Aimin Song

Unidirectional electron flow in a nanometer-scale semiconductor channel: A self-switching device

By tailoring the boundary of a narrow semiconductor channel to break its symmetry, we have realized a type of nanometer-scale nonlinear device, which we refer to as self-switching device (SSD). An applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel depending on the sign of V. This results in a diode-like characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually zero to more than 10 V, by simply adjusting the channel width. The planar and two-terminal structure of the SSD also allows SSD-based circuits to be realized by only one step of lithography.

An investigation of the conductivity of peptide nanotube networks prepared by enzyme-triggered self-assembly

We demonstrate that nanotubular networks formed by enzyme-triggered self-assembly of Fmoc-L3 (9-fluorenylmethoxycarbonyl-tri-leucine) show significant charge transport. FT-IR, fluorescence spectroscopy and wide angle X-ray scattering (WAXS) data confirm formation of beta-sheets that are locked together viapi-stacking interactions. Molecular dynamics simulations confirmed the pi-pi stacking distance between fluorenyl groups to be 3.6-3.8 A. Impedance spectroscopy demonstrated that the nanotubular xerogel networks possess minimum sheet resistances of 0.1 MOmega/sq in air and 500 MOmega/sq in vacuum (pressure: 1.03 mbar) at room temperature, with the conductivity scaling linearly with the mass of peptide in the network. These materials may provide a platform to interface biological components with electronics.

Influence of processing conditions on the stability of poly(3-hexylthiophene)-based field-effect transistors

Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene)-2,5-diyl were fabricated under different process conditions. The devices displayed drastic differences in their ambient-air stability. Whereas it took only about 10min in air for the off current to increase by one order of magnitude in OFETs prepared with chloroform and hexamethyldisilazane, a 120min exposure to air caused only a slight degradation of OFETs prepared using 1,2,4-trichlorobenzene, n-octadecyltrichlorosilane, and a heat treatment. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed.

Room-temperature operation of a unipolar nanodiode at terahertz frequencies

We report on the room-temperature electrical rectification at 1.5 THz of a unipolar nanodiode based on symmetry breaking in a nanochannel. The exploitation of its nonlinear diodelike characteristic and intrinsically low parasitic capacitance enables rectification at ultrahigh speed. The zero-voltage threshold and unique planar layout make the nanodiode suitable for building large arrays. This is the highest speed reported in nanorectifiers to date.

Operation of InGaAs/InP-Based Ballistic Rectifiers at Room Temperature and Frequencies up to 50 GHz

Novel semiconductor rectifiers based on ballistic electron transport are fabricated from a high electron-mobility InGaAs/InP wafer. Because the device sizes are sufficiently small, operations at room temperature are achieved. Furthermore, the devices are shown to work not only at least up to 50 GHz but also with a sensitivity roughly the same as commercial microwave diodes, despite the fact that the devices have not yet been optimized. Aspects of using the devices in microwave applications are discussed in terms of the physical mechanism of the novel rectifying effect.

Room-temperature operations of memory devices based on self-assembled InAs quantum dot structures

Memory devices have been fabricated in high-electron-mobility transistors with embedded InAs quantum dots (QDs). We show that memory operations can be fully controlled by gate biases at room temperature, without the need for light excitations to erase memory states. Real-time measurements indicate a charge retention time of a few minutes. Neither such retention time nor the self-consistent simulations can justify the picture that the memory effect is due to charging/discharging of intrinsic QD states. Experiments at a series of gate biases point to the presence of deep levels coexisting in the QD layer(s), which are responsible for the memory effect.

Flexible indium–gallium–zinc–oxide Schottky diode operating beyond 2.45 GHz

Mechanically flexible mobile phones have been long anticipated due to the rapid development of thin-film electronics in the last couple of decades. However, to date, no such phone has been developed, largely due to a lack of flexible electronic components that are fast enough for the required wireless communications, in particular the speed-demanding front-end rectifiers. Here Schottky diodes based on amorphous indium-gallium-zinc-oxide (IGZO) are fabricated on flexible plastic substrates. Using suitable radio-frequency mesa structures, a range of IGZO thicknesses and diode sizes have been studied. The results have revealed an unexpected dependence of the diode speed on the IGZO thickness. The findings enable the best optimized flexible diodes to reach 6.3 GHz at zero bias, which is beyond the critical benchmark speed of 2.45 GHz to satisfy the principal frequency bands of smart phones such as those for cellular communication, Bluetooth, Wi-Fi and global satellite positioning.

Gunn oscillations in a self-switching nanodiode

The feasibility of Gunn oscillations in a planar nanoscale unipolar diode or a self-switching device (SSD) is analyzed using Monte Carlo simulations. The asymmetric nanochannel that the SSD is based on is shown to induce charge domains much more receptively when compared to a reference symmetric nanochannel. The oscillation frequency reaches 130 GHz. Potential applications are discussed in terms of the ease of heat dissipation and generation of oscillations at different frequencies on a single chip, in contrast to a conventional vertical-structure Gunn diode.

Nanometer-scale two-terminal semiconductor memory operating at room temperature

Based on a nanometer-scale semiconductor channel with an intentionally broken geometric symmetry, we have realized a type of memory device that consists of only two terminals, rather than the minimum of three terminals in conventional semiconductor memories. The charge retention time is at least 10 h at cryogenic temperatures and a few minutes at room temperature. Furthermore, the simplicity of the design allows the active part of the devices to be made in a single nanolithography step which, along with the planar structure of the device, provides promising possibilities for a high integration density

Enhanced terahertz detection by localized surface plasma oscillations in a nanoscale unipolar diode

By using a two-dimensional ensemble Monte Carlo method, we have studied the terahertz-frequency response of a self-switching device (SSD), which is a semiconductor rectifier consisting of an asymmetric nanochannel. The simulations reveal that the performance can be improved by adjusting the shape and dielectric material filling of the insulating trenches that define the SSD. We show that the rectified current of the SSD has a nonmonotonic frequency dependence with a pronounced peak occurring just below the cutoff frequency. Through optimizations of the geometry, the peak current can reach twice that at low frequencies, enabling not only a higher detection sensitivity but also a degree of frequency selectivity. The effect is discussed in terms of a localized surface plasma oscillation in the asymmetric nanostructure.