Study Of Spectral And Optical Properties Of Semiconductor Quantum Dots

The beginning of the 1970s marked the new era of research on electronic structures of dimension limited to two, so-called quantum wells (QWs). The new, unusual properties of QW have attracted the attention of many research laboratories. Soon the discovery of the integer quantum Hall effect by Klaus von Klitzing's group was awarded the Nobel Prize. At present, the properties of the two-dimensional systems are well investigated and understood, and QWs have been produced and implemented fore years in numerous devices, for instance, laser diodes used in CD players or microwave receivers used in satellite television. Complete quantization of the electron's free motion is implemented by trapping it in a quasi-zero-dimensional quantum dot (QD). As a result of the strong confinement imposed in all three spatial dimensions, QD systems are similar to atoms and therefore are frequently described as the artificial atoms or superatoms. The strong quantization of electron energy, with parameters suitable for laser action, particularly in the so-called self-assembled QDs, will probably allow QD-based lasers to be able to work at higher temperatures and at lower injection currents. What is also very promising is the possibility of an application of QDs in a new generation of computers. The small dimensions and possibility of dense packing of QD matrices could permit them to be used for memory media of huge capacity. Most of the experimental techniques applied to the studies of electrical and optical properties. In this work, we focus on the spectroscopy and optical properties of semiconductor QDs and the major works and results are reported as the follows:1. We have investigated the evolution of exciton state filling spectra of InAs/GaAs QDs as a function of the excitation power density and temperature by using micro-photoluminescence (micro-PL) spectroscopy. Inaddition to the emission bands of exciton recombination corresponding to the atom-like S, P and D etc. shells of InAs QDs, it was observed for the first time that some new states, P' (between the S and P shells) and D' (between the P and D shells), appear in the state-filling spectra. The emergence of these inter-shell states is power density and temperature dependent, which is an experimental demonstration of strong exciton-exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in quantum dots.2. The atomic force microscopy (AFM) images of In_0.35Ga_0.65As template grown on GaAs(311 )B substrate shows a two-dimensionally modulated layer of closely packed connected cells. The power-dependent micro-PL spectroscopy indicates that the existence of the In_0.35Ga_0.65As template enhances the photo-absorption of excitation laser beam and the number of photo-generated excitons as well. Therefore the exciton emission from the QDs is strengthened due to efficient exciton transfer from the template into the QDs. Hence, the template has a remarkable effect on improving the optical properties of the InAs QDs.3. The temperature-dependent PL of two types of islands in ultrathin CdSe/ZnSe layers indicates that the excitons in large islands (LIs) can transfer into small islands (SIs) by tunneling at low temperature. And the hopping process of the localized excitons will gradually become the dominant mechanism of exciton transfer with increasing temperature. The micro-PL spectra at 4.2 K shows that the PL peaks of the Zn_1-xCd_xSe islands have a large red-shift of 166 meV with increasing CdSe deposition layer from 1.8 to 2.3 ML, which can't be explained by considering the change of quantum confinement potential only. We found other two important mechanisms which may lead to the large red-shift by using miro-Raman spectra. One is the increasing sheet density of large islands with lower exciton ground state energy and the other is the increased Cd composition in the Zn_1-xCd_xSe islands when increasing the CdSe deposition layer.4. Pillar-patterns were fabricated on the self-assembed CdSe QDs by using electron-beam lithography and chemical etching. This process removes the QDs outside the pillars, and therefore further reduces the dot density. We probed the recombination emission of excitonic complex in CdSe single QD by using micro-PL spectroscopy. The fine structures induced by the exchange-correlation interaction among carriers occupying the "artificial atomic shells" and the spin splitting of excitonic levels in magnetic fields were investigated.