Effect of sulfur implantation in BiFeO3 thin films

BiFeO₃ (BFO) epitaxial thin films were grown on SrTiO₃ (001) and Nb:SrTiO₃ (001) substrates using pulsed laser deposition (PLD). The structural, optical and ferroelectric properties were characterized by X-ray diffraction (XRD), UV-visible spectroscopy, spectroscopic ellipsometry and piezoresponse force microscopy (PFM), confirming the good quality of the BFO films. Sulfur (S) was incorporated in the films using the ion implantation technique with three different doses, aiming for BiFeS_0.03O₃ (1% mol. S), BiFeS_0.09O₃ (3% mol. S), BiFeS_0.3O₃ (10% mol. S), compositions. While XRD shows that the perovskite structure is preserved with a change in lattice parameters, spectroscopic ellipsometry indicates that the optical properties of the highest dose sample are strongly altered. The energy diagrams of the samples could be established with the use of a scanning Kelvin probe system combined with Ambient-Pressure Photoemission Spectroscopy (APS). Results indicated that the presence of S in BFO did not significantly modify the bandgap of the films, while the valence band level (ionization energy) is increased by about 0.3 eV. This shift leads to a strong improvement of Surface Photovoltage properties of the films (maximum from 10 mV to 137 mV), forecasting that devices based on such S-doped films would provide strongly enhanced photovoltaic performance. While the highest dose sample does not display ferroelectric properties anymore, the intermediate dose leads to ferroelectricity comparable or even better to that of the pristine samples. This paves the way for high photoferroelectric effects in S-doped BFO films.