Abstract
ZnO films were grown on c-plane sapphire by dc reactive magnetron sputtering. X-ray diffraction measurements showed that an epitaxial relationship of ZnO[101¯0]∥Al2O3[112¯0] and ZnO[21¯1¯0]∥Al2O3[11¯00] was maintained down to a substrate temperature Ts≈80°C, corresponding to <17% of the ZnO melting point. The low-temperature epitaxy was explained by the energetic species produced during sputtering and a high mobility of Zn surface atoms in low-oxygen conditions. Crystalline perfection improved with increasing Ts, decreasing growth rate, and decreasing oxygen-to-zinc flux ratio. Dense microstructures with flat surfaces were achieved at Ts≈200°C, although at Ts≈80°C films tended to transition to a columnar Zone 1 microstructure from a Zone T microstructure with increasing film thickness. In general, Ts=200–300°C and an oxygen-to-zinc flux ratio of ≈300 yielded a favorable combination of good optical transparency, crystalline perfection, flat surfaces, and dense microstructure.
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