Localized Vibrational Modes in the Dilute Nitrides
Because of the small mass of N (approximately 1/5 that of Ga or As) and the strength of Ga-N bond length, a highly localized threefold-degenerate vibrational mode exists around the N atom when it substitutes for an As atom in a GaAs host lattice. Localized vibrational modes spectroscopy is a useful technique for studying the local bonding of impurities and impurity complexes in semiconductors. We have studied the pressure dependence of the frequency of the localized vibrational modes (see figure) of substitutional nitrogen in the dilute nitride, GaNxAs1-x, within first-principles density functional theory, using a supercell approach . The theoretical pressure dependence of the frequency is found to be well fitted in the pressure range, 0≤ P ≤ 10 GPa, by ν(P) = ν0 + a1 P + a2 P2, with ν0 = 465 cm - 1, a1 = 8.92 cm-1GPa-1, and a2=-0.12 cm-1GPa-2, in good agreement with recent Raman spectroscopy measurements  . The same technique has been used to study the effect of the substitution of nearest-neighbor gallium atoms by indium  on the frequency of the localized vibrational mode of substitutional nitrogen in the dilute nitride, GaNxAs1-x. The splitting of the highly localized triply-degenerate mode (465 cm-1) into a low frequency singly-degenerate mode (447 cm-1) and a high frequency doubly-degenerate mode (485 cm-1) is obtained and compared with available Raman and FTIR spectroscopy measurements. The results are in good agreement with the experimental values. We are currently looking at the broadening of the localized vibrational mode in the dilute random alloy using a linear combination of isolated localized vibrational modes.
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