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

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 V_Ga-O_N complexes, as the compensating defects. [ABSTRACT FROM AUTHOR]