Optimizing the band gap of ferroelectric oxides is crucial for enhancing light absorption in specific spectral ranges. In thin-film heterostructures, however, light absorption is also strongly influenced by interfacial properties. Bismuth ferrite–chromite (BFCO) thin films for instance, behave as a p-type semiconductor due to intrinsic cationic bismuth vacancies. When epitaxially grown on niobium-doped ${\mathrm{SrTiO}}_3$, a p-n-like junction forms with a characteristic depletion region. In this work, we present a nondestructive approach to determine the depletion region width using surface photovoltage (SPV) measurements. Kelvin probe force microscopy under illumination was applied to BFCO films of varying thicknesses. A distinct transition in the SPV response at $67\pm 5\mathrm{nm}$ identifies the depletion region width and the extent of band bending. These results are corroborated by macroscopic SPV measurements with a Kelvin probe setup. From the depletion width, the acceptor concentration in BFCO, primarily attributed to bismuth vacancies, was estimated to be $\sim 0.01\%$ of the total number of Bi atoms in the ideal lattice. This method provides a powerful and noninvasive route to probe depletion widths and carrier concentrations in complex ferroelectric heterostructures.

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• Optoelectronics
• Photoconductivity
• Photovoltaic absorbers
• Photovoltaic effect
• Surface & interfacial phenomena
• Thin-film photovoltaics
• Complex oxides
• Ferroelectrics
• Heterostructures
• Junctions
• Solid-solid interfaces
• Atomic force microscopy
• Photoexcitation