The Benefits of High- K dielectrics for Polymer TFTs
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The Benefits of High-K dielectrics for Polymer TFTs Munira Raja§ Naser Sedghi§, Simon J. Higgins* and W. Eccleston§ § Department of Electrical Engineering and Electronics, *Department of Chemistry University of Liverpool, Liverpool L69 3BX, UK ABSTRACT An increasing range of high-K dielectric is becoming available and it is very worth considering incorporating them into polymer TFTs. One of the benefits is that if metal gates are used then aqueous anodisation provides a very simple approach that is compatible with solution based processing. The details of this process are described. High-K dielectrics reduce threshold voltage and, therefore, increase switching speed. Of particular importance is the problem of bias instability. All the results involve the fractionation and controlled doping of poly-3hexylthiophene. INTRODUCTION The demand of high performance circuits at low costs has lead to the reduction in channel length and gate oxide thickness. For this reason, high-K dielectric materials are now considered as alternative gate dielectrics to silicon dioxide (SiO2), both in terms of low cost and better performances such as high switching speeds. Aluminium oxide (Al2O3) was considered as a good competitor since it compares closely to SiO2 in terms of high band gap Eg of 8.8 eV and high band offsets obtained on silicon [1]. In addition, the high-K value of Al2O3 has a number of advantages when incorporated in thin film transistors (TFTs). The gate oxide capacitance increases with a high-K dielectric material resulting in the reduction in the threshold voltage VT. This reduction in VT also leads to lowering of the supply voltage VDD and thus resulting in lower power dissipated. High-K gate material can also increase the switching speed of a TFT as shown later. Aluminium oxide was grown using aqueous anodisation method, a simple low cost process. The set-up for anodisation process is as shown in figure 1. This process becomes even more useful if metal gates are to be used since it will be compatible with the solution based processing. The typical structure of the bottom gate polymer TFT is shown in figure 3. Poly(3hexylthiophene) (P3HT) was synthesised and fractionated using methods demonstrated by McCullough method [2] and Trznadel [3] respectively. This synthesis produces P3HT chains with high regioregularity that is further increased by fractionation. Fractionation also removes lower molecular mass chains and impurities left behind after synthesis. In order to enhance the field effect mobility, the P3HT is doped with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).
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EXPERIMENTAL DETAILS Oxide growth Aluminium oxide was grown using aqueous anodisation method, figure 1. The anode consisted of aluminium evaporated on a pre-cleaned glass substrate and the cathode was a gold electrode. The electrolyte was a mixture of ethylene glycol (CH2OHCH2OH) and tartaric acid (CHOH)2(COOH)2. Ammonium hydroxide (NH4OH) was also added to increase the pH of V VDC the electrolyte. The anodisation was performed at const
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