journal article·2026-07-01Open access

Synthesis of epitaxial TaO2 thin films on Al2O3 by suboxide molecular-beam epitaxy and thermal laser epitaxy

Yorick A. Birkhölzer, Anna S. Park, Noah Schnitzer, Jeffrey Z. Kaaret, Benjamin Z. Gregory, Tomas A. Kraay, Tobias Schwaigert, Matthew R. Barone, Brendan D. Faeth, Felix V. E. Hensling, Iris C. G. van den Bosch, Ellen M. Kiens, Christoph Baeumer, Enrico Bergamasco, Markus Grüninger, Alexander V. Bordovalos, Suresh Chaulagain, Nikolas J. Podraza, Waldemar Tokarz, Wojciech Tabis, Matthew J. Wahila, Suchismita Sarker, Christopher J. Pollock, Shun-Li Shang, Zi-Kui Liu, Nongnuch Artrith, Frank M. F. de Groot, Nicole A. Benedek, Andrej Singer, David A. Muller, Darrell G. Schlom

APL Materials· Volume 14· Issue 7

Abstract

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (11̄02) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy and thermal laser epitaxy, demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron x-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using x-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal–insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our study expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.
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Research context

OpenAlex topics: Electronic and Structural Properties of Oxides · Semiconductor materials and devices · Laser Material Processing Techniques

SciVX discovery tags are terminology-based. OpenAlex topics are source-provided classifications.

Affiliations

Department of Materials Science and Engineering, Cornell University 1 , Ithaca, New York 14853,

Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University 2 , Ithaca, New York 14853,

Kavli Institute at Cornell for Nanoscale Science 3 , Ithaca, New York 14853,

School of Applied and Engineering Physics, Cornell University 4 , Ithaca, New York 14853,

Max Planck Institute for Solid State Research 5 , 70569 Stuttgart,

MESA+ Institute for Nanotechnology, University of Twente 6 , 7500 AE Enschede,

Institute of Physics II, University of Cologne 7 , 50937 Cologne,

Department of Physics and Astronomy, University of Toledo 8 , Toledo, Ohio 43606,

Wright Center for Photovoltaics Innovation and Commercialization, University of Toledo 9 , Toledo, Ohio 43606,

Faculty of Physics and Applied Computer Science, AGH University of Krakow 10 , 30-059 Krakow,

Analytical and Diagnostics Lab, Binghamton University, State University of New York 11 , Binghamton, New York 13902,

Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University 12 , Ithaca, New York 14853,

Department of Materials Science and Engineering, The Pennsylvania State University 13 , University Park, Pennsylvania 16802,

Debye Institute for Nanomaterials Science, Utrecht University 14 , 3584 CG Utrecht,

Leibniz-Institut für Kristallzüchtung 15 , Max-Born-Str. 2, 12489 Berlin,

Funding

U.S. Department of Energy · Division of Materials Research · National Science Foundation · Gordon and Betty Moore Foundation · Exacte en Natuurwetenschappen · Deutsche Forschungsgemeinschaft · National Science Center, Poland

References (67)

Show source-provided references
1. Metal-insulator transitions10.1103/revmodphys.70.1039 ↗
2. Growth of NbO2 by molecular-beam epitaxy and characterization of its metal-insulator transition10.1557/adv.2017.505 ↗
3. Oxides which show a metal-to-insulator transition at the Neel temperature10.1103/physrevlett.3.34 ↗
4. Electrical conductivity and thermoelectric power of niobium dioxide10.1016/0022-3697(66)90094-1 ↗
5. Electron transport in single crystals of niobium dioxide10.1139/p74-297 ↗
6. In operando cryo-STEM of pulse-induced charge density wave switching in TaS210.1038/s41467-023-44093-2 ↗
7. Enhanced superconductivity in atomically thin TaS210.1038/ncomms11043 ↗
8. From Mott state to superconductivity in 1T-TaS210.1038/nmat2318 ↗
9. Some new superconducting group Va dichalcogenides10.1016/0375-9601(67)91005-5 ↗
10. Ultraefficient resistance switching between charge ordered phases in 1T-TaS2 with a single picosecond electrical pulse10.1063/5.0096850 ↗
11. Revealing the electronic structure of the current-induced metastable state in 1T-TaS210.1038/s42005-025-02367-9 ↗
12. Hidden CDW states and insulator-to-metal transition after a pulsed femtosecond laser excitation in layered chalcogenide 1T-TaS2−xSex10.1126/sciadv.aas9660 ↗
13. New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds10.1038/s41467-021-22030-5 ↗
14. Surpassing millisecond coherence in on chip superconducting quantum memories by optimizing materials and circuit design10.1038/s41467-024-47857-6 ↗
15. Millisecond lifetimes and coherence times in 2D transmon qubits10.1038/s41586-025-09687-4 ↗
16. Structure and formation mechanisms in tantalum and niobium oxides in superconducting quantum circuits10.1021/acsnano.4c05251 ↗
17. α-Ta films on c-plane sapphire with enhanced microstructure10.1063/5.0218021 ↗
18. Bulk superconductivity in transition metal oxide TaO10.1016/j.physc.2023.1354252 ↗
19. Observation of superconductivity up to 8.7 K in reduced potassium tantalate10.1002/qute.202400255 ↗
20. A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta2O5−x/TaO2−x bilayer structures10.1038/nmat3070 ↗
21. Über das system Nb2O5–Ta2O5–NbO2–TaO2–H2O–H210.1002/zaac.19522670410 ↗
22. An X-ray investigation of the tantalum-oxygen system10.3891/acta.chem.scand.08-0240 ↗
23. Structure des oxides de tantale10.1143/jjap.6.21 ↗
24. Formation of rutile-type Ta(IV)O2 by shock reduction and cation-deficient Ta0.8O2 by subsequent oxidation10.1016/0022-4596(83)90180-9 ↗
25. Preparation of TaO2 thin films using NbO2 template layers by a pulsed laser deposition technique10.1016/j.tsf.2015.12.066 ↗
26. Molecular beam epitaxy of KTaO310.1116/6.0002223 ↗
27. Epitaxial growth of TiO2 rutile thin films on sapphire substrates by a reactive ionized cluster beam method10.1143/jjap.32.3561 ↗
28. Ellingham diagrams of binary oxides10.1063/5.0216426 ↗
29. Preparation and evaluation of alexandrite, forsterite, and topaz substrates for the epitaxial growth of rutile oxides10.1063/5.0312234 ↗
30. Topotaxial formation of Mg4Ta2O9 and MgTa2O6 thin films by vapour-solid reactions on MgO (001) crystals10.1016/j.jeurceramsoc.2003.07.004 ↗
31. Silicon-doped β-Ga2O3 films grown at 1 μm/h by suboxide molecular-beam epitaxy10.1063/5.0139622 ↗
32. Cooperative determination of the melting point of alumina10.1351/pac197021010115 ↗
33. High-temperature infrared properties of sapphire, AlON, fused silica, yttria, and spinel10.1016/s1350-4495(98)00011-5 ↗
34. Film deposition by thermal laser evaporation10.1063/1.5111678 ↗
35. Thermal laser evaporation of elements from across the periodic table10.2351/7.0000348 ↗
36. State of the art, trends, and opportunities for oxide epitaxy10.1063/5.0196883 ↗
37. Strain-induced superconductivity in RuO2(100) thin-films10.1038/s43246-025-00856-6 ↗
38. Image registration of low signal-to-noise cryo-STEM data10.1016/j.ultramic.2018.04.008 ↗
39. Very-high dynamic range, 10,000 frames/second pixel array detector for electron microscopy10.1017/s1431927622000174 ↗
40. Reconstructing state mixtures from diffraction measurements10.1038/nature11806 ↗
41. PtychoShelves, a versatile high-level framework for high-performance analysis of ptychographic data10.1107/s1600576720001776 ↗
42. Electron ptychography achieves atomic-resolution limits set by lattice vibrations10.1126/science.abg2533 ↗
43. Mixed-state electron ptychography enables sub-angstrom resolution imaging with picometer precision at low dose10.1038/s41467-020-16688-6 ↗
44. Bayesian optimization for multi-dimensional alignment: Tuning aberration correctors and ptychographic reconstructions10.1017/s1431927622011692 ↗
45. Laboratory-based in situ and operando tricolor x-ray photoelectron spectroscopy10.1126/sciadv.adw6673 ↗
46. VerSoX B07-B: A high-throughput XPS and ambient pressure NEXAFS beamline at Diamond Light Source10.1107/s1600577524001346 ↗
47. Progress in spectroscopic ellipsometry: Applications from vacuum ultraviolet to infrared10.1116/1.1569928 ↗
48. Overview of variable-angle spectroscopic ellipsometry (VASE): II. Advanced applications10.1117/12.351667 ↗
49. Overview of variable-angle spectroscopic ellipsometry (VASE): I. Basic theory and typical applications10.1117/12.351660 ↗
50. High-strain-induced local modification of the electronic properties of VO2 thin films10.1021/acsaelm.2c01176 ↗
51. X-ray spectroscopy of ultra-thin oxide/oxide heteroepitaxial films: A case study of single-nanometer VO2/TiO210.3390/ma8085255 ↗
52. Structural phase transitions of NbO2: Bulk versus surface10.1021/acs.chemmater.0c04566 ↗
53. HAXPES reference spectra of Ta and Ta2O5 with Cr Kα excitation10.1116/6.0002551 ↗
54. Resonant inelastic x-ray scattering studies of elementary excitations10.1103/revmodphys.83.705 ↗
55. Ordering of hidden multipoles in spin-orbit entangled 5d1 Ta chlorides10.1103/physrevb.100.045142 ↗
56. Resonant inelastic x-ray incarnation of Young’s double-slit experiment10.1126/sciadv.aav4020 ↗
57. Quasimolecular Jtet = 3/2 moments in the cluster Mott insulator GaTa4Se810.1103/PhysRevLett.133.046501 ↗
58. Resonant inelastic X-ray scattering of tantalum double perovskite structures10.1016/j.cap.2018.05.014 ↗
59. Mott-Hubbard exciton in the optical conductivity of YTiO3 and SmTiO310.1103/PhysRevB.78.075122 ↗
60. Probing orbital fluctuations in RVO3 (R = Y, Gd, or Ce) by ellipsometry10.1103/PhysRevB.86.125128 ↗
61. Spin-orbit coupling and crystal-field splitting in Ti-doped Ca2RuO4 studied by ellipsometry10.1103/physrevb.106.085103 ↗
62. Temperature dependence of the electronic structure of the Jeff = 12 Mott insulator Sr2IrO4 studied by optical spectroscopy10.1103/PhysRevB.80.195110 ↗
63. Avalanche breakdown in GaTa4Se8−xTex narrow-gap Mott insulators10.1038/ncomms2735 ↗
64. First principles study of the strain effect on band gap of λ phase Ta2O510.1016/j.commatsci.2019.03.044 ↗
65. Gravitational waves detector mirrors: Spectroscopic ellipsometry study of Ta2O5 films on SiO2 substrates10.1016/j.tsf.2010.12.086 ↗
66. Suitability of binary oxides for molecular-beam epitaxy source materials: A comprehensive thermodynamic analysis10.1063/5.0013159 ↗
67. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides10.1107/S0567739476001551 ↗