Stability, diffusivity, and vibrational properties of monatomic and molecular hydrogen in wurtzite GaN

The stability and diffusivity of monatomic (H+ and H-) and molecular (H-2) hydrogen in wurtzite GaN are studied via first-principles calculations. Stable configurations are identified and the formation energies are studied as a function of the Fermi level. Diffusion barriers in the direction parallel to [0001] (parallel toc) and perpendicular to [0001] (perpendicular toc) are calculated. For H+ the diffusion barriers are slightly higher than in the zinc-blende phase and modestly anisotropic (0.85 eV perpendicular toc, 0.94 eV parallel toc). For H- the diffusion barriers are lower than in zinc-blende GaN, with values of 1.99 eV and 2.17 eV for parallel toc and perpendicular toc, respectively. The diffusion barriers for H-2 are relatively high (2.0 eV for parallel toc and 2.2 eV for perpendicular toc), and we propose that diffusion of H-2 is more likely to proceed by dissociation followed by diffusion of monatomic H+. The vibrational frequency of the molecule in wurtzite GaN is redshifted from the free molecule; for wurtzite GaN the frequency is 129 cm(-1) lower than in free H-2. Finally, we find that the H-2(*) complex is only slightly higher in energy than interstitial H-2, and we calculate its vibrational frequencies.