Characterization of in-depth cavity distribution after thermal annealing of helium-implanted silicon and gallium nitride

Single-crystalline silicon wafers covered with sacrificial oxide layer and epitaxially grown gallium nitride layers were implanted with high-fluence helium ions (2–6×1016cm−2) at energies of 20–30keV. Thermal annealings at 650–1000°C, 1h were performed on the Si samples and rapid thermal annealings at 600–1000°C, 120s under N2 were performed on the GaN samples. The as-implanted samples and the near-surface cavity distributions of the annealed samples were investigated with variable angle spectroscopic ellipsometry. In-depth defect profiles and cavity profiles can be best described with multiple independent effective medium sublayers of varying ratio of single-crystal/void. The number of sublayers was chosen to maximize the fit quality without a high parameter cross-correlation. The dependence of the implantation fluence, oxide layer thickness and annealing temperature on the cavity distribution was separately investigated. The ellipsometric fitted distributions were compared and cross-checked with analyses of transmission electron micrographs where the average surface cavity was determined sublayer by sublayer. The in-depth profiles were also compared with simulations of He and vacancy distributions. •He implanted and annealed Si and GaN measured by spectroscopic ellipsometry•Effective medium approximation models developed•Cavity formation as function of oxide thickness, ion dose, annealing temperature•Cavity in-depth distributions compared with transmission electron micrographs