Silicate-free growth of high-quality ultrathin cerium oxide films on Si(111)

Ultrathin Ce${}_2$O${}_3$ layers have been grown on Si(111) by reactive metal deposition in an oxygen background and characterized by x-ray standing waves, x-ray diffraction, x-ray photoelectron spectroscopy, and low-energy electron diffraction to elucidate and quantify both atomic structure and chemical composition. It is demonstrated that highly ordered, mostly $B$-oriented, epitaxial ceria films can be achieved by preadsorption of a monolayer of atomic chlorine, effectively passivating the substrate and thereby suppressing cerium silicate and silicon oxide formation at the interface.

Figure 1

Low-energy electron diffraction patterns recorded for (a) clean Si(111)-($7\times 7$), (b) Cl/Si(111)-($1\times 1$), and (c) cerium oxide reactively grown on Cl/Si(111)-($1\times 1$) in an O${}_2$ background of $5\times {10}^{-7}$ mbar.

Figure 2

Ce $3d$ XPS data obtained after cerium oxide growth at a substrate temperature of 500${}^{\circ }$C on (a) clean Si(111)-($7\times 7$) and $1\times {10}^{-7}$ mbar O${}_2$ partial pressure, (1); (b) Cl-passivated Si(111) and $1\times {10}^{-7}$ mbar O${}_2$ partial pressure, (1-Cl); and (c) Cl-passivated Si(111) and $5\times {10}^{-7}$ mbar O${}_2$ partial pressure, (5-Cl). Film thicknesses are a few nanometers in all cases.

Figure 3

Si $1s$ XPS data obtained (a) before and (b) after growth of a 2.5-nm thin cerium oxide film at 500${}^{\circ }$C substrate temperature and $1\times {10}^{-7}$ mbar O${}_2$ partial pressure [i.e., according to (1-Cl)].

Figure 4

(a) X-ray diffraction and (b) x-ray reflectivity data [open (black) circles] and theoretical fit [solid (red) lines] obtained after cerium oxide growth at $1\times {10}^{-7}$ mbar O${}_2$ backfilling and a substrate temperature of 500${}^{\circ }$C on bare Si(111) and subsequent capping [i.e., following recipe (1-capped)]. In (b), the curves are shown with a small offset, for clarity.

Figure 5

XRD data obtained after cerium oxide growth at $1\times {10}^{-7}$ mbar O${}_2$ backfilling and a substrate temperature of 500${}^{\circ }$C on Cl/Si(111) and subsequent exposure to air [i.e., following recipe (1-Cl-uncapped)]: (a) (00$L$)-CTR, (b) (0$KL$) scans at two different $L$ values used to determine the average grain size. The (red) solid line in (a) represents a calculation following kinematic diffraction theory.

Figure 6

Si$\left(01L\right)$ crystal truncation rod data obtained in GIXRD geometry after cerium oxide growth on Cl/Si(111) at $1\times {10}^{-7}$ mbar O${}_2$ backfilling and a substrate temperature of 500${}^{\circ }$C and subsequent exposure to air [i.e., following recipe (1-Cl-uncapped)].

Figure 7

Structural model for a single Ce${}_2$O${}_3$ bixbyite trilayer on Si(111). The parameter $d_{\text{int}}$, which is accessible (modulo the Si layer spacing $d_{111}$) in XSW experiments in (111) Bragg reflection, denotes the average distance between the Ce atoms of the trilayer closest to the substrate and the outermost Si bilayer.

Figure 8

Structural model for three trilayers of cubic (bixbyite) Ce${}_2$O${}_3$ compressively strained on Si(111), assuming volume conservation of the film. The (111) diffraction planes of the substrate-generated standing-wave field are indicated on the left, demonstrating the expected decrease in the Ce coherent fraction with film thickness and vertical lattice mismatch, respectively.

Figure 9

Argand diagrams showing the simulated (111) XSW results depending on relative vertical lattice mismatch and the number of oxygen-metal-oxygen trilayers of Ce${}_2$O${}_3$(111) grown on Si(111), which are assumed to adopt the bixbyite structure exhibiting a tetragonal distortion of (a) 2% and (b) 4% along $\left[111\right]$ with respect to the bulk. $\Delta \Phi$ denotes the layerwise shift in coherent position for an increasing number of trilayers, while ${\Phi }_0$ marks the coherent position of the Ce atoms in the lowest (first) trilayer, which can be deduced from the experimental result.

Figure 10

XSW data (open symbols) and theoretical fit according to the dynamical theory of x-ray diffraction (solid lines) using Ce $L\alpha$ fluorescence obtained in Si(111) Bragg reflection for $(4.9\pm 0.5)$-TL-thin cerium oxide films on (a) Si(111) and (b) Cl-passivated Si(111) for growth at oxygen partial pressures of $1\times {10}^{-7}$ mbar [(1), (1-Cl)] and $5\times {10}^{-7}$ mbar [(5), (5-Cl)], respectively.

Figure 11

XSW data (data points) and theoretical fit according to the dynamical theory of x-ray diffraction (solid lines) using Cl $K\alpha$ fluorescence obtained in Si(111) Bragg reflection for (a) Cl/Si(111)-($1\times 1$) and (b) for $(4.9\pm 0.5)$-TL-thin Ce${}_2$O${}_3$(111) films grown on Cl-passivated Si(111) at oxygen partial pressures of $1\times {10}^{-7}$ mbar (1-Cl) and $5\times {10}^{-7}$ mbar (5-Cl), respectively. All yields are shifted upward by $0.5$ units, for clarity.

Figure 12

Cl $1s$ XPS data obtained (a) before and (b) after growth of a 10-nm-thick cerium oxide film at 500${}^{\circ }$C substrate temperature and $5\times {10}^{-7}$ mbar O${}_2$ partial pressure [i.e., according to recipe (5-Cl)].