Fabrication and characterization of dielectric strontium titanium oxynitride single crystal

Perovskite-type oxides (ABO₃) are the most widely used ferroelectric materials and their excellent dielectric properties have attracted much attention not only in the electroceramic industry but also in fundamental research.^1–10) The polarization mechanism of dielectric properties and the origin of ferroelectricity are closely related to the covalency of A–O and B–O.^11–18) Therefore, the dielectric and ferroelectric properties of perovskite-type oxides change depending on the A- and B-site cations. Various combinations of A- and B-site cations in ABO₃ have been studied to improve dielectric, ferroelectric, and piezoelectric properties, even seventy years after the discovery of ferroelectric perovskite-type oxides. On the other hand, the anion substitution of the O-site is also attractive for controlling the covalency between cations and anions. Recently, several researchers have reported the anion substitution of dielectric perovskite-type oxides, e.g., Ba_1−xK_xTiO_3−xF_x,^19–22) (Ba_xSr_1−x)_1+yTi_1−yO_3−zN_z,²³⁾ and SrTaO₂N.^24–29) However, there have been few research studies owing to the difficulty in the synthesis of such oxides, and thus the substitutional effect of anions on the dielectric properties remains unknown.

The study presented herein was focused on strontium titanium oxynitride (SrTiO₃:N), which is SrTiO₃ substitutionally doped with nitrogen. Nitrogen-doped SrTiO₃ has been recently attracting attention as a photocatalyst that responds to visible light. Thus, the optical properties and photocatalytic activity of SrTiO₃:N particles and thin films have been reported by many researchers.^30–33) However, the dielectric properties of SrTiO₃:N have remained unknown. Since nitrogen has a smaller electronegativity than oxygen, the covalency between Ti and O/N in SrTiO₃:N should be higher than that between Ti and O in pure SrTiO₃. The high covalency weakens the short-range repulsion forces against long-range dipolar interaction.^{11–13,19–22)} Therefore, substitution with nitrogen may enhance the ionic polarizability of SrTiO₃.

In this paper, we show a fabrication method and the dielectric properties of strontium titanium oxynitride single crystals. Strontium titanium oxynitride single crystals were prepared by annealing SrTiO₃ and Nb-doped SrTiO₃ single crystals in gaseous ammonia, and the substitutional effect of nitrogen on their dielectric properties was discussed.

As raw materials for oxynitride single crystals, (100)-oriented SrTiO₃ and 0.1 at. % Nb-doped SrTiO₃ (SrTiO₃:Nb) single crystals purchased from Shinkosha were used. The sample was 10 × 10 × 0.5 mm³, and both of its surfaces were polished to a mirror finish. The single crystals were annealed in gaseous ammonia (NH₃) at 1000 °C using a tubular electric furnace. The flow rate of NH₃ gas was 0.1 L/min, and the annealing time was varied from 1 to 24 h.

The crystal structures of the nitrided samples were evaluated by powder X-ray diffraction (XRD) with Cu Kα rays (Rigaku RINT-2000) after milling the samples. Microstructures were observed by scanning electron microscopy (SEM; JEOL JCM-6000 NeoScope). UV–vis transmittance spectra of the samples were recorded on a UV–vis spectrophotometer (JASCO V-570). The dopant nitrogen ions in the samples were semi-quantitatively measured by X-ray photoelectron spectroscopy (XPS) with Al Kα ray (ULVAC-PHI PHI5000 VersaProbe II). The diameter of the irradiated area was 100 µm and the constant pass energy was 58.7 eV. The photoelectrons of the N 1s orbital were recorded with a takeoff angle of 45°. The dielectric properties were measured using an impedance analyzer (Agilent 4294A) in the frequency range from 1 kHz to 10 MHz after depositing Au electrodes on the top and bottom surfaces by DC sputtering. Resistivity was measured using a digital ultrahigh resistance meter (ADC 8340A).

Figure 1 shows photographs of the SrTiO₃ single crystals annealed in NH₃ gas for various annealing times. The SrTiO₃ crystals annealed in NH₃ gas changed from colorless to yellow, and the yellow color became deeper with increasing annealing time. XRD measurement indicated that the obtained samples had a perovskite structure without impurities, as shown in Fig. 2. The positions of diffraction peaks seemed to be nearly unchanged before and after annealing treatment, suggesting that the lattice parameters were hardly changed by the nitridation. In addition to the ionic radius of N³⁻ (1.46 Å) being close to that of O²⁻ (1.38 Å), the amount of nitrogen substitution might not be large enough to change the lattice parameters. Figure 3(a) shows the UV–vis transmittance spectra of the samples. The absorption edge of the transmittance shifted to the long wavelength side with increasing annealing time. Miyauchi et al. have reported the electronic densities of states (DOSs) of SrTiO_3−2xN_x and Sr_1−xLa_xTiO_3−xN_x calculated by first-principles calculation.³⁰⁾ According to their calculation results, the energy level of N 2p orbitals in SrTiO_3−2xN_x and Sr_1−xLa_xTiO_3−xN_x was above that of O 2p orbitals, and the mixed orbitals of N 2p and O 2p compose the valence band of SrTiO_3−2xN_x and Sr_1−xLa_xTiO_3−xN_x, whereas the valence band for pure SrTiO₃ consists of O 2p orbitals. Therefore, the band gap of N-doped SrTiO₃ should be narrower than that of pure SrTiO₃. The shift of the absorption edge in our study corresponded to the narrowing of the band gap, suggesting that the nitrogen substitution of oxygen sites in SrTiO₃ progressed with the annealing in NH₃ gas flow.

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If the nitrogen is substitutionally doped into oxygen sites without forming oxygen vacancies, N-doped SrTiO₃ should display p-type conductivity to maintain the charge balance in the crystal. However, it is generally very difficult to obtain p-type conductivity in an oxide semiconductor with a wide band gap because the upper level of the valence band is deep and the acceptor level is thermodynamically unstable. Therefore, most of N-doped oxides have oxygen vacancies to maintain charge balance in the crystal.³⁰⁾ From this viewpoint, it is reasonable to think that N-doped SrTiO₃ has the chemical composition SrTiO_3−3xN_2x, which has oxygen vacancies. Figure 4 shows SEM images of the SrTiO₃ crystals annealed in NH₃ gas for 24 h. Many microcracks along the 〈100〉 direction were observed in the samples annealed for more than 12 h. The samples annealed for more than 24 h were so brittle that they can be broken by hand. Shkabko et al. have reported that stacking faults originated from the oxygen vacancies along the 〈100〉 direction in SrTiO_3−3xN_2x thin films.³⁴⁾ In addition, oxygen vacancies may cause the reduction in dielectric permittivity. Therefore, it is necessary to reduce the number of oxygen vacancies to prevent reductions in mechanical strength and dielectric permittivity.

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To reduce the number of oxygen vacancies by nitrogen doping, the doping with high-valence cations may be effective. In this study, a Nb-doped SrTiO₃ crystal was annealed in NH₃ gas for 12 h and thus a SrTiO₃ crystal co-doped with nitrogen and niobium (SrTiO₃:N,Nb) was obtained. The chemical formula of the obtained SrTiO₃:N,Nb was expected to be SrTi_1−2yNb_2yO_3−3x+yN_2x, indicating that the reduction in the number of oxygen vacancies is proportional to that in the amount of doped Nb. By annealing the Nb-doped SrTiO₃ crystal in NH₃ gas, the crystal was changed from dark blue to yellow. The transmittance spectra shown in Fig. 3(b) indicate a shift of the absorption edge to higher wavelengths, corresponding to the narrowing of the band gap. In addition, the red absorption at approximately 800 nm decreased with SrTiO₃:Nb nitridation, suggesting the oxidation of Ti ions. The resistivity of the SrTiO₃:N,Nb crystal was 6 × 10¹² Ω·cm, whereas that of the pre-annealed SrTiO₃:Nb crystal was 8 × 10⁻² Ω·cm. This result indicates that the SrTiO₃:Nb crystal changed from a semiconductor to an insulator with the nitridation. Figure 5 shows XPS N 1s signals of the SrTiO₃:N and SrTiO₃:N,Nb crystals annealed in NH₃ gas for 12 h. A larger peak at 395 eV and a smaller peak at 398 eV were detected for the SrTiO₃:N and SrTiO₃:N,Nb crystals, while no peak of nitrogen was detected for the pre-annealed SrTiO₃ and SrTiO₃:Nb crystals. The N 1s peak at 398 eV was attributed to nitrogen being substituted for oxygen in the perovskite structure, and the peak at 395 eV to the N³⁻ state of the Ti–N bond.²³⁾ Therefore, nitrogen ions certainly substituted for oxygen ions and bound covalently to Ti ions. The N 1s peaks of SrTiO₃:N,Nb crystal were higher than those of the SrTiO₃:N crystal, indicating that the doping with niobium was effective for increasing the amount of doped nitrogen.

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Figure 6 shows the dielectric permittivities and loss tangents of the SrTiO₃, SrTiO₃:N, and SrTiO₃:N,Nb crystals. The relative permittivities of the SrTiO₃:N and SrTiO₃:N,Nb crystals were respectively 340 and 410 at 1 MHz, which were higher than that of the pure SrTiO₃ crystal (310). In particular, for the SrTiO₃:N,Nb crystal, specimens of two thicknesses (0.46 and 0.25 mm) were prepared by polishing, and it was confirmed that the difference in permittivity between the two specimens was less than 2%. These results suggested that the composition was nearly uniform in the thickness direction, and that the permittivity increased with increasing amount of doped nitrogen. Furthermore, the loss tangent at a low frequency decreased with the nitrogen doping because the substitution by nitrogen was effective for decreasing the donor concentration in the crystal. These results showed the effectivity of nitrogen doping for enhancing the dielectric permittivity. As mentioned in Sect. 1, the covalency between Ti and O/N in SrTiO₃:N should be higher than that between Ti and O in pure SrTiO₃, weakening the short-range repulsion forces against long-range dipolar interaction. Therefore, the ionic polarization, which is the dominant polarization mechanism of dielectric permittivity for pure SrTiO₃,⁷⁾ might increase with nitrogen substitution. In contrast, oxygen vacancies in SrTiO₃:N might suppress dielectric permittivity.

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In further study, the short-range repulsion forces of SrTiO₃:N should be simulated by first-principles calculation, and the ionic polarization of SrTiO₃:N should be quantitatively evaluated. Far-infrared ellipsometry and THz time-domain spectroscopy are powerful tools for measuring dielectric properties in the THz region for estimating ionic polarization.^35–38) Therefore, we think that the substitutional effect of nitrogen will be clarified by combined first-principles calculation and THz dielectric measurement.

In this study, N-doped SrTiO₃ single crystals were fabricated by annealing SrTiO₃ single crystals in NH₃ gas. The nitrided SrTiO₃ single crystals were yellow corresponding to the narrowing of the band gap. The N-doped SrTiO₃ was assumed to have the chemical composition SrTiO_3−3xN_2x, which contained oxygen vacancies. Since oxygen vacancies might reduce mechanical strength and dielectric properties, a SrTiO₃ crystal co-doped with nitrogen and niobium (SrTiO₃:N,Nb) were fabricated to reduce the number of oxygen vacancies. The semiconducting SrTiO₃:Nb crystal changed to a dielectric SrTiO₃:N,Nb crystal with a resistivity of 6 × 10¹² Ω·cm with annealing in NH₃ gas. Furthermore, XPS measurement indicated that niobium doping was effective for increasing the amount of doped nitrogen. The dielectric permittivity increased with the amount of doped nitrogen, showing the effectivity of nitrogen doping for enhancing the dielectric permittivity by nitrogen doping in perovskite oxides. In further study, the substitutional effect of nitrogen should be clarified by combined first-principles calculation and THz dielectric measurement.

This work was supported by JSPS KAKENHI Grant Number 26420676.