Characteristics of Si and Mg doping in a-plane GaN grown on r-plane sapphire

Non-polar a-plane GaN without triangular pits was grown on r-plane sapphire substrates using metalorganic chemical vapor deposition (MOCVD) without a low-temperature GaN or a high-temperature AlN buffer layer. The root mean square (RMS) surface roughness of a-plane n-GaN and p-GaN are 0.681 and 1.686 nm, respectively, indicating the very good surface morphologies in comparison with that of conventional c-plane GaN epilayer. However, photoluminescence (PL) intensity of a-plane-doped GaN is lower than that of c-plane-doped GaN. It is believed that the degradation of optical efficiency is attributed to the crystal imperfections of a-plane GaN due to the intrinsic crystal structure as well as the additional strain of Si or Mg doping process. Additionally, the electron concentration of a-plane n-GaN is slightly lower than that of c-plane n-GaN, whereas the hole concentration of a-plane p-GaN is higher than that of c-plane p-GaN. It may be due to the lower activation energy of Mg acceptors of a-plane GaN than c-plane GaN as well as the difference of growth condition between c-plane and a-plane n-GaN.