1258 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 6, JUNE 1999 Pentacene Organic Thin-Film Transistors for Circuit and Display Applications Hagen Klauk, Student Member, IEEE, David J. Gundlach, Student Member, IEEE, Jonathan A. Nichols, and Thomas N. Jackson, Member, IEEE Abstract— We have fabricated organic thin-film transistors (TFT’s) using the small-molecule polycyclic aromatic hydrocar- bon 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 /V-s, on/off current ratio as large as 10 , and subthreshold slope as low as 0.7 V/dec, all record values for organic transistors fabricated on nonsingle-crystal substrates. I. INTRODUCTION T HIN film transistors (TFT’s) using organic semiconduc- tors as the active material are of interest for a number of applications. Used as pixel-access devices in active-matrix displays, organic TFT’s could complement liquid-crystal light valves or organic light emitting diodes [1] to allow inexpensive display fabrication on flexible, rugged, light-weight polymeric substrates. Used as switching devices for logic gates and memory arrays, organic transistors could permit the fabrication of very low cost integrated circuits on flexible, large-area substrates for applications like smart cards, smart price and inventory tags, and large-area sensor arrays [2], [3]. The performance of organic thin-film transistors (TFT’s) has improved dramatically over the last twelve years [4]–[8], and optimized organic TFT’s now show electrical characteristics similar to those obtained with hydrogenated amorphous silicon (a-Si:H) devices. For example, carrier field-effect mobilities larger than 1 cm /V-s have been obtained in pentacene organic TFT’s [9]. In addition to a large carrier mobility, a large on/off current ratio is required for TFT’s to be useful as pixel-addressing devices in active-matrix displays. Small TFT subthreshold slope and near-zero threshold voltage are also important to reduce the power consumption of an integrated circuit or display. Finally, to address organic light emitters in all-organic emissive displays, TFT’s must be able to drive fairly large drain currents. We have fabricated organic TFT’s using the small-molecule polycyclic aromatic hydrocarbon pentacene as the active ma- terial. Pentacene TFT’s have been fabricated previously in our Manuscript received November 4, 1998; revised February 12, 1999. The review of this paper was arranged by Editor J. N. Hollenhorst. This work was supported by Opticom ASA and the Defense Advanced Research Projects Agency (DARPA). The authors are with the Center for Thin Film Devices, and Electronic Materials and Processing Research Laboratory, The Pennsylvania State Uni- versity, University Park, PA 16802 USA. Publisher Item Identifier S 0018-9383(99)04608-0. Fig. 1. Schematic cross section of a pentacene thin-film transistor on a glass substrate. laboratory and have shown excellent electrical characteristics, including field-effect mobility as large as 1.5 cm /V-s, on/off current ratio larger than 10 , and subthreshold slope as low as 1.6 V/dec [9]. For simplicity, these early devices used a single- crystal silicon wafer as the substrate and gate electrode, with thermally grown silicon dioxide serving as the gate insulator. To allow TFT integration into circuits or displays, selective gate electrodes and a gate insulator that can be deposited at temperatures compatible with the substrate are required. Low deposition temperatures are of particular concern if devices are to be fabricated on light-weight, flexible polymeric substrates. In this work, silicon dioxide deposited by reactive ion-beam sputtering at a substrate temperature of 80 C was used as the gate dielectric, and pentacene TFT’s with excellent electrical characteristics were obtained. II. DEVICE FABRICATION All transistors were fabricated on borosilicate glass (Corning 7059) using the device structure shown in Fig. 1. Nickel was used for the gate electrodes since it shows excellent adhesion to the substrate and to the gate dielectric layer. Silicon dioxide was deposited to form the gate dielectric layer. Palladium was used for the source and drain contacts, since its large work function leads to improved carrier injection into the organic material. To form the active TFT layer, pentacene was thermally evaporated in vacuum at a pressure near 10 Pa with a deposition rate near 1 ˚ A/s. During the pentacene deposition, the substrate was held at 60 C to improve molecular ordering in the pentacene film, which leads to larger carrier mobility and better device characteristics [10]. Molecular ordering also benefits from a smooth substrate. This is illustrated in Fig. 2(a) which shows an atomic force microscopy (AFM) image of a pentacene film deposited onto the smooth surface of an oxidized silicon wafer. The thermally grown silicon dioxide has a peak-to-valley roughness of 8 ˚ A and an RMS roughness of 1 ˚ A, and the deposited pentacene 0018–9383/99$10.00  1999 IEEE