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.

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Transport electron/phonon coupling parameters and Eliashberg spectral functions a tr 2 F( hx) are determined for group-IV transition-metal (TM) nitrides TiN, ZrN, and HfN, and the rare-earth (RE) nitride CeN using an inversion procedure based upon temperature-dependent (4 < T < 300 K) resistivity measurements of high-crystalline-quality stoichiometric epitaxial films grown on MgO(001) by magnetically-unbalanced reactive magnetron sputtering. Transport electron/phonon coupling parameters k tr vary from 1.11 for ZrN to 0.82 for HfN, 0.73 for TiN, and 0.44 for CeN. The small variation in k tr among the TM nitrides and the weak coupling in CeN are consistent with measured superconducting transition temperatures 10.4 (ZrN), 9.18 (HfN), 5.35 (TiN), and <4 K for CeN. The Eliashberg spectral function describes the strength and energy spectrum of electron/phonon coupling in conventional superconductors. Spectral peaks in a 2 F( hx), corresponding to regions in energy-space for which electrons couple to acoustic hx ac and optical hx op phonon modes, are centered at hx ac ¼ 33 and hx op ¼ 57 meV for TiN, 25 and 60 meV for ZrN, 18 and 64 meV for HfN, and 21 and 39 meV for CeN. The acoustic modes soften with increasing cation mass; optical mode energies remain approximately constant for the TM nitrides, but are significantly lower for the RE nitride due to a lower interatomic force constant. Optical/acoustic peak-intensity ratios are 1.15 6 0.1 for all four nitrides, indicating similar electron/phonon coupling strengths a tr ( hx) for both modes. V C 2013 AIP Publishing LLC. [http://dx