Critical Considerations in Polymer Thin-Film Transistor (TFT) Dielectrics
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Critical Considerations in Polymer Thin-Film Transistor (TFT) Dielectrics Munira Raja§, Giles Lloyd§, Naser Sedghi§, Simon J. Higgins* and William Eccleston§ § Department of Electrical Engineering and Electronics, *Department of Chemistry University of Liverpool, Liverpool L69 3BX, UK. ABSTRACT We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion. INTRODUCTION There is persistent evidence of high gate leakage in conjugated polymer Thin-Film Transistors (TFTs) [1]. High quality thin gate oxide is of great importance for applications in dynamic circuits when, for example, charges stored on an MOS DRAM capacitor, or the stored charge on the gate of a MOSFET, leaks away. This reduces the maximum possible circuit clock frequency. With a dielectric of lower leakage, thickness can be reduced and the threshold voltage VT, the supply voltage VDD, and hence the power consumption can be similarly reduced. This is particularly true if a high dielectric constant material such as aluminium oxide (Al2O3) is used as the dielectric, since this material also has a large energy gap (8eV). Despite its near-ideal properties with silicon MOSFETs, thermally grown silicon dioxide (SiO2) suffers increased leakage current when used in polymeric devices [1]. There is a particular need, in addition to high dielectric constant K and low leakage current, for a dielectric that can be easily grown and is compatible with other fabrication steps. The dielectric should have large potential barriers to both the gate conductor and polymer, a low defect concentration in the bulk, and at the dielectric-polymer interface. Al2O3 is a good contender in all these respects. It was grown, in these experiments, using both plasma and aqueous anodisation methods. Plasma anodisation produces dielectrics with lower levels of unwanted impurity, but involves more complicated growth procedures under vacuum. Aqueous anodisation is a simpler process more compatible with the ultra low cost market envisaged for polymer devices and cir
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