Luminescence and EPR Study of Lithium-Diffused ZnO Crystals
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Luminescence and EPR Study of Lithium-Diffused ZnO Crystals N. Y. Garces, Lijun Wang, M. M. Chirila, L. E. Halliburton, and N. C. Giles Physics Department, West Virginia University Morgantown, WV 26506-6315, U.S.A. ABSTRACT Zinc oxide (ZnO) crystals grown by the seeded chemical vapor transport method have been studied using photoluminescence (PL), thermoluminescence (TL), and electron paramagnetic resonance (EPR) techniques. Lithium acceptors were diffused into the crystals during anneals in LiF powder at temperatures in the 750 to 850°C range. After a lithium diffusion, EPR was used 3+ 3+ 2+ to monitor neutral lithium acceptors and neutral shallow donors, as well as Ni , Fe , and Cu impurities unintentionally present. Excitonic and deep-level PL emissions were also monitored before and after these diffusions. Two broad overlapping TL emission bands were observed at 117 and 145 K when a Li-diffused crystal was illuminated at 77 K with 325-nm light and then rapidly warmed to room temperature. The two TL bands have the same spectral dependence (the peak in wavelength is 540 nm when the intensity of the light reaches a maximum). These 2+ 2+ “glow” peaks occur when electrons are thermally released from Ni and Fe ions and recombine with holes at neutral lithium acceptors.
INTRODUCTION Zinc oxide is an emerging candidate for light emission applications in the near ultraviolet [1]. However, production of the p-type material that is needed for efficient current-injection devices is proving to be a major challenge. At the present time, nitrogen appears to be the best choice of p-type dopant for these devices, but the systematic incorporation of large concentrations of active nitrogen acceptors during growth remains an unsolved problem. On the other hand, lithium is easily incorporated on zinc sites in ZnO [2]. It is, however, a deep acceptor because in its neutral charge state the hole is localized on only one of the four oxygen neighbors and is accompanied by significant lattice relaxation. Lithium is not the ultimate acceptor for ZnO devices, but it is expected to serve a useful role in providing semi-insulating substrates for homoepitaxial film growth. More generally, we note that at this present stage of development, basic studies of all the possible acceptors in ZnO are needed so that a complete understanding of their optical and electrical properties can be firmly established. In the present paper, we focus on the behavior of donors and acceptors in high quality bulk ZnO crystals following the post-growth thermal diffusion of lithium acceptors into the material. EXPERIMENTAL DETAILS The ZnO crystals used in the present study were grown at Eagle-Picher (Miami, OK) by the seeded chemical vapor transport method. Our samples were thin c plates with dimensions of 4 x 3 3 x 0.5 mm . During the lithium diffusion treatments, the sample was “buried” in LiF powder in a small ceramic boat. The boat was placed in a small horizontal furnace open to the surrounding air and heated to temperatures between 750 and 850°C for ti
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