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

Abstract

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 ηe−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, ηe−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.

Graphene Quantum Strain Transistors

“…The length of the naked graphene channel is defined by gold films directly deposited on top of exfoliated graphene. It was previously reported that such gold films, after annealing, can dope the graphene underneath without disrupting its ballistic transport 32,33 . These gold-covered graphene regions act as contacts to the naked graphene channel.…”

“…These gold-covered graphene regions act as contacts to the naked graphene channel. Few-nm sharp p-n junctions form between these graphene contacts and the naked channel 32,33 . The large aspect ratio of the channel W/L 1 ensures that quantum transport is not affected by the atomic disorder at the edges of the exfoliated crystal, such that a smooth boundary condition in the y-direction is justified 30 .…”

“…The suspension height of the channel above the doped-silicon back-gate is t vac = 50 nm. The device's suspension enables its in situ thermal annealing, so as to reach the ballistic transport regime in both the channel 36 and suspended contacts 33,37 . A standard dc transport circuit (Fig.…”

“…1(a)-(b)). The very long injection length of electrons from gold into graphene 33,44 ensures that the electrons entering the naked graphene channel come from the graphene electrodes, and not directly from the gold film. The ∆µ contact in the graphene contacts arises from the charge transfer due to the work function difference between the metal and graphene 29 .…”

Interaction-Driven Giant Orbital Magnetic Moments in Carbon Nanotubes

“…Theory says that metallic nanotubes can develop a band gap due to symmetry breaking of the underlying graphene lattice from strains, twists and curvature. The magnitude of this gap is predicted to be around tens of milli-electron volts but zero field gaps of an order of a magnitude larger have been reported [19][20][21]. Perhaps most intriguingly though, in the single particle picture, these gaps should vanish at the Dirac field as the nanotube quantization line is pushed to the Dirac point of the underlying graphene band structure resulting in a truly metallic nanotube.…”