The Optical Response of Semiconductor Self-Assembled Quantum Dots

This dissertation, "The Optical Response of Semiconductor Self-assembled Quantum Dots" by Zhifeng, Wei, 魏志鋒, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled THE OPTICAL RESPONSE OF SEMICONDUCTOR SELF-ASSEMBLED QUANTUM DOTS Submitted by Wei Zhifeng for the degree of Doctor of Philosophy at The University of Hong Kong in May 2006 Self-assembled quantum dots (QDs) are one type of tiny coherent islands which spontaneously form via the Stranski-Krastanow (S-K) growth mode during the heteroepitaxial growth of lattice-mismatched thin films. They have attracted considerable interest in recent years because of their atom-like properties and many potential applications in novel electronic and photonic devices. A detailed investigation on the optical properties of semiconductor QDs was presented in this thesis project. The variable-temperature photoluminescence (PL) spectra of two kinds of InGaAs QDs with and without a GaAs cap layer were comparatively investigated. It was found that the optical properties of the buried QDs with the cap layer differ significantly from those of the surface QDs. The growth of cap layer can improve the dots uniformity indicated by the narrow PL peak, greatly enhance the luminescence efficiency, and result in a large blueshift of the emission peak due to the strain environment change of the QDs. The cap layer also improves the thermal stability of the QD emission. The spontaneous emission mechanisms of two quaternary self-assembled GaInAsN QDs with different compositions and growth thicknesses have been studied. Adopting a newly developed localized-states luminescence model, the temperature behavior of the emissions from the two GaInAsN QDs has been quantitatively interpreted. The physical pictures of the spontaneous emissions of GaInAsN QDs were established. For the self-assembled QDs, a strained thin layer underneath the QDs always exists and this two-dimensional thin layer is called the wetting layer (WL). A quantitative study on the role of WL in the luminescence process of InAs/GaAs self- assembled QDs has been conducted. It has been shown that the WL acts as an efficient bridge not only in the relaxation of carriers from the barrier layer to the QDs but also in the thermal escaping of the carriers already captured by the QDs. Since the knowledge of electronic structure and optical transitions of self- assembled QDs is essentially important for the infrared detection applications of the QDs, a precise PL spectroscopic probe of the electronic structures of InAs/GaAs QDs with various silicon doping concentrations has been carried out. The theoretical predicted blueshift of the fundamental transitions of the QDs has been observed as the Si doping concentration and thus the number of electrons loaded into the dot is increased. It was also found that the Si δ-doping at the central regions of barrier layers during the growth of multi-layered QDs can significantly improve the luminescence efficiency of the samples at higher temperatures. Growth and optical characterizations of the electrically coupled InAs and InP QD stacking structures have been done. The study provides a possible way to produce a new type of broad-band QD light emitting diodes via the so-called band-engineering method. DOI: 10.5353/th_b3709820 Subjects: Quantum dots Semiconductors - Optical properties