  Low energy optical modes of MBE-grown In1-xGaxN thin films with different
values of x are investigated using Raman spectroscopy. We also studied the
influence of Raman excitation frequency using red and green lasers on
scattering intensity of various Raman modes. For those In1-xGaxN alloys whose
bandgap energy is close to the red laser, a huge enhancement in the intensities
of A1(LO) mode and its 2A1(LO) replica is observed when excited with red laser
as compared to the green laser excitation. We found that the energies of
longitudinal optical modes (A1 (LO) and 2A1 (LO)) vary nonlinearly unlike the
E2 mode with increasing Ga atomic fraction. A Raman mode ~ 540 cm-1 was
observed in all In1-xGaxN films with low energy red laser excitation but was
absent with green laser excitation. We attribute this mode to A1(TO) mode of
the underneath GaN buffer layer.
