Diffusion of Ge in GaAs at SiO2-encapsulated Ge–GaAs interfaces

Abstract

The diffusion of Ge in semi-insulating GaAs was investigated for annealing temperatures in the range 650–800 °C. The samples consisted of electron-beam or metalorganic chemical-vapour deposition deposited Ge–GaAs interfaces encapsulated with SiO2. Ge diffusion profiles, measured with secondary-ion mass spectroscopy (SIMS), had a characteristic plateau and steep slope at the diffusion front, similar to those obtained for Si and Sn diffusion at SiO2-encapsulated GaAs. A saturated-surface concentration of 2 × 1019 atoms∙cm−3 was observed for anneals above 750 °C, likely the solid solubility of Ge in GaAs at these temperatures. Based on the Greiner–Gibbons substitutional-pair diffusion model (Appl. Phys. Lett. 44, 750 (1984)), the Ge-pair diffusion coefficients were calculated from the SIMS data. They ranged from 1 × 10−13 to 3 × 10−12 cm2∙s−1 but varied depending upon the annealing time and sample structure. A new model that eliminates the necessity for paired-Ge diffusion was introduced. It proposed that dissociation and out-diffusion of GaAs into the SiO2 produced a nonequilibrium flux of vacancies into the GaAs. Calculated profiles using a position-dependent Ge diffusion coefficient were shown to fit the SIMS data independent of the type of Ge diffusing species, whether single ions or pairs.