Study of Wide-Band Semiconductors Using Magnetic Resonance Spectroscopies.

Native and extrinsic defects in wide-band-gap semiconductors play a very important role in doping, compensation, phase stability and performance of relevant optoelectronic devices. There is a need of knowing the nature, and the atomic and electronic structures of such defects. In this dissertation these issues have been addressed using Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) techniques. The study of the micro-structure of point defects, their interactions with the lattice, their mutual interactions and their role in the growth of diamond, boron nitride and gallium nitride is reported. Two major defects in diamond films, prepared by different Chemical Vapor Deposition (CVD) processes, have been identified and investigated by EPR spectroscopy. The first is a native defect whose nature was attributed to clusters of vacancies. In this case the line shape was found to be the superposition of two components: a narrower Lorentzian and a broader Gaussian, suggesting exchange narrowing and a non-uniform distribution of the paramagnetic defects which were found to affect also the temperature dependence of the spin-lattice relaxation rate. The second is an extrinsic impurity identified as substitutional nitrogen. The results obtained for this defect were found to be similar to those reported in natural diamond. These results indicate a trigonal distortion of the defect whose dynamical properties were investigated by analyzing the EPR spectrum at high temperatures. The different allotropic forms of boron nitride were investigated by EPR and NMR. The dominant paramagnetic center is the nitrogen vacancy whose structure and relaxation properties have been studied by EPR. BN films, doped with C, were prepared by reactive sputtering, in order to investigate the role of carbon in the stabilization of the electron in the nitrogen vacancy. ^{11} B NMR studies on pyrolitic hexagonal boron nitride and boron nitride thin films have been carried out. In both materials the central transition broadened by a second order quadrupole perturbation was observed. The results were interpreted on the basis of charge transfer from B to N. The study of the spin-lattice relaxation rates shows that the dominant nuclear spin-lattice relaxation mechanism is related to the presence of paramagnetic centers, i.e. nitrogen vacancies. EPR studies on auto-doped n-type zinc-blende GaN films revealed, for the first time, a resonance attributed to delocalized electrons in a band of shallow donors due to nitrogen vacancies.