Heteroepitaxy of Ge on Cube-Textured Ni(001) Foils Through CaF 2 Buffer Layer
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Heteroepitaxy of Ge on Cube-Textured Ni(001) Foils Through CaF2 Buffer Layer L. Chen1, Z.-H. Lu1, T.-M. Lu1, I. Bhat2, S.B. Zhang1, A. Goyal3, L.H. Zhang4, K. Kisslinger4, and G.-C. Wang1 1
Department of Physics, Applied Physics and Astronomy, and Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3950 2 Electrical, Computer and Systems Engineering Department, and Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3950 3 TapeSolar Inc. and U. at Buffalo, Research and education in eNergy, Environment, and Water (RENEW) Institute, Buffalo, NY 14260 4 Brookhaven National Lab, Center for Functional Nanomaterials, Upton, NY 11973 ABSTRACT Epitaxial Ge films are useful as a substrate for high-efficiency solar cell applications. It is possible to grow epitaxial Ge films on low cost, cube textured Ni(001) sheets using CaF2(001) as a buffer layer. Transmission electron microscopy (TEM) analysis indicates that the CaF2(001) lattice has a 45o in-plane rotation relative to the Ni(001) lattice. The in-plane epitaxy relationships are CaF2[110]//Ni[100] and CaF2[ͳ10]//Ni[010]. Energy dispersive spectroscopy (EDS) shows a sharp interface between Ge/CaF2 as well as between CaF2/Ni. Electron backscatter diffraction (EBSD) shows that the Ge(001) film has a large grain size (~50 Pm) with small angle grain boundaries (< 8o). The epitaxial Ge thin film has the potential to be used as a substrate to grow high quality III-V and II-VI semiconductors for optoelectronic applications. INTRODUCTION Single crystal Ge substrates have many superior properties suitable for optoelectronic applications. An example is the use of Ge as a substrate to grow GaAs-based, high efficiency multiple-junction solar cells [1]. However, a single crystal Ge wafer is more costly than a single crystal wafer of Si. Researchers have shown that rather than using wafers, heteroepitaxial Ge thin films can be grown on single crystal substrates such as Si(100) [2,3,4] and Si(111) [5], which can then be used as a substrate for subsequent film growth. A different approach is to use an epitaxial buffer layer grown on a flexible cube-texture metal substrate instead of a single crystal substrate. The flexible substrate is made by the rolling-assisted biaxially textured substrate (RABiTS) method [6]. A biaxially textured foil is not single crystal, but has preferred crystallographic orientations in both the in-plane and out-of-plane directions. Under this approach, epitaxial Ge thin films have been successfully grown on biaxial buffer layers such as CeO2 on Ni-W foil [7] or CaF2 on flexible cube-textured Ni foil substrate [8,9]. Epitaxial Ge thin films have also been shown to grow on biaxial buffer layer CeO2 on ion-beam assisted deposited (IBAD) biaxial MgO on Hastelloy foil C-276 [7,10]. Because hastelloy alloy foil is not textured, IBAD of biaxial MgO film was grown first to induce biaxial texture in the CeO2. In this paper we report a the
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