Time-resolved studies of carrier and spin dynamics in quantum dots and wide band-gap materials

Time-resolved studies using ultra-short laser pulses unveil interesting aspects of carrier and spin dynamics in compound semiconductors. Here, thermally-activated carrier transfers between CdTe/ZnTe quantum dots (QDs) and ultrafast spin-relaxations in bulk GaN are reported. Carrier transfer among CdTe/ZnTe self-organized QDs was studied using time-resolved photoluminescence measurements. The carriers in the high-energy ground states of small QDs are confirmed to transfer to the lower-energy ground states of larger QDs, even at 10 K. The energy dependence of the PL decay time changes uniquely with increasing temperature. The change in the energy dependence of the PL decay time can be explained by thermally-activated carrier transfer. Excitonic spin-relaxations in hexagonal GaN and cubic GaN are observed. The A-band free exciton in hexagonal GaN shows a sub-picosecond spin-relaxation of 0.47 ps at 150 K. The acceptor-bound exciton in hexagonal GaN shows spin-relaxation times of 1.40 – 1.14 ps at 15 – 50 K. Meanwhile, the spin-relaxation times in cubic GaN at 15 – 75 K are found to be longer than 5 ns. The long nanosecond spin-relaxation time in cubic GaN is consistent with the observation that spin-relaxation time becomes longer for wider-band-gap zincblende semiconductors.