Surface Photovoltage at Nanostructures on Si Surfaces: Ab Initio Results

Abstract

Charge transfer photoinduced by steady light absorption on a silicon surface leads to formation of a surface photovoltage (SPV). The dependence of this voltage on the structure of surface adsorbates and on the wavelength of light is studied with a combination of ab initio electronic structure calculations and the reduced density matrix for the open excited system. Our derivations provide time averages of surface electric dipoles, which follow from a time-dependent density matrix (TDDM) treatment using a steady state solution for the TDDM equations of motion. Ab initio calculations have been carried out in a basis set of Kohn-Sham orbitals obtained by a density functional treatment using atomic pseudopotentials. Applications have been done to a H-terminated Si(111) surface and for adsorbed Ag, with surface coverage ranging from 0 to 3/24 of a monolayer. Calculations done also for amorphous Si agree with measured values of the SPV versus incident photon frequency for H-terminated a-Si. Surface adsorbates are found to enhance light absorption and facilitate electronic charge transfer at the surface. Specifically, Ag clusters add electronic states in the energy gap area, provide stronger absorption in the IR and visible spectral regions, and open up additional pathways for surface charge transfer. Our treatment can be implemented for a wide class of photoelectronic materials relevant to solar energy capture.

Computational studies of the optical properties of silicon compounds bonding to silver atoms and with group III and V substituents

“…The oscillator strengths were substantial for the sp 3 rings, with no sign of strengthening significantly with increased chain size or more silvers. Our results for solver clusters adsorbed on Si slabs [14], however, suggest that deposition of the silver containing compounds on solid surfaces could be good absorbers of sunlight.…”

“…The three sets of compounds were chosen for their optical properties in the vapor phase, but in an elementary way, they can be interpreted as deposition precursors or as examples of either a pure silicon surface, a silicon surface with an adsorbed metal cluster, or a surface of doped silicon [14,15]. We consider structures with bonding that show the coordination symmetry of silicon in either vapor or solid phases.…”

Generalized Response Theory for a Photoexcited Many-Atom System

“…An application to absorption of light by a semiconductor within a one-electron treatment of the p-region is given in Ref. 24. The RDOp matrix elements are ρ ss ð Þ αβ ¼ α h jρ ss ð Þ β j i, with diagonal elements providing steady population of states, and off-diagonal elements accounting for quantum coherence.…”

“…This presentation derives some new relations giving density and current responses to applied fields in terms of time-correlation functions (TCFs) of an excited system and also illustrates some of the general results on optical properties and conductivity of excited silicon slabs, from our recent work. [22][23][24][25][26][27][28][29][30][31] Light absorption moves the physical system away from thermal equilibrium (eq) and into a new state which for steady light excitation can reach a steady state (ss) involving dissipation in a medium. This results from continuous light excitation in the primary or p-region of interest concurrent with its de-excitation as the medium, or s-region, absorbs energy and is electronically excited in a time scale much faster than the alternative de-excitation by light emission.…”

Optical properties of the Si(111):H surface with adsorbed Ag clusters

“…Experimentally, observations on such systems reveal a sharp transition, corresponding to the shift from semiconducting to metallic behavior at the Ag/Si interface. In ongoing research, theoretical treatments of the Ag/Si(111) surface are being used in the development of ab initio descriptions of the surface photovoltage created on electronic excitation [19,20].…”