Gallium nitride films of high n-type conductivity grown by reactive sputtering

Abstract

The origin of n-type conductivity in as grown, undoped GaN has been debated over several decades, with contradictory views. This work deals with reactively sputtered, undoped GaN films in which a large decrease of resistivity from ∼105 to ∼2 × 10−3 Ω cm is seen, as the nitrogen percentage in argon–nitrogen sputtering atmosphere is decreased from 100% to 10%, resulting in high electron concentration ∼1020 cm−3 at the lower end of nitrogen pressure. The electrical behavior of these films has been correlated with their composition and chemical state to understand the nature of point defects which influence n-type conductivity. Powder x-ray diffraction and phi scans of the undoped GaN films grown by sputtering of GaAs at 700 °C on c-sapphire reveal epitaxial growth of single phase GaN. SIMS results show the absence of arsenic in the films and uniformly distributed gallium and nitrogen along with oxygen impurity, across their thickness. XPS data reveal the decrease of N/Ga ratio, as the nitrogen partial pressure is decreased along with the increase of uncoordinated Ga, indicating the formation of nitrogen vacancies. In the high resistivity films grown at higher nitrogen percentage in sputtering atmosphere, oxygen impurities appear to be primarily responsible for creation of n-type carriers, which are compensated by Ga vacancies and possibly VGa–ON complexes. The drastic enhancement of n-type conductivity and carrier concentration in the low resistivity films grown at lower nitrogen percentage, which reveal significantly smaller nitrogen content, is attributed dominantly to the increase in nitrogen vacancies and uncoordinated Ga.