Cadmium sulfide with a direct bandgap of about 2.43eV is a suitable window material for CdS/CdTe and $CdS/CulnSe_2$ solar cells, which show higher than 10% solar efficiencies. In this thesis, CdS thin films have been prepared by electrodeposition method and by vacuum evaporation method, and their microstructural, electrical and optical properties have been studied. Their structures and mirostructures have been investigated by X-ray diffraction and scanning electron microscope, respectively. The electrical and optical properties have been examined by electrical resistivity measurement, the Hall mdeaurement and optical transmittance spectra. This thesis consists of three parts. The first part describes the electrodeposited CdS films and the second and third parts describe undoped and In-doped CdS films, respectively.
CdS thin films have been electrodeposited on ITO/glass substrates in the aqueous solution containing $CdSO_4$ and $Na_2S_2O_3$ $5H_2O$. CdS films electrodeposited at electrode potentials between -600mV and -665mV were homogeneous and uniform. Crystalline CdS films were obtained at deposition temperature above 80℃, while amorphous films were obtained below 70℃. The grain size of the crystalline CdS films increased as the solution temperature increased.
Undoped CdS films have been evaporated in vacuum on glass substrates under various deposition conditions. The resistivity of the evaporated CdS films strongly decreased as substrate temperature decreased and the films with high deposition rate showed lower resistivity compared to the films with low deposition rate. The combination of high deposition rate and very low substrate temperature resulted in the increase of resistivity of the CdS films. The optical transmittance of CdS films increased as substrate temperature decreased and then decreased with further decrease in substrate temperature. The transmittance strongly depended on deposition rate at low substrate temperature (<100℃), while that was independent of deposition rate at high substrate temperature (>100℃). In particular, high transmittance can be extended to lower substrate temperature by reducing deposition rate. Low optical reflectance can be obtained by lowering substrate temperature. The results indicate that CdS films of low resistivity and high transmittance can be produced by vacuum evaporation at low substrate temperature and low deposition rate.
In-doped CdS films have been evaporated using CdS and indium metal on glass substrates under various deposition conditions. As indium concentration increased, the grain size of the CdS films decreased and the orientation of the films varied from the texture of (002) plane to that of (110) plane. The electrical conductivity, carrier concentration and Hall mobility increased with increasing indium concentration and then decreased with further increase in indium concentration. In-doped CdS films became degenerate semiconductors when the carrier concentration exceeded about $2\times10^{18}cm^{-3}$ and the optical band gap increased with increasing carrier concentration owing to the move-up of Fermi level above conduction band. the optimum indium concentration turned out to be $3\times10^{20}cm^{-3}$, which showed the lowest resistivity of $5\times10^{-3\Omega}$-cm and the largest optical band gap of 2. 62eV.
