6.1 CONCLUSIONS
The purpose of this chapter is to summarize the results obtained in this work. In the light of the experimental investigation and analysis on the surface morphological, structural, optical and electrical studies of pure and Cd doped CoS thin films of different compositions and thicknesses deposited by cost effective and convenient spray pyrolysis technique, the following conclusions may be drawn:
In this work, it is observed that the deposition of Co1-xCdxS films depends on various parameters such as substrate temperature, concentration of the solution, distance between spray nozzle and the glass substrates and deposition time etc. Film deposited at 523 K substrate temperature is found to be good one in terms of their uniformity of thickness and color.
The SEM micrographs of the Co1-xCdxS films of different Cd concentrations deposited on glass at substrate temperature 523 K and Co0.8Cd0.2S films annealed at 673, 773 and 873 K for 1 hour were taken. The SEM micrographs of as-deposited films show that the film surface becomes increasingly coarse as Cd increases. In as- deposited states the film was very dense with no observable voids or pinholes. The surface could be described as a conglomerate random roughness, which was a characteristic of an amorphous nature. The overall morphology of the layers seemed to be due to the growth and clustering of initial nuclei. With the increase of annealing temperature the surface homogeneity, quality, crystallinity of the annealed films increased. With the increase of annealing temperature the defects and surface roughness of the films also reduced.
Chapter-VI Conclusions and Suggestions for Future Work The elemental compositions of Co1-xCdxS thin films have been analyzed by EDX which revealed the presence of Co and S in the system. It is also clear from the EDX result that the Cd ions are acting as dopants in the Co1-xCdxS structure. It is worth to mention that the as-deposited Co1-xCdxS film is stoichiometric.
XRD analysis reveals that the as-deposited Co1-xCdxS thin film is amorphous in nature and the annealed Co0.8Cd0.2S films show the formation of crystalline in nature.
Average grain size of the films increases from 15 to 61 nm with increasing annealing temperature. Structure of the material is identified as cubic.
The absorbance spectra of the as-deposited Co1-xCdxS (0≤ x ≤1) thin films show that the absorbance is low in the wavelength range from 500 -1100 nm and high in the ultraviolet region. Transmittance of the deposited films increases in the near infrared region. Transmittance of the prepared thin films decreases with increasing incorporation of Cd in the solution. Direct band gap ( Eg ) of the films varies between 2.20 to 3.10 eV with the increase of Cd in the Co1-xCdxS system, which indicates that presence of Cd in the films greatly affects the optical band gap. The measured values of the Eg are found in good agreement with the value reported by others. The ‘k’ and
‘σopt’ of the films increases in the higher energy level with the increasing Cd in the system. The ‘n’ decreases with the increase of Cd in the system. The value of the ε r
of the dielectric constant is higher than that of the ε i. The dielectric loss increases in the high photon energy level.
Transmittance of the annealed Co0.8Cd0.2S thin films decreases as the annealing temperature increases. The direct Eg of the annealed Co0.8Cd0.2S films varies between 3.25 to 3.45 eV with increasing of annealing temperature, which indicates that annealing temperature greatly affects the Eg of the films. This is in agreement with the observed increase in the crystallite size as the annealing temperature increases, which further consolidates the suggestion that annealing enhances the crystallinity of the films. The ‘k’ and the ‘σopt’ increase with the increase of annealing temperature. The ‘n’ and dielectric loss decreases with the increase of annealing temperature.
Chapter-VI Conclusions and Suggestions for Future Work The electrical resistivity decreases and the conductivity of the as-deposited
Co1-xCdxS films increase with the increase of temperature range from 303 to 408 K.
This indicates that the behavior of Co1-xCdxS film is semiconductor. The electrical conductivity increases with increasing Cd concentration. Activation energy of as- deposited Co1-xCdxS films increases with increasing Cd in the solution. Activation energy of the films varies in the range of 0.25 to 0.50 eV.
6.2 SUGGESTIONS FOR FUTURE WORK
This is the first time that Co1-xCdxS thin films have been prepared in our laboratory by spray pyrolysis technique. In recent years Co1-xCdxS thin film has a variety of optoelectronic device applications, such as photo luminescent, photoconductor devices and photodiodes, photovoltaic cells etc. Hence to get better performance of this material the following research work may be extended as follows:
1. The Co1-xCdxS thin films may be characterized with respect to different substrate temperature.
2. The Hall Effect and temperature dependence Hall Effect may be studied of the Co1-xCdxS thin films.
3. Study of thermoelectric power of Co1-xCdxS thin films.
4. Study of photoluminescence (PL) properties of Co1-xCdxS the films.
5. Study of luminous and solar transmittance as well as reflectance of the as- deposited Co1-xCdxS films for selective surface application.
6. AC measurement of Co1-xCdxS thin films should be studied.
7. Mechanical properties of Co1-xCdxS thin films may also be also be investigated.
8. Study of magnetic properties of Co1-xCdxS thin films.