Optical Studies of Single Quantum Dots
Abstract: This thesis presents spectroscopic studies of single self-assembled InP quantum dots (QDs). The electronic properties of these QDs have been studied by photoluminescence (PL) and scanning tunnelling luminescence (STL). The QDs were grown in the Stranski-Krastanow mode and were embedded in GaInP. This material is slightly n-type, giving a Fermi-level close to the conduction band edge at low temperature. Excess electrons therefore accumulate in the QDs; the number of electrons depending strongly on the QD size. In larger QDs, up to 20 electrons are accumulated at equilibrium. The quantized energy level spacing increases as the QD size decreases, while the Fermi-level remains constant. In this way, the QD allows fewer and fewer electrons to accumulate as the size is decreased, until the dot is sufficiently small for the quantization to push the electron ground state above the Fermi-level, the QD is neutral. Embedding the QDs in a Schottky diode and applying a reverse bias lowers the Fermi-level, and the electron accumulation is thus reduced. The depopulation of electronic states was monitored by PL and PL excitation spectroscopy. In addition, the electron accumulation was controlled by introducing additional heterostructures into the QD sample, pinning the Fermi level to the conduction band edge of GaAs in the vicinity of the QDs. The overgrowth of the QDs was studied in detail using transmission electron microscopy (TEM) and scanning tunnelling microscopy (STM). It was shown that the QDs act as seeds for the GaInP overgrowth. The dependence of emission on the cap layer thickness was studied for thinly overgrown QDs (0-30 nm) using STL. For thicker layers (20-100 nm), the cap layer induced strain was studied using micro-PL. A QD sample, capped with 100 nm, was etched from the top and the emission from single QDs was measured as a function of cap thickness. It was shown that the strain-induced energy is size-dependent, with significantly larger shifts for fully developed QDs than for small QDs. The small InP QDs were used to generate non-classical states of light. Triggered single photons on demand were obtained by exciting a QD with a pulsed laser and anti-bunched photon emission from several emission lines of the QD spectrum was studied under continuous excitation. By time-correlating single photons emitted from different QD states, the emission lines could be assigned, and a quantum cascade of three photons was demonstrated for a tri-exciton recombining via the bi-exciton and the exciton to the QD ground state.
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