Optically detected magnetic resonance of indirect excitons in an ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots

Abstract

The energy level structure as well as the exciton recombination and spin dynamics are studied in a dense ensemble of (In,Al,Ga)As/(Al,Ga)As quantum dots (QDs). The band alignment in the QDs is shown to have type-I, indirect character with the lowest electron state at the $X$ valleys of the conduction band and the top hole state in the $\mathrm{\ensuremath{\Gamma}}$ point of the valence band, so that indirect excitons are formed in the QDs. Time-resolved photoluminescence and magnetic-field-induced circular polarization allow us to distinguish electron states belonging to the QDs and the wetting layer. Suppression of the exciton migration within the QD ensemble and along the wetting layer in the magnetic field is found. A pronounced effect of applied microwave radiation on the recombination and spin polarization of the indirect excitons is observed in longitudinal magnetic fields. Optically detected magnetic resonance (ODMR) is detected in both the intensity and the circular polarization degree of the QD emission. The ODMR resonance corresponds to the $g$ factor of 1.97, associated with $X$-valley electrons. The spin relaxation time of the $X$-valley electrons is measured to be $600\ifmmode\pm\else\textpm\fi{}25$ ns.