Electron mobility characteristics of n-channel metal-oxide-semiconductor field-effect transistors fabricated on Ge-rich single- and dual-channel SiGe heterostructures

Abstract

Strained Si (ε-Si) grown on Si-rich relaxed Si1−xGex buffers (single-channel heterostructures) can be used to fabricate n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) with enhanced performance over bulk Si. However, single-channel heterostructures grown on Ge-rich Si1−xGex buffers (i.e., x⩾0.5) exhibit much larger hole mobility enhancements than those on Si-rich buffers, and the highest hole mobilities have been attained in heterostructures where a compressively strained Ge (ε-Ge) layer is grown beneath the ε-Si cap (ε-Si/ε-Ge dual-channel heterostructures). In this article, we report on n-MOSFET mobility characteristics in single- and dual-channel heterostructures grown on Ge-rich Si1−xGex buffers. Single-channel n-MOSFETs were fabricated on virtual substrates with Ge contents as high as 70%, and electron mobility enhancements of 1.4–1.6 were observed. For dual-channel heterostructures, electron mobility enhancements of 1.7–1.9 were attained when the ε-Si cap was thick enough to confine electrons. Despite the high intrinsic electron mobility of bulk Ge, dual-channel n-MOSFETs with extremely thin Si caps (∼3 nm) exhibited mobility significantly below that of bulk Si. We speculate that the low extracted mobility in such heterostructures results from the difference in conduction band minima between Ge and Si.