LOW-TEMPERATURE GROWTH AND CRITICAL EPITAXIAL THICKNESSES OF FULLY STRAINED METASTABLE GE1-XSNX (X-LESS-THAN-OR-SIMILAR-TO-0.26) ALLOYS ON GE(001)2X1

Epitaxial metastable Ge1-xSnx alloys with x up to 0.26 (the equilibriu m solid solubility of Sn in Ge is <0.01) were grown on Ge(001)2 x 1 by low-temperature molecular beam epitaxy. Film,growth temperatures T-s, in these experiments were limited to a relatively narrow range around 100 degrees C by the combination of increased kinetic surface roughen ing at low temperatures and Sn surface segregation at high temperature s. 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 Ill stacking faults and microtw ins, while the terminal sublayer is amorphous. Based upon reflection h igh energy electron diffraction and cross-sectional transmission elect ron microscopy (XTEM) analyses, critical epitaxial thicknesses t(epi), defined as the onset of amorphous,growth, were found to decrease from 1080 Angstrom for pure Ge to similar or equal to 35 Angstrom for allo ys with x=0.26, TEM and XTEM analyses revealed no indication of misfit dislocations (except in Ge0.74Sn0.26 samples) and high-resolution x-r ay reciprocal lattice mapping showed that epitaxial Ge1-xSnx layers we re essentially fully strained. From an analysis of t(epi)(x) results, surface morphological evolution leading to epitaxial breakdown is cont rolled by kinetic roughening for alloys with x less than or similar to 0.09 and by strain-induced roughening at higher Sn concentrations. We propose that the thermal activation required for the cross-over, repo rted here for the first time, from kinetic to strain-induced roughenin g is partially overcome by the fact that kinetic roughening provides l ocal surface chemical potential gradients over lateral length scales w hich are sufficiently small to initiate strain-induced roughening even at these low temperatures. (C) 1998 American Institute of Physics.