Investigation of optically active E1 transversal optic phonon modes in AlxGa1-xN layers deposited on 6H-SiC substrates using infrared reflectance

We report an investigation, performed in the full composition range x = 0-1, of the change in infrared reflectivity spectra of AlxGa1-xN layers deposited on 6H-SiC substrates. We have found two different transverse E-1 (TO) phonon frequencies that can be assigned to AlN-like and GaN-like modes. The composition dependences of these frequencies can be well approximated by linear functions and the oscillator strengths scale like the corresponding Al and Ga mole fractions, respectively. On a purely experimental basis, this establishes evidence of a two-mode behavior for this controversial alloy system. The frequencies of the impurity mode of Ga in AlN (622 cm(-1)) and of the impurity mode of Al in GaN (643 cm(-1)) were determined. (C) 1998 American Institute of Physics. [S0003-6951(98)00639-1].