Defect studies in n-type indium nitride

Abstract

Sparked by the discovery of its narrow bandgap, indium nitride (InN) has recently attracted major scientific interest as a promising material in opto- and high-speed electronics. Industrial application, however, is hampered by the material's strong propensity for n-type conductivity and high defect concentrations. Despite major research efforts in that area, relatively little is known about the properties of defects in InN. The main goal of this thesis was to study the formation and characteristics of native point defects in n-type InN and investigate their role for the material's electrical properties. Positron annihilation spectroscopy was used as the main experimental technique for defect characterization. By combining density functional theory calculations with experimental positron annihilation methods, the dominant vacancy-type positron traps in common InN material were identified. As-grown and n-doped InN layers that were deposited by different growth methods, as well as irradiated and annealed material with varying carrier concentrations, were analyzed to investigate the formation and evolution of point defects in different environments. The data were compared to results from complementary techniques in order to study the interplay of point and extended defects, and their role in limiting the conductivity in n-type InN. It is found that isolated indium (In) vacancies and their complexes are the dominant vacancy-type positron traps in InN. Isolated In vacancies are introduced in high-energy particle irradiation of InN, and anneal out at moderate temperatures if not stabilized by one or more nitrogen (N) vacancies. In-N vacancy complexes are the dominant vacancy-type positron traps in as-grown InN. In state-of-the-art material with low free electron concentrations, their concentration is at or below the detection limit of positron annihilation methods. For increasing free electron concentration, an enhanced formation of these vacancy complexes is observed, which act as significant source of compensation and scattering centers in highly n-type material. Additionally, a high density of negatively charged defects with only small effective open-volume is found that is tentatively attributed to negatively charged N vacancies. Estimated defect densities are significantly higher than what is expected from thermodynamic considerations, which suggests that alternative defect formation mechanisms determine the point defect densities in the material.