Interface phonon modes in the [AlN/GaN]20 and [Al0.35Ga0.65N/Al0.55Ga0.45N]20 2D multi-quantum well structures
The observation of interface (IF) phonon modes in the recorded Raman spectra of c-plane oriented [AlN/GaN]20 and [Al0.35Ga0.65N/Al0.55Ga0.45N]20 multi-quantum well (MQW) structures grown via plasma assisted molecular beam epitaxy is reported. The nominal shift in the IF phonon mode of E1 symmetry for [Al0.35Ga0.65N/Al0.55Ga0.45N]20 compared to that of the [AlN/GaN]20 MQW structure is understood on the basis of change in dielectric constants (εm) of the surrounding medium. The presence of buffer layers in [Al0.35Ga0.65N/Al0.55Ga0.45N]20 MQW over a sapphire substrate is also understood by characterizing the IF phonon mode of A1 symmetry. The observed IF phonon modes in the spectra are attributed to the relaxation of Raman selection rules away from the Brillouin zone centre because of the breakdown of the translational symmetry of surface potential due to the presence of the periodic interfaces and surface modulations in the superlattice structures of MQWs. The corresponding required edge lengths (L) of 2D plates, for the observation of the breakdown of the surface potential, are computed from the simulated dispersion relation curve of IF modes. The integral multiples of uniformly distributed platelets, originated due to the horizontal uneven irregularities on the surface of superlattices, are matched with the calculated L values.