| Summary: | 博士 === 國立中央大學 === 物理與天文研究所 === 89 === This dissertation is devoted to the electronic and optical properties of InxGa1-xAs self-assembled quantum dots. The main focus of this dissertation can be divided into two parts. First, we present optical investigations with regard to the physical features of the confined states in InxGa1-xAs quantum dots. Three optical spectroscopes have been employed: photoluminescence, photocurrent and electroreflectance. These spectroscopes in principle can all be utilized to probe the interband transitions in the InxGa1-xAs quantum dots, but possess characteristic features specific to the different physical mechanisms involved in each. The general features of the quantum-dot photoluminescence, including the state-filling effect and its interplay with carrier dynamics, and the temperature effects on carrier distributions, are comprehensively discussed. The photoluminescence spectroscopy was further utilized to study the tuning of confined energy levels in InAs self-assembled dots via rapid thermal annealing. Intense and sharp interband transitions were observed, which demonstrates unambiguously that the investigated quantum dots retained their optical quality and zero-dimensional properties even after the strongest condition of interdiffusion. Photocurrent spectroscopy was used to investigate both temperature and electric-field effects on the InAs dots. The path for thermal escapes of photogenerated electron-hole pair from the dot states is clarified. Low-temperature photocurrent also revealed a clear feature of field-induced escapes via direct tunneling out of the quantum dots. The applied electric field not only leads to an energy shift due to quantum-confined Stark effects, but also causes a size selective tunneling. A more detailed study of electric-field effects on the quantum dot interband transitions was presented by electroreflectance spectroscopy. Asymmetric Stark shifts in transitions energies were observed, implying that the optically excited electron-hole pairs exhibit built-in dipole moments in the quantum dots.
After having the idea of confined states in the InxGa1-xAs self-assembled dots, in the second part of this dissertation, we present how to manipulate and corral electrons in these confined states. The quantum dots were incorporated into a space-charge structure, so that the charging of quantum dots can be achieved by suitably applied bias voltage, forming charged quantum dots. We developed a novel spectroscopic technique, called electron-filling modulation reflectance (EFR), to study the charging of InxGa1-xAs self-assembled dots. The EFR technique is essentially a new kind of electroreflectance, but possessing characteristic features that are more similar to the conventional space-charge techniques, such as capacitance-voltage and admittance spectroscopes. Electron distribution and level occupation in quantum dot ensemble were investigated by combining the EFR with the capacitance-voltage spectroscopy. We used the capacitance-voltage characteristics to construct the electronic structures of the investigated In0.5Ga0.5As quantum dots. The Coulomb-charging energy required for adding electrons into the dots were also deduced from the capacitance-voltage characteristics. The electron level occupations were investigated by monitoring the measured EFR intensity. We found that the electron distribution in the dot ensemble was inhomogeneous near the Fermi level, which was attributed to the correlated charge transfer among different dots. The temperature effects on electron thermal population in the dots are demonstrated. We also present a combination of EFR with admittance spectroscopy to study the charging of InAs quantum dots. Charging dynamics of the InAs dots were characterized by the admittance spectroscopy. Clear features for different electronic shells of the InAs dots were resolved, enabling a separate investigation of the electron escape behaviors in different dot shells. The interband transitions of charged quantum dots were obtained from EFR measurements. We demonstrate clear Pauli blocking of the transition strength caused by the electrons being charged into the quantum dots. Remarkable energy modification due to the formation of negatively charged exciton was observed. The experimental determined energy shifts were finally compared with the theoretical calculation of Coulomb interactions in a quantum dot with a parabolic confining potential.
|
|---|
