MOCVD growth and characterisation of au-catalysed InP nanowires (2011)

The tunable bandgap of InP nanowires compared to bulk InP offers a wide range of applications in optoelectronic and high-speed electronic devices including photodetectors, lasers, light-emitting diodes, transistors and solar cells. The aforementioned devices demand nanowires with good morphology, high-quality crystal structure and controllable optical properties. This can be realised by tailoring growth parameters to achieve nanowires of uniform diameter along their length and structurally controlled nanowires where crystal defects including rotational twins and stacking faults can be minimised. This thesis deals with the growth of Au-catalysed InP nanowires via the vapour-liquid-solid (VLS) mechanism using metal organic chemical vapour deposition (MOCVD) technique. This work details various strategies adopted to produce morphologically controlled high-quality nanowires with desired properties. This thesis focuses on the effect of growth parameters that include growth temperature, V/Ill ratio, Au catalyst size, growth rate and pre-growth annealing temperature on nanowire morphology, crystal quality and optical properties. The nanowires' morphology, shape and size are characterised carefullyby using scanning electron microscopy (SEM), while substrate-surface morphology is examined by atomic force microscopy (AFM). Transmission electron microscopy (TEM) was used to investigate nanowire crystal structures (zinc-blende or wurtzite), while high-resolution TEM is performed to examine defects (twins or stacking faults) in single nanowires. The optical properties are characterised using continuous-wave and time-resolved photoluminescence (PL) spectroscopy. Growth temperature and V/IllI ratio are found to have very significant effect on nanowire morphology, where irregular nanowires are produced at lowgrowth temperatures and low VillI ratios whereas straight (Ll Lr-oriented nanowires are produced at higher-growth temperature and/or higher VillI ratios. Increasing the temperature and V/III ratio further, however, increases nanowire tapering, where nanowire bases are wider than tips. Thus an intermediate combination of temperature and V/III ratios are chosen in order to obtain the best compromised vertically aligned InP nanowires with minimal tapering. TEM confirms that higher growth temperatures or higher V/llI ratios promotes the formation of wurtzite nanowires, while zinc-blonde nanowires are favourable to grow at lower-growth temperatures and lower V/III ratios. PL spectra show a blue-shift with increasing growth temperature andI or VillI ratio, with the bandgap attributed to zinc-blende and wurtzite nanowires. The mixed phase of zinc-blende and wurtzite nanowires show emission energy between these two. Au nanoparticle size is also found to strongly affect the InP nanowire crystal structure and optical properties. TEM studies confirmed that nanowires can be grown predominantly in the zinc-blende phase or completely wurtzite phase irrespective of the nanoparticle size. Interestingly, nanowires grown in between these two regions (mixed zinc-blende/wurtzite region), exhibit diameter dependent crystal structures, with the smaller Au nanoparticle size exhibiting wurtzite crystal structures whereas larger Au nanoparticle size show a zinc-blende dominated structure. Continuous-wave photoluminescence measurements under different excitation intensities show that nanowires grew preferably in wurtzite rather than in zinc-blende crystal structures with decreasing diameters. Time-resolved photoluminescence studies reveal that wurtzite nanowires have longer carrier lifetimes than zinc-blende nanowires. InP nanowire morphology, crystal structures and optical properties are also affected by growth rate. Low growth rates result in a wurtzite phase with fewer stacking faults and, in contrast, higher growth rates produce zinc-blend nanowires with high density of twin defects. Nonetheless, the tapering effect can be reduced with growth rate. The PL spectrum shows an emission peak for the WZ-dominated phase at the lowest growth rate with this peak shifting to lower energies as the nanowire gradually changes to a twinned zinc-blende structure. Lowering the pre-growth annealing temperature results in several interesting features and changes in the nanowires. The tapering parameter markedly decreases, this leading to a better nanowire shape and stacking fault-free wurtzite structure for (111)-oriented nanowires grown on (lll)B substrates. This study shows that there are many growth parameters that can influence the morphology-crystal structure-optical relationships in InP nanowires. By controlling growth parameters, InP nanowire morphology can be optimised, thus producing nanowires with minimal tapering and uniform diameters, while their crystal structures can be tuned to obtain desired properties. This will allow InP nanowires of known crystal structures to be designed and grown for future nanowire-device applications.