Molecular beam homoepitaxial growth of MgO(001)
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We describe homoepitaxial growth and detailed in situ characterization of MgO(OOl). We have used, for the first time, high-speed Auger electron spectroscopy as a real-time probe of film composition during growth. Excellent short-range and long-range crystallographic order are achieved in films grown to a thickness of several hundred angstroms in the substrate temperature range of 450 °C to 750 °C. Moreover, the films become more laminar as the growth temperature increases, suggesting that MgO grows homoepitaxially by the step-flow growth mechanism at elevated temperature. The surfaces of films grown at 650° and 750 °C are smoother than those obtained by cleaving MgO(OOl).
I. INTRODUCTION Most experimental effort in the epitaxial growth of oxides has centered around high-Tc superconducting films. Considerably less effort has been directed toward nonsuperconducting transition metal oxides, although the field is growing. Early work by Bando et al.1^ on oxides of Fe, Ni, and Co has been built upon by more recent work from Gao, 56 You,7 and Yamamoto.8 Surprisingly little has been done on alkali and alkalineearth oxides. Although there is considerable literature on MgO film growth on a variety of lattice mismatched substrates, there are, to the best of our knowledge, only two prior studies on homoepitaxy of MgO(OOl). Booth, Kingery, and Bowen used chemical vapor deposition (CVD) to grow single-crystalline films of MgO on MgO(OOl) from MgCl 2 , CO 2 , and H2 precursor gases.9 The resulting films had low impurity concentrations and dislocation densities that were approximately two orders of magnitude lower than those found in melt-grown material. More recently, Yadavalli, Yang, and Flynn used molecular beam epitaxy (MBE) to grow MgO on cleaved MgO(OOl) at surprisingly low growth temperatures.10 These authors report homoepitaxy down to a temperature of 140 K when using electron-beam evaporated MgO as the source material, and good surface quality based on reflection high-energy electron diffraction (RHEED) patterns down to 200 K. MgO epitaxy is of specific scientific and technical interest for at least two reasons. First, MgO has been used as a substrate for high-r e superconducting film epitaxy. However, polished MgO wafers tend to have very high dislocation and vacancy defect densities. The homoepitaxial growth of an MgO buffer layer prior to high-Tc superconducting film growth can, in principle, generate a substrate of much higher crystallographic quality, which in turn will result in higher superconducting film quality. Second, there is a growing interest in the surface chemistry of oxides. Such chemistry is of considerable 2944
J. Mater. Res., Vol. 9, No. 11, Nov 1994
http://journals.cambridge.org
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importance in environmental science, heterogeneou catalysis, waste remediation, and ceramic processing.11"13 However, unlike the situation with metals and semiconductors, oxide surface chemistry has been hampered by a lack of high-quality surfaces. The highest quality MgO(OOl) substrates studied to
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