Influence of compositional variations of quaternary barrier layers on the optical properties of an InGaN SQW

The optical properties of c-plane oriented group III-nitride layers were investigated using spatially and spectrally-resolved cathodoluminescence (CL) at liquid helium temperature. The characterized set of samples was grown on sapphire substrates with Si-doped GaN buffer layers. A nominally 3 nm thick InGaN SQW was embedded into quaternary AlGaInN barriers of varying In content due to different TMIn-fluxes ranging from 3 sccm up to 50 sccm during the pulsed MOVPE growth and finally capped by a p-doped GaN layer. In all samples the NBE emission exhibits an inhomogeneous distribution with an emission line shifted to shorter wavelengths (354.4 nm). In absolute contrast to this, with increasing In content the CL from the quaternary layers is shifted from 330 nm to the spectral position of the broad DAP emission band (380 nm). For lower In content distinct luminescence contributions from the InGaN SQW which are evenly distributed between 440 nm and 480 nm were found, whereas with an increase of the In content in the barriers the emission behavior of the InGaN SQW merges into a monomodal distribution with an emission band centered at about 445 nm causing a reduction of FWHM from 333 meV down to 140 meV.