Resonant Raman-Active Localized Vibrational Modes In Al(Y)Ga(1-Y)N(X)As(1-X) Alloys: Experiment And First-Principles Calculations

The localized vibrational modes associated with substitutional aluminium and nitrogen atoms in Al(y)Ga(1-y)N(x)As(1-x) have been studied within first-principles density functional theory using a supercell approach. Localized vibrational modes related to N-Al(m)Ga(4-m) (1 <= m <= 4) complexes have been identified, which reveal the formation of N-Al(4) units well above random abundance, in qualitative agreement with a large calculated value (391 meV) of the Al-N bond formation energy. We determine the resonant Raman-active modes from the selection rule obtained by calculating the electron-phonon coupling strength and optical transition matrix elements and compare them with resonant Raman spectroscopy measurements. The localized modes from Raman scattering measurements with frequencies around 325, 385, 400, 450, 500, and 540 cm(-1) are found to be in good agreement with the calculated modes (326, 364, 384, 410, 456, 507, and 556 cm(-1)). The modes are classified as follows: the two modes at 326 and 556 cm(-1) belong to the N-AlGa(3) configuration; there are three modes which belong to N-Al(2)Ga(2) with frequencies at 326, 364, and 507 cm(-1); the N-Al(3)Ga configuration gives rise to modes whose frequencies are 384 and 456 cm(-1); and the mode at a frequency of 410 cm(-1) belongs to the N-Al(4) complex. The comparison of line intensities from samples before and after rapid thermal annealing allows us to experimentally distinguish vibrational modes associated with different clusters, in agreement with the theoretical assignments.