Tuning optical absorption in perovskite (K,Na)NbO3 ferroelectrics

Abstract

The ability to tailor the electronic band structure and optical absorption by appropriate cationic substitution in perovskite oxide ferroelectrics is essential for many advanced electronic and optoelectronic applications of these materials. Here, we explored weak (Ba,Ni)-doping for reducing optical bandgaps in (K,Na)NbO₃ ferroelectric films and ceramics. The optical absorption in the broad spectral range of (0.7-8.8) eV was investigated in polycrystalline doped, pure, and oxygen deficient films, in doped epitaxial films grown on different substrates, and in doped ceramics. By comparing optical properties of all films and ceramics, it was established that 1-2 at% of cationic substitutions or up to 10 at % of oxygen vacancies have no detectable effect on the direct (∼4.5 eV) and indirect (∼3.9 eV) gaps. Concurrently, substantial sub-gap absorption was revealed and ascribed to structural band tailing in epitaxial films and ceramics. It was suggested that owing to fundamental strain-property couplings in perovskite oxide ferroelectrics, inhomogeneities of lattice strain can lead to increased sub-gap absorption. The uncovered structurally induced sub-gap optical absorption can be relevant for other ferroelectric ceramics and thin films as well as for related perovskite oxides.

Fig. 1. Typical (a) XPS survey spectrum and (b) XRF spectrum. The characteristic lines of K, Na, Nb, Ba, Ni, and O are marked on the plots. (c)–(h) Details of the selected characteristic XPS lines.

Fig. 2. High resolution XRD (a) grazing incidence and (b)–(g) ω−2Θ scans in the films on (a) SiO₂/Si and (b)–(g) STO and LSAT substrates. In (c), the pure KNN film on STO is shown for comparison. In (a)–(c), perovskite peaks are indexed. In (d)–(g), details of the scans around the (d) (001), (e) (002), (f) (003), and (g) (004) peaks are shown. In (a) and (d)–(g), the peaks from the films and substrates are marked by “f” and “s”, correspondingly.

Fig. 3. Absorption coefficient as a function of photon energy in the polycrystalline films of (a) doped BN-KNN, (b) pure KNN, (c) oxygen-deficient KNN, and (d) in doped BN-KNN ceramics. In (e), absorption coefficient in the films of different compositions is shown for ease of comparison. In (f), absorption coefficient in the polycrystalline BN-KNN film and ceramics is shown for ease of comparison.

Fig. 4. Absorption coefficient as a function of photon energy in the epitaxial BN-KNN films on (a) STO and (b) and (c) LSAT substrates. The nominal films thickness is (a, d) and (b, d) 50 nm and (c, d) 150 nm. In (d), absorption coefficient in the BN-KNN epitaxial and polycrystalline films is shown for ease of comparison.

Fig. 5. Second derivative of the imaginary part of the dielectric function as a function of photon energy in the (a)–(c) polycrystalline films of (a) doped BN-KNN, (b) pure KNN, (c) oxygen-deficient KNN, (d) BN-KNN ceramics, and (e)–(g) BN-KNN epitaxial films on (e) STO and (f) and (g) LSAT.

Fig. 6. Tauc plots for direct optical gap in the (a)–(c) polycrystalline films of (a) doped BN-KNN, (b) pure KNN, and (c) oxygen-deficient KNN; in (d) BN-KNN ceramics, and (e)–(g) BN-KNN epitaxial films on (e) STO and (f) and (g) LSAT. Straight lines show fits.

Fig. 7. Formal Tauc plot for direct gap in the range of weak absorption in ceramics. Dashed line shows the level of α = 10⁴ cm⁻¹. Straight lines are possible fits.

Fig. 8. Tauc plots for indirect optical gap in the (a)–(c) polycrystalline films of (a) doped BN-KNN, (b) pure KNN, and (c) oxygen-deficient KNN; in (d) BN-KNN ceramics, and (e)–(g) BN-KNN epitaxial films on (e) STO and (f) and (g) LSAT. Straight lines show fits.

Fig. 9. (a) Absorption coefficient and (b) logarithm of absorption coefficient ln(α) as a function of photon energy in different BN-KNN samples as marked on the plots. In (b), straight lines show fits.