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 $\text{x=0–1,}$ of the change in infrared reflectivity spectra of ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers deposited on 6H–SiC substrates. We have found two different transverse $E_1(\mathrm{TO})$ 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.