Study of residual stress in reactively sputtered epitaxial Si-doped GaN films

Abstract

Si-doped GaN films were grown epitaxially on c-sapphire by rf magnetron reactive co-sputtering of GaAs and Si at various partial pressures of N2 in Ar–N2 growth atmosphere. Energy dispersive x-ray spectroscopy revealed ∼2 at.% Si in all the films, but the N/Ga ratio decreased substantially as N2 percentage was reduced from 100% to 10%. The lattice parameters (a and c) were obtained independently to determine the in-plane and out-of-plane components of strain in films, which were analyzed to deduce the hydrostatic and biaxial strain contributions. High resolution x-ray diffraction revealed the dominant presence of edge dislocations (∼1012 cm−2) in the films grown at 30%–100% N2, which decreases and becomes comparable to the density of screw dislocations (∼1011 cm−2) at lower N2 percentages. The films grown at 75%–100% N2 displayed compressive biaxial stress along with large hydrostatic strain and micro-strain due to the incorporation of nitrogen at interstitial locations and grain boundaries. Both hydrostatic strain and micro-strain decrease in the films grown below 75% N2, in which, a reversal of biaxial stress to tensile is seen due to the prevalence of in-plane tensile stress generated during coalescence. The decrease of tensile stress in films grown below 30% N2, accompanied by increase in micro-strain and hydrostatic strain are ascribed to Ar incorporation and voided morphology. The presence of Si does not have a significant influence on residual stress of films and appears to be masked by the dominant growth-related intrinsic effects.