Environmental Sensitivity of Pentacene Thin-Film Transistors
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Environmental Sensitivity of Pentacene Thin-Film Transistors M. Breban, S. Mezhenny, D.R. Hines, D.B. Romero*, V.W. Ballarotto and E.D Williams The Laboratory for Physical Sciences, 8050 Greenmead Dr., College Park, MD 20740 The Department of Physics, University of Maryland, College Park, MD 20742 *Dept. of Electrical Engineering, University of Maryland, College Park, MD 20742
ABSTRACT By alternating the gate voltage polarity of a pentacene thin film transistor, we show that the drain current is stabilized and thus the bias stress effect is overcome. This allows for controlled testing of the device sensitivity to environmental conditions. We find that the conductivity of the device decreases on the time scale of seconds when the device is exposed to water vapor, which is manifested through a decrease in mobility and a shift in the threshold voltage. Simple recombination modeling suggests that trapping is the responsible mechanism. However, the effects of water vapor can be reversed by exposing the device to dry nitrogen flow. The time scale for recovery is on the order of 10s of minutes.
INTRODUCTION The sensitivity of organic thin film transistors (TFT) to factors such as chemical impurities, charges at interfaces and grain boundaries is related to still incompletely understood mechanisms of transport. The interaction between pentacene and a chemical agent may change the intrinsic conductivity, and thus chemical sensing can be engineered by monitoring these changes. However, conductivity measurements are complicated by the bias stress effect. This common but undesired property of TFT manifests itself as a slow decrease in the drain current when the device is turned on for long periods.1, 2, 3 It is generally accepted that the current decay is due to the loss of carriers in the conduction channel. The mechanisms that are believed to lead to bias stress effects are trapping within the gate dielectric, trapping at the semiconductor/dielectric interface, trapping in the semiconductor, ion migration, or structural modifications.1, 2, 4, 5 Therefore, it is imperative that bias stress be eliminated or at least controlled for organic devices to be useful. We present here a simple method for stabilizing the drain current in a pentacene TFT so that the sensitivity of such a device can be quantitatively assessed. In particular, effect of water vapor on the conductivity of pentacene is investigated. EXPERIMENTAL The devices used here are fabricated with the source and drain electrode on top of the active layer with a degenerately doped n-type Si substrate as the gate electrode. Pentacene, purchased from Aldrich without further purification, is vacuum deposited (~0.4 Å/s) at a pressure of 10-8 Torr through a shadow mask on to a 300 nm thermal oxide layer. The active layer is typically 50 nm thick. The source and the drain electrodes (100 nm thick) are formed by vacuum deposition
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Fig. 1. Drain current of pentacene TFT as a function of time. The bottom trace shows the decrease in drain current when the device
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