Optical Properties of III-V Semiconductor Nanowire Structures
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The present thesis deals with the characterisation of the optical properties of GaAsbased III-V semiconductor nanowires (NW) and with the exploration of the potential of such NWs for applications in light emitting devices. For this purpose, a host of microphotoluminescence (μ-PL) spectroscopy techniques are utilised such as excitation powerdependent, temperature-dependent, polarisation-resolved and time-resolved μ-PL, as well as optical pumping. In addition, transmission electron microscopy (TEM) is performed on the same single NWs in order to compliment the optical data with detailed information on the structure and, in some studies, also on the composition of the studied NWs. The wurtzite (WZ) crystal phase of GaAs is a relatively new finding, exclusive to NWs. Typically encountered as the predominant crystal phase in Au-catalysed NWs grown with molecular beam epitaxy (MBE), its optical and electronic properties have caused controversy in the NW research community for a long time. One of the important achievements of the present thesis is the successful application of the correlated μ-PL – TEM characterisation method to single, high crystal phase purity, high optical quality WZ GaAs NWs grown with Au-catalysed MBE. The results have shed light on important characteristics of the WZ GaAs phase such as the free exciton emission energy which was found to be 1.515 eV, the temperature dependence of the band gap which differs from that of conventional zinc blende (ZB) GaAs, the typical PL recombination lifetimes at low (~ 3-4 ns) and room temperature (~ 1 ns) and the overall optical quality judged by the optical brightness at room temperature compared to that at low temperature (90x decrease from low to room temperature, see Article 1). In addition, the band offsets at the crystal phase heterojunction between the ZB and the WZ GaAs phases were determined to be ~ 120 meV at low temperature, using self-catalysed GaAs NWs with an axial GaAsSb insert, which were found to provide a crystal phase heterojunction between quasi-bulk ZB and WZ GaAs at the upper boundary of the insert (Article 2). It should be emphasised that the overall optical quality and structural homogeneity of the fabricated NWs are of utmost importance for the feasibility of NW-based optoelectronic devices. While self-assembled Au-catalysed MBE growth of GaAs NWs can yield high optical quality, it suffers from several major drawbacks. Firstly, there is a significant variation in the structural and optical properties between different NWs from the same growth batch. Furthermore, the diameter of NWs whose surface has been passivated with a radial AlGaAs shell is inhomogeneous along the NW length due to tapering and anti-tapering effects, which is particularly problematic for applications in NW-based waveguides and lasers. Finally, using Au as the catalyst for NW synthesis is undesirable if growth is to be performed on Si substrates, which, in turn, is a major prerequisite for the compatibility of NW-based optoelectronic devices with the existing Si-wafer integrated circuit (IC) technology. Thus, an alternative technique that uses Ga as catalyst, called self-catalysed GaAs NW growth, has seen a tremendous rise in interest over the past few years. Self-catalysed NW growth by MBE was the method of choice for NW synthesis during the latter half of the present PhD project. The optimisation of the growth parameters, more specifically of the V/III ratio, used for self-catalysed growth of GaAs NWs with respect to the optical quality and crystal purity of the ZB GaAs phase has been the central subject of Article 3. In addition, the optical determination of the Sb composition in axial GaAsSb inserts incorporated in GaAs-core and GaAs/AlGaAs core-shell NWs and the examination of the PL recombination mechanisms taking place in such inserts have been the topics of Articles 2 and 4 in the thesis. Combined, the insight gained through those works has culminated in the successful fabrication of an optically pumped single-NW laser device utilising a number of axial GaAsSb-based compositional superlattices as the active medium (see Article 5). The multi-quantum well character of the active medium and the single-NW principle of operation of this NW laser result in that it possesses most of the benefits of a quantum-confined laser such as low threshold and narrow linewidth. At the same time, typical drawbacks of quantum-confined lasers such as the lower gain saturation point characteristic for shorter gain media with a limited number of states, are efficiently circumvented by fabricating a large number of wells. Single-mode operation of this NW laser was observed from low to room temperature, and in addition, compositional tunability of the GaAsSb-based superlattice has allowed to push the laser wavelength into the 980 nm range which is in demand for optical pumping of commercially available optical amplifiers. Thus, the NW laser reported in Article 5 has a great potential for applications and provides a solid basis for future development towards fabrication of an electrically driven NW laser with similar properties.