Self-consistent ensemble Monte Carlo simulations show terahertz oscillations in single-walled carbon nanotubes

Abstract

We investigate electrical transient and stationary transport properties of semiconducting single-walled zigzag carbon nanotubes (CNTs), using a transient ensemble Monte Carlo (MC) simulator that self-consistently solves the semiclassical transport and Poisson equations. We developed the ensemble MC simulator to obtain time and space dependencies of the CNT electron concentration, velocity, and current profiles self-consistently with electrical potential distribution on the tube. Our calculated MC results indicate that self-induced terahertz CNT current oscillations on the tube and at the contacts emerge under several direct current biases. We associate these terahertz CNT oscillations with intersubband scatterings that cause the transfer of electrons from the first subband to the second, intrasubband scatterings and the nonlinear dispersion curves of each subband. The slow-moving electrons in the second subband bunch together locally on the tube, whereas the fast-moving first subband electrons move beyond the bunch and leave a relatively positive charged region behind. Also, intrasubband scatterings and subband curves give rise to low and high density electron regions by creating dispersion. These relatively low and high density electron regions create a charge dipole that then perturbs the electrical potential, resulting in a propagating domain, and thus current oscillations at tens of terahertz. After we investigate the physics of these calculated terahertz oscillations, we propose methods to modulate and shift the main oscillation frequency by varying the applied bias, tube length, or the diameter.