Carrier-confinement effects in nanocolumnar GaN∕Al_xGa_1−xN quantum disks grown by molecular-beam epitaxy

Carrier confinement effects in nanocolumnar Al_xGa_1−xN∕GaN multiple quantum disks have been studied by photoluminescence, as a function of the Al content and quantum disk thickness. Experimental emission energies are compared to theoretical calculations based on a one-dimensional Schrödinger-Poisson solver, including spontaneous and piezoelectric polarizations, surface potentials, and strain. An inhomogeneous biaxial (in-plane) strain distribution within the GaN quantum disks, pseudomorphically grown on strain-free Al_xGa_1−xN nanocolumns, results from a reduction of the accumulated elastic energy at the disk free surface (GaN-air boundary). This strain reduction annihilates partially the piezoelectric field, giving rise to a specific carrier confinement mechanism (strain confinement), that depends on the disk thickness. This strain confinement mechanism is the origin of the luminescence quenching in very thin GaN quantum disks, as well as the main source of the emission linewidth broadening.