Nonlinear Chemical Sensitivity Enhancement of Nanowires in the Ultralow Concentration Regime.

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).

Surface State Dynamics Dictating Transport in InAs Nanowires.

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 (∼1013 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 (∼ 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.

Surface Properties from Transconductance in Nanoscale Systems.

Because of the continued scaling of transistor dimensions and incorporation of nanostructured materials into modern electronic and optoelectronic devices, surfaces and interfaces have become a dominant factor dictating material properties and device performance. In this study, we investigate the temperature-dependent electronic transport properties of InAs nanowire field-effect transistors. A point where the nanowire conductance becomes independent of temperature is observed, known as the zero-temperature-coefficient. The distribution of surface states is determined by a spectral analysis of the conductance activation energy and used to develop a carrier transport model that explains the existence and gate voltage dependence of this point. We determine that the position of this point in gate voltage is directly related to the fixed oxide charge on the nanowire surface and demonstrate the utility of this method for studying surface passivations in nanoscale systems by characterizing (NH4)2Sx and H2 plasma surface treatments on InAs nanowires.

Electron states in semiconductor quantum dots.

In this work, the electronic structures of quantum dots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications.

Probing the gate--voltage-dependent surface potential of individual InAs nanowires using random telegraph signals.

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.

Nano-optical probing of exciton wave-functions confined in a GaAs quantum dot.

We have enhanced the performance of near-field scanning optical microscopy (NSOM) in terms of the spatial resolution and the sensitivity in signal detection. A careful preparation of an aperture-NSOM probe provides us with a spatial resolution as high as 30 nm in fluorescence imaging spectroscopy. We have applied this technique to map out the center-of-mass wave functions of an exciton confined in a GaAs quantum dot (a monolayer-high island formed in a quantum well). The spatial profile of the exciton emission, which reflects the shape of the island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. A theoretical calculation of the spatial distribution of the polarization field quantitatively reproduced the experimental result. Furthermore, mapping of an excited state wave-function with a node structure is also demonstrated. The novel technique can be extensively applied to wave-function engineering in the design and fabrication of quantum devices.