Growth of metastable Ge1−xSnx/Ge strained layer superlattices on Ge(001)2×1 by temperature-modulated molecular beam epitaxy

Abstract

Single-crystal metastable diamond-structure Ge1−xSnx/Ge strained-layer superlattices (SLS) with x up to 0.24 (the equilibrium solid solubility of Sn in Ge is <0.01) have been grown on Ge(001)2×1 substrates using temperature-modulated molecular-beam epitaxy with maximum growth temperatures Ts≤150 °C. In situ reflection high energy electron diffraction combined with postdeposition high-resolution x-ray diffraction (HR-XRD) and cross-sectional transmission electron microscopy results show that the Ge1−xSnx(001)2×1 alloy and Ge(001)2×1 spacer layers are commensurate. In fact, the alloy layers are essentially fully strained with an average in-plane lattice constant mismatch of (1±2)×10−5 and an average tetragonal strain in the growth direction of (1.39±0.03)×10−2 as determined from HR-XRD reciprocal-space lattice maps obtained using asymmetric (113) reflections. ω broadening of the zero-order SLS peak was only 30.1 arc sec FWHM, indicating that the degree of mosaicity in these structures is negligible. The intensities and positions of the satellite reflections and finite-thickness interference fringes in HR-XRD 004 rocking curve ω-2θ scans are in good agreement with simulated patterns obtained using a dynamical scattering model.