Selective Nucleation and Growth of Large Grain Polycrystalline GaAs
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Selective Nucleation and Growth of Large Grain Polycrystalline GaAs C. G. Allen1, J. D. Beach1, A. A. Khandekar2, J. C. Dorr1, C. Veauvy1, R. T. Collins1, T. F. Kuech2, R. M. Caputo1, R. E. Hollingsworth3, C. K. Inoki4, and T. S. Kuan4 1 Physics Department, Colorado School of Mines, 1500 Illinois Golden, CO 80401, U.S.A. 2 Department of Chemical Engineering, University of Wisconsin-Madison, 1415 Engineering Dr., Madison, WI 53706, U.S.A. 3 ITN Energy Systems Inc., 8130 Shaffer Parkway, Littleton, CO 80127-4107, U.S.A. 4 Physics Department, University at Albany, Albany, NY 12222, U.S.A. ABSTRACT A method for depositing large grained polycrystalline GaAs on lattice mismatched substrates through controlled nucleation and selective growth is presented. The process was developed on Si wafers. Nucleation site formation began with nanolithography to create submicron holes in photoresist on Si. Ga metal was electrochemically deposited into the holes. Subsequent arsine anneals converted the gallium deposits into GaAs. Photoluminescence and electron diffraction verified conversion to GaAs. Metal-Organic Chemical Vapor Deposition (MOCVD) enlarged the seed crystals to coalescence without creating additional nucleation sites within the patterned field. Having successfully demonstrated the approach, subsequent work has been directed at lower cost, alternative ways to define initial nucleation sites, such as, microcontact lithography and direct decomposition of triethyl gallium to Ga metal in the MOCVD chamber. INTRODUCTION In typical thin-film deposition techniques for polycrystalline semiconductors, uncontrolled nucleation occurs simultaneously with growth. This tends to lead to small, randomly oriented grains and high densities of grain boundaries. These grain boundary defects have proven particularly problematic in polycrystalline III-V semiconductors where surface defect states degrade minority carrier properties. Developing the ability to control the location and properties of nucleation sites, may ultimately allow larger grain sizes and higher quality material to be grown. (See Ref. 1 and references there in). Here, we present approaches that position GaAs seed crystals on silicon substrates with micron length scale separations. These crystallites then act as nucleation sites for subsequent thin film growth. The ability to fabricate semiconductor crystallites at predefined locations has potential applications in areas ranging from photonics crystals to novel detectors and photovoltaics. EXPERIMENTAL APPROACH AND RESULTS We have demonstrated a process, using near-field scanning optical lithography and electrochemical deposition, that defines nucleation sites for subsequent GaAs growth [2]. In this method, an array of micron to submicron holes was patterned into a photoresist
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(a.) (b.) Figure 1. AFM images of (a.) submicron holes patterned into photoresist using NSOM lithography, and (b.) Ga selectively deposited into the holes by electrochemical deposition. (PR) layer spin coated onto silicon (Fig. 1a). Ele
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