Optical investigation of the exciton transfer between growth islands of different well widths in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum wells

The exciton population and the transfer of excitons was investigated for growth islands in quantum wells (QW’s) by cw-photoluminescence (PL), time-resolved PL, and photoluminescence excitation spectroscopy (PLE). For high-quality GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As QW’s with 13 nm QW thickness we found a strong dependence of the exciton population of three observed different island regions on excitation wavelength and excitation intensity. In a wide range of excitation wavelengths, the intensity ratios of the corresponding exciton transitions vary strongly with excitation intensity and, in general, do not reflect the area ratios of these islands. This behavior is caused by the different relaxation processes at the different excitation wavelengths when the lowest-energy states of the island regions are populated. By resonantly exciting the subbands in the QW, the created excitons remain localized and relax within the island regions. By nonresonantly exciting the exciton continuum of the confined subbands, the islands with the lowest energy are effectively populated within a time smaller than the luminescence decay-time constant. We determined the area ratios of the islands and the normalized exciton-transfer rates. A quantitative analysis of the intensity ratios of the cw measurements and time-resolved PL measurements revealed a surprisingly long exciton-transfer–time constant in the order of 2 ns, which is dependent on the potential-energy difference between neighboring islands.