Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy

Electron mobilities approaching bulk limits in "surface-free" GaAs nanowires

Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core-shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core-shell nanowires exhibited electron mobilities up to 3000 cm(2) V(-1) s(-1), reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Conductivity control of as-grown branched indium tin oxide nanowire networks

Branched indium tin oxide (ITO) nanowire networks are promising candidates for transparent conductive oxide applications, such as optoelectronic electrodes, due to their high porosity. However, these branched networks also present new challenges in assessing conductivity. Conventional four-point probe techniques cannot separate the effect of porosity on the long-range conductivity from the intrinsic material conductivity. Here we compare the average nanoscale conductivity within the film measured by terahertz time-domain spectroscopy (THz-TDS) to the film conductivity measured by four-point probe in a branched ITO nanowire network. Both techniques report conductivity increases with deposition flux rate from 0.5 to 3.0 nm s(-1), achieving a maximum of ~ 10 (Ω cm)(-1). Modeling the THz-TDS conductivity data using the Drude-Smith model allows us to distinguish between conductivity increases resulting from morphological changes and those resulting from the intrinsic properties of the ITO. In particular, the intrinsic material conductivity within the nanowires can be extracted, and is found to reach a maximum of ~ 3000 (Ω cm)(-1), comparable to bulk ITO. To determine the mechanism responsible for increasing conductivity with flux rate, we characterize dopant concentration and morphological changes (i.e., to branching behavior, nanowire diameter and nucleation layers). We propose that changes in the electron density, primarily due to changes in O-vacancy concentration at different flux rates, are responsible for the observed conductivity increase. This understanding will assist balancing structural and conductivity requirements in applications of transparent conductive oxide networks.

Enhanced luminescence properties of InAs-InAsP core-shell nanowires

Utilizing narrow band gap nanowire (NW) materials to extend nanophotonic applications to the mid-infrared spectral region (>2-3 μm) is highly attractive, however, progress has been seriously hampered due to their poor radiative efficiencies arising from nonradiative surface and Auger recombination. Here, we demonstrate up to ~ 10(2) times enhancements of the emission intensities from InAs NWs by growing an InAsP shell to produce core-shell NWs. By systematically varying the thickness and phosphorus (P)-content of the InAsP shell, we demonstrate the ability to further tune the emission energy via large strain-induced peak shifts that already exceed >100 meV at comparatively low fractional P-contents. Increasing the P-content is found to give rise to additional line width broadening due to asymmetric shell growth generated by a unique transition from {110}- to {112}-sidewall growth as confirmed by cross-sectional scanning transmission electron microscopy. The results also elucidate the detrimental effects of plastic strain relaxation on the emission characteristics, particularly in core-shell structures with very high P-content and shell thickness. Overall, our findings highlight that enhanced mid-infrared emission efficiencies with effective carrier confinement and suppression of nonradiative recombination are highly sensitive to the quality of the InAs-InAsP core-shell interface.

Enhanced minority carrier lifetimes in GaAs/AlGaAs core-shell nanowires through shell growth optimization

The effects of AlGaAs shell thickness and growth time on the minority carrier lifetime in the GaAs core of GaAs/AlGaAs core-shell nanowires grown by metal-organic chemical vapor deposition are investigated. The carrier lifetime increases with increasing AlGaAs shell thickness up to a certain value as a result of reducing tunneling probability of carriers through the AlGaAs shell, beyond which the carrier lifetime reduces due to the diffusion of Ga-Al and/or impurities across the GaAs/AlGaAs heterointerface. Interdiffusion at the heterointerface is observed directly using high-angle annular dark field scanning transmission electron microscopy. We achieve room temperature minority carrier lifetimes of 1.9 ns by optimizing the shell growth with the intention of reducing the effect of interdiffusion.

Raman spectroscopy of self-catalyzed GaAs(1-x)Sb(x) nanowires grown on silicon

Thanks to their wide band structure tunability, GaAs(1-x)Sb(x) nanowires provide exciting perspectives in optoelectronic and energy harvesting applications. The control of composition and strain of these ternary alloys is crucial in the determination of their optical and electronic properties. Raman scattering provides information on the vibrational properties of materials, which can be related to the composition and strain. We present a systematic study of the vibrational properties of GaAs(1-x)Sb(x) nanowires for Sb contents from 0 to 44%, as determined by energy-dispersive x-ray analyses. We find that optical phonons red-shift with increasing Sb content. We explain the shift by alloying effects, including mass disorder, dielectric changes and ionic plasmon coupling. The influence of Sb on the surface optical modes is addressed. Finally, we compare the luminescence yield between GaAs and GaAs(1-x)Sb(x), which can be related to a lower surface recombination rate. This work provides a reference for the study of ternary alloys in the form of nanowires, and demonstrates the tunability and high material quality of gold-free ternary antimonide nanowires directly grown on silicon.

Direct observation of charge-carrier heating at WZ-ZB InP nanowire heterojunctions

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.