The extensive photo response on metal/n-Si clarified by the zero-gap with inter-band phonon scatterings

Abstract

UVA to NIR with multi-directional photo responses have been found on metal (Au)/n-Si device. A reasonable explanation has not been found in various physical models of Si-devices for the phenomena. We approached a zero-gap at X (reciprocal point) in two conduction bands of Si to analysis the optical response with the inter-band phonon scatterings. The calculation of the quantum efficiency between X-$\Gamma$ and X-W successfully simulated the sensitivities in visible region (1.1 to 2.0 eV), the carrier density profile well fitted the response in NIR (0.6 to 1.0 eV). Filling up the zero-gap by doping electrons ($\sim 10^{18}$/cm$^3$) at around X, a lower limit of 0.6 eV arose in the measurement below Si-band gap of 1.17 eV. Indirect/direct transitions of inter conduction bands: X-W, X-K and $\Gamma$-L in the 1st Brillouin Zone/Van Hove singularity at L point, synchronizing with phonon scattering, gave a variety of directional photo-responses. The carrier scattering model for the inter bands (X-W, X-K and $\Gamma$-L) were consistent with the directional dependence of photo-currents under UVA (3.4 eV) and Visible (3.06 eV) excitations. Band to band scatterings assisted to extend the available optical range and increase its variety of directional responses. Utilizing this principle, a new frontier will be opened in the photo-conversion system by using indirect-transition semiconductors and thus, it will be released from those band gaps and directivity limitations.