Growth control of corundum-derivative MnSnO₃ thin films by pulsed-laser deposition

Linked to the recent progress in the physics of complex oxides, there has been a growing interest in corundum-derivative compounds.^1–9 In the ternary ABO₃ systems, periodic arrangement of two cations (A and B) produces the nonpolar ilmenite (IL)-type structure (space group: $R3¯$⁠) and the polar LiNbO₃ (LN)-type structure (R3c), as shown in Fig. 1.^10–12 IL-type oxides containing 3d transition metals have been investigated extensively for their rich magnetic ordering phenomena originating from spin interactions in a triangular lattice geometry.⁵ In LN-type oxides that lack the inversion symmetry (Fig. 1(b)), ferroelectricity can appear with a large spontaneous polarization,^1,7 e.g., 71 μC/cm² for LiNbO₃ and 50 μC/cm² for LiTaO₃ (Ref. 13). By adding 3d transition metals to the polar LN structure, multiferroic functions can also be induced.^2–4,7,9 Exploration of such interesting physical properties in the corundum-derivative oxides sharing basic AO₆ and BO₆ octahedral units relies on the controlled formation of the two ordered structures by appropriate synthetic techniques.

Since the early stage of bulk research on ABO₃ oxides, high-pressure techniques have been widely used to study the structural stability of the IL, LN, and also perovskite-type ABO₃.^11,14–16 Soft chemistry techniques, e.g., ion exchange,⁶ using precursors are also effective for obtaining metastable target phases by suppressing the formation of thermodynamically stable phases. Bulk samples prepared by those techniques, however, generally have powder or small sintered pellet forms, making it difficult to characterize their optical and electronic properties. With the aim of expanding opportunities for studying their unique physical properties, we attempted the phase control of corundum-derivative oxides in the thin-film form.^17,18 We selected MnSnO₃ as the target material.^3,14,16 Recently, A. Aimi et al., have reported a structural transformation from the IL-type to the LN-type MnSnO₃ under the application of 7 GPa.³ Associated with this structural transformation, spontaneous polarization emerges in addition to the antiferromagnetic properties originally present in the IL-type MnSnO₃,¹⁴ leading to multiferroic functions in the LN-type MnSnO₃.³ In this study, we report the successful growth of IL-type MnSnO₃ thin films by pulsed-laser deposition and the critical role of oxygen gas pressure P_O2 on the occurrence of the IL-type ordering. The P_O2 dependence of lattice parameters suggests that the structural transformation to the LN-type (or disordered corundum-type) MnSnO₃ can be driven by P_O2.

MnSnO₃ films were grown on double-side polished Al₂O₃ (0001) substrates at a substrate temperature of 820 °C by pulsed-laser deposition using a KrF excimer laser (repetition rate = 10 Hz). A mixed-phase Mn₂SnO₄-SnO₂ target was prepared by a spark-plasma-sintering technique at 1100 °C and 80 MPa in a vacuum using a stoichiometric mixture of MnO₂ and SnO₂ powders pre-fired at 900 °C in air. Prior to the deposition, Al₂O₃ substrates were annealed at 900 °C in an oxygen flow with a tube furnace to obtain an atomically smooth surface with a step-and-terrace structure. The films were characterized by x-ray diffraction (XRD) using Cu K_α radiation, x-ray photoelectron spectroscopy (XPS) using Al K_α radiation, and optical transmittance spectroscopy. For energy calibration of the XPS spectra, the C 1s peak at 284.8 eV was used. As a control sample, a c-axis oriented film of the IL-type MnTiO₃ was fabricated on Al₂O₃ (0001) by pulsed-laser deposition at 850 °C and P_O2 = 1 × 10⁻¹ Torr.^19,20

Figure 2(a) shows an out-of-plane XRD pattern for a Mn-Sn-O film grown at P_O2 = 4 × 10⁻⁴ Torr, measured with a condition maximizing the film diffraction intensity. In the IL structure, (0003n) diffraction peaks (n: natural number) are allowed while only (0006n) are allowed in the LN structure due to the different symmetry (Fig. 1). All the diffraction peaks from the Mn-Sn-O film are consistent with (0003n) of the IL structure, representing that AO₆ and BO₆ layers are alternately stacked along the c-axis direction (Fig. 1(a)). From the XRD reciprocal space mapping shown in Fig. 2(b), the lattice parameters of the Mn-Sn-O film are calculated to be a = 5.352 Å and c = 14.40 Å, which are close to a = 5.358 Å and c = 14.505 Å of the IL-type MnSnO₃ bulk (JCPDS, PDF No. 00-033-0913). In addition to the lattice parameters, we determined the valence of Mn ions by XPS. Figure 2(c) shows Mn 2p core-level XPS spectra of the Mn-Sn-O and MnTiO₃ (Mn²⁺ reference) films. The overall spectral shape and the satellite peak around 647 eV (Refs. 21 and 22) clearly demonstrate the dominant Mn²⁺ valence state in the Mn-Sn-O film. On the basis of these results, we conclude the formation of IL-type MnSnO₃ films.

The effects of P_O2 on the IL-type ordering in MnSnO₃ films are summarized in Fig. 3. Out-of-plane XRD patterns, displayed in Fig. 3(a), show that the (0003) diffraction peak (indicated by the red arrows) appears at P_O2 ≤ 1 × 10⁻³ Torr but not at 1 × 10⁻¹ Torr, while basic extinction rules for corundum derivatives are maintained with the appearance of the (0006) diffraction peak. In addition, Mn₂SnO₄ is segregated in the film grown at a low P_O2 of 1 × 10⁻⁴ Torr. From the (0003) diffraction intensities in these films, the optimal P_O2 for stabilizing the IL-type MnSnO₃ is found to be 4 × 10⁻⁴ Torr (Fig. 2). In Fig. 3(b), the in-plane orientation relationships between the film and substrate investigated by the in-plane ϕ scans for MnSnO₃ $(101¯4)$ and Al₂O₃ $(101¯4)$ are compared. In view of the three-fold symmetry in the Al₂O₃ substrate shown in the top panel, the MnSnO₃ film fabricated at P_O2 = 4 × 10⁻⁴ Torr has the identical three-fold symmetry (the middle panel of Fig. 3(b)). This indicates the formation of a single-crystalline MnSnO₃ film, whose in-plane orientation aligns to that of the substrate. In contrast, twin domains of MnSnO₃, which are rotated by Δϕ = 60° in the (0001) plane, are generated in the non-optimal P_O2 conditions as seen in the six-fold symmetry of the diffraction peaks (P_O2 = 1 × 10⁻³ and 1 × 10⁻⁴ Torr). Figure 3(c) shows the diffraction intensity ratio of MnSnO₃ (0003) to (0006), which reflects the degree of the IL-type ordering of AO₆ and BO₆ layers along the c-axis direction. For fair comparison, we separately measured the MnSnO₃ (0003) and (0006) intensities because they were maximized at slightly different incident ω angles. Figure 3(d) shows the diffraction intensity ratio of MnSnO₃ $(101¯4)$ at ϕ = 60° (the in-plane orientation matched domain) to that at ϕ = 120° (the rotated domain). This ratio is defined as the in-plane orientation matching, and the intensities were calculated by averaging the three diffraction intensities corresponding to each domain. Both the out-of-plane IL-type ordering (Fig. 3(c)) and in-plane orientation matching (Fig. 3(d)) are well developed at P_O2 = 4 × 10⁻⁴ Torr. These P_O2 dependences highlight the narrow P_O2 window for obtaining single-crystalline IL-type MnSnO₃ films. The lower bound of P_O2 seems to be limited by the segregation of Mn₂SnO₄, which likely comes directly from the mixed-phase Mn₂SnO₄-SnO₂ target. Considering that the IL-type ordering disappears in the high P_O2 region, we speculate that the oxidation states of A and B cations in the plume affect the layer stacking processes. The ablation of appropriate AO₆ and BO₆ precursors and moderate (or no additional) oxidation at the substrate surface may be the key to forming the IL-type ordering.

Note here that the appearance/disappearance of the IL-type ordering is discussed for the samples with comparable film qualities; the strong MnSnO₃ (0006) peak remains even at a high P_O2 of 1 × 10⁻¹ Torr (the top panel in Fig. 3(a)). Mn 2p core-level XPS spectra also keep the overall spectral signatures of Mn²⁺ against the P_O2 variation (see Fig. S1 in supplementary material). Although the satellite peak^21,22 becomes weak at high P_O2, the valence of Mn ions (i.e., Mn²⁺) is probably maintained for the sake of the robust Sn⁴⁺ character even under a low P_O2 of 4 × 10⁻⁴ Torr and charge neutrality in the A²⁺B⁴⁺O₃ structure. The growth rate is also not changed much in the varied P_O2 range (Fig. S2), and thus the kinetic effect is not the primary origin. Therefore, the P_O2 dependences presented in Fig. 3 indicate that the modification of the out-of-plane atomic ordering (AO₆ and BO₆ layer stacking) occurs in the MnSnO₃ films.

This hypothesis is supported by the decrease of the c-axis length with increasing P_O2, shown in Fig. 4(a). The c-axis length of the films grown under the low P_O2 region (≤ 4 × 10⁻⁴ Torr) is comparable to that of the IL-type MnSnO₃ bulk (the upper dashed line). As P_O2 increases, the c-axis length decreases, approaching that of the LN-type MnSnO₃ bulk³ (the lower dashed line). Although further characterization is needed to determine the microscopic structure of the LN-like MnSnO₃ film, this result agrees with the disappearance of the MnSnO₃ (0003) diffraction peak at high P_O2 (Fig. 3(a)). The incomplete shrinkage of the c-axis length may mean that both LN-type and IL-type orderings partially coexist as small domains. To examine this structural modification via the associated changes in physical properties, we measured ultraviolet-visible absorbance spectra (the inset of Fig. 4(b)). As shown in Fig. 4(b), the optical bandgap E_g decreases with increasing P_O2. The simultaneous deceases in the c-axis length and E_g suggest their strong correlation. In MnSnO₃ bulk samples, the application of a high pressure (7 GPa) induces the structural transformation from the IL-type to the LN-type MnSnO₃.³ This is reasonable because the LN-type MnSnO₃ has a reduced unit-cell volume than that of the IL-type MnSnO₃ (Ref. 3 and JCPDS, PDF No. 00-033-0913). Although the decrease of E_g in the LN-like MnSnO₃ films with reduced lattice parameters is opposite to Vegard’s law (the relations on the increase of E_g with reducing lattice parameters generally used for isostructural semiconductor alloy systems), a consistent theoretical proposal was reported for a sister compound ZnSnO₃ that E_g becomes smaller for the LN-type ZnSnO₃ than for the IL-type ZnSnO₃.²³ We therefore attribute the decreased E_g to the LN-type (disordered corundum-type) MnSnO₃. The P_O2-induced structural transformation in the thin-film form can be an effective approach to study the ferroelectric and multiferroic properties expected for the high-pressure phase LN-type MnSnO₃.³ 

We have fabricated single-crystalline ilmenite-type MnSnO₃ films on Al₂O₃ (0001) by pulsed-laser deposition. As demonstrated by the optical bandgap measurement, the single-crystalline film is useful for characterizing the physical properties. By constructing multilayer-based device structures on the films, detection of the antiferromagnetic transition^24–26 and multiferroic properties may also become possible. Moreover, we demonstrated the possibility of controlling the corundum-derivative structures by P_O2. The selective formation of nonpolar IL-type and polar LN-type phases with a simple P_O2 tuning may have advantages over conventional high-pressure techniques used in bulk studies. Understanding of the effect of P_O2 on the initial growth as well as control experiments using substrates with different symmetry and lattice constants will be important to apply the present finding to the phase control of other corundum-derivative oxides.

See supplementary material for Mn 2p core-level XPS spectra of MnSnO₃ films grown at various P_O2 (Fig. S1), and P_O2 dependence of the growth rate of Mn-Sn-O films (Fig. S2).

The authors thank K. Harata for the target synthesis and K. Omura for the XPS measurement. This work was performed under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposals No. 17G0417 and 18G0406). This work was partly supported by JSPS KAKENHI (JP15H02022), JST CREST (JPMJCR18T2), and the Futaba Research Grant Program of the Futaba Foundation.