Electrical, Optical, and Thermal Transport Properties of Oxygen-Deficient Amorphous WO_x (2.5 < x < 3) Films

Abstract

Oxygen-deficient tungsten oxide (WO x ) is known as an active material for various future applications such as smart displays, photocatalysts, Li-ion battery, and so on. WO x exhibits versatile properties depending on the valence state of W, which can vary from +6 to +4. Therefore, clarifying the relationship between x, the valence state of W ion, and the material properties of WO x is crucial for discovering more unique device applications. In case of crystalline WO x , since WO x has many different phases, the valence state of W cannot be modulated continuously from +6 to +4. On the other hand, there is no phase boundary in amorphous (a-) WO x , the valence state of W ion can be continuously modulated against x, and the effect of valence state of W on the material properties of WO x can be thoroughly examined. Here, we report the electrical, optical, and thermal properties of a-WO x films with several valence states of +6 (d0), +5 (d1), and +4 (d2) for x ranging from 2.511 to 2.982. Although the +6 dominant films were electrical insulators with optical transparency in the visible region, we found that both optical transmissivity and electrical resistivity decreased drastically with increase in the +5 concentrations, which also enhances the thermal conductivity because heat can be carried by additional conduction electrons. As the +4 state became dominant in the film, the resistivity slightly increased, whereas the low visible transmission was maintained. These results suggest that the redox of tungsten between +6 and +5 is attributed to all versatile properties of a-WO x , which would be of great use for developing unique devices in the future.

Coexistence of High Electron Conduction and Low Heat Conduction in Tungsten Oxide Epitaxial Films with 1D Atomic Defect Tunnels

“…S3 and S4). The κ values of the WOx films (x ≤ 2.92) are close to that of amorphous WOx film 22 , which shows minimum thermal conductivity 23 among the same chemical composition. On the other hand, the measured σave in the in-plane direction of WO2.97 was 0.07 S cm −1 and it increased to ~500 S cm −1 with decreasing x to 2.92.…”

“…The volume fractions of W 6+ and W 5+ are dynamically changed with decreasing x (Figure a). The composition x in WO x was determined from ([W 6+ ] × 3 + [W 5+ ] × 2.5 + [W 4+ ] × 2)/([W 6+ ] + [W 5+ ] + [W 4+ ]) …”

“…(a) Cross-plane thermal conductivity κ extracted from the TDTR measurement. The κ of amorphous WO x films and epitaxial WO x films is also plotted. (b) In-plane electrical conductivity σ.…”

“…(b) In-plane electrical conductivity σ. The σ of amorphous WO x films and epitaxial WO x films is also plotted. (c) Schematic illustration of the TDTR measurement of the WO x films.…”

“…The volume fractions of W 6+ and W 5+ are dynamically changed with decreasing x (Figure 2a). The composition x in WO x was determined from ([ 22 The film thickness was evaluated to be ∼40 nm in all cases from the analyses of the X-ray reflectivity. Coherent epitaxial growth was confirmed by measuring the X-ray reciprocal space mappings (RSMs) (Figure S2); the in-plane lattice parameter is fixed to that of the LaAlO 3 substrate.…”