Characterization of carrier transport properties in strained crystalline Si wall-like structures as a function of scaling into the quasi-quantum regime

TL;DR

This study investigates how strain and quantum effects in narrow crystalline Si wall-like structures influence carrier transport, revealing increased mobilities and conductivity as the structures scale down into the quasi-quantum regime.

Contribution

It provides a microscopic model explaining mobility enhancements due to strain and quantum effects in scaled Si nanostructures, supported by experimental measurements.

Findings

Increased carrier mobility in narrower Si walls due to strain effects.

Observation of quasi-quantum saturation of hole mobility at small scales.

Strain-induced band splitting and effective mass reduction explain transport improvements.

Abstract

We report the transport characteristics of both electrons and holes through narrow constricted crystalline Si "wall-like" long-channels that were surrounded by a thermally grown SiO2 layer. Importantly, as a result of the existence of fixed oxide charges in the thermally grown SiO2 layer and the Si/SiO2 interface, the effective Si cross-sectional wall widths were considerably narrower than the actual physical widths, due the formation of depletion regions from both sides. These nanostructures were configured into a metal-semiconductor-metal device configuration that was isolated from the substrate region. Dark currents, dc-photo-response, and carrier "time-of-flight" response measurements using a mode-locked femtosecond laser, were used in the study. In the narrowest wall devices, a considerable increase in conductivity was observed as a result of higher carrier mobilities due to lateral constriction and strain. The strain effects, which include the reversal splitting of light- and heavy- hole bands as well as the decrease of conduction-band effective mass by reduced Si bandgap energy, are formulated in our microscopic model for explaining the experimentally observed enhancements in both conduction- and valence-band mobilities with reduced Si wall thickness. The role of the biaxial strain buffing depth is elucidated and the quasi-quantum effect for the saturation hole mobility at small wall thickness is also found and explained. Specifically, the enhancements of the valence-band and conduction-band mobilities are found to be associated with different aspects of theoretical model.