Excellent optical quality versus strong grain boundary effect in a double-layer ZnO structure

ZnO samples with a double-layer structure and top nanorod arrays on the bottom film layer were grown by metal-organic chemical vapor deposition at a temperature range from 340 to 400 degrees C. The ZnO nanorods show excellent optical quality and no obvious defect related emission can be detected below 40 K except for I-6 line and the surface bound exciton emission. The free exciton emission and its phonon replicas dominate the near band edge emission between 40 and 295 K. For the film layer, the temperature-dependent Hall measurements showed that the conduction region is degenerate. In the conduction region, the carrier mobility is mainly limited by the grain boundary effect, which can be weakened by thermal annealing. The conduction mechanism in this region before and after annealing can be fitted by a uniform and a non-uniform conduction model, respectively. The results indicate that grain boundary effects strongly limit the mobility and consume large amounts of carriers by the trap states. Furthermore, we propose a qualitative model to explain the expansion of the conduction regions by annealing. It reveals a mechanism for the improvement of electrical properties of polycrystalline thin films by annealing treatments.