  Strain-engineering in SiGe nanostructures is fundamental for the design of
optoelectronic devices at the nanoscale. Here we explore a new strategy, where
SiGe structures are laterally confined by the Si substrate, to obtain high
tensile strain avoiding the use of external stressors, and thus improving the
scalability. Spectro-microscopy techniques, finite element method simulations
and ab initio calculations are used to investigate the strain state of
laterally confined Ge-rich SiGe nano-stripes. Strain information is obtained by
tip enhanced Raman spectroscopy with an unprecedented lateral resolution of ~
30 nm. The nano-stripes exhibit a large tensile hydrostatic strain component,
which is maximum at the center of the top free surface, and becomes very small
at the edges. The maximum lattice deformation is larger than the typical values
of thermally relaxed Ge/Si(001) layers. This strain enhancement originates from
a frustrated relaxation in the out-of-plane direction, resulting from the
combination of the lateral confinement induced by the substrate side walls and
the plastic relaxation of the misfit strain in the (001) plane at the SiGe/Si
interface. The effect of this tensile lattice deformation at the stripe surface
is probed by work function mapping, performed with a spatial resolution better
than 100 nm using X-ray photoelectron emission microscopy. The nano-stripes
exhibit a positive work function shift with respect to a bulk SiGe alloy,
quantitatively confirmed by electronic structure calculations of tensile
strained configurations. The present results have a potential impact on the
design of optoelectronic devices at a nanometer length scale.
