87
4
Electronic Properties of StrainEngineered Semiconductors
It wasn’t until the early 1980s when engineers and scientists started to
realise that strain could be a powerful tool to modify the band structure of
semiconductors. The band structure determines several important characteristics, in particular, its electronic and optical properties. The deformation
potential theory, which defines the concept of strain-induced energy shift of
the semiconductor, was first developed to account for the coupling between
the acoustic waves and electrons in solids by Bardeen and Shockley [1]. It
has been stated that the local shift of energy bands by the acoustic phonon would be produced by an equivalent extrinsic strain; hence, the energy
shifts by both intrinsic and extrinsic strain can be described in the same
deformation potential framework. Piezoresistance coefficients are widely
used due to their simplicity in representing the semiconductor transport
properties under strain. The first experimental work that reported strain
effects on semiconductor transport was by Smith [2], who measured the
piezoresistance coefficients for n- and p-type strained bulk silicon and germanium in 1954.
Strained Si technologies have been widely studied as a new promising
scaling vector (mobility scaling) to improve on-state drive current without degrading off-state leakage current. Mobilities of both electrons and
holes can be improved by applying stress to induce appropriate strain in
the channel, e.g., tensile strain for n-MOSFETs and compressive strain for
p-MOSFETs. In this chapter, the physics of strained Si is reviewed using
electronic band structures, and the simple piezoresistive (PR) model is also
introduced to quantify mobility enhancement induced by strain. Uniaxial
or biaxial tensile strain changes the electronic band structure of Si, leading
to carrier repopulation and band splitting between subvalleys, resulting in
a change in effective carrier mobility. Strain enhances the carrier mobility,
which is given by μ = qτ/m*, by reducing the conductivity effective mass (m*)
or increasing the relaxation time (τ). The biaxial tensile strain also improves
hole mobility by reducing hole conductivity effective mass and suppressing intervalley scattering. With strain, the hole conductivity effective
mass becomes anisotropic due to band warping, and holes preferentially
occupy higher energy light-hole (LH) valleys due to energy splitting. The
4
Electronic Properties of StrainEngineered Semiconductors
It wasn’t until the early 1980s when engineers and scientists started to
realise that strain could be a powerful tool to modify the band structure of
semiconductors. The band structure determines several important characteristics, in particular, its electronic and optical properties. The deformation
potential theory, which defines the concept of strain-induced energy shift of
the semiconductor, was first developed to account for the coupling between
the acoustic waves and electrons in solids by Bardeen and Shockley [1]. It
has been stated that the local shift of energy bands by the acoustic phonon would be produced by an equivalent extrinsic strain; hence, the energy
shifts by both intrinsic and extrinsic strain can be described in the same
deformation potential framework. Piezoresistance coefficients are widely
used due to their simplicity in representing the semiconductor transport
properties under strain. The first experimental work that reported strain
effects on semiconductor transport was by Smith [2], who measured the
piezoresistance coefficients for n- and p-type strained bulk silicon and germanium in 1954.
Strained Si technologies have been widely studied as a new promising
scaling vector (mobility scaling) to improve on-state drive current without degrading off-state leakage current. Mobilities of both electrons and
holes can be improved by applying stress to induce appropriate strain in
the channel, e.g., tensile strain for n-MOSFETs and compressive strain for
p-MOSFETs. In this chapter, the physics of strained Si is reviewed using
electronic band structures, and the simple piezoresistive (PR) model is also
introduced to quantify mobility enhancement induced by strain. Uniaxial
or biaxial tensile strain changes the electronic band structure of Si, leading
to carrier repopulation and band splitting between subvalleys, resulting in
a change in effective carrier mobility. Strain enhances the carrier mobility,
which is given by μ = qτ/m*, by reducing the conductivity effective mass (m*)
or increasing the relaxation time (τ). The biaxial tensile strain also improves
hole mobility by reducing hole conductivity effective mass and suppressing intervalley scattering. With strain, the hole conductivity effective
mass becomes anisotropic due to band warping, and holes preferentially
occupy higher energy light-hole (LH) valleys due to energy splitting. The