Band Alignment and Photoresponse of LaFeO₃-Based Heterojunctions

Epitaxial La⁢Fe⁢O₃ (LFO)-based photocells, of different thicknesses (50–200 nm) have been grown on cubic LSAT (001) [(La⁢Al⁢O₃)₀.₃⁢(Sr₂⁢Ta⁢Al⁢O₆)₀.₇] single-crystal substrates, using a La₂/₃Sr₁/₃⁢Mn⁢O₃ (LSMO) bottom electrode and different top electrodes [Pt,Ba₀.₉₅⁢La₀.₀₅⁢Sn⁢O₃ (BLSO)], to determine their photoresponse. The measured short-circuit photocurrent first increases with LFO thickness and then decreases, and it is larger or smaller, when Pt or BLSO electrodes are used, respectively. The corresponding open-circuit voltage displays the opposite trend, being smaller or larger for Pt and BLSO, respectively, which is in excellent agreement with the electronic band alignments determined by x-ray photoelectron spectroscopy and also consistent with the rectifying character of the dark current-voltage data. It turns out that the films display a complex microstructure containing different variants, strained and relaxed, of the orthorhombic La⁢Fe⁢O₃, that evolves with film thickness and entails the presence of strain gradients and, possibly, flexoelectric fields. We discuss these data and propose that the unavoidable grain boundaries between differently textured La⁢Fe⁢O₃ crystallites play a significant role on the observed responsivity (2.6 × 10−4 A W−1), found to be larger than in related LFO-based structures.