Charge doping and large lattice expansion in oxygen-deficient heteroepitaxial ${\mathbf{WO}}_{\mathbf{3}}$

Tungsten trioxide $\left({\mathrm{WO}}_3\right)$ is a versatile material with widespread applications ranging from electrochromics and optoelectronics to water splitting and catalysis of chemical reactions. For technological applications, thin films of ${\mathrm{WO}}_3$ are particularly appealing, taking advantage from a high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this paper, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of ${\mathrm{WO}}_3$ thin films. By performing x-ray diffraction and low-temperature electrical transport measurements, we find changes in the ${\mathrm{WO}}_3$ lattice volume of up to 10% concomitantly with a resistivity drop of more than five orders of magnitude at room temperature as a function of increased oxygen deficiency. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of ${\mathrm{WO}}_3$ single-crystal thin films can all be purposely tuned by controlling the oxygen vacancy formation during growth.

Figure 1

Growth of ${\mathrm{WO}}_3$ thin films. (a) RHEED intensity oscillations during film growth at different values of $p_{{\mathrm{O}}_2}$ and (inset) heterostructure schematic, along with initial, and final RHEED diffraction patterns at $p_{{\mathrm{O}}_2}=50\mu \mathrm{bar}$. (b) Photograph of the samples, (c) surface topography by atomic force microscopy, (d) line profile along the black line in (c), and (e) surface roughness calculated as the root mean square of the topographic signal.

Figure 2

X-ray diffraction characterization. (a) $\theta -2\theta$ scans around the (001) and (002) peaks of the ${\mathrm{SrTiO}}_3$ substrate. The dotted line highlights the position of the ${\mathrm{WO}}_3$ (001) peak. (b) Comparison of the rocking curves around the ${\mathrm{WO}}_3$ (001) peak to the one around the substrate (001) peak (black dashed line). (c) Film $c$-axis parameter and substrate lattice constant (dashed line), (d) number of ${\mathrm{WO}}_3$ unit cells extracted by simulating the finite-size oscillations with a kinematic scattering model. (e) Film thickness and (f) roughness obtained from reflectivity measurements (crosses) and the $\theta -2\theta$ data (circles).

Figure 3

Resistivity measurements. (a) Four-probe resistivity vs temperature curves. The dashed lines are best fits to the Arrhenius model for activated transport in Eq. (1). (b) Room-temperature resistivity and (c) activation energy for transport extracted from the fits.

Figure 4

DFT calculations of heteroepitaxial ${\mathrm{WO}}_3$ films with oxygen vacancies. (a) Left panels: in-plane band structure for ${\mathrm{WO}}_3$ with 0%, 2%, and 4% of oxygen vacancies. The calculations are performed using $2\times 2\times 4$ supercells with an oxygen vacancy on the ${\mathrm{WO}}_2$ planes. The band of the oxygen defect states is indicated by the orange dash-dotted line, and the Fermi level is represented by the green dashed line. Right panels: density of states projected on the oxygen $2p$ (red) and tungsten $5d$ (gray) orbitals. For ${\mathrm{WO}}_{2.94}$ the horizontal solid lines show the Fermi level corresponding to carrier densities of $n_{1.5\mathrm{K}}=5\times {10}^{21}{\mathrm{cm}}^{-3}$ (green) and $1\times {10}^{18}{\mathrm{cm}}^{-3}$ (violet). (b) Computed $2\times 1\times 1$ supercell of ${\mathrm{WO}}_{2.94}$ showing the real-space charge-density isosurface of the defect band (orange). The oxygen vacant site at the bottom is indicated by the dashed red circle. (c) Mobility as a function of carrier density (circles) at 1.5 K from the DFT calculations. The density at the Fermi level is indicated by the white circle. The diamond represents the experimental data from Hall effect measurements for the sample grown at $p_{{\mathrm{O}}_2}=20\mu \mathrm{bar}$. (d) Resistivity vs temperature curves for ${\mathrm{WO}}_{2.94}$ from the DFT calculations (the green dashed line corresponds to the Fermi level).