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.
Fully strained single-crystal Ge1−xSnx alloys (x⩽0.22) deposited on Ge(001)2×1 by low-temperature molecular beam epitaxy have been studied by Raman scattering. The results are characterized by a Ge–Ge longitudinal optical (LO) phonon line, which shifts to lower frequencies with increasing x. Samples capped with a 200-Å-thick Ge layer exhibit a second Ge–Ge LO phonon line whose position remains close to that expected from bulk Ge. For all samples, capped and uncapped, the frequency shift ΔωGeSn of the Ge–Ge LO phonon line from the Ge1−xSnx layer, with respect to the position for bulk Ge, is linear with the Sn fraction x (ΔωGeSn=−76.8x cm−1) over the entire composition range. Using the elastic constants, the Grüneisen parameter, and the shear phonon deformation parameter for Ge, we calculate the contribution of compressive strain to the total frequency shift to be Δωstrain=63.8x cm−1. Thus, the LO phonon shift in Ge1−xSnx due to substitutional-Sn-induced bond stretching in fully relaxed alloys is estimated to be Δωbond =−140.6x cm−1.
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