Determination of critical layer thicknesses in IV-IV-alloy systems using reflection high energy electron diffraction intensity oscillations: Ge(100)/GexSn1−x

Abstract

The critical layer thickness, Θ2D, denoting the maximum thickness of a film that can be grown in a laminar fashion before further deposition leads to islands, is an important materials parameter for a heteroepitaxial system. Technologically interesting films are restricted to thicknesses below Θ2D. Of similar importance is the knowledge of the critical layer thickness Θc beyond which the strain in a film is relieved. The analysis of the damping of reflection high energy electron diffraction (RHEED) intensity oscillations provides a fast and convenient way to determine Θ2D. It is shown for the first time that strain and relaxation of the film can be obtained from an analysis of the periods of the oscillations. Since such measurements take place during growth, questions of metastability, such as the influence of deposition rates, temperature, and other growth parameters, can be easily addressed. The RHEED method is applied to the Ge(100)/GexSn1−x system which has been predicted to exhibit a direct band gap for compositions x≊0.4–0.8 [D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994 (1987)]. At room temperature all alloys with x<0.85 grow in the Stranski–Krastanow growth mode. In the range of compositions where a direct band gap is predicted Θ2D is less than 14×1015 cm−2 (≊30 Å); for pure Sn, Θ2D=2×1015 cm−2. In all cases the films are not pseudomorphic.