Morphology and electrical transport in pentacene films on silylated oxide surfaces
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A study comparing the morphology and electrical transport properties of pentacene films on underlayers of different self-assembled monolayers (SAMs) is presented. The SAMs studied as underlayers were phenyltrichlorosilane, n-octadecyltrichlorosilane, and t-butyldiphenylchlorosilane. Pentacene thin films were grown by vacuum sublimation on SiO2 surfaces treated with self-assembled monolayers. During deposition, substrates were held at a temperature of 70 °C. The morphologies of the films at different stages of deposition were studied by atomic force microscopy, and the transport properties of the films were characterized by I-V measurements in a simple field-effect transistor (FET) structure. The SAM underlayers strongly influence the film morphology in the first few molecular layers and hence significantly impact the electrical transport in the resulting FETs.
I. INTRODUCTION
Within the general area of organic electronics, organic thin film transistors (OTFTs) are of great interest as switching devices for flexible displays, large-area sensors, smart cards, and similar applications where their low fabrication temperature and potentially lower fabrication cost provides advantages compared to current technologies. Among the candidate materials for OTFT applications, pentacene is the leading contender. Pentacene exhibits drain current on/off ratios >106 and fieldeffect mobilities greater than 1 cm2/V·s.1–3 The morphology and electrical characteristics of the pentacene film are known to be sensitive to the surface preparation prior to pentacene thin film deposition.4 An increase in transistor field-effect mobility by a factor of two to five or even more is typically reported for pentacene deposited on n-octadecyltrichlorosilane (OTS)-treated gate dielectric.5,6 The exact reasons for this increase are not completely understood. One theory is that the lower energy of the OTS-treated surface weakens interactions between pentacene molecules and the underlying substrate, thus allowing the first few molecular layers of pentacene to grow uninhibited and improving the crystalline quality of the film. Because most of the charge transport in a fieldeffect transistor occurs close to the semiconductordielectric interface, a change in the molecular ordering of the initial layers can have a profound effect on how the transistor behaves. Shtein et al. report that OTS treatment a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0255 J. Mater. Res., Vol. 19, No. 7, Jul 2004
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suppressed morphology variation and increased the oncurrent in the saturation regime.7 Their x-ray diffraction (XRD) studies indicated that an increased density of flatlying molecules at the pentacene-oxide interface accompanies the improvement in hole mobility. The improvement obtained with OTS leaves open the possibility that an even larger increase in mobility and other parameters of interest can be obtained with the right underlayer. It is also possible that the m
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