 291-294