Dynamics of charge and spin excitations in InGaAs/GaAs quantum dots

Semiconductor quantum dots are objects with dimensions of a few to a few tens of nanometers. Confinement on such a small scale, of the order of the electron de Broglie wavelength, confers upon the charge carriers inside quantum dots a discrete energy spectrum. These atomic-like energy levels give rise to several technologically interesting properties. For example, laser diodes using quantum dots as their active medium offer the possibility of very low threshold currents, due to the near impossibility of coupling to electromagnetic frequencies apart from the lasing mode. The combination of discrete excitations and the high degree of spatial separation from the surrounding environment provided by quantum dots also makes it possible to maintain the electrons inside them in coherent states for much longer than with many other material systems, making quantum dots very suitable candidates for quantum information technologies. In particular the study of individual quantum dots has received a great deal of attention in recent years, with a view toward developing devices for quantum computing and quantum key distribution. In this context, an individual quantum dot is the critical component of many envisaged devices. In this thesis, several of the basic properties of individual self-assembled quantum dots are investigated. Chapter 1 gives a brief introduction to the technology of self-assembled quantum dots, and outlines several of the milestones that have been achieved in the field of single quantum dot research. Time-correlated single photon counting, the central technique employed in this work, has been used to investigate the transient properties of photo-excited charges in quantum dots. This technique uses a sensitive photodetector, capable of registering single incident photons, to compare the arrival times of photons from the dots with a signal from a reference, allowing statistics that diagnose the transient luminescent properties to be built up. The reference may be a voltage pulse produced every time the pulsed excitation laser fires, which could allow the spontaneous emission time to be determined, or the reference may be from another detector exposed also to the photoluminescence signal from the sample, which might permit one to examine the degree to which events inside the single dot can occur simultaneously. Chapter 2 gives more detail into the experimental techniques employed during this work. The experimental work presented in this thesis is divided into three sections, (i) a study of the dependence of the spontaneous emission lifetime of a dot on its emission wavelength, (ii) an investigation into luminescence properties at high excitation powers, and (iii) a study of the transient interaction between electron spins in a quantum dot and the optically oriented spins of the nuclei of the dot. All these parts concern measurements of processes taking place within 10 ns after the arrival of a picosecond laser pulse. Chapter 3 describes measurements of the recombination lifetimes of several dots from a single sample, each with a different emission wavelength. Due to the self assembly process, each dot possesses dimensions, composition, and geometry that differ slightly from those of all the others. This accounts for the different emission wavelengths and makes it desirable to understand how variations in these properties affect the luminescence behaviour. It was found that the emission lifetime gets systematically longer, as the resonant wavelength gets larger, which is opposite to observations by others for dots that, like ours, are wider than the exciton Bohr radius. We find that our dots are so much larger than the Bohr radius that the emission wavelength is uncorrelated with the lateral dimension of the dots, but is correlated closely with the height. This is shown by examination of the diamagnetic coefficients of the dots as a function of emission wavelength. We argue that the oscillator strengths of these dots must be treated as similar to those of narrow quantum wells, such that the lifetime gets longer as the structure gets thicker, i. e. as the dots get higher. In Chapter 4, we examine photoluminescence transience at high excitation powers. Two findings in particular are described, (i) the exciton luminescence becomes delayed as the excitation power is increased, (ii) very strong background emission develops from the sample as the laser intensity is turned up. Three models are considered to account for these findings, (1) multiexciton emission, (2) dressed exciton emission, in which the background emission is actually from the exciton, but spectrally broadened by the presence of numerous charge carriers surrounding the dot, and (3) dressed exciton emission where the background emission is primarily from outside the dot, perhaps from a two-dimensionally confined continuum of states. In this latter case, the exciton emission from the dot is again broadened by the external carriers, becoming sharp only when sufficient of the external carriers have recombined. This leads to an apparent delay to the dot emission, brought about by the spectral filtering of the monochromator used in the experiments. This third mechanism is argued to be the most suitable explanation for the observations. Chapter 5 details the study of spin excitations in quantum dots. Spins were excited in photoluminescence experiments using circularly polarized laser light. Photoluminescence decays were recorded for the cases co- and counter-polarized with the pump laser, and these were combined to give the transience of the spin state of the photo-excited exciton. Normally in such experiments, one expects a monotonic decay of the photoluminescence polarization, but with several of the dots studied in this work, the polarization was found to first decay, before rising again after a few nanoseconds. This phenomenon is linked by several experiments to the transfer of spins from the optically orientated nuclear spin reservoir, mediated by the hyperfine interaction. It is confirmed that the nuclei are indeed aligned by the polarized excitation light by recording small Overhauser splittings of the spectra of individual quantum dot emission lines. These splittings are displacements of the emission energy (about 5 µeV) of the co- and counter-polarized emissions, relative to one another, when the circularly polarized excitation is used. The splitting changes direction, when the helicity of the excitation light is reversed. Next, the observed Overhauser effect is shown to exhibit power dependence, disappearing as the excitation power is reduced. Finally, the rising polarization transients, observed at relatively high power, are replaced by simple monotonic decays as the excitation is reduced to the level at which the Overhauser effect is known to be just vanishing. This last observation establishes clearly the role of the optically aligned nuclear spins in the observation of the rising polarization transients. A rate equation model is developed and used to demonstrate theoretically the feasibility to cause rising polarization transients by hyperfine interactions with optically oriented nuclear spins. The model shows that it should be possible to use measurements such as the reported transients to determine the degree of nuclear spin alignment in nuclear orientation experiments