Critical examination of growth rate for magnesium oxide (MgO) thin films deposited by molecular beam epitaxy with a mole
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The authors report a study of molecular beam deposition of MgO films on amorphous SiO2 and (0001) GaN surfaces over a large range of temperatures (25–400 C) and molecular oxygen growth pressures (107–104 Torr). This study provides insight into the growth behavior of an oxide with volatile metal constituents. Unlike other materials containing volatile constituents (e.g., GaAs, PbTiO3), all components of MgO become volatile at normal epitaxial growth temperatures (250 C). Consequently, defining which species is the adsorption controller becomes ambiguous. Different growth regimes are delineated by the critical substrate temperature for Mg re-evaporation and the Mg:O flux ratio. These regimes have impact on phase purity, quartz crystal microbalance calibration, and film microstructure. The universal decay in deposition rate above growth 105 Torr O2 is also considered. By introducing a third flux of inert argon gas, rate reduction is attributed to increased molecular scattering and not oxidation of the metal source.
I. INTRODUCTION
MgO films grown by molecular beam epitaxy (MBE) have received much scientific interest as a model system for studying oxide surface science1,2 and heteroepitaxial interfaces with nonoxide substrates.3–6 MgO epilayers have also been investigated for technological applications including interfacial layers,7,8 magnetic tunnel junctions,9–11 and surface passivation of solid-state devices.12–14 MgO epitaxy readily occurs due to low surface binding energies that permit rapid diffusion across surface terraces and strong incorporation at step edges.15,16 Film stoichiometry and phase purity is easily maintained because the precursors (O2 gas and Mg metal) are relatively volatile at modest growth temperatures (200 C), Mg metal has a single oxidation state, and the point defect formation energies are large. The self-regulating stoichiometric growth of MgO by MBE closely resembles the adsorption-controlled growth of GaAs17,18 and PbTiO3.19 However, unlike GaAs and PbTiO3, all of MgO’s constituents are volatile at growth temperatures 200 C. Because all components are volatile, defining which species is the adsorption controller becomes ambiguous and the ability to control the growth rate with a single flux is lost. A complete study of MgO deposition as a function of oxygen pressure, magnesium flux, and growth temperaa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0096
670
http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 4, Apr 2010 Downloaded: 14 Mar 2015
ture has not been reported previously. Yang and Flynn published one data set covering the deposition rate of MgO as a function of growth conditions for (001) homoepitaxy.20 At the 250 C isotherm explored in their work, an oxygen growth pressure of 105 Torr maximized the deposition rate. At this growth pressure, the deposition rate increased linearly with Mg flux and decreased with substrate temperature. Thermodynamic calculations by Vassent et al.21 confirm that Mg volati
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