Superconducting phase of ${\mathrm{Ti}}_{x}{\mathrm{O}}_{y}$ thin films grown by molecular beam epitaxy

Superconducting phase of ${Ti}_{x} O_{y}$ thin films grown by molecular beam epitaxy

Yasemin Ozbek, Cooper Brooks, Xuanyi Zhang, Athby Al-Tawhid, Vladmir A. Stoica, Zhan Zhang, and Divine P. Kumah

Phys. Rev. Materials 6, 064805 – Published 27 June 2022

Abstract

We investigate the complex relationship between the growth conditions and the structural and transport properties of ${Ti}_{x} O_{y}$ thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure $P_{O x}$ for films grown on $(0001) − {Al}_{2} O_{3}$ substrates at $850^{\circ}$ C. A cubic $c − {TiO}_{1 \pm δ}$ buffer layer is formed for low O/Ti ratios, while a corundum $cr − {Ti}_{2} O_{3}$ layer is formed under higher-oxidizing conditions. Metallicity is observed for $c − {TiO}_{1 - δ}$ buffer layers. The superconducting $γ − {Ti}_{3} O_{5}$ Magnéli phase is found to nucleate on a $c − {TiO}_{1 - δ}$ buffer for intermediate $P_{O x}$ conditions, and an insulator-superconducting transition is observed at 4.5 K $(T_{C}^{onset} = 6 K)$ for 85 nm thick films. Strain relaxation of $γ − {Ti}_{3} O_{5}$ occurs with increasing film thickness and correlates with a thickness-dependent increase in $T_{C}$ observed for ${Ti}_{x} O_{y}$ thin films.

Received 2 March 2022
Accepted 8 June 2022
Corrected 28 November 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.064805

Physics Subject Headings (PhySH)

Electrical conductivity Growth Superconductors

Epitaxy Film deposition X-ray diffraction

Condensed Matter, Materials & Applied Physics

Corrections

28 November 2022

Correction: The previously published Figure 3 contained incorrect axis labeling along the ordinate of part (c) and has been replaced.

Authors & Affiliations

Yasemin Ozbek ¹, Cooper Brooks ¹, Xuanyi Zhang¹, Athby Al-Tawhid¹, Vladmir A. Stoica², Zhan Zhang³, and Divine P. Kumah ^1,*

¹Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
²Department of Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
³Advanced Photon Source, Lemont, Illinois 60439, USA

^*dpkumah@ncsu.edu

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Vol. 6, Iss. 6 — June 2022

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Figure 1
Atomic structure of cubic $c$ -TiO, corundum $cr - {Ti}_{2} O_{3}$ , $γ − {Ti}_{3} O_{5}$ , and ${Ti}_{4} O_{7}$ along projections in the $α − {Al}_{2} O_{3}$ [10 $\bar{1}$ 0] direction (left panel), the [11 $\bar{2}$ 0] direction (middle panel), and the [0001] direction (right panel).
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Figure 2
Evolution of the surface RHEED pattern for the growth of the 85 nm thick ${Ti}_{x} O_{y}$ film on (0001)-oriented $α − {Al}_{2} O_{3}$ at $P_{O 2} = 2 \times 10^{- 7}$ Torr and a growth rate of 2.5 Å/min (sample E). Diffraction pattern of (a) the initial ${Al}_{2} O_{3}$ surface and (b) after 1 ML ${Ti}_{x} O_{y}$ along the [11 $\bar{2} 0$ ] azimuth and after 85 nm along (c) the [11 $\bar{2} 0$ ] and (d) [10 $\bar{1} 0$ ] directions.
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Figure 3
(a) Comparison of sheet resistance for MBE-grown ${Ti}_{x} O_{y}$ films as a function of growth oxygen pressure and film thickness. The inset shows the drop in resistivity at low temperatures for the superconducting samples (samples D and E). (b) Resistivity as a function of magnetic field applied in plane for the 85 nm thick ${Ti}_{x} O_{y}$ film (sample E) grown at $P_{O x} = 2 \times 10^{- 7}$ Torr. (c) Relationship between critical magnetic field and $T_{c}$ and the corresponding Werthamer-Helfand-Hohenberg fit.
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Figure 4
(a) Comparison of specular x-ray diffraction around the $α − {Al}_{2} O_{3}$ (0006) Bragg peak for sample A (25 nm, $P_{O x} = 3 \times 10^{- 7}$ Torr), B (25 nm, $P_{O x} = 4 \times 10^{- 8}$ Torr), D (45 nm, $P_{O x} = 2 \times 10^{- 7}$ Torr), and E (85 nm, $P_{O x} = 2 \times 10^{- 7}$ Torr) ${Ti}_{x} O_{y}$ films grown by MBE. (b) Corresponding reflections for samples D and E at higher $q$ .
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Figure 5
(a) Schematic structures of samples A, B, D, and E determined from x-ray diffraction measurements and (b) the corresponding phase diagram showing the relationship between the ratio of the growth oxygen pressure $P_{O x}$ to the Ti flux rate and the film thickness.
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Figure 6
Structural and transport properties of an 80 nm thick ${Ti}_{x} O_{y}$ film grown at a Ti flux rate of 1.8 Å/min in $P_{O 2} = 2 \times 10^{- 7}$ Torr (sample F). (a) Reflection high-energy diffraction for 80 nm ${Ti}_{x} O_{y}$ on (0001)-oriented ${Al}_{2} O_{3}$ . (b) Optical micrograph of the film indicating structural domains. High-resolution synchrotron x-ray diffraction raster maps around Bragg peaks observed at diffraction conditions fixed for (c) the $c$ -TiO (111) Bragg peak and (d) the ${Ti}_{4} O_{7}$ ( $2 \bar{2} 0$ ) Bragg peak indicating structural phase separation. (e) Specular diffraction along the substrate $00 L$ direction at different points on the sample. (f) Resistance versus temperature. The inset shows the superconducting transition at low temperature.
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Physical Review Materials

Superconducting phase of TixOy thin films grown by molecular beam epitaxy

Yasemin Ozbek, Cooper Brooks, Xuanyi Zhang, Athby Al-Tawhid, Vladmir A. Stoica, Zhan Zhang, and Divine P. Kumah

Phys. Rev. Materials 6, 064805 – Published 27 June 2022

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Superconducting phase of ${Ti}_{x} O_{y}$ thin films grown by molecular beam epitaxy