Reducing Leakage Currents in n-Channel Organic Field-Effect Transistors Using Molecular Dipole Monolayers on Nanoscale Oxides

Abstract

Leakage currents through the gate dielectric of thin film transistors remain a roadblock to the fabrication of organic field-effect transistors (OFETs) on ultrathin dielectrics. We report the first investigation of a self-assembled monolayer (SAM) dipole as an electrostatic barrier to reduce leakage currents in n-channel OFETs fabricated on a minimal, leaky ∼10 nm SiO2 dielectric on highly doped Si. The electric field associated with 1H,1H,2H,2H-perfluoro-octyltriethoxysilane (FOTS) and octyltriethoxysilane (OTS) dipolar chains affixed to the oxide surface of n-Si gave an order of magnitude decrease in gate leakage current and subthreshold leakage and a two order-of-magnitude increase in ON/OFF ratio for a naphthalenetetracarboxylic diimide (NTCDI) transistor. Identically fabricated devices on p-Si showed similarly reduced leakage and improved performance for oxides treated with the larger dipole FOTS monolayer, while OTS devices showed poorer transfer characteristics than those on bare oxide. Comparison of OFETs on both substrates revealed that relative device performance from OTS and FOTS treatments was dictated primarily by the organosilane chain and not the underlying siloxane-substrate bond. This conclusion is supported by the similar threshold voltages (VT) extrapolated for SAM-treated devices, which display positive relative VT shifts for FOTS on either substrate but opposite VT shifts for OTS treatment on n-Si and p-Si. Our results highlight the potential of dipolar SAMs as performance-enhancing layers for marginal quality dielectrics, broadening the material spectrum for low power, ultrathin organic electronics.

Revealing molecular conformation–induced stress at embedded interfaces of organic optoelectronic devices by sum frequency generation spectroscopy

“…Representative model devices for the embedded interface between an organic semiconductor/gate insulator are depicted in Fig. 1 (A to C), in which a self-assembly monolayer (SAM) is inserted between the organic semiconductor and gate insulator to form a high-compatibility interface (19)(20)(21)(22)(23)(24)(25)(26). This model device suffers from serious stability problems (20,21,(27)(28)(29)(30)(31).…”

Structural Characterization of Alkylsilane and Fluoroalkylsilane Self-Assembled Monolayers on SiO2 by Molecular Dynamics Simulations

“…4 These are ob-2 viously desired qualities in particular for manufacturing gate dielectrics in organic field-effect transistors (OFET), for which the deposition of an organic semiconductor directly onto the untreated standard SiO 2 substrate leads to ill reproducible results in terms of threshold/onset voltage and charge carrier mobilities due to the presence of defects. 5,6 The effect of SAM functionalization on OFET performances is often remarkable since it leads to controllable shifts of the threshold voltage (which depends on the chemical nature of the SAM itself), it allows to modulate the charge carrier mobility of the overlying organic semiconductor, [7][8][9][10][11][12][13][14][15] and even to control the band gap opening in highly conductive materials such as graphene. 16 The microscopic origin of the modulating effect of SAM on OFET performances is a matter of intense theoretical studies.…”

Engineering gate dielectric surface properties for enhanced polymer field-effect transistor performance

“…[5][6][7] The surface hydroxyl groups, if left untreated, would adversely affect device performance and stability due to their hydrophilicity and incompatibility with organic semiconductor materials. 8,9 Accordingly, a great many different self-assembled monolayers (SAMs) have been utilized to modify the SiO 2 surface to render it hydrophobic and compatible with organic semiconductors, [10][11][12][13][14][15] leading to improvements in OFET performance.…”