Enhanced nonradiative recombination in AlxGa1-xN-based quantum wells thinner than the critical layer thickness determined by X-ray diffraction

The optical properties of AlxGa1-xN-based quantum wells (QWs) with various thicknesses are investigated. When the AlxGa1-xN thickness in Al0.8Ga0.2N/AlN QWs exceeds 6 nm, the photoluminescence lifetime is drastically shortened even at cryogenic temperatures, which indicates that nonradiative recombination processes are enhanced. Interestingly, the thicknesses for the degradation of the optical properties of AlxGa1-xN on AlN (0001) are about two orders of magnitude thinner than the critical layer thicknesses for lattice relaxation determined by a conventional X-ray diffraction method. To avoid the degradation of the QW optical properties, AlyGa1-yN (y > x) underlying layers are effective.