Excimer Laser Assisted Deposition of GaAs, AlAs, and [Al,Ga]As from Lewis Acid-Base Adducts
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EXCIMER LASER ASSISTED DEPOSITION OF GaAs, AlAs, AND [AI.Ga]As FROM LEWIS ACID-BASE ADDUCTS J.J. ZINCK, P.D. BREWER, J.E. JENSEN, G.L. OLSON AND L.W. TUTT Hughes Research Laboratories, Malibu. CA 90265 ABSTRACT Laser-assisted deposition of GaAs, AlAs and [AIGa]As thin films on Ge(I00) substrates from trimethylgallium-trimethylarsenic and trimethylaluminumtrimethylarsenic Lewis acid-base adduct source materials is reported. A parametric study has been performed in which reactive gas pressure, substrate temperature, laser fluence, laser wavelength (248 nm or 193 nm). and orientation of the laser beam with respect to the substrate have been varied. In the case of irradiation parallel to the substrate, stoichiometric films of GaAs and [AIGa]As have been obtained. The data suggest that for irradiation perpendicular to the substrate a competition exists between desorption and photodeposition, which adversely affects film stoichiometry under the conditions studied. INTRODUCTION Laser-assisted reactions are being actively investigated as a means for depositing compound semiconductor films at reduced temperatures and with a degree of spatial control not available with conventional thermal methods. Particular attention has been directed toward the laser-assisted deposition of III-V materials. For example, InP has been deposited via ArF (193 nm) excimer laser dissociation of organometallic starting materials [I]. and GaAs has been deposited using a cw Ar laser beam for spatially localized heating [2-4]. In this paper, we discuss recent work on the excimer laser-assisted deposition of GaAs, AlAs, and fAI.Ga]As from organometallic Lewis acid-base adducts. specifically, trimethylgalliumtrimethylarsenic (GaAs adduct) and trimethylaluminum-trimethylarsenic (AlAs adduct). Adducts offer several potential advantages over conventional gas source materials. These include, for example, the ability to deposit stoichiometric films using a single reactant gas. and the use of wavelengths longer than 193 nm to promote the deposition reaction. Previously. we have demonstrated that stoichiometric films of GaAs could be produced by irradiation of the GaAs adduct at its equilibrium vapor pressure (approximately 7 Torr at 25°C) in a static gas cell at 248 nm [5]. In the present work we have utilized a low pressure metalorganic chemical vapor deposition (MOCVD) system, and have performed a systematic variation of reaction parameters in order to establish conditions under which stoichiometric films could be obtained: the ultimate goal being the low temperature growth of epitaxial layers with controlled composition. Experimental parameters which were varied include: reactant gas pressure, substrate temperature, laser fluence, laser wavelength (193 or 248 nm) and orientation of the incident laser pulse with respect to the substrate (parallel or perpendicular). In addition, we have extended the earlier studies to include an investigation of the AlAs adduct. EXPERIMENTAL The low pressure MOCVD system is shown schematically in Fig. 1. The reaction vesse
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