Application of ultrafast spectroscopy to the study of size-controlled, self-assembled indium gallium arsenide/gallium arsenide quantum dots.

摘要

The delta-function-like density of states in self-assembled semiconductor quantum dots (QD) makes them highly promising for novel device applications, such as QD lasers. However, the threshold of state-of-the-art QD lasers remains highly temperature dependent, thus degrading their performance. A goal of present research is to reduce this temperature dependence by engineering the QD electronic states through controlled crystal growth. Another potential problem with QD lasers is a limited modulation rate, which may arise through restricted carrier energy relaxation. This phenomenon, which was predicted early in the study of QDs, is associated with the discrete nature of the QD energy states and the limited energy available in phonons, the principle source of energy relaxation in semiconductors. This “phonon bottleneck” remains controversial and has not yet been experimentally verified unambiguously. The problems of energy state engineering and the phonon bottleneck indicate that a systematic study of the carrier dynamics, which are directly related to both these phenomena and to device performance, is therefore of continuing interest.; In this dissertation, a temperature and density dependent study of the carrier dynamics in self-assembled, size-controlled quantum dots (SAQD) is presented. The dynamics are inferred from measurements of both excitonic ground state and first excited state emission in the In(Ga)As SAQD ensembles. The investigations encompass a set of four samples that differ in QD size and environment.; All of our observations indicate that the carrier dynamics in SAQDs are size-dependent. Therefore, it is possible to engineer the electronic structure so as to improve performance of QD-based devices. This, and the ability to control the QD size through manipulation of growth conditions, suggests that the electronic structure of SAQDs can be optimized for laser applications.