Dominant free exciton emission in ZnO nanorods.

Nanostructured ZnO is considered to be a promising building block in the design of nanoscale optoelectronic devices. It usually shows dominant donor-bound exciton (DX) emission at low temperatures. In this study, ZnO nanorods with high crystallinity and optical quality were grown by metal-organic chemical vapor deposition on a-plane sapphire (1120) substrates. Dominant free exciton (FX) emission at a low temperature (14 K) was observed by photoluminescence spectroscopy. It was attributed to both the enhancement of the FX emission induced by the high crystalline quality of the nanorods and the suppression of the DX emission induced by hydrogen out-diffusion. The latter reason is believed to be more important from the analysis of the hydrogen distribution in the nanorods through photoluminescence spectroscopy and secondary ion mass spectrometry. A slow cooling process during the deposition is suggested to result in a better optical quality. These results can promote our understanding of the optical properties of ZnO nanostructures.