ZnO nanorods by hydrothermal method for ZnO/GaN LEDs

Zinc oxide (ZnO) has been attractive for optoelectronics application due to its wide band gap (E_g=3.37 eV) and large exciton binding energy (~60 meV) characteristics. However, p-type doping of ZnO is still controversial and problematic. Therefore, there is considerable interest in fabrication of n-ZnO/p-GaN heterojunction LEDs. In this work, we fabricated the LEDs consisting of n-ZnO nanorod arrays on p-GaN substrate. ZnO nanorod arrays were fabricated by a hydrothermal method. Hydrothermal methods have the advantage that they are simple, inexpensive and environmentally friendly. However, nanorods fabricated by hydrothermal methods typically have large numbers of defects due to the low growth temperature (90degC). The defect related photoluminescence (PL) is significantly affected by annealing, and under suitable conditions it can be entirely eliminated. As-grown nanorods exhibit UV emission and large yellow defect emission which is likely due to the presence of OH groups. The PL spectra can be significantly improved by annealing the nanorods at 200degC under Ar flow. Therefore we investigated the influence of argon annealing of ZnO nanorods on the performance of ZnO/GaN LEDs, as well as the influence of annealing environments. Devices with ZnO rod length ~ 250 nm were fabricated and the results obtained are discussed.