Shape engineering of InP nanostructures for optoelectronic applications

Abstract

III-V semiconductor nanostructures have been the research focus in the past two decades thanks to the superior properties of the materials themselves and the unique properties produced by reducing the dimension to the nanoscale. In particular, III-V nanowires have drawn much more attention and have been extensively applied in a wide range of devices including solar cells, light-emitting diodes, lasers, transistors, and photodetectors. Despite these great successes, these one-dimensional (1D) nanostructures still face many challenges in terms of synthesis, assembly, fabrication and the ability to form complex nanoarchitectures. Surprisingly, it has been demonstrated that nanostructures with more complex two- and three-dimensional (2D and 3D) shapes can provide possible solutions. These shape-engineered nanostructures can improve material properties and device functionality. Furthermore, recent intensive investigation of nanostructure networks shows a great demand on the shape flexibility, uniformity, structural and optical qualities of epitaxially grown III-V nanostructures. This dissertation presents the shape engineering of InP-based nanostructures grown by metal organic chemical vapour deposition (MOCVD) from 1D nanowires to more sophisticated 2D and 3D shapes. Shape transformation mechanism, crystal structure and optical properties were thoroughly investigated to understand the growth mechanism of these complex III-V nanostructures for electronic and optoelectronic applications. Among the various growth techniques, selective area epitaxy (SAE) has the advantages of producing uniform nanostructure arrays with a high degree of controllability in pattern geometry, such as shape, dimension, site position and spacing, and thus has been used in this work. InP nanostructures grown on {111}A InP substrates was first investigated. Two key growth parameters, temperature and V/III ratio, were studied to optimise the growth conditions. We found that a higher growth temperature was crucial to obtain high quality nanostructures. Under optimal growth conditions, the highly uniform arrays of wurtzite (WZ) InP nanostructures with tunable shapes, such as nanowires, nanomembranes, prism- and ring-like nanoshapes, were simultaneously achieved. Their side facets can be dominated by {101̅0} and/or {112̅0}, making WZ InP a good candidate for tailoring the shape of the nanostructures. In-depth investigation of shape transformation with time and opening geometry/dimension revealed that the shape was essentially determined by pattern confinement and minimisation of total surface energy. A theoretical model was proposed to explain the observed behaviour. Structural and optical characterisation results demonstrated that all the different InP nanostructures grown under optimal conditions have perfect wurtzite (WZ) crystal structure regardless of their shape and strong and homogeneous photon emission. Moreover, we investigated the shape evolution from branched nanowires to vertical/inclined nanomembranes and crystal structure of InP nanostructures grown on InP substrates of different orientations, including {100}, {110}, {111}B, {112}A and {112}B. Two growth models were proposed to explain these observations regarding shape transformation and phase transition. A strong correlation between the growth direction and crystal phase was revealed. Specifically, WZ and zinc-blende (ZB) phases form along the <111>A and <111>B directions, respectively, while crystal phase remains the same along other low-index directions. The polarity-induced crystal structure difference was explained by thermodynamic difference between the WZ and ZB phase nuclei on the {111}A and {111}B planes. Growth from the openings was essentially determined by pattern confinement and minimisation of the total surface energy, similar to growth on {111}A InP substrates. Accordingly, a novel type-II WZ/ZB nanomembrane homojunction array was obtained by tailoring growth directions through alignment of the openings along certain crystallographic orientations. Finally, the incorporation of InAsP quantum well was carried out on pure WZ InP nanostructure templates grown on {111}A InP substrates with two different shapes, i.e. nanowires and nanomembranes. InAsP quantum wells grew both axially and laterally on the InP nanowires and nanomembranes. While the axial quantum well was of ZB phase, the lateral one grown on side facets had a WZ phase. When sidewalls of nanowires and nanomembranes were the nonpolar {11̅00} facets, the radial quantum well selectively grew on the sidewall located at the semi-polar <112̅>A side of the axial quantum well, leading to the shape evolution of nanowires from hexagonal to triangular cross section and destroying the symmetry of nanomembranes. In comparison, nanomembranes with {112̅0} sidewalls are shown to be an ideal template for growing InP/InAsP heterostructures thanks to the high symmetry and uniformity of quantum well nanomembrane array. EDX results showed that quantum well composition was highly dependent on the crystal facet. Moreover, quantum well nanomembranes with {112̅0} sidewalls gave strong and uniform photon emission around 1.3 µm, showing superior optical properties compared with quantum wells incorporated in InP nanowires and nanomembranes with {11̅00} facets.