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We report on the atypical temperature evolution of the phonon modes in undoped, pseudomorphic GaAs1−xNx layers, 0 ≤ x ≤ 4.2%, on GaAs substrates. The modes were characterized by micro-Raman backscattering for temperatures ranging from 90 to 430 K. The dominant Ga-As LO mode of GaAsN is dramatically decreased at 90 K, and intensifies with increasing temperature, in contrast to the expected temperature evolution of first order phonon modes. The TO mode is not Raman active in the zinc blende lattice in the backscattering geometry, but emerges as a result of the symmetry lowering in GaAsN. Both the disorder activated TO mode and the Ga-N local vibrational mode (LVM) are prominent at 90 K, and their intensities decrease with increasing temperature. Hence the relative intensity of the nitrogen induced phonon modes with respect to the dominant LO mode is a strong function of the sample temperature. These results provide evidence for a coupling of the Ga-As LO mode in GaAsN to electrons at low temperature. The coupling increases with increasing N incorporation and decreases with increasing temperature. We explain this observation with the formation of nitrogen nano-clusters which have a high local electron density. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) |
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