Polarity-controlled growth of high-quality ZnO epitaxial films

Polarity selection of ZnO films grown on both polar and non-polar substrates was investigated by using molecular beam epitaxy. It was revealed that surface structure and spontaneous polarization of substrates are key factors in polarity determination of subsequent ZnO films. Zn-polar ZnO film was obtained on Ga-terminated Ga-polar GaN/sapphire template while N-terminated N-polar one is essential for O-polar ZnO epitaxy. This polarity control technique can also be applied to centrally-symmetric substrates. In this case, the formation of a uniform surface polarity on substrate plays a crucial role in the unipolar ZnO film growth. As an illustrative example, 0-polar and Zn-polar films were steadily prepared on sapphire (0001) using different substrate pretreatments and nitridation temperatures. Clear 3x3 and 4x4 surface reconstructions were observed on these samples, respectively.