| dc.description.abstract | 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
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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. | en_US |