Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

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.

Thermooptical evidence of carrier-stabilized ferroelectricity in ultrathin electrodeless films

“…The experiments performed here validate this mechanism in the films of PSTO. We note that other ultrathin ferroelectric films (of KNbO 3 , NaNbO 3 , KTaO 3 , BaTiO 3 , and PbTiO 3 ) also exhibit similar behavior, highlighting its universal character 48–52 .…”

First‐principles study of structural, electronic, elastic, and optical properties of cubic KNbO3 and KTaO3 crystals

“…Secondly, the accuracy of the results of the theorectical methods depends on the relevant parameter sets and computational costs that also influence the reliability of the calculations. Thirdly, the experimental bandgaps for KNbO 3 and KTaO 3 actually yield the ranges of 3.10–3.95 and 3.42–3.76 eV , respectively. In fact, the different sample preparation conditions and measurement techniques can induce different experimental results of the bandgaps.…”

“…The bandgaps are computed using LDA, PBE, HSE06, and GW0 calculations, respectively, and the detailed results are compared with the available experimental data in Table . The present indirect bandgaps based on LDA and PBE are about 55.7 and 56.2% smaller than the experimental data for KNbO 3 (3.24 eV), and about 43.2 and 42% smaller for KTaO 3 (3.64 eV), respectively.…”

“…Perovskite‐type oxides ABO 3 belong to the very important family of materials having interesting properties of superconductivity, ferroelectricity, magnetism, metal–insulator transition, high photocatalytic activity, and piezoelectric anisotropy suitable for numerous applications . Among these systems, KNbO 3 and KTaO 3 have been paid special attention because of the excellent properties . Theoretically, first‐principles are regarded as important and popular methods for the calculations of the structures and properties of materials, and many excellent results have been reported within first‐principles tools for KNbO 3 and KTaO 3 crystals , most of which are based on the local density approximation (LDA) or generalized gradient approximation (GGA).…”

Optical effects induced by epitaxial tension in lead titanate

“…We stress that many of the previously observed epitaxy-induced changes in the optical properties in ferroelectrics point to insufficient fundamental understanding of these properties. [30][31][32][33][34] Because epitaxy proves to be a remarkably powerful tool for engineering these properties, more experimental and theoretical investigations in the field are highly desirable.…”