Cr2O3 and Fe2O3 are both wide-gap insulators with bulk band gaps of 2.2 and 3.3 eV, respectively. CrxFe2-xO3 alloys might be expected to exhibit band gaps that scale monotonically with x between these two end-member values, as occurs with Si-Ge alloys. We show that contrary to this expectation, epitaxial films of CrxFe2-xO3 prepared by molecular beam epitaxy exhibit a direct gap that bows downward to a minimum of ~1.8 eV at x = 0.5, as determined by optical absorption spectroscopy. Photoconductivity measurements of the band gap are in progress and thus far agree with the optical absorption data quite well. DFT + U calculations of the optical excitation energies have also been carried out and reproduce the experimental trend rather well, provided the magnetic structures of the alloys are accurately accounted for. Band gap lowering this far into the visible is of significant interest for solar light harvesting and photocatalysis.

Optical absorption and photoconductivity in epitaxial α-CrxFe2-xO3 films

245th National Meeting (2013)

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