Novel configuration of Mg-H complexes in GaN
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Novel configuration of Mg-H complexes in GaN Sukit Limpijumnong∗ , John E. Northrup, and Chris G. Van de Walle Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304. ABSTRACT We describe a novel configuration of the Mg-H complex in GaN, which is not the lowestenergy configuration at T =0 but is stabilized at elevated temperatures by the large entropy associated with a set of low-energy rotational excitations. We focus on a comparison with two types of experimental results: (1) vibrational spectroscopy using polarized light [B. Clerjaud et al., Phys. Rev. B 61, 8328 (2000)]; and (2) ion-channeling [W. Wampler et al., J. Appl. Phys. 90, 108 (2001)]. New results on the predicted quantum-mechanical delocalization of the hydrogen atom and on the effect of replacing hydrogen with deuterium are reported. INTRODUCTION Continued progress in nitride-based devices has been hampered by the limited carrier concentrations in p-type material. While various alternative dopants (such as Be) [1, 2, 3] are being explored, most efforts are still focused on improving the doping efficiency with Mg, the most widely used acceptor. Hydrogen is known to play an important role in p-type doping, and is abundantly present in the growth environment of many growth techniques used for GaN. Hydrogen passivates the acceptors during growth, rendering the as-grown material highly resistive. A post-growth anneal dissociates the Mg-H complexes, and removes the hydrogen from the vicinity of the acceptors, resulting in p-type conductivity. This process can be experimentally investigated by monitoring the local vibrational modes associated with the Mg-H complex. The stretching mode of this complex occurs at 3125 cm−1 , and disappears during the activation process [4]. Despite the established role of hydrogen in the doping process, the details of the microscopic configuration of the Mg-H complex have proved elusive. A frequency of 3125 cm−1 is too high to be associated with a Mg-H bond. Previous computational studies [5, 6] showed that H prefers to bond with a neighboring N atom rather than with the Mg acceptor itself. The vibrational frequencies were calculated for various configurations, and it was found that hydrogen in an antibonding position (ABN ) provided the best agreement with the experimental frequency, whereas the bond-center (BC) configuration (H inserted in the bond between Mg and N) is energetically less favorable and yields a much higher vibrational frequency (by several 100 cm−1 ). Since then, discussions of experimental results on Mg-H complexes [7, 8] have focused on the ABN configuration. While those experiments have helped to rule out the BC configuration, they did not confirm the ABN configuration, and in fact raised some questions about that assignment. Clerjaud et al. [7] performed multitransmission infrared spectroscopy with polarized light. They found that the N-H bond is not aligned along the c-axis. In fact, they found that the N-H bond (or, at least the electric dipole induced by the mode) forms an angle of 130±5◦ with the c-axi
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