An efficient direct band-gap transition in germanium by three-dimensional strain

Abstract

Complementary to the development of highly three-dimensional (3D) integrated circuits in the continuation of Moore's law, there has been a growing interest in new 3D deformation strategies to improve device performance. To continue this search for new 3D deformation techniques, it is essential to explore beforehand -using computational predictive methods -which strain tensor leads to the desired properties. In this work, we study germanium (Ge) under an isotropic 3D strain on the basis of first-principle methods. The transport and optical properties are studied by a fully ab initio Boltzmann transport equation and many-body Bethe-Salpeter equation (BSE) approach, respectively. Our findings show that a direct band gap in Ge could be realized with only 0.34% triaxial tensile strain (negative pressure) and without the challenges associated with Sn doping. At the same time a significant increase in refractive index and carrier mobility -particularly for electrons -is observed. These results demonstrate that there is a huge potential in exploring the 3D deformation space for semiconductors -and potentially many other materials -in order to optimize their properties.

Germanium Nanocrystal Properties from Photoluminescence

“…For a Sn fraction of 0.07, the lattice mismatch to Ge is about 1%. At a Sn fraction of 0.023, therefore, the direct Ge NC BG energy at 720 meV 46 is lower than the indirect BG. Thus, this direct Ge NC BG behavior has a relatively modest strain requirement compared with that for 1D strain.…”

“…In a recent publication, 46 more tractable strain conditions are indicated for direct gap behavior in Ge are obtained with simultaneous strain in all three dimensions. The conclusion by these authors is that for greater than 0.34% of 3D tensile strain, Ge has a lower energy direct BG than the indirect BG.…”