Optical and structural characterisation of III-V semiconductor qauntum wire and quantum dot structures
This thesis describes an extensive study of the optical and structural properties of GaAs/AlGaAs V-groove quantum wire and InAs/GaAs self-organised quantum dot structures. For the optical characterisation, the spectroscopic techniques of photoluminescence (PL), photoluminescence excitation (PLE), cathodoluminescence (CL) and electroluminescence (EL) have been used. In addition important information concerning the structural properties of the quantum wires and quantum dots has been obtained by high resolution transmission electron microscopy (TEM) measurements. Initial characterisation and optimisation of the quantum wire structures was conducted using CL and TEM imaging. Further optical characterisation of optimised structures was performed using a micro-focusing PL set-up which produced a laser spot size of -'2µm, allowing individual wires to be studied. Excitation under both high and low power density conditions revealed important information concerning the quality of the growth and the nature of the one-dimensional` confinement within the structure. Structures were grown with the quantum wires placed in the intrinsic region of a p-i-n junction. EL spectroscopy measurements allowed the observation of subband filling effects and an enhanced luminescence intensity for the quantum wire for low forward bias currents. This latter behaviour has important implications for device applications. An investigation of these p-i-n samples in magnetic fields up to 14T, revealed further evidence for 1D confinement in the quantum wires and 2D confinement of carriers in a vertical quantum well. This vertical quantum well, a feature that arises automatically during the growth, appears to channel carriers into the quantum wire, providing a possible mechanism for the enhanced wire electroluminescence intensity observed. In addition, evidence is presented for a possible excitonic-free carrier transition which is observed for high carrier densities in the wires. TEM structural analysis of self-organised InAs/GaAs quantum dots has shown that the dots can show a high degree of surface ordering, aligning themselves on the step edges of the underlying substrate. Excitation under high laser power densities using the micro-PL set-up has shown subband separations of around 75meV, which is very promising for room temperature opto-electronic device applications. In addition, very narrow line width emission has been observed from individual quantum dots on a small sub-micron etched mesa which contains -100 dots. The electronic structure of the quantum dots has been probed using resonant PL and PLE. These techniques have revealed important information regarding carrier relaxation mechanisms that exist in the quantum dots. In PLE features are observed at approximately 60 and 90meV from the detection energy and which move rigidly in energy when the detection energy is varied. This behaviour is attributed to carrier relaxation by the emission of multiple LO- phonons, a process that appears to bypass any `phonon bottleneck'. Similar features are also observed in resonantly excited PL spectra. Two distinct carrier relaxation mechanisms are demonstrated by this technique: a non-resonant mechanism from the upper excited state and a resonant mechanism involving the emission of multiple LO phonons, from the first excited state.