Solid-phase epitaxial growth of the correlated-electron transparent conducting oxide $\mathrm{SrV}{\mathrm{O}}_3$

$\mathrm{SrV}{\mathrm{O}}_3$ thin films with a high figure of merit for applications as transparent conductors were crystallized from amorphous layers using solid phase epitaxy (SPE). Epitaxial $\mathrm{SrV}{\mathrm{O}}_3$ films crystallized on $\mathrm{SrTi}{\mathrm{O}}_3$ using SPE exhibit room-temperature resistivities as low as $5.2\times {10}^{–5}$ and $2.5\times {10}^{–5}\mathrm{\Omega }\mathrm{cm}$, residual resistivity ratios of 2.0 and 3.8, and visible light transmission maxima of 0.89 and 0.52 for film thicknesses of 16 and 60 nm, respectively. $\mathrm{SrV}{\mathrm{O}}_3$ layers were deposited at room temperature using radiofrequency sputtering in an amorphous form and subsequently crystallized by heating in a controlled gas environment. The lattice parameters and mosaic angular width of x-ray reflections from the crystallized films are consistent with partial relaxation of the strain resulting from the epitaxial mismatch between $\mathrm{SrV}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$. A reflection high-energy electron diffraction study of the kinetics of SPE indicates that crystallization occurs via the thermally activated propagation of the crystalline/amorphous interface, like SPE phenomena in other perovskite oxides. Thermodynamic calculations based on density functional theory predict the temperature and oxygen partial pressure conditions required to produce the $\mathrm{SrV}{\mathrm{O}}_3$ phase and are consistent with the experiments. The separate control of deposition and crystallization conditions in SPE presents possibilities for the crystallization of transparent conductors in complex geometries and over large areas.

Figure 1

(a) Solid-phase epitaxial growth of $\mathrm{SrV}{\mathrm{O}}_3$. (i) Amorphous $\mathrm{SrV}{\mathrm{O}}_3$ deposited at room temperature on $\mathrm{SrTi}{\mathrm{O}}_3$ (001). (ii) Crystallized $\mathrm{SrV}{\mathrm{O}}_3$ after heating and the epitaxial relationship between $\mathrm{SrV}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$. (b) Oxygen transport and exchange through the free surface crystallization in a reducing gas atmosphere. (c) Transparent conductor figures of merit (FOMs) for 16- and 60-nm-thick $\mathrm{SrV}{\mathrm{O}}_3$ synthesized by solid-phase epitaxy (SPE; this paper), and reported FOMs for $\mathrm{SrV}{\mathrm{O}}_3$ films grown by hybrid molecular beam epitaxy (hMBE; diamonds) [8], pulsed laser deposition (PLD; upward [4], right [20], and downward [12] pointing triangles), and sputtering (circles) [14].

Figure 2

(a) θ-2θ diffraction pattern showing 00L x-ray reflections for a crystallized epitaxial $\mathrm{SrV}{\mathrm{O}}_3$ film and the $\mathrm{SrTi}{\mathrm{O}}_3$ substrate, acquired at x-ray wavelength 1.5406 Å. (b) Diffraction profile near the 002 reflections. (c) Reciprocal space map in the region of the $\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{SrV}{\mathrm{O}}_3$ 113 x-ray reflections. $Q_z$ and $Q_{xy}$ represent the components of the x-ray wave vector along the surface normal and [110] in-plane directions, respectively. (d) X-ray photoelectron spectroscopy (XPS) core level spectra for V $2p$ and O $1s$ in the amorphous precursor film (red) and a crystallized $\mathrm{SrV}{\mathrm{O}}_3$ film (blue). The spectrum for the crystallized layer is shifted vertically by 0.1.

Figure 3

(a) Scanning transmission electron microscope high-angle annular dark-field (STEM-HAADF) image of the 60 nm film crystallized at 750 °C for 3 h. (b) Average image intensity along the B site columns $\pm 3$ nm from the interface. (c) Energy dispersive x-ray fluorescence spectroscopy (EDS) intensity profile for Ti and V measured normal to the interface at the location of the black arrow in (a).

Figure 4

(a) Electrical resistivity of crystallized 16 nm (open circles) and 60 nm (open squares) $\mathrm{SrV}{\mathrm{O}}_3$ films. (b) Optical transmission spectrum for 60-nm-thick amorphous (open triangles) and crystallized 16 nm (open circles) and 60 nm (open squares) $\mathrm{SrV}{\mathrm{O}}_3$ films. (c) Optical absorption coefficient spectra for amorphous and crystallized $\mathrm{SrV}{\mathrm{O}}_3$ films calculated from the transmission spectra in (b) under assumptions described in the text.

Figure 5

(a) X-ray scattering intensity from amorphous $\mathrm{SrV}{\mathrm{O}}_3$, collected with x-ray wavelength 1.54 Å. (b) X-ray reflectivity curves for amorphous (red) and crystallized (blue) $\mathrm{SrV}{\mathrm{O}}_3$, collected with an x-ray wavelength of 1.5406 Å. X-ray reflectivity models with parameters given in the text are shown as solid lines. The reflectivity of the crystallized layer has been multiplied by 0.1. (c) Atomic force microscopy (AFM) height map of amorphous $\mathrm{SrV}{\mathrm{O}}_3$. (d) AFM height map of crystallized $\mathrm{SrV}{\mathrm{O}}_3$.

Figure 6

Reflection high-energy electron diffraction (RHEED) patterns acquired after crystallization at 735 °C with the incident beam oriented along (a) $\langle 110\rangle$ and (b) $\langle 010\rangle$ directions. Integrated intensity profiles are shown below each pattern. (c) Schematic definition of crystallization time $t$, at which the crystallization interface reaches the ∼1 nm depth probed by the RHEED experiment. (c) Temperature dependence of the crystallization velocity with a fitted activation energy of 2.74 eV.

Figure 7

Convex hull energy for the Sr-V-O system as a function of temperature and oxygen partial pressure calculated using density functional theory (DFT). Higher convex hull energy values indicate a stronger driving force for reaction of $\mathrm{SrV}{\mathrm{O}}_3$ to energetically favored competing phases.