Composition dependent determination of band offsets in ZnCdSe/ZnSe and ZnSe/ZnSSe SQW by optical means

MBE-grown ZnCdSe/ZnSe and MOVPE-grown ZnSe/ZnSSe single quantum wells (SQW) were investigated by photoreflectance (PR) and photoluminescence excitation (PLE) spectroscopy. For various compositions a set of quantum well structures with different well thicknesses was grown. The PR spectra exhibit sharp signatures of which the transition energies have been accurately determined by a line shape fit. The peaks in the PLE spectra and correspondin,gly the PR signatures can be attributed to ground and excited excitonic states by comparison with the excitonic absorption model of Toyozawa. With an envelope function formalism including all strain effects a fitting procedure was developed which allows the evaluation of the band alignment. This procedure overcomes the problems of the uncertainties in the hole masses by simultaneously fitting all the transitions observed at one composition by varying also the hole masses. It is the accurate knowledge of the structural parameters of the samples and the precise determination of the transition energy which gives the composition dependent offset a high reliability.