Distribution of visible luminescence centers in hydrogen-doped ZnO
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ZnO crystals have been investigated by scanning cathodoluminescence microscopy and spectroscopy at 80 K following hydrogen incorporation by plasma exposure. The intensity of the ZnO near-band-edge (NBE) emission is greatly enhanced while the defect-related green emission is quenched following plasma treatment. These effects are attributed to the passivation of zinc vacancies by hydrogen. The green and yellow intensities and their intensity ratios to the NBE vary with excitation depth for both undoped and H-doped ZnO crystals. The intensities of the green and yellow emissions exhibit sublinear dependencies on electron beam excitation density while the NBE intensity increases linearly with the excitation density. These saturation effects with increasing excitation density must be taken into account when assessing defects in ZnO by luminescence characterization. I. INTRODUCTION
ZnO is a promising material for optoelectronic applications due to its large exciton binding energy (60 meV), low refractive index (1.9), and stability under intense ultraviolet exposure. Although it is relatively simple to grow high-quality single crystal n-type ZnO, fabricating stable p-type material is proving to be difficult despite an intense research effort. Along with native defects such as oxygen vacancy (VO), zinc interstitial (Zni), and zinc vacancy (VZn), hydrogen has been proposed to be an important source of n-type conductivity in ZnO.1 Firstprinciples investigations based on density functional theory have revealed that hydrogen in ZnO, unlike in other semiconductors, acts exclusively as a shallow donor impurity,1 which has recently been confirmed by experiments.2 Hydrogen can also have a pronounced effect on the optical and electronic properties of ZnO,2–4 especially it has a very high diffusivity in ZnO and is a common impurity usually present during the processing of ZnO crystals.5 Understanding the properties of hydrogen in ZnO is pivotal to lowering the background n-type conductivity levels and improving the possibility of producing p-type ZnO. It has been reported that the near-band-edge (NBE) luminescence efficiency in ZnO is significantly enhanced by the incorporation of hydrogen,6,7 but there is little systematic data available on its effects on defect-related visible emissions. Two commonly observed defect-related emissions in ZnO are: (i) ubiquitous green luminescence that has been assigned to native defects8,9 and (ii) yellow luminescence attributed to the presence of Al or Li impurities.10 The aim of the present study is to evaluate the role of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.383 2912
J. Mater. Res., Vol. 26, No. 23, Dec 14, 2011
http://journals.cambridge.org
Downloaded: 19 Mar 2015
hydrogen incorporation in the defect-related and NBE emissions in ZnO. Cathodoluminescence (CL) in a scanning electron microscope (SEM) is a powerful technique used extensively to study structural defects and the optical properties of semiconductor materials and devices
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