The key role of polarity in the growth process of (0001) nitrides

A direct method for the determination of the polarity of (0001) wurtzite GaN films is presented, using a combination of ion channeling and convergent beam electron diffraction experiments (CBDE). The samples were grown by metal organic chemical vapor deposition (MOCVD) on (0001) Al2O3. Depending on the nitridation conditions of the buffer, the samples are flat or rough. It is found that flat samples are Ga-terminated. The rough samples exhibit numerous pyramidal defects. In that case, a mixture of N- or Ga-terminated domains is put in evidence. These results unambiguously demonstrate that the nitride layer quality is closely related to the control of the polarity from the very first stages of the growth.