Epitaxial growth of high quality WO3 thin films

We have grown epitaxial WO3 films on various single-crystal substrates using radio-frequency (RF) magnetron sputtering. While pronounced surface roughness is observed in films grown on LaSrAlO4 substrates, films grown on YAlO3 substrates show atomically flat surfaces, as demonstrated by atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements. The crystalline structure has been confirmed to be monoclinic by symmetric and skew-symmetric XRD. The dependence of the growth modes and the surface morphology on the lattice mismatch is discussed. Tungsten trioxide (WO3) is a well-known electrochromic material which changes color under an applied electric field. It is also very sensitive to NOx exposure, and hence it is used to fabricate gas sensors. Both of these applications require WO3 to be grown in a thin film form. Various methods have been used to prepare WO3 thin films, including thermal evaporation, chemical vapor deposition (CVD), 9,13 sputtering, 6,8,14-17 and pulsed laser deposition (PLD). Films prepared on glass or Si substrates usually have amorphous or polycrystalline structure and rough surfaces. Growth of epitaxial films of WO3 on single-crystal substrates such as SrTiO3, MgO, and sapphire has been reported as well. However, because of large lattice mismatch the surface morphology of these films has been inadequate for superlattice growth or for surface-sensitive experiments such as electrolyte gating, which generally require atomically flat surfaces and interfaces with a root-mean-square (rms) roughness less than 1 nm. Our main goal here has been to develop a method, relatively simple if possible, of fabricating atomically smooth WO3 films suitable for such experiments. In the present study, we synthesized epitaxial WO3 thin films on single-crystal LaSrAlO4 (LSAO) and YAlO3 (YAO) substrates using RF magnetron sputtering technique. X-ray diffraction (XRD) measurements show that in either case the BNL-108357-2015-JA

films are epitaxially oriented with respect to the substrates. Both LSAO and YAO substrates as-purchased come with atomically flat surfaces, as we verified by atomic force microscopy (AFM) scans before growth. However, only films grown on YAO substrates with the surfaces polished perpendicular to the crystallographic [110] direction (for brevity, (110) YAO in what follows) have atomically flat surfaces, as demonstrated by AFM and XRD measurements. Our data indicate that the lattice mismatch between the film and the substrate plays a key role in controlling the growth mode and the surface morphology.
The WO3 can be viewed as a cubic ReO3 structure with eight WO6 octahedra centered at the eight corners. The center of the cube is empty, and hence the structure is easily distorted and tilted upon heating or cooling, with concomitant symmetry lowering. Five different crystal structures of WO3 have been observed below 1,000 K. [27][28][29] At room temperature, the most stable structure is γ-monoclinic with the following lattice parameters: a1 = 7.306 Å, b1 = 7.540 Å, c1 = 7.692 Å, and β = 90.88°; note that this unit cell contains eight WO3 formula units.
LSAO substrate has a tetragonal structure with an in-plane lattice constant a = 3.754 Å. At room temperature, the lattice mismatch between the substrate and the WO3 film, defined as ε = (as-af)/as, is -0.4% in one direction and 2.7% in the other.
In the present study, WO3 films were deposited in an RF magnetron sputtering system at a growth temperature varied from 550 °C to 850 °C . The pressure during growth was kept at 60 mTorr with an O2/Ar ratio 4:1. The growth rate was kept at approximately 1 nm/minute. The film thickness was determined using X-ray reflectivity measurements. Fig. 1 shows XRD patterns for seven WO3 films deposited on (001) LSAO substrates at various growth temperatures. All these films were grown at the same RF power (60 W) and for the same time (1 hour), and are of similar thickness, determined to be around 60 nm. For the γ-monoclinic WO3, a peak around 23.1° is expected, corresponding to the out-of-plane lattice constant c=7.692 Å. For films grown at a temperature ≤ 650 °C , the XRD pattern indeed shows a single peak at about 23.1°, suggesting that the films are epitaxial and the structure is monoclinic. However, as the growth temperature increases to 700 °C and above, extra peaks show up in the XRD patterns, while the main peak near 23.1° diminishes and finally disappears at 850 °C . Although the WO3 films grown on LSAO at temperatures ≤ 650 °C are epitaxially aligned with respect to the substrate, their surfaces are not atomically flat, as can be seen from a typical AFM image shown in Fig. 2. The rms surface roughness is about 1.2 nm, and the entire film surface is covered by grains with a diameter of about 100 nm, indicating a threedimensional (island) growth mode. 30 We have grown dozens of films on LSAO substrates at various temperatures with thicknesses ranging from 10 nm to 100 nm, the RMS roughness falls in the range of 1 to 10 nm. These films are epitaxial but fall short of our goal of fabricating WO3 samples with atomically flat surfaces. In an attempt to improve the surface morphology, we have deposited WO3 films on (110) YAO substrates. Sputtering was done at temperatures between 750 °C and 850 °C , with other conditions similar to what we have used before in the growth of WO3 films on LSAO. In Fig. 3, we show a typical AFM image of a WO3 film grown on a (110) YAO substrate. One can clearly discern steps and terraces. The average RMS roughness is only 1.6 Å, indicating that the film surface is indeed atomically flat. In the bottom panel in Fig. 3, we show the height-profile scan obtained from a horizontal cut across the AFM image. The step heights are found to be either 3.5 Å or 7 Å, corresponding to one-half or one unit cell of WO3, indicating a two-dimensional (layer-by-layer or step-flow) growth mode.   4 shows wide angle X-ray diffraction patterns of WO3 films with thickness ranging from 4 nm to 84 nm, grown on (110) YAO substrates. A very high crystal quality is apparent from very pronounced finite-thickness (Laüe) fringes, which testify that the film surfaces and the substrate-film interfaces are perfect and parallel on the atomic scale and the film is singlecrystalline throughout its whole thickness. Only the peaks corresponding to the (00n) family of crystallographic planes of WO3 can be seen over the entire scan range (5° < 2θ < 85°). The out-of-plane lattice constant calculated from the WO3 Bragg peak (84nm film) is 7.73(2) Å, which matches the known monoclinic structure. The in-plane lattice constants have been determined to be 7.31 Å and 7.51 Å from skew-symmetric XRD measurements of (202) and (222) reflections, as shown in  Comparison of films grown on LSAO and YAO substrates suggests that it is possible to grow epitaxial films on substantially mismatched (ε > 2%) substrates, if the growth conditions (especially the growth temperature) are appropriately adjusted.
However, the growth mode and thus the surface morphology may be very different. For III-V semiconductors, it is known that the lattice mismatch plays a crucial role in determining the film growth mode and surface morphology. [31][32][33] In general, the growth mode depends on the competition of the free energy of a film/epilayer (σf) and the surface energy of a substrate (σs). For a lattice-mismatched system, σf consists of the total surface energy of the epilayer and the strain energy that results from the lattice mismatch. The island morphology always provides a larger surface energy than that of a flat film. However, the strain energy stored in islands is always less than that stored in a flat film. Thus, in the case of a sufficiently large lattice mismatch, even though the surface energy of the epilayer favors a flat film morphology, the total free energy of the film may still favor an island morphology if the reduction in strain energy is large enough to offset the increase in surface energy. 34 Furthermore, it has been shown that films grown under a tensile strain tend to crack much more readily than those under the compressive strain. 31 In the present study, the in-plane area of the unit cell of γ-monoclinic WO3 is 2.3% smaller than that of the (001) LSAO (quadrupled) while being 1.0% larger than that of a (110) YAO. Thus, one would indeed expect the stretched WO3 films on LSAO to crack more readily, and have a rougher surface, than the compressed WO3 films on YAO substrates.
In summary, we have grown atomically flat epitaxial WO3 thin films on (110) YAO substrates using RF magnetron sputtering. We have also shown that films grown on the other substrates such as (001) LSAO may have an epitaxial orientation, if the growth temperature is in the range of 550 °C to 650 °C . However, WO3 films on LSAO always show island morphology with a much larger rms surface roughness. We ascribe this to the facts that WO3 has a larger lattice mismatch with LSAO than with YAO, and that the tensile strain in WO3 films on LSAO makes them crack more readily than the compressed WO3 films on YAO.