Effect of cation off-stoichiometry on optical absorption in epitaxial LaFeO3 films

Abstract

We have investigated the effect of La and Fe deficiency on the c-axis lattice parameter and optical absorption in La_1−xFe_1−yO_3−δ films deposited by oxide MBE. La deficient films display minimal changes in optical and structural properties compared with stoichiometric LaFeO₃, even with relatively large cation vacancy concentrations (∼22%). Fe deficient films show a linear increase in c-axis parameter with increasing Fe vacancy concentration. The optical absorption coefficient is reduced for Fe deficient films over the spectral range from 1.2 to 5.0 eV. This leads to a relatively simple and effective method to quantify the approximate Fe deficiency in LaFeO₃. Minimal changes in optical and structural properties are observed when cation deficient films are exposed to strongly oxidizing (ozone annealing) conditions. This indicates a strong accompaniment of charge balancing oxygen vacancies to maintain a nominal Fe³⁺ valence state, acting to stabilize the band structure even in the presence of significant cation off-stoichiometry.