Engineering the Optical Properties of InGaAs Nanowire Arrays for Optoelectronic Device Applications

Abstract

III-V semiconductor nanowires have attracted significant research interest in the optoelectronic field thanks to the excellent optical properties of III-V semiconductors and the one-dimensional structure of nanowires. Amongst III-V nanowires, ternary alloys allow tuning of the bandgap as a function of alloy composition. However, despite progress, it remains challenging to control the alloy composition and the structure of promising ternary nanowires such as InGaAs. Amongst nanowire growth methods, selective area epitaxy (SAE) results in highly uniform nanowire arrays with a constant composition over the entire array. To effectively incorporate InGaAs nanowire into optoelectronic devices is essential to understand the role of growth parameters and pattern geometry on their morphology, composition and crystal structure. Furthermore, decreasing InGaAs nanowires surface recombination velocity (SRV) is important to enabling their use for many applications. In this dissertation, we investigate the growth of InGaAs nanowires via SAE on GaAs substrate. We first establish optimal growth parameters to achieve highly uniform GaAs nanowire arrays with enhanced optical properties. These optimally grown GaAs nanowires have a twinned zinc-blende (ZB) crystal structure with taper-free {110} side facets that result in a low SRV of 35000 cm/s. This is an order of magnitude lower than that reported for defect-free GaAs nanowires grown by vapour-liquid-solid technique. Using time-resolved photoluminescence (TRPL) and cathodoluminescence (CL) measurements, we untangle the local correlation between structural and optical properties, demonstrating the superior role of the side-facets in determining recombination rates over that played by twin defects. The low SRV of these nanowires enable us to demonstrate, for the first time, low-temperature lasing from unpassivated GaAs nanowires, and also efficient room-temperature lasing after their passivation. We extend these findings to achieve highly uniform In1-xGaxAs nanowire arrays over an extensive range of Ga concentrations, from 0.1 to 0.91. We observe that the Ga content always increases with increasing Ga/(Ga+In) precursors ratio, group-V flow and growth temperature. The increase in Ga content is supported by a blue shift in the PL peak emission. The geometry of the nanowire arrays also plays an important role in the resulting PL peak emission. Increasing the nanowire pitch size from 0.6 to 2 micometer in a patterned array red-shifts the PL peak emission by up to 120 meV. Irrespective of these growth and geometry parameters, the Ga content determines the crystal structure, resulting in a predominantly wurtzite (WZ) structure for XGa<0.3 and a predominantly ZB phase for XGa>0.65. Finally, we introduce the engineering of highly performing InGaAs nanowires by effectively suppressing their surface states with an InP passivation shell. We identify optimal InP shell growth conditions and thickness to increase minority carrier lifetime significantly. The PL intensity of these passivated InGaAs nanowires is up to three orders of magnitude higher than that of their bare counterparts. Moreover, strong PL emissions and a long minority carrier lifetime of up to 13 ns were measured with these passivated nanowires at room temperature. Optimal passivation of InGaAs nanowires with an emission wavelength of 1530 nm results in an ultra low SRV of 280 cm/s. The crystal structure of these nanowires also plays an important role in the luminesce intensity, revealing significantly lower emission intensities in WZ sections of the nanowires than in mixed WZ-ZB and ZB ones. Overall, we demonstrate that optimally SAE grown GaAs nanowires have the lowest SRV reported for GaAs, despite the presence of twin defects. We grow highly uniform InGaAs nanowire arrays in a wide range of compositions. Passivation of the InGaAs nanowires with an InP shell makes them ideal candidates for efficient nanowire-based devices. Show more