Monte Carlo simulations of electron transport in coupled Si quantum wells

Abstract

Coupled wide and narrow tensile strained Si X2-valley quantum wells could provide the basis for an SiGe transistor operating in a velocity modulation mode. Ideally, charge would be rapidly switched between high- (wide) and low- (narrow) mobility channels under the action of an applied gate bias, with little or no modulation of the total channel charge density. With this application in mind, the Monte Carlo technique has been used to simulate in-plane electron transport for a back-doped Si/Si0.85Ge0.15 double-well structure. The equilibrium band profile from the Schottky or oxide top gate to the SiGe virtual substrate is such that electrons are confined to the narrow well when the gate is unbiased. The calculated in-plane mobility is sensitive to the transverse field applied across the quantum well structure - the maximum:minimum mobility ratio is 13:1 at 77 K. At 77 K, a lower mobility in the narrow channel (4000 cm2 V s-1) is mainly due to surface roughness and Coulomb scattering by supply layer impurities. The simulations also predict that mobility modulation would be far less effective at 300 K; scattering by acoustic and inelastic g phonons (LO, TA, LA modes), and consequent distribution across the available subbands (icons/Journals/Common/le" ALT="le" ALIGN="TOP"/>9) of the entire well structure reduces confinement to either the wide or narrow well, and hence the predicted maximum:minimum mobility ratio is less than 2:1.