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.
