Trapping of environmental charges in surface states typically dominates electrical transport in nanostructured field-effect transistors (FETs) applied as sensors. Such surface effects produce exceptional sensitivity, yet time dependencies on experimental timescales simultaneously results in hysteresis of FET conductance and signal instability. Whereas hysteresis is usually suppressed by means of chemical surface treatments, here we study it as a source of information for ion sensing. Ion-sensitive FETs were prepared by coupling InAs nanowires to fluorosilicone membranes doped with Na+ ionophores. From cyclic transfer characteristics in electrolytes of varying concentration, potentiometric and hysteretic calibration curves were obtained. The observed hysteresis was attributed to changes in membrane capacitance by redox reactions between ionized donor-like traps at the InAs surface and electroactive membrane constituents. Hysteresis was correlated to the ion potential through a model and demonstrated a filtering effect that stabilized the hysteretic response against potential drifts. Furthermore, the model elucidated the ability to modulate ion sensitivity by controlling the initial density of ionized traps via electrostatic polarization by the gate. In this mode of active operation, we demonstrate enhancement above the Nernstian limit with linear calibrations of (-77.5 ? 3.2 to -80.7 ? 3.0 mV/dec) despite the presence of nonequilibrium ion fluxes.

Much recent attention has been focused on the development of field-effect transistors based on low-dimensional nanostructures for the detection and manipulation of molecules. Because of their extraordinarily high charge sensitivity, InAs nanowires present an excellent material system in which to probe and study the behavior of molecules on their surfaces and elucidate the underlying mechanisms dictating the sensor response. So far, chemical sensors have relied on slow, activated processes restricting their applicability to high temperatures and macroscopic adsorbate coverages. Here, we identify the transition into a highly sensitive regime of chemical sensing at ultralow concentrations (<1 ppm) via physisorption at room temperature using field-effect transistors with channels composed of several thousand InAs nanowires and ethanol as a simple analyte molecule. In this regime, the nanowire conductivity is dictated by a local gating effect from individual dipoles, leading to a nonlinear enhancement of the sensitivity. At higher concentrations (>1 ppm), the nanowire channel is globally gated by a uniform dipole layer at the nanowire surface. The former leads to a dramatic increase in sensitivity due to weakened screening and the one-dimensional geometry of the nanowire. In this regime, we detect concentrations of ethanol vapor as low as 10 ppb, 100 times below the lowest concentrations previously reported. Furthermore, we demonstrate electrostatic control of the sensitivity and dynamic range of the InAs nanowire-based sensor and construct a unified model that accurately describes and predicts the sensor response over the tested concentration range (10 ppb to 10 ppm).

Much recent attention has been focused on the development of field-effect transistors based on low dimensional nanostructures for the detection and manipulation of molecules. Because of their extraordinarily high charge sensitivity, InAs nanowires present an excellent material system in which to probe and study the behaviour of molecules on their surfaces and elucidate the underlying mechanisms dictating the sensor response. So far, chemical sensors have relied on slow, activated processes restricting their applicability to high temperatures and macroscopic adsorbate coverages. Here, we identify the transition into a highly sensitive regime of chemical sensing at ultra-low concentrations (< 1 ppm) via physisorption at room temperature using field-effect transistors with channels comprised of several thousand InAs nanowires and ethanol as a simple analyte molecule. In this regime, the nanowire conductivity is dictated by a local gating effect from individual dipoles leading to a non-linear enhancement of the sensitivity. At higher concentrations (> 1 ppm), the nanowire channel is globally gated by a uniform dipole layer at the nanowire surface. The former leads to a dramatic increase in sensitivity due to weakened screening and the one-dimensional geometry of the nanowire. In this regime, we detect concentrations of ethanol vapour as low as 10 ppb, 100 times below the lowest concentrations previously reported. Furthermore, we demonstrate electrostatic control of the sensitivity and dynamic range of the InAs nanowire-based sensor and construct a unified model that accurately describes and predicts the sensor response over the tested concentration range (10 ppb to 10 ppm).

Because of their high aspect ratio, nanostructures are particularly susceptible to effects from surfaces such as slow electron trapping by surface states. However, nonequilibrium trapping dynamics have been largely overlooked when considering transport in nanoelectronic devices. In this study, we demonstrate the profound influence of dynamic trapping processes on transport in InAs nanowires through an investigation of the hysteretic and time-dependent behavior of the transconductance. We observe large densities (similar to 10(13) cm(-2)) of slow surface traps and demonstrate the ability to control and permanently fix their occupation and charge through electrostatic manipulation by the gate potential followed by thermal deactivation by cryogenic cooling. Furthermore, we observe a transition from enhancement- to depletion-mode and a 400% change in field-effect mobility within the same device when the initial gate voltage and sweep rate are varied, revealing the severe impact of electrostatic history and dynamics on InAs nanowire field-effect transistors. A time-dependent model for nanowire transconductance based on nonequilibrium carrier population dynamics with thermally activated capture and emission was constructed and showed excellent agreement with experiments, confirming the effects to be a direct result of the dynamics of slow surface traps characterized by large thermal activation barriers (similar to 700 meV). This work reveals a clear and direct link between the electrical conductivity and the microscopic interactions of charged species with nanowire surfaces and highlights the necessity for considering dynamic properties of surface states in nanoelectronic devices.

The conductance of semiconductor nanowires is strongly dependent on their electrostatic history because of the overwhelming influence of charged surface and interface states on electron confinement and scattering. We show that InAs nanowire field-effect transistor devices can be conditioned to suppress resonances that obscure quantized conduction thereby revealing as many as six sub-bands in the conductance spectra as the Fermi-level is swept across the sub-band energies. The energy level spectra extracted from conductance, coupled with detailed modeling shows the significance of the interface state charge distribution revealing the Coulomb landscape of the nanowire device. Inclusion of self-consistent Coulomb potentials, the measured geometrical shape of the nanowire, the gate geometry and nonparabolicity of the conduction band provide a quantitative and accurate description of the confinement potential and resulting energy level structure. Surfaces of the nanowire terminated by HfO2 are shown to have their interface donor density reduced by a factor of 30 signifying the passivating role played by HfO2.

Nanowire-based field-effect transistors (FETs) have demonstrated considerable promise for a new generation of chemical and biological sensors. Indium arsenide (InAs), by virtue of its high electron mobility and intrinsic surface accumulation layer of electrons, holds properties beneficial for creating high performance sensors that can be used in applications such as point-of-care testing for patients diagnosed with chronic diseases. Here, we propose devices based on a parallel configuration of InAs nanowires and investigate sensor responses from measurements of conductance over time and FET characteristics. The devices were tested in controlled concentrations of vapour containing acetic acid, 2-butanone and methanol. After adsorption of analyte molecules, trends in the transient current and transfer curves are correlated with the nature of the surface interaction. Specifically, we observed proportionality between acetic acid concentration and relative conductance change, off current and surface charge density extracted from subthreshold behaviour. We suggest the origin of the sensing response to acetic acid as a two-part, reversible acid-base and redox reaction between acetic acid, InAs and its native oxide that forms slow, donor-like states at the nanowire surface. We further describe a simple model that is able to distinguish the occurrence of physical versus chemical adsorption by comparing the values of the extracted surface charge density. These studies demonstrate that InAs nanowires can produce a multitude of sensor responses for the purpose of developing next generation, multi-dimensional sensor applications.

The charge distribution and potential profile of p-n-junctions in thin semiconductor nanowires (NWs) were analyzed. The characteristics of screening in one-dimensional systems result in a specific profile with large electric field at the boundary between the n- and p- regions, and long tails with a logarithmic drop in the potential and charge density. As a result of these tails, the junction properties depend sensitively on the geometry of external contacts and its capacity has an anomalously large value and frequency dispersion. In the presence of an external voltage, electrons and holes in the NWs can not be described by constant quasi-Fermi levels, due to small values of the average electric field, mobility, and lifetime of carriers. Thus, instead of the classical Sah-Noice-Shockley theory, the junction current-voltage characteristic was described by an alternative theory suitable for fast generation-recombination and slow diffusion-drift processes. For the non-uniform electric field in the junction, this theory predicts the forward branch of the characteristic to have a non-ideality factor. several times larger than the values 1 < eta < 2 from classical theory. Such values of. have been experimentally observed by a number of researchers, as well as in the present work.

We report the demonstration of hybrid high-T-c-superconductor-semiconductor tunnel junctions, enabling new interdisciplinary directions in condensed matter research. The devices are fabricated by our newly developed mechanical-bonding technique, resulting in high-T-c-superconductor-semiconductor tunnel diodes. Tunneling-spectra characterization of the hybrid junctions of Bi2Sr2CaCu2O8+delta combined with bulk GaAs, or a GaAs/AlGaAs quantum well, exhibits excess voltage and nonlinearity, similarly to spectra obtained in scanning-tunneling microscopy, and is in good agreement with theoretical predictions for a d-wave-superconductor-normal-material junction. Additional junctions are demonstrated using Bi2Sr2CaCu2O8+delta combined with graphite or Bi2Te3. Our results pave the way for new methods in unconventional superconductivity studies, novel materials, and quantum technology applications. DOI: 10.1103/PhysRevX.2.041019

The structure of wurtzite and zinc blende InAs–GaAs (001) core–shell nanowires grown by molecular beam epitaxy on GaAs (001) substrates has been investigated by transmission electron microscopy. Heterowires with InAs core radii exceeding 11 nm, strain relax through the generation of misfit dislocations, given a GaAs shell thickness greater than 2.5 nm. Strain relaxation is larger in radial directions than axial, particularly for shell thicknesses greater than 5.0 nm, consistent with molecular statics calculations that predict a large shear stress concentration at each interface corner.

The conductance due to scattering by a single repulsive Coulomb impurity is measured as a function of gate voltage in backgated InAs nanowires by analysis of random telegraph noise. Comparison with a quantum mechanical theory for carrier response and scattering reveals that the large dielectric mismatch between the nanowire and its surroundings enhances the Coulomb interaction, produces a nonlinear screening process that weakens dielectric response, and enhances the self-consistent Coulomb-impurity barrier of a single repulsive impurity, as nanowire diameter is reduced. Consequently, the scattering rate by such an impurity is enhanced by nearly two orders magnitude for 30 nm diameter InAs nanowires. A dramatic asymmetry of scattering by repulsive and attractive impurities, where the latter produce majority carriers, explain how a single repulsive impurity can control the conductance of a 1 mu m long InAs nanowire. Relevance to proposed nanoelectronic and sensing devices is discussed.

Indium-arsenide–gallium-arsenide (InAs–GaAs) core-shell, wurtzite nanowires have been grown on GaAs (001) substrates. The core-shell geometries (core radii 11 to 26 nm, shell thickness >2.5 nm) exceeded equilibrium critical values for strain relaxation via dislocations, apparent from transmission electron microscopy. Partial axial relaxation is detected in all nanowires increasing exponentially with size, while radial strain relaxation is >90%, but undetected in nanowires with both smaller core radii <16 nm and shell thicknesses <5 nm. Electrical measurements on individual core-shell nanowires show that the resulting dislocations are correlated with reduced electron field-effect mobility compared to bare InAs nanowires.

We report a novel method for probing the gate-voltage dependence of the surface potential of individual semiconductor nanowires. The statistics of electronic occupation of a single defect on the surface of the nanowire, determined from a random telegraph signal, is used as a measure for the local potential. The method is demonstrated for the case of one or two switching defects in indium arsenide (InAs) nanowire field effect transistors at temperatures T=25-77 K. Comparison with a self-consistent model shows that surface potential variation is retarded in the conducting regime due to screening by surface states with density Dss≈10(12) cm(-2) eV(-1). Temperature-dependent dynamics of electron capture and emission producing the random telegraph signals are also analyzed, and multiphonon emission is identified as the process responsible for capture and emission of electrons from the surface traps. Two defects studied in detail had capture activation energies of EB≈50 meV and EB≈110 meV and cross sections of σ∞≈3×10(-19) cm2 and σ∞≈2×10(-17) cm2, respectively. A lattice relaxation energy of Sℏω=187±15 meV was found for the first defect.

Summary form only given: Semiconductor nanowires produced by metal-catalyzed vapour liquid solid (VLS) growth continue to attract strong interest for applications in electronics, optoelectronics, and biological sensing. Charge transport in nanowires has been shown to be strongly influenced by ionized impurities, surface roughness scattering, and charged surface states. Recently we discovered that dynamic capture of electrons producing repulsively charged traps can strongly modulate the conductance of backgated InAs nanowires, and shift threshold voltage by more than 100 mV, and that the modulation in conductance depends sensitively on nanowire diameter. To obtain high performance field effect transistors and sensors, scattering from charged centers and surface roughness should be reduced as much as possible. With the goal of improving surface properties in mind, we recently we began investigation of growth of InAs/GaAs core/shell heterostructure nanowires. We discuss preliminary results from this study along with some device-oriented applications.

We report on measurements of the infrared photo-response of InAs-based p-i-n diodes in the spectral region above 1.8 mu m. These photodiodes were fabricated from arrays of InAs quantum dots grown in nano-patterned template structures using a combination of block copolymer lithography and molecular beam epitaxy. The devices studied were comprised of a single layer of quantum dots. The temperature dependence of the current versus voltage for these devices is presented and discussed. Finally, a model is presented that can explain the key characteristics of the measured current versus voltage curves as a function of both temperature and applied electric field.

Catalyst-assisted growth of semiconductor nanowires has opened up several new and exciting possibilities for low-dimensional semiconductor structures. The authors review progress on the realization of quantum dots in semiconductor nanowires, and their characterization by transport spectroscopy. Emphasis is placed on the wide range electronic properties exhibited due to flexibility of the growth process in terms of nanostructure composition and size. Particular attention is placed on studies of spin in few-electron quantum dots.

Nature Nanotechnology 5, 737–741 (2010); published online: 19 September 2010; corrected after print: 6 December 2010. In the version of this Letter originally published, a label in Figure 1a was incorrect. There were also two minor text errors. These errors have now been corrected in the HTML and PDF versions of the text.

We report on selective area growth of InAs quantum dots on GaAs substrates patterned with a hexagonal array of 20 nm pores using block copolymer lithography. We discuss the mechanisms of growth, highlighting the variation in the resulting morphology as a function of nucleation enhancing AlGaAs layers. We also evaluate the optoelectronic performance of p-i-n photodiodes based on single layer nanopatterned grown InAs quantum dot devices. At low to moderate reverse biases, we observe room temperature photoresponse in both near- and mid-IR regimes. At high biases, we observe strong avalanche effects in the mid-IR range with a gain factor of ∼4000.

We report on selective area growth of InAs and GaAs quantum dots (QDs) on GaAs through ∼20 nm SiO2 windows prepared by block copolymer lithography. We discuss the mechanisms of growth through these masks, highlighting the variation of the resulting morphology (dot size, spacing, uniformity, and areal density) as a function of growth parameters. We have obtained highly uniform arrays of InAs and GaAs QDs with mean diameters and areal densities of 20.6 nm and 1×1011 cm−2, respectively. We have also investigated the optical characteristics of these QDs as a function of temperature and drawn correlations between the optical response and their crystalline quality.

An InAs/GaAs quantum dot saturable absorber mirror was used to mode lock a Nd:YVO4 solid-state laser at the operation wavelength of 1064.6 nm. Multi-watt average power was obtained during stable cw mode locking with pulses as short as 24 ps, and with a repetition rate of 65 MHz.