Study On The Influence Of Defect Distribution And Process Design On The Performance Of GaN-based Devices

GaN, as a third-generation semiconductor material, has shown great prospect in applications of short wavelength blue and ultraviolet (UV) light-emitting devices (LEDs), microwave devices and high-power semiconductor devices, due to its unique properties such as wide direct bandgap (3.39 eV), high breakdown electrical field (3MV/cm), high electron saturation velocity (3×10'cm/s), high thermal conductivity, high thermal stability, and so on. Many countries have put a lot of manpower, material and financial resources to study it.In this dissertation, we studied defect and impurity distributions in free-standing bulk GaN substrate, the effect of surface treatment on the performance of GaN- and AlGaN/GaN-based Schottky barrier diodes, and the breakdown characteristics of AlGaN/GaN-based Schottky barrier diodes fabricated on Si substrate. The main conclusions are listed as follows:1. Cathodoluminescence (CL) spectroscopy and mapping technique were used to study defect and impurity distributions in free-standing bulk GaN substrates prepared by hydride vapor phase epitaxy. It was found that, in the bulk GaN substrates investigated, dislocation clusters appearing as dark cores in the CL map were surrounded by bright disk-like regions with higher luminescence efficiency than that of the outside areas. This large-area luminescence non-uniformity disappeared in homoepitaxial GaN grown on top of the GaN substrate. Schottky barrier diodes fabricated on the homo-epilayer exhibited low average reverse leakage current, while dislocation clusters duplicated from the original bulk GaN substrate still limited device yield.2. The effect of oxygen plasma treatment on the performance of GaN Schottky barrier diodes is studied. The GaN surface is intentionally exposed to oxygen plasma generated in an inductively coupled plasma etching system before Schottky metal deposition. The reverse leakage current of the treated diodes is suppressed in low bias range with enhanced diode ideality factor and series resistance. However, in high bias range the treated diodes exhibit higher reverse leakage current and corresponding lower breakdown voltage. The X-ray photoelectron spectroscopy analysis reveals the growth of a thin GaOx layer on GaN surface during oxygen plasma treatment. Under sub-bandgap light illumination, the plasma-treated diodes show larger photovoltaic response compared with that of untreated GaN diodes, suggesting that additional defect states at GaN surface are induced by the oxygen plasma treatment.In addition, the effect of O2 plasma treatment, ultra-thin dielectric inter-layer, CF4+O2 plasma treatment, and thermal annealing on the performance of AlGaN/GaN Schottky barrier diodes was also studied.3. AlGaN/GaN-based planar Schottky barrier diodes with various spacings between ohmic and Schottky contacts were fabricated on silicon substrate. The effect of SiO2 and SiNx/SiO2 surface passivation on the breakdown characteristics of the Schottky barrier diodes was studied. SiO2 and SiNx/SiO2 surface passivation are found both effective to suppress the reverse leakage current and increase the breakdown voltage of the diodes. The diodes with SiO2 surface passivation exhibits a maximum breakdown voltage (VBR) of-600 V at a contact spacing of 20μm, producing a high VBR2/RON value of>112 MW·cm-2. The breakdown voltage of the diodes fabricated on Si substrates tends to saturates as the contact spacing exceeds a critical value, which.is believed caused by electrical breakdown occurring at the Si/GaN interface.