the pursuit of electric-field-controlled magnetism. [2] BFO is part of a larger family of ferroelectric oxides, important compounds that are relevant in the fields of optical modulation, [3] data storage, [4,5] actuation, [6] and sensing. [7] While many of these established applications are based on bulk samples (either single crystals or ceramics), there has recently been a substantial drive to exploit and enhance the desirable functional properties of ferroelectric oxides in thin film form. [8][9][10] Epitaxial strain [11] is a powerful tool in this context: tuning the structure of thin films by strain can bring about, for example, strong modification of ferroelectric polarization and transition temperatures, [12] spin configurations, [13] and magnetic and magneto-transport properties. [14] A wide range of remarkable functionalities has been shown in BFO thin films, [15] such as a tuneable ferroelectric domain structure, [16] strongly strain-dependent magnetic structure, [17] and electric-field sensitive magnetic functionalities. [18,19] While the bulk parent compound of BFO is rhombohedral (R'), a peculiar aspect of thin film BFO is the formation, under strong (≈4%) compressive strain, of a giant axial ratio tetragonal-like (T') polymorph. [20,21] This change in structure is concomitant with a modification of the oxygen coordination (from octahedral to square pyramidal) and drastic changes in optical [22] and piezoelectric responses, [23] and magnetic properties. [24] From an optical point of view, BFO has a band gap that is rather low for ferroelectric oxides, [25] and it exhibits a significant bulk photovoltaic effect, [26] pushing it into the realm of optical research in the hope of using it in energy harvesting, photocatalysis or optical devices. [22] Given the significant general interest of ferroelectrics for photovoltaic applications, [27] it is important to obtain a stronger understanding of the influence of growth parameters and other factors on the optical properties of BFO thin films. In addition, significant size effects, related to microstrain and oxygen content, have been shown to influence the optical response of BFO nanoparticles. [28] Although a large body of literature regarding optical properties (particularly the band gap) for BFO in the form of thin films exists, there is a large dispersion in the data, with reported values for R' BFO ranging from 2.65 to 2.82 eV. For A detailed structural and optical band gap characterization study for more than 40 epitaxial bismuth ferrite (BiFeO 3 -BFO) thin films, measured by X-ray diffraction, atomic force microscopy, and optical transmission spectroscopy, is reported. The films are grown in different deposition systems to varying thicknesses (10-140 nm), on several substrates, and under different growth and cooling conditions. Using the results and literature data, first it is shown that the band gap measured by transmission is systematically lower than the gap found by ellipsometry, suggesting that sufficient caution must be exercised when comparing...
