Optical Study of p-Doping in GaAs Nanowires for Low-Threshold and High-Yield Lasing

Sub-Picosecond Carrier Dynamics Explored using Automated High-Throughput Studies of Doping Inhomogeneity within a Bayesian Framework

Bottom-up production of semiconductor nanomaterials is often accompanied by inhomogeneity resulting in a spread in electronic properties which may be influenced by the nanoparticle geometry, crystal quality, stoichiometry, or doping. Using photoluminescence spectroscopy of a population of more than 11 000 individual zinc-doped gallium arsenide nanowires, inhomogeneity is revealed in, and correlation between doping and nanowire diameter by use of a Bayesian statistical approach. Recombination of hot-carriers is shown to be responsible for the photoluminescence lineshape; by exploiting lifetime variation across the population, hot-carrier dynamics is revealed at the sub-picosecond timescale showing interband electronic dynamics. High-throughput spectroscopy together with a Bayesian approach are shown to provide unique insight in an inhomogeneous nanomaterial population, and can reveal electronic dynamics otherwise requiring complex pump-probe experiments in highly non-equilibrium conditions.

Improving Quantum Well Tube Homogeneity Using Strained Nanowire Heterostructures

Bottom-up grown nanostructures often suffer from significant dimensional inhomogeneity, and for quantum confined heterostructures, this can lead to a corresponding large variation in electronic properties. A high-throughput characterization methodology is applied to >15,000 nanoskived sections of highly strained GaAsP/GaAs radial core/shell quantum well heterostructures revealing high emission uniformity. While scanning electron microscopy shows a wide nanowire diameter spread of 540_-60⁺⁶⁰ nm, photoluminescence reveals a tightly bounded band-to-band transition energy of 1546_-3⁺⁴ meV. A highly strained core/shell nanowire design is shown to reduce the dependence of emission on the quantum well width variation significantly more than in the unstrained case.

Holistic Determination of Optoelectronic Properties using High-Throughput Spectroscopy of Surface-Guided CsPbBr3 Nanowires

Optoelectronic micro- and nanostructures have a vast parameter space to explore for modification and optimization of their functional performance. This paper reports on a data-led approach using high-throughput single nanostructure spectroscopy to probe >8000 structures, allowing for holistic analysis of multiple material and optoelectronic parameters with statistical confidence. The methodology is applied to surface-guided CsPbBr3 nanowires, which have complex and interrelated geometric, structural, and electronic properties. Photoluminescence-based measurements, studying both the surface and embedded interfaces, exploits the natural inter nanowire geometric variation to show that increasing the nanowire width reduces the optical bandgap, increases the recombination rate in the nanowire bulk, and reduces the rate at the surface interface. A model of carrier recombination and diffusion ascribes these trends to carrier density and strain effects at the interfaces and self-consistently retrieves values for carrier mobility, trap densities, bandgap, diffusion length, and internal quantum efficiency. The model predicts parameter trends, such as the variation of internal quantum efficiency with width, which is confirmed by experimental verification. As this approach requires minimal a priori information, it is widely applicable to nano- and microscale materials.

Understanding the role of facets and twin defects in the optical performance of GaAs nanowires for laser applications

GaAs nanowires are regarded as promising building blocks of future optoelectronic devices. Despite progress, the growth of high optical quality GaAs nanowires is a standing challenge. Understanding the role of twin defects and nanowire facets on the optical emission and minority carrier lifetime of GaAs nanowires is key for the engineering of their optoelectronic properties. Here, we present new insights into the microstructural parameters controlling the optical properties of GaAs nanowires, grown via selective-area metal-organic vapor-phase epitaxy. We observe that these GaAs nanowires have a twinned zinc blende crystal structure with taper-free {110} side facets that result in an ultra-low surface recombination velocity of 3.5 × 104 cm s-1. This is an order of magnitude lower than that reported for defect-free GaAs nanowires grown by the vapor-liquid-solid technique. Using time-resolved photoluminescence and cathodoluminescence 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 surface recombination velocity of these taper-free {110} side facets enable us to demonstrate, for the first time, low-temperature lasing from bare (unpassivated) GaAs nanowires, and also efficient room-temperature lasing after passivation with an AlGaAs shell.

Measuring, controlling and exploiting heterogeneity in optoelectronic nanowires

Fabricated from ZnO, III-N, chalcogenide-based, III-V, hybrid perovskite or other materials, semiconductor nanowires offer single-element and array functionality as photovoltaic, non-linear, electroluminescent and lasing components. In many applications their advantageous properties emerge from their geometry; a high surface-to-volume ratio for facile access to carriers, wavelength-scale dimensions for waveguiding or a small nanowire-substrate footprint enabling heterogeneous growth. However, inhomogeneity during bottom-up growth is ubiquitous and can impact morphology, geometry, crystal structure, defect density, heterostructure dimensions and ultimately functional performance. In this topical review, we discuss the origin and impact of heterogeneity within and between optoelectronic nanowires, and introduce methods to assess, optimise and ultimately exploit wire-to-wire disorder.

Facet-Related Non-uniform Photoluminescence in Passivated GaAs Nanowires

The semiconductor nanowire architecture provides opportunities for non-planar electronics and optoelectronics arising from its unique geometry. This structure gives rise to a large surface area-to-volume ratio and therefore understanding the effect of nanowire surfaces on nanowire optoelectronic properties is necessary for engineering related devices. We present a systematic study of the non-uniform optical properties of Au-catalyzed GaAs/AlGaAs core–shell nanowires introduced by changes in the sidewall faceting. Significant variation in intra-wire photoluminescence (PL) intensity and PL lifetime (τ_PL) was observed along the nanowire axis, which was strongly correlated with the variation of sidewall facets from {112} to {110} from base to tip. Faster recombination occurred in the vicinity of {112}-oriented GaAs/AlGaAs interfaces. An alternative nanowire heterostructure, the radial quantum well tube consisting of a GaAs layer sandwiched between two AlGaAs barrier layers, is proposed and demonstrates superior uniformity of PL emission along the entire length of nanowires. The results emphasize the significance of nanowire facets and provide important insights for nanowire device design.

Nonpolar GaAs Nanowires Catalyzed by Cu5As2: Insights into As Layer Epitaxy

Controlled synthesis of GaAs nanowires (NWs) with specific phases and orientations is important and challenging, which determines their electronic performances. Herein, single-crystalline GaAs NWs are successfully synthesized by using complementary metal-oxide semiconductor compatible Cu₂O catalysts via chemical vapor deposition at an optimized temperature of 560 °C. In contrast to typically Au catalyzed GaAs NWs, the Cu₂O catalyzed ones are found to grow along nonpolar orientations of zincblende <110> and <211> and wurtzite <1̅100> and <2̅110>. The Cu₂O catalysts are found to change into orthorhombic Cu₅As₂ after the NW growth, which is also significantly distinguished from the Au-Ga catalyst alloy in the literature. The Cu₅As₂ alloy plays the epitaxy role in the nonpolar GaAs NW growth due to the lattice matching with the nonpolar planes of GaAs, which is verified by the atomic stack model. These nonpolar oriented GaAs NWs have minimized stacking faults, promising for the other semiconductor synthesis as well as electronic applications.

Heterostructure and Q-factor engineering for low-threshold and persistent nanowire lasing

Continuous room temperature nanowire lasing from silicon-integrated optoelectronic elements requires careful optimisation of both the lasing cavity Q-factor and population inversion conditions. We apply time-gated optical interferometry to the lasing emission from high-quality GaAsP/GaAs quantum well nanowire laser structures, revealing high Q-factors of 1250 ± 90 corresponding to end-facet reflectivities of R = 0.73 ± 0.02. By using optimised direct-indirect band alignment in the active region, we demonstrate a well-refilling mechanism providing a quasi-four-level system leading to multi-nanosecond lasing and record low room temperature lasing thresholds (~6 μJ cm-2 pulse-1) for III-V nanowire lasers. Our findings demonstrate a highly promising new route towards continuously operating silicon-integrated nanolaser elements.

Characterization, Selection, and Microassembly of Nanowire Laser Systems

Semiconductor nanowire (NW) lasers are a promising technology for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy density, and subsequently printed in arrays onto a secondary substrate. Statistical analysis of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Analysis on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.

Threshold reduction and yield improvement of semiconductor nanowire lasers via processing-related end-facet optimization

Both gain medium design and cavity geometry are known to be important for low threshold operation of semiconductor nanowire lasers. For many applications nanowire lasers need to be transferred from the growth substrate to a low-index substrate; however, the impact of the transfer process on optoelectronic performance has not been studied. Ultrasound, PDMS-assisted and mechanical rubbing are the most commonly used methods for nanowire transfer; each method may cause changes in the fracture point of the nanowire which can potentially affect both length and end-face mirror quality. Here we report on four common approaches for nanowire transfer. Our results show that brief ultrasound and PDMS-assisted transfer lead to optimized optoelectronic performance, as confirmed by ensemble median lasing threshold values of 98 and 104 μJ cm^-2 respectively, with nanowires transferred by ultrasound giving a high lasing yield of 72%. The mean threshold difference between samples is shown to be statistically significant: while a significant difference in mean length from different transfer methods is seen, it is shown by SEM that end-facet quality is also affected and plays an important role on threshold gain for this nanowire architecture.