Selective exciton formation in thin GaAs-AlGaAs quantum wells

We have found experimentally, that the exciton luminescence rise times in GaAdAlGaAs quantum wells oscillate as a function of incident laser energies. Guided by MonkCarlo simulations we interpret these results as the occurrence of selective LO-phonon assisted exciton formation. In the photoexcitation of semiconductors excitons can be generated either directly with high momentum or by free electrons and holes [I]. In 11-VI semiconductors the process of direct exciton formation is enhanced by the large exciton binding energy and by the strong electron-phonon coupling. The relaxation of these hot excitons gives rise to sharp resonances in the excitation and luminescence spectra [2]. In 111-V semiconductors, where the exciton binding energy is much smaller, the photoexcited electrons and holes relax independently within their (sub)bands and subsequently form excitons by interaction with optical and acoustic phonons or by carrier-carrier scattering. So in this model the exciton dynamics are governed by carrier cooling, exciton formation and exciton relaxation and will be reflected in the risetime of the luminescence after laser excitation. The decay time is a measure for the exciton life time. We have measured in a standard up-conversion experiment [3] this rise time for a narrow quantum well as a function of the laser energy. The sample consisted of 10 wells each 26 A wide. In such a well there is only one electron, h-hole and 1-hole level. The results are given in fig. I and show a remarkable oscillating behaviour. From this we draw as a first conclusion, that the position in k-space where the carriers land after the LO-phonon cascade might be of vital importance. d(flv# 26 A X-0.40 SCH-SQW Fig. 1 Experimental exciton luminescence rise times at T=8K as a function of laser energy. Note the zero suppression of the scale.