Photoabsorbance and Photovoltage of Crystalline and Amorphous Silicon Slabs with Silver Adsorbates

Abstract

The optical properties of silicon surfaces are affected by their atomic structure and in particular by whether their lattice is crystalline or amorphous. Silver atoms adsorbed on the Si surface enhance the absorption of light and electronic charge transfer at the surface, and the size and shape of the adsorbed Ag clusters play a big role in the photovoltaic properties of Si. We have modeled the photoabsorbance and photovoltage of a nanostructured Si(111) surface with a slab terminated with hydrogen (H) atoms on both surfaces to compensate for dangling bonds, without and with a periodic lattice of adsorbed Ag cluster. Similar structures were also constructed with amorphous lattices to compare the properties of the structures. The optical properties of these structures are investigated using density functional theory to generate a basis set of orbitals and to construct equations of motion for a reduced density matrix from which properties have been obtained in a unified way. Density of electronic states, band gap, and intensity of light absorption with and without silver adsorbates are presented. Light absorbance and surface photovoltages have been calculated in terms of the reduced density matrix. The absorbance in the region around visible light and surface photovoltage (SPV) created by steady light absorption and charge redistribution are calculated for Si slabs containing one, three, or four adsorbed Ag atoms. The ratio of averaged values of absorption flux densities over photon energies in the IR and visible region generally show an increase in absorption with increasing size of a Ag cluster. The changes of absorbance due to silver adsorbates were not large but should be observable. Crystalline Si slabs absorb light mainly at high photon energies, while amorphous Si structures show broader absorption with less intensity. In the case of SPVs, we found that addition of silver adsorbates enhances the SPV of both c-Si and a-Si slabs with a very large increase for c-Si and smaller ones for a-Si. The a-Si structures also show broader SPV spectra compared to the corresponding c-Si structures.