ATOMIC DIFFUSION AT POLYCRYSTALLINE-SILICON/GALLIUM-ARSENIDE INTERFACES

A study of the metallurgical and electrical stability of thin film polycrystalline-silicon/gallium arsenide (poly-Si/GaAs) interfaces by Rutherford backscattering, secondary ion mass spectroscopy and transmission electron microscopy is described. Considerable interdiffusion occurs at heavily As or P-doped poly-Si/GaAs (5-30 at%) interfaces whereas interdiffusion is negligible at GaAs interfaces with undoped poly-Si or As and B-doped poly-Si with dopant concentrations of less than 1 at%. Interfacial reaction is not detected after heat treatments to 1050$\sp\circ$C. The out-diffusion of Ga and As into heavily-doped poly-Si occurs via grain boundaries and the Si diffusion in the GaAs is characterized by steep slopes at the diffusion front which do not fit to simple solutions of the diffusion equation. It is proposed that the Si diffusion is enhanced by the diffusion of vacancies from the interface created by the disproportionation of the substrate. The diffusion profiles can be simulated with a position-dependent Si diffusion coefficient determined from the local vacancy concentration. Excellent fits to the data are obtained if it is further assumed that the vacancy diffusion is concentration-dependent and dominated by divacancy diffusion. This model was shown to apply equally well to the diffusion of P and In in GaAs also studied from poly-Si and to the diffusion of Ge, Sn and Si at SiO$\sb 2$/Ge/GaAs interfaces where the disproportionation of the substrate occurs by out-diffusion into the SiO$\sb 2$. Based on the divacancy diffusion model the enhancement in the diffusivity of Si, Ge, Sn, P and In in GaAs encapsulated with poly-Si or SiO$\sb 2$ may be as high as 10$\sp 4$ above the equilibrium diffusion coefficients.

The electrical stability of poly-Si/GaAs was correlated to interdiffusion. At interfaces where Si diffusion was detected the contacts were ohmic (contact resistivity 1.4 $\times$ 10$\sp{-4}$ $\Omega$-cm$\sp 2$ or poorly rectifying. At poly-Si:B(0.5 at%)/GaAs interfaces where no out-diffusion of Ga and As was detected a rectifying contact (barrier height of 0.83 eV) was obtained after heat treatments to 950$\sp\circ$C for 10 sec.