Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms |
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| Authors: | Marina Tyunina Lide Yao Dagmar Chvostova Alexandr Dejneka Tomas Kocourek Miroslav Jelinek Vladimir Trepakov Sebastiaan van Dijken |
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| Affiliation: | 1.Microelectronics and Materials Physics Laboratories, University of Oulu, PO Box 4500, FI-90014 Oulun Yliopisto, Finland;2.NanoSpin, Department of Applied Physics, Aalto University School of Science, PO Box 15100, FI-00076 Aalto, Finland;3.Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic;4.Ioffe Physical-Technical Institute RAS, 194021 St. Petersburg, Russia |
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| Abstract: | Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices. |
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| Keywords: | epitaxial growth ferroelectric nanofilms optical properties |
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