Optoelectronic properties of valence-state-controlled amorphous niobium oxide

We first investigated the valence states and local structures of a-NbO_x films with respect to the change of ${{P}_{{{\text{O}}_{\text{2}}}}}$, by Nb K-edge x-ray absorption fine structure (XAFS) measurements on 200 nm thick a-NbO_x films. The XAFS measurements were performed with a fluorescence mode at the NW10A beamline of the Photon Factory in the High Energy Accelerator Research Organization (KEK-PF), Japan⁶, where the energy resolution of the beamline is estimated as ca. 4 eV at 19 keV. Figure 2(a) shows normalized x-ray absorption near-edge structure (XANES) spectra; the Nb K-edge feature clearly changed depending on ${{P}_{{{\text{O}}_{\text{2}}}}}$. The ${{P}_{{{\text{O}}_{\text{2}}}}}$ dependence of the edge energy (E₀), which was defined as the energy giving the half value of the normalized continuum absorption in arbitrary units, is summarized in the inset of figure 2(a). E₀ clearly shifted to lower energy with decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$, indicating the reduction of the valence state of Nb ion [17]. We also observed the valence-state change of Nb ion by x-ray photoemission spectroscopy (XPS, figure 3). For a-NbO_x films fabricated under ${{P}_{{{\text{O}}_{\text{2}}}}}$  =  1.5 Pa, only the Nb⁵⁺ 3d peak was observed in the core level spectra, whereas, for a-NbO_x films fabricated under lower ${{P}_{{{\text{O}}_{\text{2}}}}}$, the Nb⁴⁺ 3d peak appeared along with the Nb⁵⁺ one, and the peak intensity ratio of Nb⁴⁺/Nb⁵⁺ increased with decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$, supporting the hypothesis that the electronic configuration of Nb ion changed from [Kr]4d ⁰ (Nb⁵⁺) to [Kr]4d¹ (Nb⁴⁺) by decrease of ${{P}_{{{\text{O}}_{\text{2}}}}}$. Figure 2(b) shows k³-weighted extended XAFS (EXAFS) oscillations of the a-NbO_x films, along with those of polycrystalline Nb₂O₅ and NbO₂ for comparison. The EXAFS oscillation shape of a-NbO_x films systematically changed from a Nb₂O₅-like pattern to NbO₂-like pattern by decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$; in the high wave vector region (k  ⩾  6), the peak separation, which is a characteristic change from a NbO₂-like to Nb₂O₅-like pattern, was observed in a-NbO_x films with increasing ${{P}_{{{\text{O}}_{\text{2}}}}}$. Figure 2(c) summarizes the Fourier-transformed (FT) magnitudes of the k³-weighted EXAFS oscillations as a function of the phase-uncorrected interatomic distance (R). The FT magnitudes at R  ⩾  2.5 Å were significantly suppressed, and only the first shells for the Nb–O bonding survived due to the amorphous structure. The R of Nb−O bonding increased and got closer from that of polycrystalline Nb₂O₅ to NbO₂ with decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$ (the inset of figure 2(c)), suggesting the transformation of the O²⁻ coordination state around Nb ion from Nb₂O₅-like to NbO₂-like structure. The a-NbO_x films with Nb–O polyhedra seem comparable to the glass state as in silicate glasses with building block SiO₄; the a-NbO_x films with a short range order of building units can be considered as a non-crystalline material, although they have no long range order (amorphous phase). These features can also be seen in the XANES spectra (figure 2(a)), where the pre-edge shoulder, as indicated by a black arrow, was observed for a-NbO_x films fabricated at high ${{P}_{{{\text{O}}_{\text{2}}}}}$. To clearly show the change, the normalized pre-edge spectra for a-NbO_x films and those for polycrystalline Nb₂O₅ and NbO₂ are compared in supplementary figure S1⁶. The pre-edge shoulder has been reported to depend on the coordination state of niobium compounds [18]; it was observed for Nb₂O₅, but was not observed for NbO₂ crystals. The normalized pre-edge peak was clearly observed for the a-NbO_x films fabricated at high ${{P}_{{{\text{O}}_{\text{2}}}}}$ of 1.5 and 0.7 Pa, but it abruptly disappeared in the films fabricated at lower ${{P}_{{{\text{O}}_{\text{2}}}}}$, further supporting the hypothesis that a-NbO_x films changed from Nb₂O₅-like to NbO₂-like local structure. These results indicate that the valence state of Nb ion can be controlled from 5+  to 4+  by changing ${{P}_{{{\text{O}}_{\text{2}}}}}$, along with the change of the coordination environment of Nb ions from Nb₂O₅-like to NbO₂-like local structure. It should be noted that, according to the equilibrium phase diagram of crystalline NbO_x with respect to oxygen pressure [19], the Nb₂O₅ phase is stable in a wide range, while NbO₂ is obtained only in highly reduced conduction at RT; the control of x in NbO_x is difficult in the equilibrium state. The present a-NbO_x films fabricated by PLD are considered to be in the non-equilibrium state and the x can be easily controlled by ${{P}_{{{\text{O}}_{\text{2}}}}}$ during vapor phase deposition, where the kinetic effect on the mass transport in vapor is dominant.

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Then we measured temperature dependence of electrical resistivity (ρ–T) for the valence-state-controlled a-NbO_x films, by the dc four-point probe method with van der Pauw configuration (the film thickness was fixed at 100 nm). Figure 4(a) shows ρ–T curves for the a-NbO_x films in the T range of 300–20 K. Although the a-NbO_x film at ${{P}_{{{\text{O}}_{\text{2}}}}}$  =  1.5 Pa was highly insulating (ρ was not in the measureable range), ρ at RT was largely reduced from 9.2  ×  10¹ Ω cm to 2.8  ×  10⁻² Ω cm by changing ${{P}_{{{\text{O}}_{\text{2}}}}}$ from 1.0  ×  10⁰ to 1.0  ×  10⁻³ Pa. For the temperature dependence of resistivity, an exponential increase of ρ with respect to the temperature, i.e. semiconducting behavior, was observed for all the a-NbO_x films throughout the whole temperature range. Arrhenius plots (log σ  −  1000/T) are summarized in figure 4(b); log σ does not follow a simple thermally activated behavior. At T  >  200 K, a linear slope is observed, indicating the band conduction of thermally activated charge carriers. In contrast, at T  <  200 K, the Arrhenius plots exhibit large deviations from straight lines, where log σ transforms to much weaker temperature dependence. Better straight lines are obtained in the log σ versus T ^−1/4 plots (figure 4(c)) at a low temperature of 150–20 K: log σ can be fitted well by two distinct T ^−1/4 scaling behaviours, indicating variable range hopping (VRH) conduction in the low temperature regime [20]. These results clearly support the hypothesis that the carrier transport in a-NbO_x films is largely modulated by valence-state control with oxygen-vacancy formation.

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Since the carrier transport in the a-NbO_x films should be closely related to the localized defective states, optical absorption spectra were measured by UV-vis/NIR microscopy at RT. Figure 5 shows the absorption coefficient (α) in the energy range up to 6.0 eV. A fundametal absorption edge was clearly observed for a-NbO_x film at high ${{P}_{{{\text{O}}_{\text{2}}}}}$  =  1.5 Pa, whereas an additional absorption band appeared just below the absorption edge and became larger with decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$. The inset summarizes the ${{P}_{{{\text{O}}_{\text{2}}}}}$ dependence of the optical bandgap, estimated by Tauc plots of the a-NbO_x films (supplementary figure S2⁶). For a-NbO_x films at high ${{P}_{{{\text{O}}_{\text{2}}}}}$  =  1.5 Pa, the Tauc gap (E_g) was estimated to be 3.8 eV, which is almost the same as previously reported values of ~3.4 eV for the a-Nb₂O₅ phase [21], ~3.7 eV for the Nb₂O₅ crystalline phase [22, 23]. With decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$, E_g significantly decreased and saturated at ~1.6 eV, due to the appearance of the subgap state. These results suggest that the optical bandgap of a-NbO_x films basically originates from the electronic structure of the a-Nb₂O₅ phase and the oxygen-vacancy-related subgap state is formed by decreasing ${{P}_{{{\text{O}}_{\text{2}}}}}$.

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Figure 6 summarizes the correlation between σ at RT and α_s (α at peak position ~3.1 eV of the subgap state), together with thermopower (S). S is a powerful measure to evaluate the relationship between electronic structure and carrier transport, because S basically reflects the energy differential of the density of states (DOS) at the Fermi energy [24, 25]. The inset shows thermoelectromotive force (ΔV) versus temperature gradient (ΔT) for the a-NbO_x films, where the actual temperatures at both sides of the film surface were monitored by two tiny thermocouples. S was obtained from the linear slope of the ΔV–ΔT plots; S values were always negative, indicating n-type conduction of the a-NbO_x films.

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Both S and α_s are closely correlated with σ. α_s increased with increase of σ, suggesting that σ was affected by a large amount by the appearance and increase of the subgap state. In addition, |S| monotonically decreased from 721 μV K⁻¹ to 77 μV K⁻¹ with an increase of σ; the gradient of the linear slope in S–log σ is  −182 μV K⁻¹ decade⁻¹. In general, semiconductors possessing a parabolic DOS show a linear relationship between |S| and the log of carrier concentration (n_e): |S|  =  −k_B/e·ln 10 (log n_e  +  C), where C is parameter that depends on the type of materials [26]. When considering that the electron mobility is constant for all a-NbO_x films⁷, the observed linear slope corresponds well with  −k_B/e·ln 10 (=−198 μV K⁻¹ decade⁻¹), suggesting that electrons are doped into the conduction band, and the gradient of DOS becomes moderate, similar to carrier doping in conventional semiconductors. It should be noted that only Nb d-band conduction is considered here, but other conduction mechanisms, such as narrow band conduction thourgh localized hopping and tunneling processes, may contribute to conduction in a-NbO_x films. However, the linear relation between |S| and log σ suggests that Nb d-band conduction should be dominant at RT. These results suggest that the increase of σ with respect to ${{P}_{{{\text{O}}_{\text{2}}}}}$ originates from the change of carrier concentration in the a-NbO_x films due the appearance and formation of the oxygen-vacancy-related subgap state working as an electron donor.