Field-Induced ESR Spectroscopy on Rubrene Single-Crystal Field-Effect Transistors
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1154-B08-04
Field-Induced ESR Spectroscopy on Rubrene Single-Crystal Field-Effect Transistors Hiroyuki Matsui1,2 and Tatsuo Hasegawa1 1 Photonics Research Institute (PRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan. 2 Department of Advanced Materials Science, The University of Tokyo, Kashiwa, Chiba 2778561, Japan. ABSTRACT We investigate the electron spin resonance (ESR) spectroscopy for the field-induced carriers in rubrene single-crystal field-effect transistors (SC-FETs), and compare the results with those on pentacene thin-film transistors (TFTs). We observe Lorentz-type ESR signal in rubrene SC-FETs whose linewidth is narrowed with increasing gate voltage and temperature. It demonstrates that the ESR linewidth is determined by motional narrowing effect as we reported on pentacene TFTs. Based on the observations, we discuss the multiple trap-and-release (MTR) processes in the two systems with and without grain boundaries. INTRODUCTION Organic field-effect transistors (FETs) have attracted considerable world-wide attentions for their applications toward large-area, low-cost, flexible, and light-weight electronic devices [1,2]. One of the most fundamental issues in organic FETs is to reveal the microscopic nature of charge transport in organic semiconductors, the understanding of which should be key to bringing out the ultimate limit of device characteristics for molecular semiconducting materials [3-5]. Recently, Marumoto and co-workers reported electron spin resonance (ESR) of organic MIS diodes and organic transistors [6,7]. ESR method is quite useful in that it provides a microscopic probe for charge carriers like unpaired electrons and holes [8,9]. Not only to examine the number of carriers and the orientation of molecules, it is possible also to know about carrier dynamics as we discussed in the previous paper [10,11]. By using high-mobility pentacene thin-film transistors (TFTs), we successfully observed the motional narrowing of their ESR spectra; the linewidth of single-Lorentz-type ESR spectra is narrowed as the mobility increases with the variation of gate voltage and temperature. From the analyses of motional narrowing [12], we found that the carrier dynamics is explained by multiple trap-and-release (MTR) model with the average trapping time of a few nanoseconds and the average trap depth of 10 ~ 15 meV [13-15]. However, the origin of the shallow traps and their relation to grain boundaries still remain open questions. In this paper, we compare the field-induced ESR of single-crystal FETs (SC-FETs), where grain boundaries are not included, with that of pentacene TFTs. We use rubrene as a channel material of SC-FETs because it is known as high-mobility (up to 40 cm2/Vs) material for single-crystal devices [16,17]. We first show the feature of field-induced ESR at room temperature with a focus on gate-voltage- and angle-dependence. We also show the temperaturedependence of ESR spectra, and point out that the quite similar motional narrowing
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