Reid J. Chesterfield

Gated four-probe measurements on pentacene thin-film transistors: Contact resistance as a function of gate voltage and temperature

We describe gated four-probe measurements designed to measure contact resistance in pentacene-based organic thin-film transistors (OTFTs). The devices consisted of metal source and drain electrodes contacting a 300-A-thick pentacene film thermally deposited on Al2O3 or SiO2 dielectrics with a p-doped Si substrate serving as the gate electrode. Voltage-sensing leads extending into the source-drain channel were used to monitor potentials in the pentacene film while passing current during drain voltage (VD) or gate voltage (VG) sweeps. We investigated the potential profiles as a function of contact metallurgy (Pt, Au, Ag, and Ca), substrate chemistry, VG, and temperature. The contact-corrected linear hole mobilities were as high as 1.75cm2∕Vs and the film sheet resistance and specific contact resistance were as low as 600kΩ∕◻ and 1.3kΩ-cm, respectively, at high gate voltages. In the temperature range of 50–200K, the pentacene OTFTs displayed an activated behavior with activation energies of 15–30meV. Importa...

Variable temperature film and contact resistance measurements on operating n-channel organic thin film transistors

We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activa...

Transport properties of single-crystal tetracene field-effect transistors with silicon dioxide gate dielectric

Single-crystal organic field-effect transistors (SX-OFETs) with channel lengths of 1 and 100μm have been fabricated by adhering thin crystals of tetracene to freshly ashed SiO2 substrates containing countersunk gold contacts. The intrinsic transport properties of the tetracene single crystals, corrected for potential contact effects by using a standard four-probe configuration, have been measured from room temperature down to 4.2K. These OFETs exhibit mobilities as high as 0.1cm2V−1s−1, subthreshold swings of <500mV∕decade, and Ion∕Ioff ratios in excess of 109. The larger devices (L=100μm,W=1000μm) show thermally activated mobilities over the temperature range 200K<T<300K, but thermally induced cracks in the crystal prevent this analysis from being extended to lower temperatures. The smaller devices have a greater probability of surviving to low temperatures without a crack permeating the active channel, and representative devices have been investigated over the full range 4.2K<T<300K. The transport mecha...

High mobility top-gated pentacene thin-film transistors

A common device geometry for measuring the electrical characteristics of organic semiconductors is the thin-film organic field-effect transistor (OTFT). Mostly for reasons of cost, convenience, and availability, this usually involves depositing the organic material on a prefabricated gate structure such as Si∕SiO2, the surface chemistry of which is often modified with self-assembled monolayers. The interactions between these surfaces and the deposited organic can have a profound effect on thin-film growth and the resulting electrical characteristics since most of the charge transport in these structures occurs near the organic-insulator interface. An alternative to this traditional technique is to assemble the transistor on top of the organic semiconductor. We have used chemical-vapor deposition of the polymeric dielectric material parylene to create pentacene OTFTs with gate electrodes both on top of and below the semiconductor film, with field-effect mobilities as high as 0.1cm2∕Vs and Ion∕Ioff ratios g...

Hydrostatic-pressure dependence of the photoconductivity of single-crystal pentacene and tetracene

Pentacene and tetracene show readily observable photoconductivity when illuminated with light in the blue part of the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of both materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a carrier-mobility increase under pressure is most likely to cause the increase in photoconductivity in the case of pentacene. For tetracene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results.

Hydrostatic-pressure dependence of organic thin-film transistor current versus voltage characteristics

We report results of electrical output and transfer characteristics for two top-contact pentacene thin-film transistors under hydrostatic pressure at room temperature. Strong reversible increases of the drain current and the field-effect hole mobility with increasing pressure were observed, in particular for a device with relatively low current at atmospheric pressure.