Hydrogen is an important impurity in ZnO, and it is believed to act as a shallow donor and to passivate acceptors in the material. H is readily associated with other defects in ZnO, forming complexes with characteristic localized vibrational modes (LVMs). The H-related peaks observed in the IR absorption spectra of ZnO is thus highly dependent on the concentration of other impurities and native defects.
In this work, H-related defects in hydrothermally (HT) grown ZnO single crystals have been investigated by Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and four point resistivity measurements.
Due to a high concentration of Li acceptors, the LVM of a OH-Li complex dominates the IR spectra of the as-grown samples. Several other H-related peaks are however also observed. The results presented in this work indicate that the 3577 cm-1 peak associated to the OH-Li defect exhibits a complex annealing dependency, which may be explained by a process involving diffusion and recapture of H. Also, dissociation of the defect occurs at substantially lower temperatures than the previously reported thermal stability of 1200 °C. The absorption cross section of the OH-Li signal has been estimated to be 1.27 x 10^(-17) cm.
A group of IR absorption peaks at 4216, 4240 and 4246 cm-1 have also beenobserved in the IR spectra of as-grown samples. By comparison with SIMS measurements, the previous identification of these peaks as internal electronic transitions of substitutional Ni_(Zn) impurities has been verified. The absorption cross section of the peaks has been found to be 2.91 x 10^(-17) cm.
Several other H-related peaks appeared in the IR spectra recorded after annealing of the samples in H2 and/or D2 atmospheres, caused by diffusion of H/D into the crystals. Two IR absorption peaks at 3303 and 3321 cm-1 were assigned to the LVMs of a defect complex labeled H-X, consisting of two O-H bonds associated to an unknown impurity atom. The 3321 cm-1 mode is oriented along the c-axis of ZnO, while the 3303 cm-1 mode is oriented at an angle with the c-axis. The H-X complex is thermally stable up to ~ 600 °C and the activation energy fordissociation was estimated to be 2.8 eV.
An IR absorption peak at 2783 cm-1 was also observed after hydrogenation. This peak was assigned to the LVM of a defect complex labeled H-Y, involving a single O-H bond oriented at an angle with the c-axis. Also, a pair of peaks at 3347 and 3374 cm-1 were observed in both as-grown and hydrogenated samples after annealing at ~ 500 °C. These peaks were assigned to the LVMs of two O-H bonds associated to the same defect, labeled H-Z. The H-X defect is to our knowledge not reported in the literature. The H-Y and H-Z defects have previously been identified as OH-Ni_(Zn) and (OH)2-Cu_(Zn) complexes, respectively. However, the SIMS and IR absorption data presented in this work indicate that both these assignments should be revisited.
The H2 and/or D2 gas anneals were also followed by a substantial drop in resistivity, which was found to be stable after annealing at 200 °C. The resistivity however increased markedly after subsequent annealing at higher temperatures (>500 °C). The increased carrier density after hydrogenation is presumably causedby a combination of thermally stable H donors like HO and H passivation of acceptors present in the as grown samples, forming neutral complexes like OH-LiZn.