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

The spectral-charge properties of the 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 studied by computer simulation. The presence of trap states in the TiO₂ film, which contribute to the localization of charge carriers, is taken into account. The simulation has been 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 Multyphysics software package. We have calculated the distribution of the generation rates and concentration of charge carriers in the heterostructure, the distribution of charge density and electric potential on the wavelength l of solar radiation incident on the TiO₂ film, and also on the energy of trap states E_t, which has been set inside the band gap, counting from the bottom of the conduction band. The integral density of solar radiation 1 kW/m² is assumed to be the same for all wavelengths. Nonmonotonic dependences of the generation rate of charge carriers in TiO₂ in the wavelength range l = 325–375 nm are revealed. It has been established that for relatively shallow traps (E_t = 0.2–0.3 eV) a positive charge with the density of 1.6 mC/cm³ is formed in the volume of a TiO₂ film, which weakly depends on the radiation wavelength l. As the trap energy increases, the volume charge density in the TiO₂ film decreases and changes sign, reaching –3.4 mC/cm³ at E_t = 0.8 eV and l = 900 nm. The surface charge density on the titanium dioxide film is negative, its value increases with increasing the trap energy E_t and the radiation wavelength l, reaching –2.8 × 10^–4 μC/cm² at E_t = 0.8 eV and l = 900 nm. We explain the results obtained by the interrelation between the interference processes in TiO₂ of the incident and reflected waves from the interface, the separation of charge carriers generated by sunlight at this interface, and also by the localization of electrons on the surface states of TiO₂.

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