Organic Field Effect Transistors Based on Multilayer Films via Molecular Layer Epitaxy
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Organic Field Effect Transistors Based on Multilayer Films via Molecular Layer Epitaxy Yuval Ofir1, Offer Schwartsglass2, Joseph Shappir2 and Shlomo Yitzchaik1* 1 Department of Inorganic and Analytical Chemistry. 2School of Engineering, the Hebrew University of Jerusalem, Jerusalem 91904, Israel. ABSTRACT
A Self-Assembly oriented technique from the vapor-phase, Molecular Layer Epitaxy (MLE), was utilized for the buildup of organic multilayers as the active channel in organic field effect transistors (OFET). Carrier gas-assisted chemical vapor deposition (CVD) of 1,4,5,8naphthalene-tetracarboxylic-dianhydride (NTCDA) and an aliphatic spacer are used in a pulsed mode for the covalent attachment of a single monolayer at a time resulting in an ordered dense multilayer film. The MLE approach uses a template layer to promote coupling between the substrate and the precursors deposited from the vapor phase. Interlayer epitaxy is governed by self-limiting vapor-phase condensation reactions while intra-layer ordering is achieved via horizontal π-stacking. Resulting multilayers were characterized by means of contact angle, variable angle spectroscopic ellipsometry (VASE), AFM, absorbance in the UV-vis.-NIR and FTIR. Multilayer structures are also built on a silicon substrate with predefined gold electrodes, using a self assembled template layer on the electrodes and on the thin gate oxide, thus allowing for the buildup of a multilayer structure covering both the electrodes and the channel area while enhancing the nature of the contact between the multilayer and the source and drain electrodes. Resulting OFET devices show n-type conductivity with a mobility of 0.031 cm2 V-1 s-1 for a 6nm thickness MLE film, thus justifying the utilization of the technique in OFETs research and applications. INTRODUCTION
Organic semiconductors attract a significant attention as a potential substitution for regular elemental components in electronic devices as well as an object for fundamental studies in organic solid state physics. A very special interest is given to organic field-effect transistors (OFETs)1 and organic light-emitting diodes (OLEDs)2 which may find applications in a variety of large-area electronics, such as displays, sensors, and electronic barcodes. Although there is a wide spread research on OFET’s it is widely accepted that they will not substitute standard silicon semiconductor technology mainly due to low charge carrier mobility and low on/off ratio. Herein we describe a new methodology based on molecular layer epitaxy (MLE)3,4 for the fabrication of OFET’s with a layer thickness of few nanometers being the same size as the transistor channel. The MLE technology is a vapor-phase oriented technique that allows the buildup of organic heterostructures via epitaxial growth of subsequent layers by interlayer covalent bonding (Fig. 1). First a template layer is deposited on a substrate, such as metal or oxide (Fig. 1, step i), exposing a desired functionality on the surface toward the interface. Then discrete pulses o
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