Optimization of the heteroepitaxy of Ge on GaAs for minority-carrier lifetime
Growth of Ge on GaAs at reasonably high temperatures, which produces better crystallinity in the Ge, presents serious difficulties due to the dissociation of the GaAs substrate. In this paper, we describe the growth of a low-temperature buffer layer of Ge on GaAs that considerably reduces the effects of decomposition of the GaAs during high-temperature growth of Ge. Using this approach, we present the first report of highly specular, mass-transport-limited, high-temperature growth of Ge on GaAs that is comparable to the homoepitaxy of Ge, although with a reasonably high residual n-type ( 1018 cm-3) doping level. The factors affecting the structural, electrical and optical properties of Ge on GaAs, using such an epitaxial growth technique, were studied. Lifetime variations from very low values to about 0.45 ?s were measured by a microwave technique as a function of growth conditions. Significantly, the removal of the surface oxide on the GaAs substrate prior to low-temperature buffer-layer growth, terminating the flow of germane (GeH4) during the ramp to high growth temperatures, thinner buffer layers, and high-temperature growth of Ge, were found to be important for obtaining long lifetimes.