Infrared Spectroscopy of Hydrogen in ZnO
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Infrared Spectroscopy of Hydrogen in ZnO M.D. McCluskey and S.J. Jokela Department of Physics, Washington State University, Pullman, WA 99164-2814, U.S.A. ABSTRACT Zinc oxide (ZnO) has shown great promise as a wide band gap semiconductor with optical, electronic, and mechanical applications. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen’s usual role as a passivating impurity. The structures of such hydrogen complexes, however, have not been determined. To address this question, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H2) or deuterium (D2) gas. Using infrared (IR) spectroscopy, we have observed O-H and O-D stretch modes at 3326.3 cm-1 and 2470.3 cm-1 respectively, at a sample temperature of 14 K. These frequencies are in good agreement with the theoretical predictions for hydrogen and deuterium in an antibonding configuration, although the bond-centered configuration cannot be ruled out. The IR-active hydrogen complexes are unstable, however, with a dissocation barrier on the order of 1 eV.
INTRODUCTION Zinc oxide (ZnO) is a wide-bandgap semiconductor that has attracted tremendous interest as a blue light emitting material, a buffer layer for GaN-based devices [1], and a transparent conductor [2]. The stability of excitons in ZnO results in a very high quantum efficiency at temperatures of 300 K and higher, making it an ideal active material for the emission of blue to UV light in high-temperature environments. Theoretical work has predicted ferromagnetism above room temperature for Mn-doped ZnO, an important requirement for spintronic devices [3]. In addition to these device applications, ZnO has a potential application as a scintillator detector, due to the high cross section for nuclear reactions between 64Zn and fast neutrons [4]. As-grown ZnO is nearly always n type. Recent theoretical work has demonstrated that hydrogen is a shallow donor in ZnO [5], raising the possibility that hydrogen donors may be introduced into the bulk during growth or processing [6,7]. Experimental results on muonium implanted into ZnO [8] and electron-nuclear resonance measurements on lightly doped, n-type ZnO [9] have provided evidence that hydrogen is indeed a shallow donor. In order to determine the microscopic structure of hydrogen donors, in previous work, we used infrared (IR) spectroscopy to measure the local vibrational modes (LVMs) arising from these complexes [10]. By comparing with ab initio calculations [5], it was proposed that the O-H complex has an antibonding orientation. However, the bond-centered orientation could not be ruled out. Two possible models for the O-H complex are shown in Fig. 1. We have also performed polarized IR spectroscopy at room temperature [11]. These measurements indicate that the dipole of the O-H complex lies at an angle of approximately 112° to the c axis of wurtzite ZnO. No dipoles were observed that were oriented parallel to the c ax
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