Shape engineering of InP nanostructures for optoelectronic applications

Abstract

III-V semiconductor nanowires 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. Despite the great successes, these one-dimensional (1D) nanostructures still face many challenges. Surprisingly, it has been demonstrated that nanostructures with more complex two- and three-dimensional (2D and 3D) shapes can provide possible solutions. This dissertation presents the shape engineering of InP-based nanostructures from 1D nanowires to more sophisticated 2D and 3D shapes. First, 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. 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 {1-100} facets, the radial quantum well selectively grew on the sidewall located at the semi-polar <11-2>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 {11-20} 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 {11-20} sidewalls gave strong and uniform photon emission around 1.3 um.