Modeling and Simulation of Electron Transport through Nanoscale Heterojunctions

Abstract

The thesis discusses some transport aspects of nanoscale heterojunctions, the archetypal devices which constitute a considerable part of the rapidly growing nanotechnology and nanoscience today. It discusses two distinct types of nanoscale heterojunctions, namely weakly correlated and strongly correlated heterojunctions. The weakly correlated heterojunction employs normal metal for its leads, where the particles behave like typical 3D electron gas and the standard density functional theory allows rigorous ab initio analysis for such systems. On the other hand, the strongly correlated heterojunction employs superconductors for its leads, therefore appropriate models need to be used to describe the essential physics from which the transport properties are derived.

The weakly correlated heterojunction consists of two normal metals and a carbon nanotube (CNT) in between, a ubiquitous system in nanoscale experimental devices. Despite of all its novel and great promises, a full exploitation of the device has so far been hindered by various problems, and one of them is the interface problems with the metal probes which typically produce considerable resistance. Schottky barriers formed at CNT-metal contacts have been well known to be crucial for the performance of CNT based field effect transistors (FETs). Through an extensive first principles calculations we show that an optical nanowelding process can drastically reduce the Schottky barriers at CNT-metal interfaces, resulting in significantly improved conductivity. Results presented may have great implications in future design CNT-based nanoelectronics.

The strongly correlated heterojunction consists of two superconducting leads and a quantum dot in between. A phenomenon of so-called differential conductance anomaly} is predicted to occur in such devices at high bias when the transport is theoretically linear. The phenomenon is caused by the potential symmetry which affects the pinning mechanisms of the localized level by the superconducting gaps of the leads. Due to this, we anticipate a counter intuitive phenomenon where the linear conductivity may be increasing as the coupling strength between the leads and the quantum dot is reduced. The phenomenon can be used to investigate the symmetry across the quantum dot which would otherwise be impossible to probe using other methods. A recent experiment may already indicate the existence of such effects.

We then consider another hybrid superconducting system and study the effect of electron tunneling under external microwave radiations. The microwave radiations stimulate interlevel quantum transitions on the multilevel quantum dot. We develop a method to combine Floquet theory and nonequilibrium Green's function in order to describe supercurrent tunneling process through the heterojunction. We find that the effect of transition amplitude or the coupling between levels is reflected at the current-bias (I-V) curves only at Rabi frequency. The radiation splits the dc resonance and the separation between each splits is proportional to the coupling between the localized levels. The observation provides a possibility for an experimental inference of the interlevel coupling from simple time averaged measurements.

In all parts of the transport analysis we employ nonequilibrium Green's function method which is considered to be the most rigorous and systematic way to treat most quantum transport problems. Some other secondary and on going works are not included in this thesis in order to maintain a coherent picture of the presentation.