Photoluminescence Studies of Single Quantum Dots

Sammanfattning: This thesis presents photoluminescence (PL) spectroscopy studies of single III-V semiconductor quantum dots (QDs). The electronic properties of the QDs, as well as, the dynamics of the processes taking part inside and in the surroundings of the QDs have been studied by single quantum dot photoluminescence spectroscopy, time resolved spectroscopy, and correlation spectroscopy. A detailed overview of solid immersion spectroscopy, a technique offering superior resolution and collection efficiency over conventional PL spectroscopy, is presented. Power-dependent PL spectroscopy was used to identify the origin of the spectral lines of In(Ga)As/GaAs Stranski-Krastanow (SK) grown QDs. It was found that holes in excited states sometimes recombine with electrons in the ground state. A rate equation system was used to model the intensity behavior, showing that the presence of electrons and holes in excited states is also observable when monitoring the PL between electrons and holes in their single-particle ground states. Correlation spectroscopy measurements on In(Ga)As/GaAs SK grown QDs have shown that the light emission from a single QD is antibunched, and a QD can be used to generate single photons on demand. Blinking in the PL from In(Ga)As/GaAs SK grown QDs have been observed and investigated. The PL from a single QD, exhibiting switching between two intensity levels, was spectrally resolved. We found no spectral shift of the emission from the different states. It is only the intensity, mainly for higher energy lines, that changes. The switching is attributed to a mobile, metastable defect acting as a nonradiative recombination center, and we show that the switching can be used as a monitor of the defect. A new type of random telegraph noise have been observed in the PL from small InP/GaInP SK grown QDs. The switching occurs between two states, which have similar total intensities, but distinctly different spectra, that are not simply shifted by an electric field. The QDs can be permanently settled in one of the states by strong illumination. Measurements on QDs in a semitransparent Schottky diode, show that the quantum dots can be reversibly forced into one of the states by applying a sufficiently large reverse bias. Single GaAs nanowires, grown by means of the vapor-liquid-solid growth mode, with InAs QDs in the form of a segment incorporated into the GaAs, have been studied by PL spectroscopy. Rich spectra, consisting of sharp lines with excitation power dependency behavior very similar to that seen for SK grown QDs were observed. By reducing the excitation power density, a QD spectrum was obtained, consisting of only one single sharp line - the exciton line.