Adsorption-controlled growth and properties of epitaxial SnO films

When it comes to providing the unusual combination of optical transparency, $p$-type conductivity, and relatively high mobility, ${\mathrm{Sn}}^{2+}$-based oxides are promising candidates. Epitaxial films of the simplest ${\mathrm{Sn}}^{2+}$ oxide, SnO, are grown in an adsorption-controlled regime at $380{}^{\circ }\mathrm{C}$ on ${\mathrm{Al}}_2{\mathrm{O}}_3$ substrates by molecular-beam epitaxy, where the excess volatile ${\mathrm{SnO}}_x$ desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch ($-12\%$). The unintentionally doped films exhibit $p$-type conductivity with carrier concentration $2.5\times {10}^{16}{\mathrm{cm}}^{-3}$ and mobility $2.4{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at room temperature. Additional physical properties are measured and linked to the ${\mathrm{Sn}}^{2+}$ valence state and corresponding lone-pair charge-density distribution.

Figure 1

Phase-pure litharge $\mathrm{SnO}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3\left(1\overline{1}02\right)$ films produced via molecular-beam epitaxy. (a) Backscattered Raman Stokes spectrum and (b) XRD $\theta -2\theta$ scan establishing phase-pure litharge $\mathrm{SnO}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3\left(1\overline{1}02\right)$ films. The inset depict atomic displacement patterns corresponding to Raman-active vibrational modes (a) and the film/substrate orientational relationship (b).

Figure 2

Spiral growth of fully dense $\mathrm{SnO}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3\left(1\overline{1}02\right)$ films. (a) XDS map exhibiting diffuse wings and a decay in specular intensity that is indicative of fully dense films $(6.2\mathrm{g}/{\mathrm{cm}}^3)$ and atomically smooth surfaces (1.0 nm roughnesses). (b) AFM amplitude image showing shallow spiral growth mounds. The overlaid height-difference correlation function has a presaturation slope which is consistent with high adatom diffusivity.

Figure 3

Structural perfection of semicoherent $\mathrm{SnO}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3\left(1\overline{1}02\right)$ films. (a) STEM image acquired along the ${\mathrm{Al}}_2{\mathrm{O}}_3\left[1\overline{1}0\overline{1}\right]$ zone axis near the $\mathrm{SnO}\text{/}{\mathrm{Al}}_2{\mathrm{O}}_3$ interface. Misfit dislocations are exposed by the overlaid in-plane strain isocontours. (b) NBD pattern of the film region. Indexed reflections indicate an ${\left(001\right)}_{\mathrm{SnO}}\mid \mid {\left(1\overline{1}02\right)}_{{\mathrm{Al}}_2{\mathrm{O}}_3}$ and ${\left[110\right]}_{\mathrm{SnO}}\mid \mid {\left[11\overline{2}0\right]}_{{\mathrm{Al}}_2{\mathrm{O}}_3}$ epitaxial relationship. (c) RSM of SnO 114 and ${\mathrm{Al}}_2{\mathrm{O}}_{3}4\overline{2}\overline{2}6$ peaks evincing overlayer relaxation. (d) $\theta -2\theta$ XRD scan in the vicinity of the SnO 001 peak. (e) Superimposed XRD rocking curve scans of the SnO 001 and ${\mathrm{Al}}_2{\mathrm{O}}_{3}1\overline{1}02$ peaks, establishing substrate-limited film structural perfection. The full width at half maximum of both film and substrate $\omega$-rocking curve peaks is $0.{007}^{\circ }$ (25 arcsec). (f) Higher magnification STEM image highlighting a $0.40\pm 0.03$-nm-wide gap that develops, separating a commensurately strained monolayer of the SnO film from the remainder of the fully relaxed SnO layer. The gap, which is a signature of van der Waals epitaxy, pins dislocations as misfits near the film/substrate interface, promoting the growth of films with high structural perfection.

Figure 4

Electronic properties of litharge SnO, a model lone-pair system. (a) Theoretical SnO electronic band dispersions with states colorized and broadened according to orbital ($s$ vs $p$) and atomic (tin vs oxygen) characters. The insert shows electron and hole pockets. (b) Charge-density maps of hole-pocket states reveal a lone-pair-like distribution. (c) and (d) SnO complex dielectric function $\varepsilon \equiv {\varepsilon }_1+i{\varepsilon }_2$ resolved into ordinary $xy$ (blue) and extraordinary $z$ (red) components as determined from VASE (solid) and RPA calculations (dashed). (e) XPS scans as a function of photon energies between 400 and 1500 eV; the densities of states of SnO and ${\mathrm{SnO}}_2$ are also plotted for reference.