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SURFACE MORPHOLOGY OF CDS THIN FILMS BY VACUUM EVAPORATION DEPOSITION

D KATHIRVEL a*, N SURIYANARAYANAN b, S PRABAHAR c, S SRIKANTH c

a* Department of Physics, Kalaignar Karunanidhi Institute of Technology, Coimbatore, India

b Department of Physics, Government College of Technology, Coimbatore, India. c Department of Physics, Tamilnadu College of Engineering, Coimbatore, India

ABSTRACT

Cadmium Sulphide thin films have been deposited on to well cleaned glass substrate in a vacuum of 10-6 Torr. The thickness of the films has been determined by quartz crystal monitor method. The surface morphology studies are performed using various techniques such as Scanning electron microscope (SEM) and Atomic force Microscope (AFM). SEM and AFM are the best tools to investigate the surface smoothness and to find the grain size of the particles. The grain size is calculated for all films of different thickness. Keywords: CdS, Structural Properties, X-ray diffraction, SEM and AFM.

1. INTRODUCTION

The wide energy gap of CdS semiconductor is one of the most important properties leading to the great experimental interest in these materials. CdS is a suitable window layer for solar cells [1] and also finds applications as optical filters and multilayer light emitting diodes [2-4], photo detectors, TFETs, gas sensors and transparent conducting semiconductors for optoelectronic devices [3-5]. Various methods are used to deposit CdS thin films [6-7]. Among the vacuum evaporation is an attractive, effective method and the application at enables the deposition of thin films of larger area with good uniformity [8-9]. The properties of the CdS thin films are strongly depends on the internal structure. The wide ranges of experimental methods are available for the evaluation of structure of materials with high accuracy and precision. The present study reveals the variation of surface morphology of CdS thin films.

2. EXPERIMENTAL DETAILS

The CdS powder of purity 99% was evaporated using Tungsten conical basket (200 amps) under the pressure of 2 x 10-5 Torr on to a pre cleaned glass substrate (3.25 x 2.75 x 0.1 cm dimension). The pressure was obtained by diffusion pump backed by rotary

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pump in the coating unit and was measured using Pirani and Penning gauge. A constant rate of evaporation of the order of 1 Å / sec was maintained throughout the film fabrication. A rotary device was employed to maintain uniformity in film thickness. The thickness of the film was controlled and measured by Quartz crystal monitor and the thickness monitor in a flat circular plate approximately 0.05 inch (1.4cm) in diameter and 0.011 inch (0.28 cm) thick. A substrate heater arrangement was employed to grow the thin film at different substrate temperature. The Copper – constant and thermocouple was employed to measure the temperature inside the champer. Scanning electron microscopy is a power full tool for investigating surface topology of material. It operates in a vacuum, with a high energy beam (typically from 5 to 20eV) focused into a spot of several tens of nanometers in diameter (or fraction of nanometers in modern high-resolution instruments). Another possible characteristic of SEM have high energy electrons, since samples may be loose powders, fractures surfaces, polished sections but they must be dry without excessive out gassing. The SEM analysis was carried out using "FEI Quanta 200". The surface morphology of the films is investigated by means of scanning electron microscope (SEM) and atomic force microscopy (AFM).

3. RESULT AND DISCUSSION

3.1. SEM ANALYSIS OF CDS THIN FILMS

Scanning electron microscopy is a convenient technique to study the microstructure of thin films. Fig. 1, Fig. 2 and Fig. 3 show the SEM images of CdS thin films of different thicknesses. The SEM micrographs of different thickness are analyzed at a resolution of 20µm with 5000x magnification. The SEM micrographs of 880 Å thickness, the distributions of grains are not uniform throughout all the regions. But the films are without any void, pinhole or cracks and that they cover the substrates well. We clearly observe the small nanosized grains engaged in a fibrous-like structure, which clearly indicates the glassy nature along with amorphous phase of CdS thin films. The grains are found to be thickly packed, and inter grain spacing is reduced in the case of film thickness 930 Å shown in Fig. 2.

Fig. 1. X-ray diffractogram of CdS thin film of thickness 880 Å

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Fig. 2. X-ray diffractogram of CdS thin film of thickness 930 Å

Fig. 3. X-ray diffractogram of CdS thin film of thickness 2550 Å

From the Fig. 3, the SEM of higher thickness of 2550 Å, such a difference might be due to the presence of some amorphous phase in the films along with their predominant crystalline phase. The surface morphological study also indicates that the decrease in the Cd content improves the surface smoothness.

3.2. AFM ANALYSIS OF CDS THIN FILMS Fig. 4 & 5 shows the two dimensional and three dimensional AFM micrograph of the CdS thin films having thickness of 880 Å, 930 Å and 2550 Å. The scanning is done over an area of 1µm x 1µm.The AFM images exposed the high uniformity of the films with round-shaped nano particles, and also shows the beta phase films which consists of grain size 40-180 nm.

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(a) 880 Å

(b) 930 Å

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(c) 2550 Å

Fig. 4. Two dimensional AFM micrographs of CdS thin film of thickness (a) 880 Å (b) 930 Å & (c) 2550 Å

(a) 880 Å

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(b) 930 Å

(c) 2550 Å

Fig. 5. Three dimensional AFM micrographs of CdS thin film of thickness

(a) 880 Å (b) 930 Å & (c) 2550 Å

The maximum size of the particle is calculated by Scheorer equation. The maximum size and the root mean square of the roughness (rms) of the surface of CdS films of thickness 880 Å, 930 Å and 2550 Å are tabulated in Table 1.

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Table 1. Variation of particle size with thickness.

Sl. No. Film thickness (Å) Particle size (nm) Roughness of the surface (nm)

1 2 3

880 930

2550

57.20 60.45

165.75

12.50 13.12 23.78

From the Table 1. reveals that, the particle size and roughness of the surface of the CdS thin films increases with increase in thickness.

4. CONCLUSIONS CdS thin films prepared from Vacuum Evaporation deposition are amorphous nature in lower thickness and polycrystalline nature and the SEM micrographs of lower thickness, the distributions of grains are not uniform throughout all the regions and also, which clearly indicates the glassy nature along with amorphous phase of CdS thin films. The SEM of higher thickness of 2550 Å, such a difference might be due to the presence of some amorphous phase in the films along with their predominant crystalline phase. The surface morphological studies show that the size of the particles increases with increase in thickness of the films by means of AFM. ACKNOWLEDGEMENTS

The author would like to acknowledge the assistance of each of the following: Dr. P. Rajasekaran, Prof. and Head of Physics Department, Kalaignar Karunanidhi Institute of Technology, Coimbatore, India. Dr. K V. Kannan Nithin , Assistant Professor, Kathir College of Engineering, Coimbatore, India. REFERENCES

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