Computational study of phonon modes in short-period AlN/GaN superlattices

The phonons and infrared absorption of short period [0001] superlattices (AlN)ₙ(GaN)_8-n, with $n=3,4,5$ were calculated by using the density-functional perturbation theory in the local-density approximation. The nature of the modes is discussed in terms of quantum confinement and interface localized modes. The spectrum for e-symmetry modes shows two TO peaks, of which the lowest corresponds to two closely spaced modes confined each near one half of the GaN part of the cell, and the higher one to two closely spaced AlN like modes with one mode localized at the interface showing the strongest oscillator strength and a bulklike LO-TO splitting. For a₁ symmetry, in contrast, a GaN vibration type mode localized near the interface shows the strongest LO-TO splitting and oscillator strength, while two higher modes are found to be only weakly localized in the AlN part of the cell. Lattice constant relaxation of free-standing superlattices leads to a decreasing lattice constant with increasing number of AlN layers and hence an overall increase in phonon frequencies. Qualitative agreement with experimental data is found for the e symmetry but quantitative discrepancies exist and are interpreted as evidence of a different strain state in the experiment.