Transport in thin-body MOSFETs fabricated in strained Si and strained Si/SiGe heterostructures on insulator

Abstract

The combination of channel mobility enhancement techniques such as strain engineering, with non-classical MOS device architectures, such as ultra-thin body or multiple-gate structures, offers the promise of maximizing current drive while maintaining the electrostatic control required for aggressive device scaling in future CMOS technology nodes. Two structures that combine strain engineering and new materials with the ultrathin body silicon-on-insulator (SOI) technology are examined primarily from the point of view of hole mobility: (1) strained Si directly on insulator (SSDOI), and (2) strained Si/SiGe (with 46-55% Ge)/strained Si heterostructure-on-insulator (HOI). In SSDOI, high strain levels are required to obtain hole mobility enhancements at both low and high inversion charge densities. As the strained Si channel thickness is reduced below 8 nm, hole mobility in SSDOI decreases, as in unstrained SOI. The hole mobility of 3.9 nm-thick 30% SSDOI is still enhanced compared to hole mobility in 15 nm-thick unstrained SOI. Below 4 nm thickness, hole mobility in SSDOI decreases rapidly, which is found to be due to scattering from film thickness fluctuations.

(cont.) Comparisons between SSDOI of two strain levels indicate benefits of strain engineering down to 3 nm thickness. The hole mobility in HOI is improved compared to that in SSDOI, due to the high hole mobility in the Si1-zGez channel. The mobility enhancement is similar at low and high hole densities even at moderate strain levels. The hole mobility in HOI with SiGe channel thickness below 10 nm is observed to follow a similar dependence on channel thickness as hole mobility in SSDOI. Simulations of electrostatics in HOI and SSDOI with ultra-thin channel thicknesses indicate similarities in the confinement of the inversion charge in ultra-thin body HOI and SSDOI. This suggests that the similar reduction of hole mobility in HOI and SSDOI with 4-10 nm-thick channels is associated with an increase in phonon scattering from the reduced effective channel thickness.