Photoelectric properties of sandwich Au/Bi4Ti3O12/Si/Al heterostructures
Photoelectric properties of Au/Bi4Ti3O12/Si/Al heterostructures were investigated in continuous and modulated light in the 250-1100 nm wavelength range. Four bands, centred on 400 nm (3.1 eV), 500 nm (2.48 eV), 865 nm (1.43 eV) and 1025 nm (1.2 eV), can be observed in the normalized spectral distributions of photoconducting and photovoltaic signals. The relative amplitudes of these peaks depend on the annealing temperature, ferroelectric polarization and illumination conditions. The 1025 nm (1.2 eV) and 400 nm (3.1 eV) bands were assigned to the intrinsic carrier generation in Si and Bi4Ti3O12, respectively. The 865 nm (1.43 eV) band is due to the charge carrier excitation from some interface states acting as trapping centres. The 500 nm (2.48 eV) band was attributed to the intrinsic excitation in a third phase, probably a silicate, which is developing at the Bi4Ti3O12-Si interface during the air annealing. The frequency dependence of the ac photovoltaic signal is wavelength dependent, suggesting different recombination mechanisms in the two basic materials. To explain the experimental results a possible band structure for the interface is proposed. This band structure considers the presence of spontaneous polarization in the ferroelectric film and the existence of some trapping states at the Bi4Ti3O12-Si interface.