Acceptor Dopants in Bulk and Nanoscale ZnO
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Acceptor Dopants in Bulk and Nanoscale ZnO Matthew D. McCluskey,1 Marianne C. Tarun,1 and Samuel T. Teklemichael1 1 Department of Physics and Astronomy, Washington State University Pullman, WA 99164-2814, U.S.A. ABSTRACT Zinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nanolasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, recent theoretical and experimental work suggests that nitrogen is a deep acceptor and will not lead to p-type conductivity. In nitrogen-doped samples, a red photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the 1.4-1.5 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. In our model, the 0.46 eV level is shallow with respect to the surface valence band, raising the possibility of surface hole conduction. INTRODUCTION ZnO is an electronic material with desirable properties for a range of energy applications.1 ZnO is a wide band gap (3.4 eV) semiconductor that emits light in the near-UV region of the spectrum. The high efficiency of the emission, thanks in part to stable excitons at room temperature,2 makes ZnO a strong candidate for efficient solid-state white lighting. Reports of stimulated emission in ZnO nanowires3,4 and multicrystallite thin films5,6 suggest the feasibility of UV lasers made from this material. ZnO is already used as a transparent conductor7 in solar cells, a UV-absorbing component in sunscreens,8 and the active material in varistors.9 Researchers have also fabricated transparent transistors, invisible devices that could find widespread use in products such as liquid-crystal displays.10 Besides its fundamental optical and electrical properties, ZnO has other benefits that could make it a dominant material for energy applications. In contrast to GaN, large single crystals can be grown routinely.11 ZnO is relatively benign environmentally and is actually used as a dietary supplement in animal feed.12 From an economic perspective, the low cost of zinc versus indium provides an advantage over indium tin oxide for use as a
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