Study of transformation of defect states in GaN- and SiC-based materials and devices

The present thesis is a study of the evolution of defect states in devices based on wide bandgap semiconductors. The attention has been focused on light-emitting diodes based on GaN and Schottky diodes based on SiC, these latter a basic structure for the fabrication of high-power rectifiers and ionising particle detectors. In both cases, we studied the defects and their electronic properties by means of the following experimental techniques: current-voltage (I-V) measurements, in order to investigate the effect of imperfections on the transport properties of the material/device; capacitance-voltage (C-V) measurements, yielding the profile of concentration of charge carriers, and giving information on the influence of defects on this concentration; deep level transient spectroscopy (DLTS), a technique allowing for the identification and characterization of defect-originated electron levels in the gap. I also employed techniques, such as photocurrent spectroscopy (PC), allowing for the characterization of light absorption by the material and/or device versus varying photon energy. In both cases of SiC and GaN, the defect characterization was always interpreted in the framework of its influence on device operation. In the analysed LEDs the defect evolution was connected to the evolution of quantum efficiency, and in the SiC diodes we studied the effects of defect introduction on the charge collection efficiency (CCE) and on the leakage current of the device. Furthermore, for the interpretation of photocurrent spectra, I developed a model describing the generation of photocurrent considering the dispersion relations for the absorption coefficient and refractive index in the various device layers, as well as the internal reflection, transmission and interference phenomena involving the optical field within the device. The research yielded various interesting results: I detected many deep levels introduced by proton- and electron-irradiation in SiC. From the study of their annealing behaviour I concluded that one of these levels is related to a particular lattice defect, the carbon interstitial. By means of the analysis of the introduction rates of the levels and comparisons between proton and electron irradiation, I was able to distinguish between deep levels related to simple intrinsic defects and to defect complexes. In the case of the GaN LED, I found that the evolution of several independent properties are strongly correlated, meaning that a single degradation mechanism is responsible for the observed changes. In particular, I concluded that the degradation of the light emission intensity is due to the generation of defects in the active region of the device