Giant electron-hole transport asymmetry in ultra-short quantum transistors

Making use of bipolar transport in single-wall carbon nanotube quantum transistors would permit a single device to operate as both a quantum dot and a ballistic conductor or as two quantum dots with different charging energies. Here we report ultra-clean 10 to 100 nm scale suspended nanotube transistors with a large electron-hole transport asymmetry. The devices consist of naked nanotube channels contacted with sections of tube under annealed gold. The annealed gold acts as an n -doping top gate, allowing coherent quantum transport, and can create nanometre-sharp barriers. These tunnel barriers define a single quantum dot 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 η e−h . This asymmetry is maximized for short channels and small band gap tubes. In a small band gap device, we demonstrate the fabrication of a dual functionality quantum device acting as a quantum dot for holes and a much longer quantum bus for electrons. In a 14 nm-long channel, η e−h reaches up to 2.6 for a device with a band gap of 270 meV. The charging energies in this device exceed 100 meV. By utilizing electron-hole asymmetry in ultra-short single-walled carbon nanotube (SWCNT) transistors, McRae et al ., develop ‘two-in-one’ SWCNT quantum devices that can switch from behaving as quantum-dot transistors for holes to quantum buses for electrons by changing the transistor’s gate voltage