Low-ohmic contacts for high-frequency carbon nanotube field-effect transistors (NanoCon)

• Applicants: Dr.-Ing. Sascha Hermann; Professor Dr.-Ing. Michael Schröter
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2020 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444590424

Carbon nanotubes (CNTs) possess several appealing electronic properties, such as high current carrying capability, high carrier velocity, low distortion, low channel noise, and high temperature stability, which are all beneficial for various applications, especially analog high-frequency (HF) electronics. Most recently fabricated CNT field-effect transistors (FETs) have for the first time demonstrated HF operating frequencies close to those of silicon MOSFTEs. One of the most important parameters determining CNTFET performance is the electrical contact to the CNTs. Despite the most recent performance data, CNTFETs still suffer from high contact resistance (Rc) in the range of 100 kOhm/tube and beyond, which significantly deteriorates their performance. The current state-of-the-art contact formation approaches yield neither reliable nor sufficiently low- contact resistance values due to a lack of in-depth understanding of the impact of processing procedures and the employed material on Rc. The fact that none of the approaches for reducing Rc claimed in materials science publications has been confirmed in fabricated HF CNTFETs and the most encouraging recent HF data indicate that significant research on this topic is not only necessary but is also promising for achieving competitive HF performance in CNTFETs.This project aims at a significant reduction of Rc to values below 25 kOhm per tube by a systematic study of materials and process conditions as well as an in-depth device modeling. Dependable data and trends will be obtained by combining wafer-scale CNTFET fabrication technology and detailed electrical characterization of CNTFETs as well as test structures. Technologically, the proposed work intends to go beyond state of the art by exploring the impact of metal deposition parameters, interfacial layers and various interface modifications onto the CNT contact properties. An in-depth study on the removal of interface contaminations like resist residuals by means of different site-selective CNT contact treatments will be conducted. Also, the influence of different contact geometries going beyond typical omega contacts will be investigated. The wafer-scale fabrication approach enables an in-depth electrical study on large numbers of fabricated CNTFETs (>1000 per wafer), covering DC and HF device characteristics, Rc and Schottky barrier height extraction. To separate the impact of Rc from that of other physical effects on the speed and linearity of fabricated CNTFETs, parameters for an accurate compact CNTFET model will be extracted. This enables an accurate evaluation of the performance of a CNTFET-based technology.

• DFG Programme: Research Grants