Spectral-Charge Properties of a Titanium-Dioxide/Silicon Heterostructure Under Sunlight

The spectral-charge properties of a heterostructure of a 100-nm-thick n-type titanium dioxide (TiO2) film on a p-type silicon substrate in the solar radiation wavelength range of 300-1200 nm were studied by computer simulation. The presence of trap states in the TiO2 film that contribute to the localization of charge carriers was taken into account. The simulation was carried out using the Anderson model for semiconductor heterojunctions, the solution of the Poisson equation, the continuity equations for electrons and holes, and the Maxwell equations for electromagnetic waves in the COMSOL Multiphysics software package. The distribution of the generation rates and concentration of charge carriers in the heterostructure and the distribution of charge density and electric potential on wavelength lambda of solar radiation incident on the TiO2 film and on the energy of trap states Et, which was set inside the band gap, were calculated counting from the bottom of the conduction band. The integral density of solar radiation 1 kW/m2 was assumed to be the same for all wavelengths. Nonmonotonic dependences of the charge-carrier generation rate in TiO2 were revealed in the wavelength range lambda = 325-375 nm. A positive charge with a density of 1.6 mC/cm3 that depended weakly on the radiation wavelength lambda was formed in the TiO2 film volume for relatively shallow traps (Et = 0.2-0.3 eV). The volume charge density in the TiO2 film decreased and changed sign as the trap energy increased, reaching -3.4 mC/cm3 at Et = 0.8 eV and lambda = 900 nm. The surface charge density on the TiO2 film was negative and increased with increasing trap energy Et and radiation wavelength lambda, reaching -2.8 center dot 10-4 mu C/cm2 at Et = 0.8 eV and lambda = 900 nm. The results were explained by the interrelation between interference processes in TiO2 of incident waves and waves reflected from the interface, the separation of charge carriers generated by sunlight at the interface, and the localization of electrons on TiO2 surface states.