Lowering of stimulated emission threshold of zinc oxide by doping with thermally diffused aluminum supplied from sapphire substrate

Undoped zinc oxide (ZnO) thin films were grown on sapphire and zirconia [yttria-stabilized zirconia (YSZ)] substrate to investigate the effect of the annealing on luminescence properties. For the films grown on sapphire substrate, the electron concentration increased to 1.6×1019cm−3 as a result of in-diffusion of aluminum from the substrate during annealing and the threshold pumping power to excite stimulated emission decreased from 530kW∕mm2 (as-deposited film) to 32kW∕mm2 (film annealed at 900°C). In contrast, for ZnO films grown on YSZ substrate, electron concentration was unchanged by annealing and the stimulated emission threshold only slightly decreased after annealing. The current results indicated that electron concentration is an essential parameter for the stimulated emission threshold of ZnO.