Mobility and sub-threshold characteristics in high-mobility dual-channel strained Si/strainef SiGe p-MOSFETs

Abstract

In this work, for the first time, a comprehensive study of mobility, sub-threshold slope and off-state leakage current in high Ge content dual-channel strained Si/strained Si_1-yGe on relaxed Si_1-xGe_x p-MOSFETs is presented. Hole mobility enhancements of 3X are observed at high inversion charge densities (N _inv=10¹³ cm^-2) for the strained Si_0.3Ge_0.7 on relaxed Si_0.7Ge_0.3 (70/30) structure with 2 nm-thick cap, and 3 nm-thick gate oxide. A wide range of Ge fractions and Si cap thicknesses are studied. The mobility enhancement is dominated by the Ge fraction in the strained Si _1-yGe_y layer, while the level of strain is a second order effect. The off-state drain leakage is studied in detail. At low drain-to-gate bias (V_DG), off-state leakage is attributed to a trap assisted tunneling (TAT) mechanism at the Si surface, and is sensitive to Si cap layer thickness. At high V_DG the leakage increases with the Ge fraction in the strained Si_1-yGe_y and strain in the Si cap layer, consistent with band-to-band tunneling (BTBT). The data illustrates trade-offs critical to optimizing the structures with respect to mobility, charge control, and leakage