Confinement of Optical Phonons Observed by Raman Scattering in GaN/AlN Multiple Quantum Disk Nanocolumns

We have studied phonon modes in GaN/AlN multiple quantum disk nanocolumns, which are self-organized on Al2O3 and Si substrates by rf-plasma-assisted molecular beam epitaxy, by means of Raman scattering. The Raman spectrum drastically changes with decreasing thickness of the quantum disks. In thick quantum disks, the broad peaks reflect the density of states of the pristine phonon modes in GaN and AlN. They are activated by the folding of the Brillouin zone owing to the multiple quantum disk structure. Several peaks of the confined, quasi-confined, and interface phonon modes are observed in thin quantum disks, which are peculiar modes in the superlattice structure. Moreover, the pristine optical phonons are entirely confined to one of the constituents in the case of very thin quantum disks, where the dielectric continuum model is not applicable.