The band gaps of rare-earth titanates are commonly reported to be 0.2-0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report theoretical and experimental results on GdTiO$_3$ (GTO) indicating that the gap is significantly larger. First-principles calculations, based either on density functional theory (DFT) with a hybrid functional or on DFT+U, show that the gap is close to 2 eV. We compare these results with photoluminescence (PL) measurements, which show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. We show that the previously reported optical absorption spectra arise from excitations related to small hole polarons. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials. This work was supported by ONR and NSF.
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Authors
Lars Bjaalie
Materials Department, Univ. of California - Santa Barbara
Materials Department, University of California Santa Barbara
Amit Verma
Department of Electrical Engineering, University of Notre Dame
Burak Himmetoglu
Materials Department, University of California Santa Barbara
Anderson Janotti
Materials Department, University of California Santa Barbara
Santosh Raghavan
Materials Department, University of California Santa Barbara
Vladimir Protasenko
Department of Electrical Engineering, University of Notre Dame
Elizabeth Steenbergen
Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio
Debdeep Jena
Department of Electrical Engineering, University of Notre Dame
Susanne Stemmer
Materials Department, University of California Santa Barbara
Chris Van de Walle
University of California, Santa Barbara
Univ of California - Santa Barbara
Materials Dept., Univ of California, Santa Barbara
Materials Department, Univ. of California - Santa Barbara
Materials Department, University of California Santa Barbara
University of California Santa Barbara
Materials Department, University of California, Santa Barbara, CA 93106-5050
