Hydrogen-related excitons and their excited-state transitions in ZnO

The role of hydrogen in the photoluminescence (PL) of ZnO was investigated using four different types of bulk ZnO single crystal, with varying concentrations of unintentional hydrogen donor and Group I acceptor impurities. Photoluminescence spectra were measured at 3 K, with emission energies determined to +/-50 mu eV, before and after separate annealing in O-2, N-2, and H-2 atmospheres. Using this approach, several new hydrogen-related neutral-donor-bound excitons, and their corresponding B exciton, ionized donor, and two electron satellite (TES) excited state transitions were identified and their properties further investigated using hydrogen and deuterium ion implantation. The commonly observed I-4 (3.36272 eV) emission due to excitons bound to multicoordinated hydrogen inside an oxygen vacancy (HO), that is present in most ZnO material, was noticeably absent in hydrothermally grown (HT) ZnO and instead was replaced by a doublet of two closely lying recombination lines I-4b,I-c (3.36219, 3.36237 eV) due to a hydrogen-related donor with a binding energy (ED) of 47.7 meV. A new and usually dominant recombination line I6-H (3.36085 eV) due to a different hydrogen-related defect complex with an ED of 49.5 meV was also identified in HT ZnO. Here, I-4b,I-c and I6-H were stable up to approximately 400 and 600 degrees C, respectively, indicating that they are likely to contribute to the unintentional n-type conductivity of ZnO. Another doublet I-5 (3.36137, 3.36148 eV) was identified in hydrogenated HT ZnO single crystals with low Li concentrations, and this was associated with a defect complex with an ED of 49.1 meV. A broad near band edge (NBE) emission centered at 3.366 eV was associated with excitons bound to subsurface hydrogen. We further demonstrate that hydrogen incorporates on different lattice sites for different annealing conditions and show that the new features I-4b,I-c, I6-H, and I-5 most likely originate from the lithium-hydrogen defect complexes LiZn-HO, AlZn-HO-LiZn, and VZn-H3,4, respectively.