Structural study of polar MgO(111) epitaxial thin films grown on SrTiO${}_3$(111)

We have studied the structures of polar MgO(111) thin films grown on SrTiO${}_3$(111) substrates to elucidate the effects of polarity in oxide thin films. MgO(111) films as well as MgO(100) films have been grown by pulsed laser deposition and have been characterized by x-ray diffraction, reflection high-energy electron diffraction, and high-resolution transmission electron microscopy. The lattice expansion up to ∼3$\%$ and the enhanced structural disorder have been observed in the thin (<100 nm) MgO(111) films, indicating an enhanced formation of the $F$ center. With increasing film thickness, the bulk lattice constants have been recovered and the structural disorder has been suppressed. The present findings show that MgO(111) thick films containing a finite structural disorder can be metastably realized.

Figure 1

Crystal structure of STO along the [111] direction, where Ti${}^{4+}$ and SrO${}_3$${}^{4-}$ layers are alternately stacked.

Figure 2

RHEED images of ∼1 nm thick MgO films on (a) as-polished and (b) Ti-STO(111) deposited at $T_{\mathrm{G}}$ $=$ 500–850 °C. Electron beam was incident along the [11$\overline{2}$]${}_{\mathrm{STO}}$ azimuth. Diffraction patterns along the [$1\overline{1}0$]${}_{\mathrm{MgO}}$ and [$11\overline{2}$]${}_{\mathrm{MgO}}$ are denoted by up and down arrows, respectively.

Figure 3

HR-XRD patterns of ∼40 nm thick MgO films on Ti-STO(111) [(a), (b), (d)] and on as-polished STO(111) (c) prepared at various $T_{\mathrm{G}}$: (a) Out-of-plane (2θ-ω) and (b) in-plane (2θχ-ϕ) scan along the [$1\overline{1}0$]${}_{\mathrm{STO}}$ direction. Inset to panel (a) shows the RHEED image of ∼50 nm thick MgO film along the [11$\overline{2}$]${}_{\mathrm{MgO}}$ azimuth fabricated on Ti-STO at $T_{\mathrm{G}}$ $=$ 800 °C. Arrows indicate the $(\sqrt{3}\times \sqrt{3})\mathrm{R}{30}^{\circ }$ reconstruction pattern. Inset to panel (b) shows a full in-plane ϕ scan rocking curve at the $2\overline{2}0$${}_{\mathrm{MgO}}$ diffraction. In-plane rocking curves around Δϕ $=$ 0 deg at the $2\overline{2}0$${}_{\mathrm{MgO}}$ diffraction along the [$1\overline{1}0$]${}_{\mathrm{STO}}$ on as-polished and Ti-STO(111) are shown in panels (c) and (d), respectively.

Figure 4

(a) A cross-sectional HR-TEM image of MgO/Ti-STO(111) interface. Inset shows a 2D Fourier transform of the MgO region of (a). (b) The enlarged HR-TEM image of the square in (a) and its one-dimensional Fourier transform along the [111]${}_{\mathrm{MgO}}$ (c) and [$1\overline{1}0$]${}_{\mathrm{MgO}}$ direction (d), respectively. (e) An EDX analysis along the AB line, which is shown in the DF-STEM image of the MgO/Ti-STO(111) interface (inset).

Figure 5

(a) The FWHM of out-of-plane and (b) in-plane [$2\overline{2}0$ diffraction for MgO(111) films] rocking curves of MgO(111) and (100) films. (c) Out-of-plane and (d) in-plane lattice constants of MgO(111) and (100) films. MgO(111) and (100) films were grown on Ti-STO(111) and TiO${}_2$-STO(100), respectively. Open squares and circles correspond to the lattice constants determined by streak-to-streak distance of RHEED patterns for thinner films (<∼20 nm) or by HR-XRD measurements for thicker films ($\lesssim$∼20 nm) on Ti-STO(111) and TiO${}_2$-STO(100), respectively, while closed squares and circles correspond to those determined by HR-XRD measurements after postannealing at 900 °C under O${}_2$ flow. Open triangles in (c) denote the lattice constants according to HR-TEM image. We have estimated the out-of-plane lattice constant by averaging ∼20 lattice fringes along the in-plane direction and then by further averaging 5 lattice fringes (1.2 nm) along the out-of-plane direction. The uncertainty in the estimation of out-of-plane lattice constants by HR-TEM observation is ∼0.01 Å above 10 nm thickness and increases up to ∼0.02 Å below 2 nm. Out-of-plane (111${}_{\mathrm{out}}$) and in-plane (111${}_{\mathrm{in}}$ and 100${}_{\mathrm{in}}$) lattice constants at a smaller thickness are enlarged in the inset to panel (d). (e) Lattice volume ($V$) of the MgO(100) and (111) films calculated by the $a_{\mathrm{out}}$ and $a_{\mathrm{in}}$ in (c) and (d).