Hydrostatic strain enhancement in laterally confined SiGe nanostripes

TL;DR

This study demonstrates that lateral confinement of SiGe nanostripes induces significant tensile hydrostatic strain, enhancing optoelectronic properties without external stressors, through advanced spectro-microscopy, simulations, and calculations.

Contribution

It introduces a novel lateral confinement strategy to induce high tensile strain in SiGe nanostripes, improving scalability and device performance.

Findings

Nano-stripes show large tensile hydrostatic strain at the surface center.

Strain exceeds typical values in thermally relaxed Ge/Si layers.

Positive work function shift indicates enhanced electronic properties.

Abstract

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.