Spin-selective excitation spectroscopy of excitons and biexcitons inInxGa1−xAsquantum disks: Suppression of the spin-flip process in semiconductor zero-dimensional systems

The spin relaxation of photocreated excitons in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_y{\mathrm{Ga}}_{1-y}\mathrm{As}$ quantum disks is investigated by microscope photoluminescence experiments under spin-selective optical excitation. Complete quantization of the two-dimensional energy structure of the quantum well into comb-shaped density of states results in inhibition of the elastic spin-flip processes, allowing only a simultaneous exchange of energy and momentum for spin relaxation. This slows down the spin flip in both the thermalization and recombination of exciton and substantially increases the spin-relaxation time. For some exciton excited states, the spin-flip relaxation into another spin state is efficient, thereby allowing the second photon absorption into a biexciton state even under the selective-spin excitation.