Resonant Raman characterization of InAlGaN/GaN heterostructures

InAlGaN/GaN heterostructures and thin films with In composition ranging from 0.03 to 0.1 are characterized by means of Raman scattering excited at various energies in the ultra violet range, tuning the laser excitation energy through the band gap of In_xAl_yGa_1-x-yN. It is shown that the addition of In to the Al_yGa_1-yN alloy diminishes considerably the vibration energy of the A₁(LO) phonon mode. The phonon line is asymmetric on the low energy side, and the asymmetry increases with In content, while the main peak shifts to lower energies. A shift of the phonon energy has also been observed when the excitation energy is close to the absorption edge of the In_xAl_yGa_1-x-yN layer. The nature of this shift is discussed in relation with intrinsic and extrinsic inhomogeneities in the quaternary alloy.