Giant Electron-hole Charging Energy Asymmetry in Ultra-short Carbon Nanotubes

TL;DR

This paper demonstrates a significant electron-hole charging energy asymmetry in ultra-short suspended SWCNT transistors, enabling dual quantum device functionalities with potential room temperature applications.

Contribution

It introduces a method to create ultra-short SWCNT transistors with large electron-hole asymmetry using annealed gold contacts, revealing new quantum device capabilities.

Findings

Maximum e-h charging energy ratio of 2.6 in 14 nm channels

Asymmetry is enhanced in short channels and small band gap SWCNTs

Strong asymmetry persists at room temperature

Abstract

Making full usage of bipolar transport in single-wall carbon nanotube (SWCNT) transistors could permit the development of two-in-one quantum devices with ultra-short channels. We report on clean $\sim$10 to 100 nm long suspended SWCNT transistors which display a large electron-hole transport asymmetry. The devices consist of naked SWCNT channels contacted with sections of SWCNT-under-annealed-gold. The annealed gold acts as an n-doping top gate which creates nm-sharp barriers at the junctions between the contacts and naked channel. These tunnel barriers define a single quantum dot (QD) whose charging energies to add an electron or a hole are vastly different ($e-h$ charging energy asymmetry). We parameterize the $e-h$ transport asymmetry by the ratio of the hole and electron charging energies $\eta_{e-h}$. We show that this asymmetry is maximized for short channels and small band gap SWCNTs. In a small band gap SWCNT device, we demonstrate the fabrication of a two-in-one quantum device acting as a QD for holes, and a much longer quantum bus for electrons. In a 14 nm long channel, $\eta_{e-h}$ reaches up to 2.6 for a device with a band gap of 270 meV. This strong $e-h$ transport asymmetry survives even at room temperature.