Formation of ilmenite-type single-crystalline MgTiO3 thin films by pulsed-laser deposition

Masamichi Negishi,Atsushi Tsukazaki,Kohei Fujiwara

doi:10.1063/5.0078021

Masamichi Negishi, Atsushi Tsukazaki + Show 1 more

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https://doi.org/10.1063/5.0078021

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Abstract

Ilmenite structure is a good playground to explore magnetism in honeycomb lattices owing to the existence of an intriguing variety of magnetism in 3d transition metal compounds. Because of the absence of magnetic ions, ilmenite-type MgTiO3 is a promising candidate for d0 transparent insulators to reveal general features of TiO6 honeycomb layers. In this study, we found an optimum growth condition to synthesize ilmenite-type single-crystalline MgTiO3 thin films on Al2O3(0001) substrates by pulsed-laser deposition. By increasing oxygen pressure from roughly 10−6 to 10−1 Torr, we obtained (0001)-oriented MgTiO3 thin films with suppression of segregation of a Mg2TiO4 phase. On the triangular lattice of Al2O3, twin-domain formation is suppressed with decreasing deposition rate, resulting in the synthesis of single-crystalline MgTiO3 thin films. The bandgap of the MgTiO3 film was evaluated to be about 4.4 eV by optical absorption spectra, which implies d0 transparent insulator.

Highlights

Ilmenite-type (IL-type) crystal structure is closely related to corundum-type and LiNbO3-type (LN-type) with chemical formula ABO3 (A and B are cations and O is oxygen).[1,2,3] While all three structures are composed of layered honeycomb lattices, the combination of valences of A and B cations and whether inversion symmetry exists or not are different
Some materials are known to be stabilized as both IL-type and LN-type structures though the cation combination is identical; these two structures are controlled by growth methods, for example, using high-pressure synthesis[1,2,5,6,7,8] or thinfilm techniques.[9,10]
Samples were prepared by chemical vapor transport,[27,28] sol–gel method,[29–35] metalorganic solution deposition,[36,37] sputtering,[38–49] pulsed-laser deposition (PLD),[50,51] and the other methods.[52–54]

Summary

Introduction

Ilmenite-type (IL-type) crystal structure is closely related to corundum-type and LiNbO3-type (LN-type) with chemical formula ABO3 (A and B are cations and O is oxygen).[1,2,3] While all three structures are composed of layered honeycomb lattices, the combination of valences of A and B cations and whether inversion symmetry exists or not are different. MgTiO3 will be a useful IL-type material as a non-magnetic d0 insulator.[12] Previous studies on MgTiO3 films have focused on the dielectric properties for low-loss optical applications. In these studies, samples were prepared by chemical vapor transport,[27,28] sol–gel method,[29–35] metalorganic solution deposition,[36,37] sputtering,[38–49] pulsed-laser deposition (PLD),[50,51] and the other methods.[52–54]. The single-crystalline structure was obtained by annealing process in both sol–gel[29] and sputtering[43] methods, indicating IL-type MgTiO3 is thermodynamically stable under appropriate conditions. Another stable phase of Mg2TiO4 was detected in polycrystalline films.[43]. The bandgap of the MgTiO3 film was evaluated to be about 4.4 eV from absorption spectra, indicating a transparent insulator

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Conclusion