Photo absorption enhancement in strained silicon nanowires: An atomistic study

The absorption spectra of silicon nanowires (SiNW) were calculated using semi-empirical 10 orbital (sp3d5s*) tight binding (TB) and Density Functional Theory (DFT) methods. The effect of diameter, optical anisotropy (wave function symmetry), strain and crystallographic direction on the absorption of SiNWs were investigated. It was observed that compressive strain up to -2% can change the band edge absorption by at least one order of magnitude due to the change of symmetry of wave functions and optical dipole matrix element. The optical anisotropy also manifests itself in the different values of band edge absorption corresponding to different polarizations of incoming photons. This results in three orders of magnitude difference between the band edge absorption of [110] and [110] silicon nanowires for polarization in parallel with the axis of wires. Of interest to applications, we found that strained silicon nanowires can have 100 times larger absorption compared to bulk silicon in the same range of Infrared (IR) energy (Eg = 1.1 eV). Our computational method based on tight binding as well as DFT can successfully reproduce the experimental absorption spectra of bulk silicon for direct inter-band transitions. We compare a fully numerical and a semi-analytic method and they both satisfactorily compute the band edge absorption values in close agreement.