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First-Principles Study of Defects in GaN

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Final rept. Mar 2008-Feb 2009

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Based on first principles supercell approach, we have successfully initiated and continued our study of defects in AlN, GaN, diluted nitrides and related wide gap materials. We have also investigated and implemented an approach to calculate the x-ray absorption near edge structures XANES by using first principles pseudopotential in conjunction with ab initio multiple scattering codes. We have surpassed our original milestones in several aspects and already accomplished important results. a We have calculated native defects in AlN, namely VAl, VN, Ali, and Ni. Most of the results are in agreement with previous work by Stampfl and Van de Walle. Except for the Ni, where we have shown that Ni prefers the split-interstitial configuration. b Potential p-type dopants in AlN, Mg and Be, have been studied. We found that both dopants, when substituting for Al, give a rather deep level. This means both Mg and Be are not suitable p-type dopants in AlN. c We have calculated the Ga Frenkel pairs interstitial Ga and gallium vacancy complexes in GaN. We studied both the stability of the pair at different separations and the barriers for the pair to formdisintegrate. Our results show that the pair at 4 angstrom separation is stable and has a binding energy of about 3.3 eV. However, the barrier for the pair to recombine is only 0.2 eV, making it unlikely to be stable at room temperature. d In dilute nitrides, the nitrogen atoms are highly strained. Our calculations show that they prefer to bind with small impurity atoms such as C or Si in the form of CN and SiN molecule, substituting on the As site. The CNAs gives a vibration frequency in good agreement with the local vibration mode observed in an infrared spectroscopy measurement. The SiNAs has low formation energy and could explain the mutual passivation of shallow donor Si and isovalent N in dilute nitride.

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  • Solid State Physics

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