Inorganic Ternary Thin films: Anaysis of Optical Properties

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Electrical properties of CdS thin films:
Electrical properties of CdS thin films depend on the deposition condition. Evaporated films prepared for solar cell applications usually have resistivity in the range of 1 to 1000 -cm. They are usually n-type in conductivity dominated by the deviation from stoichiometry which influences the films to have S vacancies or Cd excess. Resistivity decreases as the thin film thickness increase. In thermal evaporation the [Cd]:[S] ratio during evaporation influences the electrical properties as does when doping. For CdS films doped with indium (CdS:In), the best electrical properties are obtained at a [Cd]:[S] ratio of 1:5 at which the films also exhibit resistivity values as low as 10-3 cm on CdS:In [1.5%)] samples.
Resistivity of doped evaporated films is insensitive to substrate temperature during deposition which contrasts the strong dependence of the resistivity of un-doped films on deposition temperature. CdS thin films evaporated for photovoltaic cell applications have carrier concentrations in the range 1016 to 1018 cm-3 and minority carrier diffusion lengths in the range 0.1 to 0.3 m and films grown at higher rates exhibit higher carrier concentration that increase with increase in film thickness. Those doped with indium increases their carrier concentration by almost three orders of magnitude up to a concentration of about 2% by weight and thereafter the carrier concentration does not increase. It is noted that at low indium concentrations, the carrier concentration and mobility decreases.
Chlorine has been used to dop CdS thin films. It has been reported that the CdS films doped with chlorine gas present very low photoconductivity at room temperature and their Hall mobility decreases with heat treatment. Those prepared by evaporating CdCl2 mixed CdS powders increases in resistivity and carrier concentration. A rapid incremental behaviour of mobility for the pure and low-doped films below 0.05% CdCl2

Cliff Orori Mosiori is a lecturer of Physics at Kenyatta University. He works as a researcher and student demonstrator/advisor for postgraduate student at Kenyatta University. He is also a travelling lecturer at the Rift Valley Institute of Science and Technology (RVIST), Kenya, in the Department of Electrical and Electronic Enginering. He is furthermore author of Physics course books, among them Digital Electronics, Optical coating, Physics of Thermal Phenomena.