Ultrafast carrier dynamics in semiconductor self-assembled quantum dots in the low carrier density regime.

Abstract

Self-assembled quantum dots are nanoscopic clusters of semiconductor atoms that exhibit atom-like properties because of their three dimensional quantum confining potentials. The quantum confinement offered by quantum dots is expected to reap benefits for many optoelectronic applications. In fact, high performance lasers and detectors based on quantum dots are already being developed. For these applications as well as for those with new functionalities, one of the most critical factors affecting performance will be relaxation processes of the carriers. Thus in order to fully exploit the benefits of self-assembled quantum dots, one must have a clear understanding of the physical mechanisms that govern carrier dynamics. Ultrafast carrier dynamics which occur on the time scales of femtoseconds and picoseconds among the quantum dots at low densities are the topics of this thesis. A femtosecond differential transmission pump-probe technique is employed to time-resolve directly the carrier distribution among an ensemble of multilayer self-assembled quantum dots. Measurements show that in multilayer structures where the barrier region is very thin, electronic coupling occurs in a time scale of hundreds of femtoseconds among the confined excited states. In a slightly longer time scale on the order of just a few picoseconds, electrons and holes relax from the high-lying states down to the low-lying dot states. When electrons and holes are captured non-geminately or separately into the excited states of different dots, the electrons experience a phonon bottleneck or the suppression of the interlevel relaxation. This bottleneck signal decays with a time constant of approximately 750 picoseconds and is attributed to thermal excitation. Temperature-dependent measurements analyzed with an ensemble Monte Carlo simulation indicate that thermal reemission and non-radiative recombination play a strong role in the carrier dynamics above 100 Kelvin. Collectively these results contribute to the ongoing efforts in the pursuit of a fuller understanding of the properties of self-assembled quantum dots.