Localization of excitons by potential fluctuations and its effect on the Stokes shift in InGaP/InP quantum confined heterostructures

We present investigations of the temperature evolution of the photoluminescence (PL) and optical absorption spectra of a series of tensile-strained InGaP/InP multiple quantum wells (MQW) samples. As in previous work on compressive-strained InAsP/InP MQW, the results support the assignment of the low temperature PL transitions to recombination from excitonic band-tail states. The energy of such transition is redshifted with respect to the free exciton recombination energy. This results in a large apparent energy difference between PL and optical absorption peaks which is, at 6 K, about 5 meV in the coherently strained samples and less than 1 meV in the partially relaxed ones. The analysis of the low temperature PL line shape which we present enables the determination of the excitonic band-gap energy for all the samples. The difference between the energy of the optical absorption transition and the excitonic band-gap energy thus determined gives a measure of the Stokes shift arising from thermalization effects alone. The values of the Stokes shift thus obtained are in better agreement with the sharpness of the optical absorption transitions which indicate samples of high crystalline quality.