Local Alloy Order in a Ge1-xSnx/Ge Epitaxial Layer
Agnieszka Anna Corley-Wiciak, Shunda Chen, Omar Concepción, Marvin Hartwig Zoellner, Detlev Grützmacher, Dan Buca, Tianshu Li, Giovanni Capellini, Davide Spirito
备注：
20 pages, 10 figures, Accepted Manuscript of a Published Paper in Physical Review
The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited using far from the equilibrium processes, as is the case of epitaxial Ge1-xSnx layers. In this work, we investigate the arrangement of Ge and Sn atoms in optoelectronic grade Ge1-xSnx epitaxial layers featuring a Sn content in the 5-14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of Sn atoms, that modify the local environments of Ge atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in Ge1-xSnx alloys, particularly with regard to the impact of local ordering of the different atomic species.
Magnetostructural Transformation and Magnetoresponsive Properties of MnNiGe1-xSnx Alloys
采用X射线衍射（XRD）、差热分析（DTA）和磁化强度测量方法研究了MnNiGe 1-xSnx（0 \leq x \leq 0.200）合金的马氏体相变和磁相变。结果表明，随着Sn替代量的增加，MnNiGe_（1-x）Sn_x的马氏体相变温度从460 K降低到100 K，马氏体中AFM螺旋结构转变为反平行AFM结构。在此基础上，获得了显著的磁场诱导PM/spiral-AFM和FM/AFM磁结构转变以及较大的正、负磁热效应。Mn Ni Ge_（1-x）Sn_x合金的磁响应效应因Sn的替代而增强。提出了MnNiGe_（1-x）Snx合金的结构和磁性相图。
We use first-principles simulation and virtual crystal approximation to reveal the unique double band inversion and topological phase transition in Ge1-xSnx alloys. Wavefunction parity, spatial charge distribution and surface state spectrum analyses suggest that the band inversion in Ge1-xSnx is relayed by its first valence band. As the system evolves from Ge to α-Sn, its conduction band moves down, and inverts with the first and the second valence bands consecutively. The first band inversion makes the system nontrivial, while the second one does not change the topological invariant of the sy…
The thermodynamic stability of germanium tin $\mathrm{Ge_{1-x}Sn_x}$ alloys is investigated across the composition range $0 \le x \le 1$ by applying density functional theory (DFT) together with the cluster expansion formalism (CE). It is known that GeSn alloys are immiscible and that non-equilibrium growth techniques are required to produce metastable films and nanostructures. Insight into the driving forces behind component segregation is gained by investigating the equilibrium thermodynamics of GeSn systems. The alloy free energy of mixing is computed by combining enthalpies from CE with en…
Epitaxial germanidation of full-Heusler Co2FeGe alloy thin films formed by rapid thermal annealing
采用快速热退火诱导锗化方法，在绝缘体上硅（SOI）衬底上外延生长Fe/Co/赝Ge（001）-on-insulator（GOI）多层膜，制备了全Heusler Co2 FeGe（CFG）合金薄膜。X射线衍射（XRD）分析结果表明，薄膜沿[001]方向外延生长，且具有面内外延结构，薄膜的面内外延关系为[100]|| GOI[100]，虽然电影稍微包含纹理成分。强的（111）和（200）超晶格衍射强度表明，CFG薄膜具有高度有序的L21结构。薄膜的横截面高分辨率透射电子显微镜图像表明薄膜具有主要的外延和轻微的织构组分，这与XRD测量结果一致。外延元件直接生长在SOI衬底的BOX层上，而不形成任何界面层。
In this Letter, we demonstrate that it is possible to form a two-dimensional (2D) silicene-like Si$_5$Ge compound by replacing the Si atoms occupying on-top sites in the planar-like structure of epitaxial silicene on ZrB$_2$(0001) by deposited Ge atoms. For coverages below 1/6 ML, the Ge deposition gives rise to a Si$_{6-x}$Ge$_{x}$ alloy (with $x$ between 0 and 1) in which the on-top sites are randomly occupied by Si or Ge atoms. The progressive increase of the valence band maximum with $x$ observed experimentally originates from a selective charge transfer from Ge atoms to Si atoms. These ac…
Future spintronics requires the realization of thin film of half-metallic ferromagnets having high Curie temperature and 100\% spin polarization at the Fermi level for potential spintronics applications. In this paper, we report the epitaxial thin films growth of half-metallic CoFeMnGe Heusler alloy on MgO (001) substrate using pulsed laser deposition system, along with the study of structural, magnetic and transport properties. The magnetic property measurements of the thin film suggest a soft ferromagnetic state at room temperature with an in-plane magnetic anisotropy and a Curie temperature…
In the paper we present the results of extensive studies of palladium-rich Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a composition range of x = 0.01-0.07 these materials are soft ferromagnets, the saturation magnetization and magnetic anisotropy of which can be tuned by its composition. Vibrating sample magnetometry was used to study temperature dependences of spontaneous magnetic moment and to establish the temperature of magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR) measurements at low temperatures in the in-plane and out-of-plane geome…
MgO-barrier magnetic tunnel junctions with half-metallic Heusler alloy electrodes attracted much attentions for spintronics applications. However, a couples of issues related to materials still remain to be resolved for practical uses. Recently, quarterly equiatomic Heusler alloys attracted attentions as advanced Heusler alloys. CoIrMnZ (Z = Al, Si, Ga, and Ge) half-metallic Heusler alloys were designed and predicted to have moderate Curie temperatures and to be a lattice-matched with the MgO barrier, being advantageous to traditional Co2 Heusler alloys [T. Roy et al., J. Magn. Magn. Mater. 49…
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The synthesis of antimonene, which is a promising group-V 2D material for both fundamental studies and technological applications, remains highly challenging. Thus far, it has been synthesized only by exfoliation or growth on a few substrates. In this study, we show that thin layers of antimonene can be grown on Ag (111) by molecular beam epitaxy. High-resolution scanning tunneling microscopy combined with theoretical calculations revealed that the submonolayer Sb deposited on a Ag (111) surface forms a layer of AgSb2 surface alloy upon annealing. Further deposition of Sb on the AgSb2 surface …
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GeSn epitaxial heterostructures are emerging as prominent candidates for the monolithic integration of light sources on Si substrates. Here we propose a judicious explanation for their temperature-dependent photoluminescence (PL) that is based upon the so far disregarded optical activity of dislocations. By working at the onset of plastic relaxation, which occurs whenever the epilayer releases the strain accumulated during growth on the lattice mismatched substrate, we demonstrate that dislocation nucleation can be explicitly seen in the PL data. Notably, our findings point out that a monotono…
Off-stoichiometric variable doping for exceptional power factors in L2$_1$ Fe$_2$VAlM$_x$ (M=Ti, W) epitaxial thin films
Instituto de Micro y Nanotecnología, IMN-CNM, CSIC (CEI UAM+CSIC)（微纳技术研究所，IMN-CNM，西班牙国家科学研究委员会（CEI UAM+CSIC））；Departamento de Química Física, Facultad de Química, Universidad de Sevilla（塞维利亚大学化学学院物理化学系）；Institute of Solid-State Physics, Technische Universität Wien（维也纳工业大学固体物理研究所）
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We calculate the electronic structure of germanium-tin (Ge$_{1-x}$Sn$_{x}$) binary alloys for $0 \leq x \leq 1$ using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition $x$ are identified, and the impact of epitaxial strain is included by constraining supercell lattice constants perpendicular to the [001] growth direction to the lattice constants of Ge, zinc telluride (ZnTe), or cadmium telluride (CdTe) substrates. It is found that application of 1% tensile strain reduces the Sn composition required to bring the (positi…
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Unlike silicene, for which the demonstration of its existence has been done through numerous independent studies, the possibility of growing epitaxial germanene remains highly controversial. It has been recently shown by scanning tunneling microscopy that the (3 x 3) surface reconstruction formed upon Ge deposition on Al(111) presents a honeycomb structure, and it was assigned to a pure germanene monolayer. Using quantitative measurements by surface X-ray diffraction compared to density functional theory calculations, we demonstrate that this Ge/Al(111) (3 x 3) reconstruction corresponds, in f…
Ultra-wide bandgap semiconductors are ushering in the next generation of high power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here it is discovered that single-crystalline layers of $α$-(AlGa)$_2$O$_3$ alloys spanning bandgaps of 5.4 - 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the $α$- to the narrower bandgap $β$-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth fro…
Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as …
