Preparation, characterization and study of amorphous gaas thin films and related compounds.
2015-11-19T09:17:12Z (GMT) by
The purpose of this thesis is to investigate the electrical and optical properties of a-GaAs thin films and also the related ternary compounds a-GaAlAs and a-GaAsP. The films were prepared by the technique of r.f sputtering in an argon atmosphere both with and without hydrogen. Experiments carried out include optical absorption, infrared absorption and electrical conductivity. The composition of the films was determined by EDAX. Information on structure and local bonding configurations for the a-GaAsP films was obtained from EXAFS measurements. Initially investigations were focussed on a-GaAs films prepared at room temperature, and in particular the effect of hydrogenation on film properties. Unhydrogenated a-GaAs has an optical gap of about 1.0 eV, although hydrogenation was found to increase this to about 1.3 eV. Infrared absorption data show that the main hydrogen mode in a-GaAs : H is the Ga-H-Ga bridging mode. It was also found that a-GaAs and a-GaAs : H films could be prepared at elevated substrate temperatures; similar measurements were also carried out on these specimens. Non-stoichiometric (As-rich) GaAs films were also prepared and characterized. Films of a-GaAlAs were successfully prepared by co-sputtering. For this system an interesting result is that the optical gap was found to decrease initially with increasing Al content before increasing at larger Al contents, in contrast to the crystalline case where the band gap increases monotonically with increasing Al content. Above 30 at.% A1, the films were found to be microcrystalline. A set of a-GaAsP films was also prepared. EXAFS measurements on these specimens reveal that Ga-P bonds in the amorphous network rapidly maximize at the expense of Ga-As bonds as the P-content increase. In other words the Ga bonding environment appears to be partially ordered. P-P and P-As "wrong" bonds are detected and, at high P-content, possibly also As-As bonds. The EXAFS results were found to correlate.