Electrical characterization and identification of deep levels in β-Ga2O3

Abstract

This thesis brings together fundamental studies presented in five papers with the focus on the investigation of defects in the wide bandgap semiconductor β-Ga2O3. A broad range of methods was applied; in particular the characterization of electrically active defects was done using capacitance spectroscopic techniques along with chemical characterization, x-ray absorption, and theoretical modelling. In Paper I we compare different metals for use as Schottky contacts on (010) and (2;¯01) oriented samples, and measure a new E4 deep level in bulk material for the first time. In Paper II we present strong arguments for attributing the dominating E2 level to Fe impurities. This assignment was reasoned by systematic correlations across a set of several samples, theoretical modelling of Fe on Ga sites, and irradiation experiments excluding the intrinsic origin of E2. Concurrently, we discovered a new level labeled E2* in close proximity to E2 and attributed its origin to intrinsic defects in β-Ga2O3. The irradiation studies were continued in Paper III, providing a systematic picture of the irradiation induced charge carrier removal and deep level generation in β-Ga2O3. In particular, we describe the kinetics of charge carrier recovery during annealing, based on a combination of experimental and theoretical work. We suggest that the origin of the carrier removal is in pinning of the Fermi level from the VGa acceptors as well as Gai and GaO donors. In its turn the carrier recovery is mediated by complex formation and passivation via H- or VO-related defects. A discussion on the generation of the deep levels E2* and E4* is also given, with the focus on their concentrations being influenced by high temperature treatments. Similar temperature effects were also observed in epitaxial material, where generation of three new deep levels, E3*, E5, and E6, occurred under reverse bias conditions and heating up to 625 K, as described in Paper IV. In Paper V we made an attempt to record the electrical and structural signatures of the defects in β-Ga2O3 simultaneously detecting capacitance and fluorescence signals upon the x-ray absorption. Altogether, this thesis may be seen as a step forward better understanding of defects in β-Ga2O3, which is currently a hot topic in semiconductor physics.