X-ray absorption study of defects in reactively sputtered GaN films displaying large variation of conductivity

Abstract

Undoped GaN films were epitaxially grown on c-sapphire by rf magnetron reactive sputtering of GaAs at different N2 percentages in Ar-N2 sputtering atmosphere. These films display a large increase in conductivity from ∼10-6 Ω-1cm-1 to ∼102 Ω-1cm-1 with decrease of N2 in sputtering atmosphere from 100% to 10%. X-ray photoelectron spectroscopy reveals a monotonous decrease of N/Ga ratio, concurrently with increase of uncoordinated/interstitial Ga in the films, with decrease of N2 percentage in sputtering atmosphere. Extended x-ray absorption fine structure measurements and x-ray absorption near edge structure (XANES) simulation at Ga K-edge demonstrate the absence of N vacancies in sputtered GaN films and point towards the presence of Ga vacancies, which increase with decrease of N2 percentage in sputtering atmosphere. XANES measurements at O K-edge reveal that O interstitials are present in most of the films, except for the film grown at low N2 percentages (∼10%), in which case, O substitution on N-site may also take place. A monotonous increase of n-type conductivity is observed with increase of Ga interstitials, which suggests that Ga interstitials are the main source of n-type conductivity in sputtered GaN films. The low conductivity of the films grown at high N2 percentages (⪆75%) is attributed to the limited presence of Ga interstitials and a strong compensation, dominated by N and O interstitials. The high conductivity GaN films grown at low N2 percentages may be partly contributed by O substituted on N site, along with Ga vacancies and possibly VGa-ON complexes, as the compensating defects.