Coherent island formation of Cu 2 O films grown by chemical vapor deposition on MgO(110)
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Cuprous oxide (Cu2O) films have been grown on single-crystal MgO(110) substrates by a chemical vapor deposition process in the temperature range 690–790 °C. X-ray diffraction measurements show that phase-pure, highly oriented Cu2O films form at these temperatures. The Cu2O films are observed to grow by an island-formation mechanism on this substrate. Films grown at 690 °C uniformly coat the substrate except for micropores between grains. However, at a growth temperature of 790 °C, an isolated, three-dimensional island morphology develops. Using a transmission electron microscopy and atomic force microscope, both dome- and hut-shaped islands are observed and are shown to be coherent and epitaxial. The isolated, coherent islands form under high mobility growth conditions where geometric strain relaxation occurs before misfit dislocation can be introduced. This rare observation for oxides is attributed to the relatively weak bonding of Cu2O, which also has a relatively low melting temperature.
I. INTRODUCTION
Cuprous oxide (Cu2O) is a unique material that offers the possibility of observing Bose–Einstein exciton condensation (BEC).1 However, despite the demonstration of Bose–Einstein statistical behavior of excitons in Cu2O,2–5 BEC of excitons in this material has not been conclusively observed. In this regard, epitaxial Cu2O thin films are of interest for several reasons. First, a single crystallike Cu2O thin film is expected to confine excitons spatially. This effectively increases the concentration of excitons, a positive step toward reaching the quantum saturation condition2 needed for BEC. Second, with strong bonding to the substrate, a compressive stress can be induced in the Cu2O film when cooling to cryotemperatures (Cu2O has a negative thermal expansion coefficient at low temperature6). A compressive stress applied in the (110) direction of bulk Cu2O crystals has been demonstrated to reduce the crystal symmetry from cubic to orthorhombic, thereby lowering the degeneracy of orthoexcitons.1 An additional benefit of lower crystal symmetry is the observation of paraexciton radiative recombination, which is otherwise forbidden.7 The ability to create and observe paraexcitons represents an additional avenue to observing exciton BEC in Cu2O. Thin films of Cu2O have been grown by several deposition techniques, including pulsed laser deposition,8–10 chemical vapor deposition (CVD),11,12 and sputtering.13,14 a)
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J. Mater. Res., Vol. 16, No. 8, Aug 2001 Downloaded: 30 Apr 2015
Typical growth temperatures in these studies were approximately 700 °C, nearly 100 °C lower than the temperatures used in the present study. Furthermore, singlecrystallike Cu2O films suitable for excitonic optical studies have not been realized using these growth techniques. This contribution analyzes issues relevant to the growth of epitaxial Cu2O films on MgO(110) singlecrystal substrates by CVD. At elevated temperatures and reduced growth rates, coherent islands
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