Red and green emissions are observed from P ion implanted ZnO. Red emission at ~680 nm (1.82 eV) is associated with the donor-acceptor pair (DAP) transition, where the corresponding donor and acceptor are interstitial zinc (Zn<sub>i</sub>) and interstitial oxygen (O<sub>i</sub>), respectively. Green emission at ~ 516 nm (2.40 eV) is associated with the transition between the conduction band and antisite oxygen (O<sub>Zn</sub>). Green emission at ~516nm (2.403 eV) was observed for ZnO annealed at 800 oC under ambient oxygen, whereas, it was not visible when it was annealed in ambient nitrogen. Hence, the green emission is most likely not related to oxygen vacancies on ZnO sample, which might be related to the cleanliness of ZnO surface, a detailed study is in progress. The observed micro-strain is larger for N ion implanted ZnO than that for P ion implanted ZnO. It is attributed to the larger straggle of N ion implanted ZnO than that of P ion implanted ZnO. Similar phenomenon is also observed in Be and Mg ion implanted GaN.
Homoepitaxial and heteroepitaxial ZnO films were grown by plasma-assisted molecular beam epitaxy (P-MBE). Homoepitaxial ZnO layers were grown on an O-face melt-grown ZnO (0001) substrate. Heteroepitaxial ZnO layers were grown on an epitaxial GaN template predeposited by metalorganic chemical vapor deposition on a c-plane sapphire substrate. There exists a residual strain in the heteroepitaxial ZnO, which is ε = -0.25%. Low-intensity excitation PL spectra of ZnO epilayers excited by a He-Cd laser exhibit only bound-exciton emission with phonon replicas. The quality of ZnO epilayers is better than that of ZnO substrate. However, under high-intensity excitation by a N2 laser, the emission due to exciton-exciton collisions dominates the PL spectrum from heteroepitaxial ZnO layer but is not observed from homoepitaxial ZnO layer.