Effects of grid bias on $ZnO/α-Al2O3(0001)$ heteroepitaxy

We studied grid-bias effects on the microstructure of epitaxial $ZnO/α-Al2O3(0001)$ films using synchrotron x-ray scattering and atomic force microscopy. ZnO films were grown by radio frequency magnetron sputtering, applying positive or negative bias on a grid that was set-up 2 cm from the substrate. We found that the epitaxial quality of the ZnO films was greatly dependent on the bias voltage; as the bias increased from 0 to +100 V, the mosaic distribution of the ZnO(0002) planes and the surface roughness continually improved. But a further increase in bias led to a falling off of the epitaxial quality. Meanwhile, negative biases deteriorated the epitaxial quality significantly. We attribute the best epitaxial quality at an optimum bias to the great flux reduction of energetic oxygen anions bombarding the film surface, thereby significantly suppressing the resputtering phenomenon. We suggest that a biased grid may be very useful for enhancing the epitaxial quality of ZnO films in sputter deposition.