Pentacene thin-film transistors on polymeric gate dielectric: device fabrication and electrical characterization J. Puigdollers * , C. Voz, I. Martin, A. Orpella, M. Vetter, R. Alcubilla Departament d’Enginyeria Electronica, Universitat Politecnica de Catalunya c/ Jordi Girona 1-3, Modul C4, Barcelona 08034, Spain Available online 9 April 2004 Abstract Pentacene thin-film transistors using polymethyl methacrylate as a gate dielectric have been fabricated. A bottom gate, inverted staggered structure was selected to study the influence of the dielectric on the device performance. Crystalline silicon wafers and polyethylenenaphtalate polymer foils were used as substrates. Pentacene thin-films were deposited by thermal evaporation in a high- vacuum system. The maximum process temperature was 170 °C, corresponding to the baking of polymethyl methacrylate. These devices showed satisfactory p-type electrical characteristics with on/off ratios exceeding 10 3 for V GS ranging from )30 to 30 V. The field-effect mobility and threshold voltage were around 0.01 cm 2 V 1 s 1 and )14 V, respectively. The polymethyl methacrylate dielectric also seems to provide some advantages of the so-called self-assembling monolayers. Ó 2004 Elsevier B.V. All rights reserved. PACS: 61.10.H; 73.61.P; 72.80.L; 85.30.T 1. Introduction Organic thin-film transistors (OTFT) have the key advantage of relatively simple and low temperature processing. Therefore, OTFTs could be competitive for applications requiring large area coverage on inexpen- sive substrates. Such applications include smart cards, radio frequency identification tags and displays. The performance of OTFTs has considerably improved during the last years [1]. Among the variety of organic semiconductors, pentacene has allowed the highest electrical performance. Pentacene TFTs have already reached field-effect mobilities over 1 cm 2 V 1 s 1 [2]. These values are comparable to those obtained with hydrogenated amorphous silicon (a-Si:H) TFTs, which are the devices commonly used in large area active matrix arrays. So far most of the pentacene TFTs have been fabri- cated using inorganic dielectrics such as thermally grown silicon dioxide or plasma-enhanced chemical vapor deposited silicon nitride. In addition, OTFTs are usually fabricated on non-flexible substrates, like crys- talline silicon or glass. However, the use of plastic sub- strates to obtain flexible devices is more challenging. Nevertheless, some plastic substrates could not be compatible with the high or moderate temperature processes used to obtain standard inorganic dielectrics. Alternative polymeric dielectrics which can be spin cast or dip coated on inexpensive substrates are more desir- able. In this paper we present pentacene TFTs using polymethyl methacrylate (PMMA) as a gate dielectric material. The thermal and mechanical stability of PMMA, together with its high resistivity (>2 · 10 15 X cm) and suitable dielectric constant (e ¼ 2:6@1 MHz, e ¼ 3:9 @ 60 Hz), make of PMMA a good can- didate as a dielectric layer for OTFTs. In addition, its low baking temperature (<170 °C) is compatible with plastic substrates. The first part of the paper deals with the influence of the substrate on the structural properties of thermally evaporated pentacene thin-films. Unlike with other inorganic semiconductors as a-Si:H, the properties of pentacene layers are strongly dependent on the base layer. In particular, we have studied the * Corresponding author. Tel.: +34-93 401 1002; fax: +34-93 401 6756. E-mail address: jpuigd@eel.upc.es (J. Puigdollers). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.054 Journal of Non-Crystalline Solids 338–340 (2004) 617–621 www.elsevier.com/locate/jnoncrysol