A study of strain-induced indirect-direct bandgap transition for silicon nanowire applications

Recently, ultralarge (>10%) strain with fully reversible elastic deformation has been experimentally achieved in silicon nanowires [H. Zhang et al., Sci. Adv. 2, e1501382 (2016)]. With this breakthrough, here in this work, based on the first principles calculation, the structural and electric properties of silicon under ultralarge strain are comparatively investigated. Unlike previous theoretical/simulation investigations on silicon nanowires with only a few nanometers, bulk silicon models are employed here to provide more realistic and comparable results to our experimentally tested samples (∼100 nm diameter). Strong anisotropic effects are induced by loading strain along all different orientations. Simultaneously, the band structures evolution demonstrates electronic anisotropy with the loading strain on three orientations. Silicon keeps an indirect bandgap under increased strain along the ⟨100⟩ orientation while transforming to a direct bandgap with strain along ⟨110⟩ and ⟨111⟩ orientations. Furthermore, ultralarge strain on these two orientations would diminish the bandgap and result into metallization. These results provide insights into understanding "elastic strain engineering" of silicon nanowire applications and demonstrate the possibility of tuning the electronic and optical properties through pure mechanical straining of functional materials.