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Textured ZnO Thin Films on Sapphire Produced by Chemical Solution Deposition

Highly textured zinc oxide films were produced on basal plane sapphire substrates by chemical solution deposition. Films with oriented growth were achieved by spin coating a 0.75 M precursor solution of zinc acetate dihydrate and ethanolamine in 2-methoxyethanol, heated at 300 °C/10 min, then at 500 °C/5 h, and finally at 850 °C/12 h. Films were characterized with x-ray diffraction (XRD) and scanning and transmission electron microscopy. The films exhibited only the ZnO line in XRD diagrams, proving a very well-developed out-of-plane texture. At temperatures above 700 °C the ZnAl2O4 spinel was observed, which formed as a reaction layer between sapphire and ZnO. Few specimens produced both in-plane and out-of-plane oriented growth of ZnO on basal plane sapphire. It was hypothesized that the substrate miscut, uncontrolled for the current experiments, could be the cause of the infrequent growth of epitaxial films.

There has been increased interest in high quality ZnO films for use in a diverse range of applications such as in high frequency surface acoustic wave filters, buffer layers for GaN growth, transparent and conductive electrodes, and solid state lasers. In the present paper, ZnO films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350–450°C. X-ray diffraction and electron microscopy results indicate that the ZnO films are epitaxially grown on Al2O3 surface with the plane parallel to the surface. Cross-sectional high resolution-transmission electron microscopy imaging of the as-grown film shows that the interface is semi-coherent and atomically sharp, with misfit dislocations relieving the misfit strain between ZnO and sapphire. In order to check the thermal stability of the as-grown ZnO films, annealing in an O2+N2 ambience at 850°C for 30 min was performed. The annealed films showed improved crystallinity. At the same time, limited reaction between ZnO and sapphire occurred, resulting in the formation of a 15–20 nm thick spinel layer at the interface.


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