High-Resolution Imaging of Coga/GaAs and Eras/GaAs Interfaces
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HIGH-RESOLUTION IMAGING OF CoGa/GaAs AND ErAs/GaAs INTERFACES Jane G. Zhu, Stuart McKeman, Chris J. Palmstrom* and C. Barry Carter Department of Materials Science and Engineering, Bard Hall, Cornell University, Ithaca, NY 14853; *Bellcore, 331 Newman Springs Road, Red Bank, NJ 07701 ABSTRACT CoGa/GaAs and ErAs/GaAs grown by molecular-beam epitaxy have been studied using high-resolution transmission electron microscopy (HRTEM). The epitactic interfaces have been shown to be abrupt on the atomic scale. Computer simulations of the HRTEM images have been obtained for different interface structures under various specimen and image conditions. Practical problems in the comparison between the simulated and experimental images are discussed. INTRODUCTION The epitactic growth of metal compounds, such as NiAI [1], CoGa [2] and NiGa [3], and rare-earth arsenides, such as ErAs [4,5], LuAs [5] and YbAs [6], on GaAs has recently been reported. These materials have potential applications for both stable contacts on compound semiconductors and three-dimensional device structures. The layers on each side of the heterojunction of these newly developed systems have different crystal structures, unlike the extensively studied semiconductor-semiconductor systems [7]. Transition metal-gallium (or aluminium) compounds, e.g., CoGa, CoAl, FeAl, NiGa and NiAl, have the CsC1 (B2) structure and can be epitactically grown on GaAs with about 2% lattice mismatch. Misfit dislocations at the interface of CoGa and GaAs have recently been investigated [8]. Most of the rare earth monoarsenides have the NaCI structure [9]. The lattice mismatch between ErAs and GaAs is about 1.6%. In this paper, the study of CoGa/GaAs and ErAs/GaAs structures by high-resolution transmission electron microscopy (HRTEM) is reported. Atomic-resolution characterization of the interface configuration is very important since the interface can dominate the electronic properties of the devices. A determination of the interface location by the difference in the background intensity in conventional transmission electron microscopy is not reliable since the images at the interface region could be modulated by artifacts of sample preparation. HRTEM studies are necessary to precisely locate the interface position. The detailed interpretation of micrographs, however, requires the comparison of simulated and experimental images. EXPERIMENTAL The CoGa/GaAs and ErAs/GaAs layers used in this study were grown in a VG-V80H MBE system. Co was evaporated by an e--gun. Er, Ga and As 4 were evaporated from conventional effusion cells. 50 nm of CoGa was grown at 450"C after the growth of a 500 nm GaAs buffer layer at 600°C. Ga was deposited to obtain a Ga-terminated surface prior to the growth of CoGa. ErAs was grown at 400'C after the growth of a 500nm-GaAs buffer layer at 600'C. The As 4 shutter was open during the period when the substrate temperature was lowered from 600"C to the CoGa or ErAs growth temperature. Details of the MBE growth can be found elsewhere [2,5]. The samples were chara
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