J.A. Nichols

An experimental study of contact effects in organic thin film transistors

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

Pentacene organic thin-film transistors for circuit and display applications

We have fabricated organic thin-film transistors (TFTs) using the small-molecule polycyclic aromatic hydrocarbon pentacene as the active material. Devices were fabricated on glass substrates using low-temperature ion-beam deposited silicon dioxide as the gate dielectric, ion-beam deposited palladium for the source and drain contacts, and vacuum-evaporated pentacene to form the active layer. Excellent electrical characteristics were obtained, including carrier mobility as large as 0.6 cm/sup 2//V-s, on/off current ratio as large as 10/sup 8/, and subthreshold slope as low as 0.7 V/dec, all record values for organic transistors fabricated on nonsingle-crystal substrates.

Analog and digital circuits using organic thin-film transistors on polyester substrates

We have fabricated and characterized analog and digital circuits using organic thin-film transistors on polyester film substrates. These are the first reported dynamic results for organic circuits fabricated on polyester substrates. The high-performance pentacene transistors yield circuits with the highest reported clock frequencies for organic circuits.

Thin-film transistors based on well-ordered thermally evaporated naphthacene films

We report on organic thin-film transistors fabricated using the small-molecule organic semiconductor naphthacene as the active layer material with device performance suitable for several large-area or low-cost electronics applications. We investigated naphthacene thin films deposited by thermal evaporation onto amorphous substrates held near room temperature. Using atomic-force microscopy and x-ray diffraction we find naphthacene films consist of a high density of submicron-sized grains with a surprisingly high degree of molecular order. Thin-film transistors fabricated using evaporated naphthacene films on thermally oxidized silicon substrates have field-effect mobility larger than 0.1 cm2/V s, current on/off ratio greater than 106, negative threshold voltage, and subthreshold slope of 1 V/decade.

Potential imaging of pentacene organic thin-film transistors

Scanning Kelvin probe microscopy (SKPM) has been used to simultaneously obtain high-resolution topographical and potential images of pentacene organic thin-film transistors (OTFTs) during device operation. SKPM images of OTFTs show large potential drops at the source with the magnitude dependent on contact metallurgy and relatively small potential drops at grain boundaries in polycrystalline pentacene films.

Improved organic thin film transistor performance using chemically modified gate dielectrics

We report on the use of silicon dioxide gate dielectric chemically-modified with vapor-deposited octadecyltrichlorosilane (OTS) monolayers for improved organic thin film transistor (OTFT) performance. To date, silicon dioxide gate dielectric chemically-modified with OTS monolayers deposited from solvent solution have demonstrated the highest reported OTFT performance using the small-molecule organic semiconductor pentacene as the active layer. Vapor treatment is an attractive alternative, especially for polymeric substrates that may be degraded by solvent exposure. Using our OTS vapor treatment we have fabricated photolithographically defined pentacene OTFTs on flexible polymeric substrates with field-effect mobility greater than 1.5 cm2/V-s. We find the performance of pentacene as well as several other small-molecule organic active layer materials can be significantly improved using silicon dioxide gate dielectric chemically-modified with vacuum vapor prime OTS. Pentacene, naphthacene, Cu-phthalocyanine, and alpha-sexithienyl OTFTs fabricated on thermally oxidized silicon substrates with photolithographically defined bottom contacts typically show a factor of 2 to 5 improvement in field-effect mobility and reduced subthreshold slope when using silicon dioxide gate dielectric vacuum vapor treated with OTS compared to OTFTs on untreated gate dielectric.

Ion-beam-deposited ultrathin transparent metal contacts

Using ion-beam sputtering we have prepared ultrathin transparent metal contacts with large broad-band optical transmittance and low electrical sheet resistance. Metal films deposited by ion-beam sputtering have exceptionally small surface roughness, and films as thin as about 20 A are continuous and conductive, and provide optical transmittance as large as 80%. Ultrathin transparent metal contacts provide a number of advantages over more commonly used conductive transparent metal oxides such as indium tin oxide. Unlike indium tin oxide, ultrathin metal contacts can be deposited at room temperature and require no post-deposition anneal, allowing thin film optoelectronic devices such as organic light-emitting diodes and photovoltaic cells to be fabricated on low-cost, lightweight, flexible polymeric substrates. Transparent metal contacts may also eliminate the oxygen-related degradation of organic thin film devices associated with indium tin oxide contacts. Using 30-A thick ion-beam-deposited transparent palladium contacts we have fabricated organic light-emitting diodes on inexpensive, flexible plastic substrates and obtained devices with good injection and emission characteristics. Finally, unlike indium tin oxide, ultrathin metal contacts provide large optical transmittance in the ultraviolet part of the spectrum, making them useful for ultraviolet photodetectors and providing the potential for increased conversion efficiency for photovoltaic cells, especially for space applications.

Fast organic circuits on flexible polymeric substrates

We have fabricated the fastest organic circuits on flexible substrates yet reported. These circuits use the small-molecule hydrocarbon pentacene as the active semiconductor material and 75 /spl mu/m thick flexible, transparent, colorless, polyethylene naphthalate (PEN) film as the substrate. Transistor arrays, inverters, ring oscillators, and other circuits with good electrical performance, yield, and uniformity were obtained. A field-effect mobility of 1 cm/sup 2// V-s was extracted from OTFT saturation characteristics, and ring oscillators had minimum propagation delay <40 /spl mu/sec per stage and <50 /spl mu/sec per stage at bias levels below 8 V.

52.4L: Late-News Paper: a-Si:H TFT Active-Matrix Phosphorescent OLED Pixel

We report a two-transistor amorphous silicon (a-Si:H) thin film transistor (TFT) active-matrix organic light emitting diode (OLED) pixel using high-efficiency phosphorescent OLED devices. The gate to source voltage (VGS) for the OLED drive transistor is less than 8 V for a pixel brightness of 100 cd/m2 and less than 12 V for 400 cd/m2. The low drive voltage is important for good TFT stability and display uniformity.

An organic thin film transistor backplane for flexible liquid crystal displays

Organic thin film transistors (OTFTs) have made impressive progress over the past decade, and it appears increasingly likely that OTFTs will find use in a number of low-cost, large-area electronic applications, such as active-matrix displays, smart cards, price and inventory tags, and large-area sensor arrays. OTFTs provide two principal advantages over TFTs based on inorganic semiconductors: they can be fabricated at lower temperature and, potentially, at significantly lower cost. Low processing temperatures allow OTFT device and circuit fabrication on polymeric or other inexpensive substrates, rather than glass. The prospect of a flexible, rugged, lightweight active-matrix display at relatively low cost has spurred a number of manufacturers and government agencies to consider plastic displays for a variety of military, medical, industrial, and consumer applications. We report here on the design and fabrication of a flexible active-matrix OTFT backplane suitable for use in flexible polymer-dispersed liquid crystal displays. 75 /spl mu/m thick flexible polyethylene naphthalate (PEN) film was used as the substrate, and OTFT and pixel arrays with good electrical performance, yield, and uniformity were obtained.