Vacancy-type defects in GaN self-assembled nanowires probed using monoenergetic positron beam

Abstract

Vacancy-type defects in GaN nanowires (NWs) and the trapping of electrons by the vacancies were studied by positron annihilation. Undoped, Si-, and Mg-doped GaN NWs were grown on Si substrates by plasma-assisted molecular beam epitaxy. The major species of vacancies in the undoped and Si-doped samples was identified as a complex between a Ga vacancy and impurities such as oxygen and hydrogen. For the Mg-doped samples, the trapping rate of positrons for such defects decreased with the increase in Mg concentration because of the downward shift of Fermi level position and a resultant shift of the vacancy charge states from neutral (negative) to positive. Under the illumination of a 325-nm He-Cd laser, positrons were found to be trapped by vacancy-type defects, which was attributed to the trapping of excited electrons by these defects.