Polarity control of ZnO films grown with high temperature N-polar GaN intermediate layers by plasma-assisted molecular beam epitaxy

ZnO epilayers were grown on nitridated c-Al2O3 substrates by plasma-assisted molecular beam epitaxy and the effect of the deposition temperature for GaN buffer and/or intermediate layers on the polarity of ZnO epilayer was investigated. First, it was found that the polarity of the ZnO epilayers grown on low temperature-grown GaN buffer and/or intermediate layers was uncertain and it became often +c polarity, i.e., Zn-polarity. This seems strange because the polarity of the GaN-underlying layer was -c polarity, i.e., N-polarity, and then the polarity of the under layer was not kept unchanged by the following epilayer. On the other hand, when the growth temperature of the GaN buffer/intermediate layer was increased above 850 degreesC, it was found that the polarity of the GaN layer was followed by the ZnO epilayers; the polarity became -c polarity, i.e., O-polarity. This reason was estimated by the effect of surface oxides on GaN which were formed just before the deposition of ZnO buffer layer on it.