Enhanced nonradiative recombination in Al_xGa_1−xN-based quantum wells thinner than the critical layer thickness determined by X-ray diffraction

<jats:title>Abstract</jats:title> <jats:p>The optical properties of Al<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Ga<jats:sub>1−<jats:italic>x</jats:italic> </jats:sub>N-based quantum wells (QWs) with various thicknesses are investigated. When the Al<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Ga<jats:sub>1−<jats:italic>x</jats:italic> </jats:sub>N thickness in Al<jats:sub>0.8</jats:sub>Ga<jats:sub>0.2</jats:sub>N/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 Al<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Ga<jats:sub>1−<jats:italic>x</jats:italic> </jats:sub>N 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, Al<jats:sub> <jats:italic>y</jats:italic> </jats:sub>Ga<jats:sub>1−<jats:italic>y</jats:italic> </jats:sub>N (<jats:italic>y</jats:italic> > <jats:italic>x</jats:italic>) underlying layers are effective.</jats:p>