The wide spread of studied material has led to some uncertainty in phonon frequencies, especially of the
LO modes. Recently, however, the coupling to plasmons in doped material and stress induced effects due to
lattice mismatch with the substrate could be separated. A1(LO) lies close to Eg in sapphire and has been
confused in Raman experiments. In 2 mm GaN/sapphire (0001) [9] modes are within ± 1 cm-1 to values in
bulk GaN (Table 1):
Table