TESIN
Abstract
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M. Reiche and S. Christiansen
Carrier transport enhancement induced by strain has led to much interest in using strained Si to improve current drive in Si MOSFETs. In a thin layer of Si pseudomorphically grown on relaxed SiGe tensile strain lifts the 6-fold degeneracy in the ellipsoidal valleys in the conduction band of silicon as well as the degeneracy between the heavy and light hole bands in the valence band. As a result, inter-valley/band scattering is suppressed, and the effective transport mass is reduced, leading to electron and hole mobility enhancements in strained silicon (Fig. 1).
Formation of strained silicon
Different types of strain generation are applied. Local strain in the channeal region of individual devices is realized by embedded SiGe junctions in combination with compressively stressed nitride liners and other techniques [2]. Global strain on wafer level is obtained either by mechanical straining and wafer bonding (uniaxial strain) [3] or by deposition of thin silicon layers on relaxed SiGe buffer layers (biaxial strain) [1,4].
Fig. 1: Mobility enhancement for electrons (left) and holes (right) in strained silicon (thickness 7 nm) grown on relaxed SiGe buffer layers of different Ge concentration [1].
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