Highly strained Ge nanostructures and direct bandgap transition induced by femtosecond laser

Germanium (Ge), characterized by its indirect bandgap energy of 0.66 eV, faces limitations in optoelectronic applications. However, applying strain transforms Ge into a direct bandgap semiconductor, potentially broadening its technological utility. This study investigates the effects of intense femtosecond (fs) laser irradiation on crystalline Ge to induce such strain and examine its consequent structural and electronic alterations. Employing micro-Raman spectroscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and spectrophotometric analyses, we aim to elucidate the underlying mechanisms of strain-induced transformations. Our findings reveal a maximum Raman shift of up to 10.5 cm-1, indicative of significant localized tensile strain. TEM analysis shows polycrystalline structures with rich defects, corroborating Raman data and suggesting strained nanostructures. XRD results point to anisotropic type of strain, which could facilitate the transition towards a direct bandgap semiconductor compared to uniaxial or biaxial strain. Optical measurements further indicate bandgap enlargement to 0.78 eV, close to the direct transition energy at 0.8 eV. These comprehensive analyses demonstrate that fs laser irradiation can effectively induce strains to transform Ge, thereby enhancing its application potential in photonic and optoelectronic devices.