Deep Level Transient Spectroscopy on Semiconductor Materials

Abstract

Deep Levels in Crystalline Semiconductors, caused by impurities and defects, are an important field of research both from pure physics as well as applications point of view. This project. concerned itself with the study of deep levels .)y establishing ti•4 standard technique of Deep Level Transient Spectroscopy (DLTS) for the first time in Pakistan. The major part of the project consisted of the study of the deep level content of green and red light emitting diodes (LEDs) prepared from Gallium Phosphide (GaP). The deep levels present in this' material were mostly inadvertant. Another part of the project concerned itself with the study of deep levels rt-ulting from deliberate doping of Silicon with. known Thin DLTS set-up has been successfully installed and expertise for the technique fully developed so that now deep level spectroscopy can be performed on any Semiconductor junction for a complete characterization of• its deep level content. The study of green LEDs has shown the presence of five electron-emitting and three hole-emitting levels. Emission rate data for these levels have been obtained. Three'of these levels have been the focus of attention. It was found that thermal emission rates of the already known 0.35 eV hole level were weakly field dependent. This field dependance was investigatged. in detail. By comparing with the previously published data on it and by fitting our results with theoretical models, important conclusion as to the origin and microscopic structure' of this level were drawn. In the case of another well known level, the nitrogen related 0.45 eV level, it was discovered that its electron capture cross-sections were strongly temperature dependent. This tempera-tIre dependance was investigated in detail. The results seem to pose a question to the established model of temperature dependance of capture cross-sections. The dominant deep levels in the green LEDs are - two mid-gap levels that have not been reported in literature previously. Detailed charactrization of these levels was accomplished. Investigating the red emitting LEDs it was discovered that he important oxygen level in GaP has strongly - dependent thermal emission rates. This field effect was studied in detail and the results are expected to modify the well-estbalished models for this level.