Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001).

Figure 1

(Color online) RHEED photographs of (a) clean and annealed YSZ (001), [(b)–(d)] EuO films on YSZ (001) after 10 min of growth at $400°\text{C}$ with $8.1–8.2\text{Å}/\text{min}$ Eu flux rates, and [(e)–(f)] with $4.2–4.3\text{Å}/\text{min}$ Eu flux rates. The oxygen pressure in the chamber was $4\times {10}^{-8}\text{mbar}$ for (b) and $2\times {10}^{-8}\text{mbar}$ for (c) and (e). No oxygen was supplied into the chamber for (d) and (f). The RHEED electron energy was 20 keV with the beam incident along the [100] direction.

Figure 2

(Color online) RHEED intensity oscillations of the specularly reflected electron beam recorded during the deposition of EuO films on YSZ (001) using oxygen pressures $\left(P_{\text{Ox}}\right)$ and Eu flux rates $\left({\Phi }_{\text{Eu}}\right)$ as indicated. The corresponding RHEED photographs after 10 min of growth are displayed in Figs. 1b, 1c, 1d, 1e, 1f.

Figure 3

(Color online) LEED photographs of epitaxial EuO films on YSZ substrate grown for 10 min at $400°\text{C}$ in the absence of oxygen in the MBE chamber using (a) a $8.2\text{Å}/\text{min}$ Eu flux rate and recorded at electron beam energy of 215 eV and (b) a $4.3\text{Å}/\text{min}$ Eu flux rate and recorded at electron beam energy of 213 eV.

Figure 4

(Color online) (a) $\text{Zr}3d\text{-Eu}4d$ core level XPS spectra and (b) $\text{Zr}4p\text{-O}2s\text{-Eu}5p$ core level and $\text{O}2p\text{-Eu}4f$ valence band XPS spectra of EuO films on YSZ (001), grown at $400°\text{C}$ in the absence of oxygen in the MBE chamber. The spectra were collected at normal emission. From top to bottom: EuO film after 10 min of growth using a $4.3\text{Å}/\text{min}$ Eu flux rate, after 20 min using $4.2\text{Å}/\text{min}$, after 10 min using $8.2\text{Å}/\text{min}$, and clean YSZ substrate.

Figure 5

(Color online) Take-off angle dependence of the $\text{Zr}3d\text{-Eu}4d$ core level XPS spectra of a EuO film on YSZ (001). Top: grazing emission, i.e., $\Theta =70°$ with respect to the surface normal. Bottom: normal emission. The film was grown at $400°\text{C}$ for 10 min with a $4.3\text{Å}/\text{min}$ Eu flux rate in the absence of oxygen in the MBE chamber.

Figure 6

(Color online) (a) $\text{Zr}3d\text{-Eu}4d$ core level XPS spectra and (b) $\text{Zr}4p\text{-O}2s\text{-Eu}5p$ core level and $\text{O}2p\text{-Eu}4f$ valence band XPS spectra of EuO films on YSZ (001), grown at $400°\text{C}$ with a $8.2\text{Å}/\text{min}$ Eu flux rate for 10 min. The spectra were collected at normal emission. From top to bottom: EuO films grown under oxygen pressures of 16, 12, 10, 8, and $4\times {10}^{-8}\text{mbar}$, and clean YSZ substrate.

Figure 7

(Color online) Take-off angle dependence of the $\text{Zr}3d\text{-Eu}4d$ core level XPS spectra of a EuO film on YSZ (001). Top: grazing emission, i.e., $\Theta =70°$ with respect to the surface normal. Bottom: normal emission. The film was grown at $400°\text{C}$ under a $4\times {10}^{-8}\text{mbar}$ oxygen pressure and a $8.2\text{Å}/\text{min}$ Eu flux rate for 10 min.

Figure 8

(Color online) Top panels: RHEED photographs of EuO films on YSZ (001) grown at $400°\text{C}$ with $8.0–8.3\text{Å}/\text{min}$ Eu flux rates under oxygen pressures of—from left to right—(a) 4, (b) 8, (c) 10, and (d) $12\times {10}^{-8}\text{mbar}$. The deposition times were 200, 200, 100, and 100 min, respectively. The RHEED electron energy was 20 keV with the beam incident along the [100] direction. Bottom panels: corresponding LEED photographs. The LEED electron energies were 369, 368, 370, and 266 eV, respectively.

Figure 9

RHEED intensity oscillations of the specularly reflected electron beam, recorded during deposition of a EuO film on YSZ (001) grown at $400°\text{C}$ using a $10\times {10}^{-8}\text{mbar}$ oxygen pressure and a $8.2\text{Å}/\text{min}$ Eu flux rate.

Figure 10

XRR curve of epitaxial EuO film on YSZ (001) grown for 200 min at $400°\text{C}$ using a $4\times {10}^{-8}\text{mbar}$ oxygen pressure and a $8.0\text{Å}/\text{min}$ Eu flux rate.

Figure 11

EuO film thickness, as determined from XRR measurements, versus the product of oxygen pressure and total deposition time.

Figure 12

(Color online) Temperature dependence of the magnetization of epitaxial EuO films on YSZ (001) grown at $400°\text{C}$ with $8.0–8.3\text{Å}/\text{min}$ Eu flux rates under various oxygen pressures as indicated. The small magnetization contribution from the substrate has been subtracted. The applied magnetic field was 1000 G. The inset shows the field dependence of the magnetization of epitaxial EuO on YSZ (001) at 5 K. The film was grown at $400°\text{C}$ with a $8.2\text{Å}/\text{min}$ Eu flux rate and a $8\times {10}^{-8}\text{mbar}$ oxygen pressure.

Figure 13

(Color online) $\text{Eu}{M}_{4,5}$ $\left(3d\to 4f\right)$ XAS spectra of EuO films grown epitaxially on YSZ (001) at $400°\text{C}$ with $8.1–8.2\text{Å}/\text{min}$ Eu flux rates under various oxygen pressures as indicated. The films are capped with a $20–40\text{Å}$ aluminum overlayer. Theoretical spectra of ${\text{Eu}}^{2+}$ and ${\text{Eu}}^{3+}$ are also shown, retrieved from Ref. 46.

Figure 14

(Color online) Top panels: RHEED photographs of EuO films on MgO (001) grown at $400°\text{C}$ with $8.1–8.2\text{Å}/\text{min}$ Eu flux rates under oxygen pressures of—from left to right—(a) 4, (b) 8, (c) 10, (d) 12, and (e) $16\times {10}^{-8}\text{mbar}$. The deposition times were all 100 min. The RHEED electron energy was 20 keV with the beam incident along the [100] direction. Bottom panels: corresponding LEED photographs. The LEED electron energy was set at 360 eV.

Figure 15

(Color online) Temperature dependence of the magnetization of epitaxial EuO films on MgO (001) grown at $400°\text{C}$ with a $8.1\text{Å}/\text{min}$ Eu flux rate under various oxygen pressures as indicated. The small magnetization contribution from the substrate has been subtracted. The applied magnetic field was 1000 G.