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Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms

View Article: PubMed Central - PubMed

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.

No MeSH data available.


Absorption coefficient α as a function of photon energy E for bulk crystals (thin curves) and epitaxial nanofilms (thick curves) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). The arrows in (a)–(c) indicate the spectral shifts. Note that data for α near the absorption edge are plotted on a logarithmic scale in (d)–(f).
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Figure 3: Absorption coefficient α as a function of photon energy E for bulk crystals (thin curves) and epitaxial nanofilms (thick curves) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). The arrows in (a)–(c) indicate the spectral shifts. Note that data for α near the absorption edge are plotted on a logarithmic scale in (d)–(f).

Mentions: The optical absorption coefficient α as a function of photon energy E for the epitaxial FE nanofilms and bulk crystals of KTO, KNO, and NNO are shown in figure 3. The energies of the near-bandgap transitions were analyzed using linear fits to Tauc plots (αE)1/2 ∝ (E − Ei) and (αE)2 ∝ (E − Ed) for indirect and direct transitions (figure 4). For a film thickness of 4–10 nm, the optical properties did not vary. The extracted data are summarized in table 2. The epitaxial nanofilms and bulk crystals exhibit main absorption maxima at energies in the range of 5–7 eV. The indirect bandgaps and direct transitions are found at Ei = 3.2–4.05 eV and Ed = 3.6–5 eV. The observations for bulk crystals are consistent with earlier reports [40]. Compared to these reference crystals, the energies of the absorption maxima and near-bandgap transitions of the nanofilms are clearly different. The change in the absorption coefficient near the absorption edge amounts one to two orders of magnitude for all films. Importantly, the changes occur above an optical absorption of more than 1000 cm−1. Since oxygen vacancies and other defects typically affect the absorption by less than 100 cm−1 [50, 51], they can be ruled out as the main source of the effect. The photon energy Eα for which the absorption coefficient equals α = 104 cm−1 is used additionally for the quantification of optical differences between the thin films and bulk crystals.


Concurrent bandgap narrowing and polarization enhancement in epitaxial ferroelectric nanofilms
Absorption coefficient α as a function of photon energy E for bulk crystals (thin curves) and epitaxial nanofilms (thick curves) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). The arrows in (a)–(c) indicate the spectral shifts. Note that data for α near the absorption edge are plotted on a logarithmic scale in (d)–(f).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036465&req=5

Figure 3: Absorption coefficient α as a function of photon energy E for bulk crystals (thin curves) and epitaxial nanofilms (thick curves) of KTO (a), (d), KNO (b), (e), and NNO (c), (f). The arrows in (a)–(c) indicate the spectral shifts. Note that data for α near the absorption edge are plotted on a logarithmic scale in (d)–(f).
Mentions: The optical absorption coefficient α as a function of photon energy E for the epitaxial FE nanofilms and bulk crystals of KTO, KNO, and NNO are shown in figure 3. The energies of the near-bandgap transitions were analyzed using linear fits to Tauc plots (αE)1/2 ∝ (E − Ei) and (αE)2 ∝ (E − Ed) for indirect and direct transitions (figure 4). For a film thickness of 4–10 nm, the optical properties did not vary. The extracted data are summarized in table 2. The epitaxial nanofilms and bulk crystals exhibit main absorption maxima at energies in the range of 5–7 eV. The indirect bandgaps and direct transitions are found at Ei = 3.2–4.05 eV and Ed = 3.6–5 eV. The observations for bulk crystals are consistent with earlier reports [40]. Compared to these reference crystals, the energies of the absorption maxima and near-bandgap transitions of the nanofilms are clearly different. The change in the absorption coefficient near the absorption edge amounts one to two orders of magnitude for all films. Importantly, the changes occur above an optical absorption of more than 1000 cm−1. Since oxygen vacancies and other defects typically affect the absorption by less than 100 cm−1 [50, 51], they can be ruled out as the main source of the effect. The photon energy Eα for which the absorption coefficient equals α = 104 cm−1 is used additionally for the quantification of optical differences between the thin films and bulk crystals.

View Article: PubMed Central - PubMed

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.

No MeSH data available.