Growth, microstructure, and strain relaxation in low-temperature epitaxial Si1−xGex alloys deposited on Si(001) from hyperthermal beams

Abstract

Epitaxial Si1−xGex(001) alloy films, with 0.15≤x≤0.30, were grown on Si(001) at temperatures Ts ranging from 300 to 550 °C using hyperthermal Si (average energy 〈ESi〉≂18 eV) and Ge (〈EGe〉≂15 eV) beams. The deposition rate was 0.1 nm s−1 and film thicknesses ranged from 30 nm to 0.8 μm. The energetic Si and Ge beams are generated by bombarding Si and Ge targets with 1 keV Kr+ ions from double-grid, multiaperture, broad ion-beam sources in a system geometry established based upon TRIM simulations of energy-dependent angular distributions of sputtered and backscattered particles. A combination of high-resolution plan-view and cross-sectional transmission electron microscopy, high-resolution x-ray diffraction, Rutherford backscattering spectroscopy, channeling, and axial angular-yield profiles demonstrated that the films are of extremely high crystalline quality. Critical layer thicknesses hc for strain relaxation in these alloys were found to increase rapidly with decreasing growth temperature. For Si0.70Ge0.30, hc ranged from 35 nm at Ts=550 °C to 650 nm at 350 °C compared to an equilibrium value of ≂8 nm. At even lower growth temperatures, hc becomes larger than critical epitaxial layer thicknesses, ≳1 μm at 300 °C. In addition, atomic force microscopy studies showed that strain-induced roughening, which occurs at elevated growth temperatures, is strongly suppressed at Ts between 300 and 400 °C with no indication of kinetic roughening.