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Controlled Growth of O-Polar ZnO Epitaxial Film by Oxygen Radical Preconditioning of Sapphire Substrate

Oxygen radicals pregrowth treatment and surface nitridation were used to eliminate Zn-polar inversion domains and control the growth of single-domain O-polar ZnO film on sapphire substrate by rf plasma-assisted molecular beam epitaxy. We found that the formation of oxygen-terminated sapphire surface prior to nitridation is crucial for achieving the anion polarity in subsequent AlN and ZnO layers, as demonstrated by formation of the 3×3 surface reconstruction during ZnO growth and ex situ polarity determination. This method, in general, can be applied to growth of other polar films, such as II-VI oxides and III-V nitrides, on sapphire substrates.

High-quality ZnO thin films have been grown on sapphire substrates by the atomic layer epitaxy (ALE) technique using diethylzinc (DEZn) and H2O as reactant gases at substrate temperatures of 170 and 400 °C. In order to investigate the effect of annealing treatment on the optical properties of ZnO films, the films have been annealed at various annealing temperatures after deposition. After the annealing treatment the optical properties of ZnO thin films were characterized by photoluminescence (PL). A strong free exciton emission with a weak defect-band emission in the visible region was observed at room temperature (RT). Full width at half-maximum (FWHM) measurement of the free exciton emission of 67.54 meV have been made at room temperature. Based on the temperature dependence of the PL spectra in the temperature range from 10 K to room temperature it has been shown that the crystalline quality of the films increases as the annealing temperature increases. Also it shows that the best result is obtained when the sample is deposited at 170 °C and annealed at 1000 °C.


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