Spectroscopic studies of InGaAs/GaAs/A1GaAs quantum dots and wires

Abstract: It is demonstrated that the photoluminescence (PL) spectra of single selfassembled lnAs/GaAs quantum dots (QDs) are very sensitive to excitation energy and crystal temperature, which is explained in terms of modulation of the effective diffusivity of the photogenerated carriers affecting the capture probability into theQD. This effect results in a population of the QD with excess electrons, and can be used as a simple tool to create and study charged exciton complexes. Charged excitons of the studied QDs are thus identified and found to be in agreement with a theoretical model.An alternative way of optically charging single QDs with extra electrons at low temperatures is also presented, relying on the presence of residual acceptor-atoms in the barrier in the vicinity of the dot. The acceptors define a distinct threshold transition energy below the barrier band gap. This threshold energy is dependent on the binding energies of the acceptors, i.e. which common acceptors in GaAs are present in the vicinity of the studied QD. Exciting with a photon energy above (below) this threshold creates negative (neutral) excitons in the QD, and is a very efficient tool to study charged exciton complexes.It has been proposed that the properties of excitons in low-dimensional quantum structures such as quantum wires (QWRs) can be described by a fractional dimension with a value depending on the confinement. This theory has been applied to V-groove GaAs /A1GaAs QWRs in order to obtain the absorption spectrum, and the achieved result is verified by comparison with a more complete model as well as with PL excitation (PLE) experiments. It is found that the fractional dimension theory offers an efficient and accurate way to determine the absorption spectrum, although situations of strong intersubband couplings are identified where the theory naturally fails, at least to some extent.Asymmetric double V-groove GaAs/A1GaAs quantum wires light-emitting diode (LED) systems have been subjected to electroluminescence (EL) and PLE measurements, revealing a very efficient charge and exciton transfer from the narrow tothe wide QWR at low temperatures, despite a 7 nm thick tunneling barrier. The tunneling efficiency is found to be comparable for both electrons and holes, and evidence for tunneling is revealed up to room temperature. Furthermore, PL and PLE studies on analogous QWR systems with various thicknesses of the barriers demonstrate, when compared with a numerical calculation of the tunneling efficiency, that the carrier transfer between the QWRs can be explained by conventional tunneling. The effect of exciton leakage often present between the quantum wells is thus not present here.Finally GaAs/A1GaAs and InGaAs/GaAs/A1GaAs V-groove quantum wire LEDs are subjected to magnetic fields for different configurations of the field with respect to the wire axis. The ground state diamagnetic shifts are determined by ELand compared with the results of a proposed two-band exciton model. It is found that this model cannot in a satisfactory way explain the experimental data, since it predicts too large shifts (about a factor of two for the GaAs/A1GaAs QWRs). One possible reason for this is the neglected valence band mixing in the exciton approach, which issupported by a better agreement for the InGaAs/GaAs/A1GaAs QWRs where the strain splits the valence bands and weakens the effects of valence band mixing.