Carrier Transport in Nanodevices

Abstract

Future VLSI scaling realization of gate lengths is expected to 70 nm and below. While we do not know all the underlying physics, we are beginning to understand some limiting factors, which include quantum transport, in these structures. The discrete nature of impurities, the fact that devices have critical lengths comparable to their coherence lengths, and size quantization will all be important in these structures. These phenomena will lead to pockets of charge, which will appear as coupled quantum dots in the device transport. We review some of the physics of these dots.

Environmental coupling and phase breaking in open quantum dots

“…While it has been known for some time that coupling a quantum system to an external environment may modify, but not obscure, its discrete characteristics, this notion has recently been emphasized for the quantum dots of interest here [1]. In these devices, current flow is thought to occur via a highly selective process in which a small number of cavity eigenstates are excited by the collimating action of the input quantum point contact [2,3]. At sufficiently low temperatures, phase coherence of the electrons is maintained over long distances and interference between the selectively excited eigenstates gives rise to the phenomenon of wavefunction scarring [2][3][4][5][6][7].…”

“…In these devices, current flow is thought to occur via a highly selective process in which a small number of cavity eigenstates are excited by the collimating action of the input quantum point contact [2,3]. At sufficiently low temperatures, phase coherence of the electrons is maintained over long distances and interference between the selectively excited eigenstates gives rise to the phenomenon of wavefunction scarring [2][3][4][5][6][7]. The scarring in turn implies that the current flow through the device is of a highly non-uniform nature, providing a remarkable manifestation of classical mechanics in the quantum transport behaviour.…”

“…For example, experimental studies have shown that the intrinsic properties of disordered quantum wires may be strongly obscured by the diffusion of phase-coherent electrons into their connecting probes [11]. In the case of the open quantum dots of interest here, environmental factors may be addressed by breaking the system up into discrete components whose interactions are defined through suitable coupling matrices [2]. With strong coupling between these components, the intrinsic properties of the isolated dot are modified by the filtering action of the quantum point contact leads.…”

Open Problems in Quantum Simulationin Ultra‐Submicron Devices

“…It is clear that the carrier density in the channel is very inhomogeneous and this will lead to carrier localization in regions approximately 10- on a side, because the exchange energy leads to band-gap narrowing in this region. Consequently, these carriers sit in small 3D quantum boxes containing 10-20 electrons per box [1]. Yet most approaches to classical device simulation do not adequately treat this fluctuation in the carrier density and doping, nor indeed the strong Coulomb correlations.…”

“…Here, A, C and R are the coupling matrices from the input contact to the device, from the device to the output contact, and from the device back to the input contact, respectively. There are at least three different cases of interest [1].…”