Dy-Doped BiFeO3 thin films: piezoelectric and bandgap tuning

Multiferroic materials, including lead-free BiFeO3, are of special interest for their challenging functional properties which can suit various applications. This paper reports the optimization of the MOCVD process used for the deposition of epitaxial thin films of Dy-doped bismuth ferrite, Bi(1-x)DyxFeO3 (with 0 <= x <= 0.11), on conductive SrTiO3:Nb(100) single crystal substrates. The tri-metallic precursor mixture thermal behaviour is assessed under working conditions up to 130 degrees C and the impact of Dy-doping on the film morphology (FE-SEM), growth rate and structure (XRD and Raman spectroscopy) is systematically reported and compared to the literature. For Dy-doping with x <= 0.11, no change of symmetry has been observed and all films show great homogeneity. Piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS) have been applied to investigate the ferroelectric and piezoelectric properties of BiFeO3 and Bi(1-x)DyxFeO3 films. Ferroelectric and piezoelectric responses are good up to a Dy-doping of 0.08 with a significant reduction of the optical bandgap: 2.25 eV (for the highest doping at x = 0.11) compared to 2.68 eV of pure BiFeO3 films.