Rapid Optimization of Pentacene Buffer for Rubrene Thin Film Transistors by Using Combinatorial MBE
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Rapid Optimization of Pentacene Buffer for Rubrene Thin Film Transistors by Using Combinatorial MBE
M. Haemori1, J. Yamaguchi1, S. Yaginuma1, K. Itaka2,3 and H. Koinuma2,3,4 1 Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 2 Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan 3 CREST, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan 4
NIMS-COMET, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
ABSTRACT By means of insertion of thin pentacene buffer layer, we have succeeded in the fabrication of highly c-axis oriented rubrene (5,6,11,12-tetraphenylnaphthacene) thin films and their field effect transistors (FETs). In the case without pentacene buffers, only amorphous rubrene films were obtained and their FETs did not show operation. After optimization of pentacene buffer by using combinatorial thickness-gradient method, we obtained the crystalline rubrene thin films and their FETs showed p-type operation with a mobility of 0.05 cm2/V·s and an on-off ratio of 106.
INTRODUCTION In these years, organic field effect transistors (OFETs) have received much attention and OFETs with high mobility are required for broader application [1,2]. Recently, rubrene (5,6,11,12-tetraphenylnaphthacene) single crystal FETs, which show the highest mobility in OFET, were reported. This FET showed p-type operation and exhibited high intrinsic mobilities of 4.4 and 15.4 cm2/V·s along the a and b axes, respectively [3,4]. Although rubrene is the well-known compounds as a yellow electroluminescence dopant, there is no report of crystalline rubrene thin films [5]. A rubrene has four phenyl groups as a side chain, and the entropy of their bulky structure is large. So, it is hard to crystallize rubrene thin films. In this study, by insertion of pentacene thin layer which is well-crystallized on most insulating substrates, crystalline rubrene thin films were fabricated successfully. By using this rubrene thin film on a pentacene buffer, the rubrene FETs were operated with p-type action. Combinatorial method for thin film and device fabrication is powerful tools to get systematic data promptly. In particular, observation of the initial growth, film thickness gradient sample
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library is much effective for time reduction of sample preparation [6]. The combinatorial thickness gradient films can be fabricated using a combinatorial mask, which travels from an edge to another edge of substrate continuously during the deposition. In-situ device chamber enables us to fabricate organic devices without exposing to the atmosphere. After fabrication of this library, we can observe the film growth using atomic force microscopy (AFM).
EXPERIMENTAL DETAILS Fabrication and characterization of rubrene thin film Organic thin films were fabricated by using the combinatorial organic molecular beam epitaxy (OMBE) apparatus. Our combinatorial OMBE apparatus equips qu
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