Low-temperature growth and critical epitaxial thicknesses of fully strained metastable Ge1−xSnx (x≲0.26) alloys on Ge(001)2×1

Abstract

Epitaxial metastable Ge1−xSnx alloys with x up to 0.26 (the equilibrium solid solubility of Sn in Ge is <0.01) were grown on Ge(001)2×1 by low-temperature molecular beam epitaxy. Film growth temperatures Ts in these experiments were limited to a relatively narrow range around 100 °C by the combination of increased kinetic surface roughening at low temperatures and Sn surface segregation at high temperatures. All Ge1−xSnx films consisted of three distinct sublayers: the first is a highly perfect epitaxial region followed by a sublayer, with an increasingly rough surface, containing 111 stacking faults and microtwins, while the terminal sublayer is amorphous. Based upon reflection high energy electron diffraction and cross-sectional transmission electron microscopy (XTEM) analyses, critical epitaxial thicknesses tepi, defined as the onset of amorphous growth, were found to decrease from 1080 Å for pure Ge to ≃35 Å for alloys with x=0.26. TEM and XTEM analyses revealed no indication of misfit dislocations (except in Ge0.74Sn0.26 samples) and high-resolution x-ray reciprocal lattice mapping showed that epitaxial Ge1−xSnx layers were essentially fully strained. From an analysis of tepi(x) results, surface morphological evolution leading to epitaxial breakdown is controlled by kinetic roughening for alloys with x≲0.09 and by strain-induced roughening at higher Sn concentrations. We propose that the thermal activation required for the cross-over, reported here for the first time, from kinetic to strain-induced roughening is partially overcome by the fact that kinetic roughening provides local surface chemical potential gradients over lateral length scales which are sufficiently small to initiate strain-induced roughening even at these low temperatures.