Influence of Chemical Dopant Technique to Reduce Schottky Barriers of Pd-Contacted CNTFETs
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Influence of Chemical Dopant Technique to Reduce Schottky Barriers of Pd-Contacted CNTFETs Damien Casterman , and Merlyne Maria De Souza EMTERC, Gateway Street, Hawthorne Building H00.26, Leicester, LE19BH, United Kingdom ABSTRACT The role of the p-type chemical dopant, SbCl6, on Palladium (Pd)-contacted carbon nanotube field effect transistors (CNTFETs) is investigated using ab initio calculations. The interaction of SbCl6 with Pd leads to the chemisorption of one chlorine atom (Cl) which separates off from the rest of the molecule leaving behind a rehybridized SbCl5 molecule. This interaction increases the local workfunction by 0.08 eV. The interaction of the molecule with the carbon nanotube (CNT) itself results in the physisorption of SbCl6 onto CNT. The SbCl6 is found to degenerately dope CNT p-type and shifts the local potential by 0.29 eV. These barriers are useful for modelling of transport of Schottky barrier CNTFETs. INTRODUCTION Carbon Nanotube Field Effect Transistors (CNTFETs) have been a subject of significant interest due to their exceptional electrical capabilities for near-ballistic operation. Nevertheless, their performance is limited by the presence of Schottky Barriers (SB) at metal/nanotube Source/Drain (S/D) contacts. As a result, the control and evaluation of the Schottky barrier heights is of significant technological interest. For example, it has been shown that the one dimensional current flow of CNT as well as the nanoscale size of the contact area limits the effect of Fermi level pinning and thins down the tunnelling barrier for carriers [1]. On the other hand, SBs have also been found to strongly depend on the local workfunctions of both metal and CN, as well as the wetting properties of the carbon atoms with metallic electrodes [2]. This local workfunction dependence is an impediment to reproducibility of devices since adsorbed species are able to critically modify it and thereby alter the SB height at metal/nanotube contacts [3-5]. Unlike conventional silicon based MOSFETs, the particular geometry of CN makes traditional doping by ion substitution delicate as it modifies the electronic properties of the nanotube by breaking its geometry which ensures ballistic transport. To address this issue, novel transistor architecture design has recently emerged using charge transfer technique via chemical dopants to modify the Fermi level in the regions near the S/D contacts to enhance the injection of carriers [6, 7]. This doping method was found to thin the SB as well. In this work, the role of one chemical dopant, hexachloroantimonate (SbCl6), which has recently been reported by IBM, is investigated in the framework of ab initio calculations. The interactions of this molecule with a slab of Palladium (Pd) as well as an (8,0) semiconducting CNT are examined in order to understand the improvement of electrical properties of Pdcontacted CNTFETs observed experimentally [7]. For the first time, the local potential shifts of
both Pd and CNT are calculated in order to provide necessary
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