The Role of Surface Chemistry in Growth and Material Properties of ZnO Epitaxial Layers Grown on a-Plane Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy

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Abstract
The role of surface chemistry in the growth and material properties of ZnO epilayers grown on a-plane sapphire by plasma-assisted molecular beam epitaxy is investigated. The surface chemistry of a-plane sapphire is controlled from O-rich to Al-rich by changing the pregrowth treatment from oxygen plasma to atomic hydrogen. Such a change in surface treatment causes a significant difference in growth mode presumably due to a difference in the surface migration of adatoms: two-dimensional growth is more favorable on an atomic-H-treated surface. Accordingly, ZnO layers grown on an atomic-H-treated surface show a smoother surface morphology consisting of larger hexagonal islands with a typical size of 2.5 µm, which should be compared with an island size of 0.2 µm on an O-plasma-treated surface. The observed surface morphology is found to be consistent with the result of X-ray diffraction analysis that shows a larger coherent length for ZnO films with an atomic-H pretreatment. Accordingly, the ZnO films with an atomic-H treatment show stronger excitonic emission with weaker deep-level emission than those on an O-treated surface. High-quality undoped ZnO epilayers with an electron mobility as high as 130 cm2V-1s-1 and an electron concentration of 1.4×1017 cm-3 are grown with good reproducibility.

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