Resonant Raman Scattering from Quasi-2D Van Hove Excitons in Strain-Engineered Germanium Microstructures

Abstract

We present a sub-μm spatial resolution Raman mapping study of strain-engineered germanium (Ge) micro-bridge structures, fabricated on a 220 nm silicon-on-insulator (Ge-SOI). Revision of the analytical geometric-strain model for such structures, to account for partial under-etching, provides good agreement with the uniaxial tensile strain, inferred from Raman peak shifts, over a wide range of geometries. The additional mechanical support, provided by the partial under-etch, leads to improved yield while still achieving a significant degree of up to ∼2.5% strain. We reveal that even relatively small variations in geometry lead to inhomogeneous strain profiles, including regions of locally enhanced strain at points of high radii of curvature. Our observation of a strongly correlated Raman intensity–strain relation is due to the strain-shifted E₁ critical point coming into resonance with our 1.96 eV pump laser. A simple model for this provides an estimate of the electron–hole lifetime, τ_e–h ∼35 fs, and exciton binding energy, E_b ∼15 meV, which are consistent with efficient coupling of the Raman process to quasi-two dimensional (2D) van Hove (vH) excitons in a region of the Ge band structure exhibiting nonparabolicity and a saddle-point topology with a logarithmic joint density of states (JDOS).