3.6 Single Crystal Uniquely Synthesized by Molecular Beams

Fig. 3-11 Schematic figure of the molecular beam epitaxy method
Single crystals are grown on a substrate after the evaporation of raw materials by electron beams and/or heaters.

Fig. 3-12 The results measured by X-rays and infra-red absorption for a ZrO2 single crystal film synthesized by the molecular beam epitaxy method
(a) Sharp peaks and diffraction angles demonstrate a good quality of ZrO2 single crystal.
(b) Another evidence of a monoclinic structure by infra-red absorption. The absorption due to the monoclinic structure was deduced from a comparison with the absorption from the polycrystalline ZrO2 including cubic and monoclinic structures as well as chemical impurities.

The synthesis of a single crystal is essential to study the fundamental properties of materials. Ordinarily, many single crystals are grown in thermal equilibrium, but this thermal equilibrium method is inapplicable to some crystals. One example of this inapplicability is the synthesis of a zirconium dioxide (ZrO2) single crystal, which is used for a crucible at high temperature and for a film used for the separation of hydrogen gas. This crystal transforms into three different crystal structures, cubic, tetragonal and monoclinic structures, at high temperature. Since the transformation from the tetragonal to monoclinic structure at 1,170 degrees is accompanied by a large and destructive volume change, the synthesis of a stable single crystal with monoclinic structure by the thermal equilibrium method has been impossible at room temperature.
In this study, we were able to synthesize a ZrO2 single crystal film on a sapphire substrate by using the molecular beam epitaxy method as shown in Fig. 3-11. The structure of the film examined by X-rays and infra-red spectroscopy is shown in Fig. 3-12. A good quality and homogeneous single crystal film is demonstrated by the sharp diffraction peaks and diffraction angles of the X-rays, and also the transmission frequencies in infra-red absorption. This molecular beam epitaxy method enabled us to synthesize new single crystals such as strontium oxide (SrO) which has been impossible to synthesize by the thermal equilibrium method. With this new method of synthesizing crystals, we can exactly measure some properties of the materials such as ionic conductivity which cannot be obtained for polycrystalline solids.

Reference
H. Asaoka et al., Epitaxial Growth of Zirconium Dioxide Films on Sapphire Substrates, Appl. Surf. Sci., 113/114, 198 (1997).

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