Doping properties of hydrogen in ZnO
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The doping properties and stability of hydrogen in zinc oxide (ZnO) crystals have been investigated by cathodoluminescence (CL) spectroscopy. Hydrogen incorporation was achieved by hydrogen plasma at 200 °C. The ZnO near-band-edge (NBE) peak is dramatically enhanced, while the green emission at 2.4 eV is quenched with increasing hydrogen incorporation. These effects are attributed to hydrogen passivating green luminescence centers, which are most likely negatively charged zinc vacancy defects. E-beam irradiation of H-doped ZnO crystals by an intense electron beam with lW power reverses the hydrogen doping process. This effect is ascribed to the dissociation of H-related defects, formation of “hidden” H2, and electromigration of H1 under the influence of the local trapped charge-induced electric field. These results highlight the potential to modify the local luminescent properties of ZnO by e-beam irradiation.
I. INTRODUCTION
Considerable research efforts have been focused on the investigation of zinc oxide (ZnO) as an alternative material to gallium nitride (GaN) in light-emitting devices because of its unique optoelectronic properties. These include tuneable emissions over a broad spectral range, high luminescence efficiency, large carrier mobility, and the largest exciton binding energy among II–VI compounds.1,2 Undoped ZnO typically exhibits n-type conductivity—a fact that, despite numerous theoretical and experimental studies, is still an issue of debate. Calculations based on density functional theory (DFT) by Van de Walle et al.3,4 indicated that interstitial hydrogen (Hi) and hydrogen trapped at an oxygen vacancy (HO) can act as two shallow donors, which can be a cause of n-type conductivity in ZnO. These results have been supported by experiments, which showed that the n-type conductivity can be enhanced via the incorporation of H from a H2 ambient.5,6 Further studies using Raman, infrared (IR) absorption and photoluminescence spectroscopy were able to identify two occupation sites: bondcentered and oxygen vacancy bound hydrogen with donor ionization energies of 53 and 47 meV, respectively.7–9 As a reactive and common impurity, understanding the role of hydrogen in ZnO is not only of fundamental interest but also of technological importance. It has been reported that the near-band-edge (NBE) luminescence efficiency in ZnO bulk and nanostructures can be significantly enhanced at the expense of deep-level (DL) emissions after hydrogen incorporation.10–12 This enhancement of the NBE emission has been attributed to H shallow donors, which give rise to an additional re-
combination channel at the NBE and passivate competitive recombination pathways.12 Although the origins of DL emissions in ZnO are still not thoroughly understood, the quenching of DL emissions suggests that hydrogen interacts with native defects in some way. There have been reports of a stable hydrogen-related complex formed by zinc vacancy (VZn) acceptor and two H atoms.13 In this paper, H is incorporated into ZnO crystals and the effects of ele
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