Chapter 5 - Characterization of native point defects in GaN by positron annihilation spectroscopy

Abstract

This chapter discusses the vacancy-type defects, utilizing the positron annihilation spectroscopy, which is used to identify vacancy defects in GaN epitaxial layers. It yields quantitative information on vacancy concentrations in the range of 1015–1020 cm–3. Positron experiments detect Ga vacancies as native defects in GaN bulk crystals. The concentration of VGa decreases with increasing Mg doping, as expected from the behavior of their formation energy as a function of the Fermi level. The trapping of positrons at the hydrogenic state around negative ions gives evidence that most of the Mg atoms are negatively charged. Ga vacancies are observed as native defects in various n-type GaN overlayers grown by molecular organic chemical vapor deposition (MOCVD) on sapphire. Their concentration is >1017 cm–3 in nominally undoped material, which show n-type conductivity due to residual oxygen. When similar doping is done with Si impurities and less oxygen is present, the concentration of Ga vacancies is lower by at least an order of magnitude. No Ga vacancies are observed in p-type or semi-insulating layers doped with Mg.