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

摘要

The transport characteristics of both electrons and holes through narrow constricted "wall-like" Silicon (Si) long-channels that were surrounded by a thermally grown SiO2 layer was investigated. 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 all sides. The physical height of the crystalline-Si structures was ~1500nm, and the widths were incrementally scaled down from ~200nm to ~20nm. These nanostructures were configured into a metal-semiconductor-metal (MSM) device configuration that was isolated from the substrate. Dark currents, dc-photo-response, and time 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. 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. Specifically, the enhancements of the valence-band and conduction-band mobilities are found to be associated with different aspects of physical mechanisms. The role of the biaxial strain buffering depth is elucidated and its importance to the scaling relations of wall-thickness is reproduced theoretically, i.e., 1/L2 for electrons and 1/L for holes.