Investigation of the influence of InGaN underlying layers on the optical properties of InGaN quantum well structures

The optical properties of InGaN/InGaN multiple quantum wells (MQWs) with InGaN underlying layers (UL) on sapphire substrates have been comprehensively investigated by highly spatially and spectrally resolved cathodoluminescence microscopy (CL) at He temperature and by temperature dependent photoluminescence spectroscopy (PL). The Indium content of the UL was systematically varied from 1% to 4% between the samples. SEM and AFM measurements were used to examine the sample morphology. The evaluation of the temperature dependent PL measurements shows a rising activation energy of nonradiative centers with increasing In content. CL investigations of the sample surface show elongated structures in the integral intensity images and peak wavelength images, which becomes more spot-like with rising In content. The peak energy of the MQW luminescence shows a blueshift with rising In content which may be caused by a possible reduction of the quantum confined Stark effect (QCSE). At the same time the FWHM of the MQW emission is reduced from 27 meV to about 18 meV when introducing ULs.