Abstract
Downscaling of the contact length Lc of a side-contacted carbon nanotube field-effect transistor (CNTFET) is challenging because of the rapidly increasing contact resistance as Lc falls below 20-50 nm. If in agreement with existing experimental results, theoretical work might answer the question, which metals yield the lowest CNT-metal contact resistance and what physical mechanisms govern the geometry dependence of the contact resistance. However, at the scale of 10 nm, parameter-free models of electron transport become computationally prohibitively expensive. In our work we used a dedicated combination of the Green function formalism and density functional theory to perform an overall ab initio simulation of extended CNT-metal contacts of an arbitrary length (including infinite), a previously not achievable level of simulations. We provide a systematic and comprehensive discussion of metal-CNT contact properties as a function of the metal type and the contact length. We have found and been able to explain very uncommon relations between chemical, physical and electrical properties observed in CNT-metal contacts. The calculated electrical characteristics are in reasonable quantitative agreement and exhibit similar trends as the latest experimental data in terms of: (i) contact resistance for Lc = ∞, (ii) scaling of contact resistance Rc(Lc); (iii) metal-defined polarity of a CNTFET. Our results can guide technology development and contact material selection for downscaling the length of side-contacts below 10 nm.
Highlights
Carbon nanotube field-effect transistors (CNTFETs) have been considered as one of the major contenders for replacing digital Si-CMOS technology beyond the 7 nm node and for complementing highly linear analogue applications.[1,2] The high expectations from the carbon nanotube field-effect transistor (CNTFET) technology have arisen mainly due to the unique intrinsic properties of single wall CNTs.[3]
The challenges in reaching small contact resistance for sub-10 nm contacts were reported in ref. 6, which calls for appropriate theoretical support
In this work we have extended an approach[20] towards nonzero bias in order to calculate the contact resistance of CNTFETs for a set of different contact metals, which represent a wide spectrum of different carbon–metal bonding strengths and electro-negativity
Summary
Carbon nanotube field-effect transistors (CNTFETs) have been considered as one of the major contenders for replacing digital Si-CMOS technology beyond the 7 nm node and for complementing highly linear analogue applications.[1,2] The high expectations from the CNTFET technology have arisen mainly due to the unique intrinsic properties of single wall CNTs.[3] One major obstacle to find access to these properties in practical electronic applications is the contact. Major achievements in the fabrication of CNTFETs allowing access to the intrinsic CNT properties are related to the progress in CNT–metal contact fabrication. Javey et al have made a breakthrough by demonstrating the first ohmic contact to a CNT,[4] and Franklin and Chen have shown promising scaling abilities when decreasing the length of a CNT– metal contact.[5] the challenges in reaching small contact resistance for sub-10 nm contacts were reported in ref. The challenges in reaching small contact resistance for sub-10 nm contacts were reported in ref. 6, which calls for appropriate theoretical support
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