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Register a new userLinear conduction in N-type organic field effect transistors with nanometric channel lengths and graphene as electrodes
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In this work we test graphene electrodes in nano-metric channel n-type Organic Field EffectTransistors (OFETs) based on thermally evaporated thin films of perylene-3,4,9,10-tetracarboxylic acid diimide derivative (PDIF-CN2). By a thorough comparison with short channel transistors made with reference gold electrodes, we found that the output characteristics of the graphene-based devices respond linearly to the applied biases, in contrast with the supra-linear trend of gold-based transistors. Moreover, short channel effects are considerably suppressed in graphene electrodes devices. More specifically, current on/off ratios independent of the channel length (L) and enhanced response for high longitudinal biases are demonstrated for L down to ~140 nm. These results are rationalized taking into account the morphological and electronic characteristics of graphene, showing that the use of graphene electrodes may help to overcome the problem of Space Charge Limited Current (SCLC) in short channel OFETs.
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We report the development of nanowire field-effect transistors featuring an ultra-thin parylene film as a polymer gate insulator. The room temperature, gas-phase deposition of parylene is an attractive alternative to oxide insulators prepared at high temperatures using atomic layer deposition. We discuss our custom-built parylene deposition system, which is designed for reliable and controlled deposition of <100 nm thick parylene films on III-V nanowires standing vertically on a growth substrate or horizontally on a device substrate. The former case gives conformally-coated nanowires, which we used to produce functional $Omega$-gate and gate-all-around structures. These give sub-threshold swings as low as 140 mV/dec and on/off ratios exceeding $10^3$ at room temperature. For the gate-all-around structure, we developed a novel fabrication strategy that overcomes some of the limitations with previous lateral wrap-gate nanowire transistors. Finally, we show that parylene can be deposited over chemically-treated nanowire surfaces; a feature generally not possible with oxides produced by atomic layer deposition due to the surface `self-cleaning effect. Our results highlight the potential for parylene as an alternative ultra-thin insulator in nanoscale electronic devices more broadly, with potential applications extending into nanobioelectronics due to parylenes well-established biocompatible properties.
The realization of both p-type and n-type operations in a single organic field effect transistor (OFET) is critical for simplifying the design of complex organic circuits. Typically, only p-type or n-type operation is realized in an OFET, while the respective counterpart is either suppressed by charge trapping or limited by the injection barrier with the electrodes. Here we show that only the presence of a top dielectric turns an n-type polymer semiconductor (N2200, Polyera ActiveInk) into an ambipolar one, as detected from both bottom and top gated OFET operation. The effect is independent of the channel thickness and the top dielectric combinations. Variable temperature transfer characteristics show that both the electrons and holes can be equally transported through the bulk of the polymer semiconductor.
We report on room temperature THz detection by means of antenna-coupled field effect transistors fabricated by using epitaxial graphene grown on silicon carbide substrate. Two independent detection mechanisms are found: plasma wave assisted-detection and thermoelectric effect, which is ascribed to the presence of junctions along the FET channel. The superposition of the calculated functional dependence of both the plasmonic and thermoelectric photovoltages on the gate bias qualitatively well reproduces the measured photovoltages. Additionally, the sign reversal of the measured photovoltage demonstrates the stronger contribution of the plasmonic detection compared to the thermoelectric mechanism. Although responsivity improvement is necessary, these results demonstrate that plasmonic detectors fabricated by epitaxial graphene on silicon carbide are potential candidates for fast large area imaging of macroscopic samples.
Polymer field-effect transistors with 2D graphene electrodes are devices that merge the best of two worlds: on the one hand, the low-cost and processability of organic materials and, on the other hand, the chemical robustness, extreme thinness and flexibility of graphene. Here, we demonstrate the tuning of the ambipolar nature of the semiconductor polymer N2200 from Polyera ActiveInk by incorporating graphene electrodes in a transistor geometry. Our devices show a balanced ambipolar behavior with high current ON-OFF ratio and charge carrier mobilities. These effects are caused by both the effective energy barrier modulation and by the weak electric field screening effect at the graphene-polymer interface. Our results provide a strategy to integrate 2D graphene electrodes in ambipolar transistors in order to improve and modulate their characteristics, paving the way for the design of novel organic electronic devices.
Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 ${mu}A/{mu}m$ at 80 K and >200 ${mu}A/{mu}m$ at 300 K) and relatively low contact resistance (1.2 to 2 $k{Omega}cdot{mu}m$ from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals, extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer h-BN between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly de-pin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.
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