AVS 70 Session TF1-TuM: Thin Films: Controlling Crystalline Phases

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. I will describe how the growth of thin films on graphene-terminated substrates enables synthesis of single crystalline, mechanically exfoliatable membranes [1,2]. Using rippled membranes of the Heusler compound GdPtSb, we demonstrate the first experimental example of flexomagnetism, that is, ferro/ferri-magnetism induced by strain gradients [3]. I will also describe evidence of superconductivity induced in another Heusler membrane via strain. More broadly, Heusler membranes provide highly tunable platform for tuning ferroic order, topological states, and correlations [4].

[1] S. Manzo, et. al., Nature Commun., 13, 4014 (2022). https://doi.org/10.1038/s41467-022-31610-y

[2] D. Du et. al., Nano Lett. 22, 21, 8647 (2022). https://doi.org/10.1021/acs.nanolett.2c03187

[3] D. Du, et. al., Nature Commun., 12, 2494 (2021). https://doi.org/10.1038/s41467-021-22784-y

[4] D. Du, e. al. APL, 122, 170501 (2023). https://doi.org/10.1063/5.0146553

1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
2. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14850, USA
3. Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University, Ithaca, New York 14853, USA

Ferroelectric photovoltaics are an emerging renewable energy technology where charge separation occurs in a single layer and the open-circuit voltage can be larger than the bandgap. The metastable P6₃cm phase of ScFeO₃ (h-ScFeO₃) is an ideal candidate for ferroelectric photovoltaics due to its polar crystal structure and narrow bandgap (1.2 eV). While h-ScFeO₃ can be grown on Al₂O₃ using a layered approach,¹ on conductive substrates, such as (111) Pt, only the ground state bixbyite phase of ScFeO₃ is stable. h-ScFeO₃ growth on a substrate with high conductivity and low epitaxial strain is critically needed for photovoltaic device fabrication.

In this work we grew h-ScFeO₃ on (111) platinum via molecular beam epitaxy (MBE). The stabilization of the h-ScFeO₃ phase was enabled by an Fe₃O₄ interlayer. This approach is adapted from reports of growth of hexagonal LuFeO₃ on oriented Fe₃O₄ via MBE.² Epitaxial (111) Pt which was deposited on c-Al₂O₃ was found to have a narrow (~0.005°) full width at half maximum (FWHM) rocking curve about the (111) peak, indicating a highly crystalline film. After depositing (111) Pt, a Fe₃O₄ layer was deposited. ScFeO₃ was then deposited on the Fe₃O₄ film with alternating depositions of Sc and Fe.¹ The films are solely (0002l) oriented of h-ScFeO₃ with clear Laue oscillations. Furthermore, rocking curves about the (0004) hexagonal ScFeO₃ peak have a FWHM of 0.07°, indicating a highly crystalline film. Films grown on conductive substrates then enabled measurements of the carrier mobility and dielectric response of the h-ScFeO₃. Overcoming the challenge of growing h-ScFeO₃ on conductive substrates will allow us to utilize these materials advantageous properties for high-efficiency photovoltaics.

1. Garten, L. M.et al. Stromataxic Stabilization of a Metastable Layered ScFeO₃ Polymorph. Chem. Mater. 33, 7423–7431 (2021).

1. Zhang, X. et al. Effect of interface on epitaxy and magnetism in h-RFeO₃/Fe₃O₄/Al₂O₃ films (R = Lu, Yb). J. Phys. Condens. Matter 29, 164001 (2017).

Cerium oxide nanostructures possess unique oxidative recycling properties, enabling their use in various engineering applications. However, predictable and precise defect engineering remains challenging. Ceria thin films offer a method to engineer defects by controlling thickness, albeit with limitations. To further enhance performance, we fabricated 2.9 nm and 0.5 nm CeO2-33 nm VO2 bilayers using atomic layer deposition (ALD), leveraging the low-temperature phase transition of VO2. We established a test vehicle utilizing X-ray Photoelectron Spectroscopy (XPS) with in-situ heating and cooling to study defect levels. The Ce3+/Ce4+ ratio was determined using Ce3d XPS scans during heating and cooling. A remarkable Ce3+/Ce4+ ratio of 5.97 (Ce3+ ~ 85%) was obtained and modulated over a temperature range from room temperature to 90 ºC. Additionally, Raman studies with in-situ heating and cooling were conducted to understand the mechanism behind this phenomenon.

Sulfides have been mostly grown using CVD, although MBE, MOCVD, pulsed laser deposition (PLD)and several other techniques have been exploited. However, large area and high-quality growth with precise and uniform thickness control and low defect densities still remain a challenge due to a large cation-sulfur vapor pressure mismatch, corrosive and reactive nature of most chalcogen precursors, high synthesis temperatures and the propensity to oxidize easily in the presence of oxygen at these high temperatures. Most growth techniques utilize H2S as the sulfur source. However, H₂S is a toxic, hazardous, and flammable gas and require high temperatures for efficient decomposition and sulfurization, resulting in high defect densities.

Here, we discuss epitaxial growth of CPs and TMDCs (specifically sulfides) using a novel hybrid PLD (Adv. Mater 2024) approach wherein we employed organo-sulfur precursors as the sulfur source. Large area epitaxial thin films of Group IV 2D chalcogenides such as TiS₂ (metallic) and ZrS₂ (semiconducting) and CPs such as BaZrS₃ and BaTiS₃ were successfully grown. This novel method utilizes precursors with optimum vapor pressures and they decompose at a lower temperatures (~250-400°C) to provide dissociated sulfur species. Structural and electrical characterization, along with low temperature transport studies reveal low defect densities and high carrier mobilities in these films. This work emphasizes on low temperature growth of high mobility refractory metal-based sulfides, especially with BEOL compatibility. Although slightly elevated temperatures were required for CPs, the interfacial roughness and optoelectronic performance were improved. The potential of these chalcogenides as candidates for transparent and conducting layers in chalcogenide-based optoelectronic devices will be discussed.

High entropy oxides (HEOs) can exhibit enhanced mechanical, refractory, catalytic, cryogenic, and dielectric properties over conventional materials [1]. The classic understanding of HEOs is founded on a high degree of configurational disorder as well as a positive enthalpy of formation. It has been thought that five discrete elements were necessary to cause this high degree of disorder (S_config >1.6R,J/mol*K). However, it has recently been shown that A₆B₂O₁₇ perovskites, where A = Zr or Hf and B = Nb or Ta can demonstrate an exceptionally high degree of disorder (S_config of 4.50R, J/mol*K) [2]. Though these materials have been synthesized by powder sintering [5] and ternary ZrTaO has been demonstrated with atomic layer deposition (ALD) [3], entropy stabilized Zr_xTa_yO_z has yet to be achieved via ALD. Herein we report the successful transformation of ALD Zr_xTa_yO_z into the entropy-stabilized form.

ZrO₂ and Ta₂O₅ were synthesized via ALD using tantalum ethoxide (Ta(OEt)₅) and zirconium chloride (ZrCl₄), respectively, with O₃ [3]. Zr_xTa_yO_z was deposited using modulated ZrCl₄/N₂/Ta(OEt)₅/N₂/O₃/N₂ super cycles [4]. Films were deposited on Si substrates and were amorphous as-deposited. Following annealing at 800°C for 30 minutes in N₂, x-ray diffractometry (XRD) spectra (Fig. 1) show a strong overlap between annealed ALD Zr₆Ta₂O₁₇ and the expected peaks for HEO stabilized A₆B₂O₁₇ [5], and no overlap with binary ZrO₂ or Ta₂O₅.

Metal/insulator/Si (MIS) electrical devices were fabricated using Ag top electrodes. 800 °C annealed entropy stabilized Zr_xTa_yO_z devices exhibited unipolar threshold voltage switching when illuminated, with a ON/OFF current ratio of 7x10³ and a ~ +0.56 MV/cm SET voltage. Switching did not occur in the dark. Unannealed amorphous Zr_xTa_yO_z devices did not exhibit switching in this voltage range but required a +1.8 MV/cm SET voltage and had an ON/OFF ration of only ~10². Switching under illumination is still under investigation and was possibly due to the formation of oxygen vacancies in the entropy stabilized matrix as opposed to the un-annealed amorphous form [6].

1. Aamlid, S. et al. J. Am. Chem. Soc. 145 (2023).
2. Voskanyan, A. et al. Scritpa Materiala 204 (2021).
3. Kukli, K. et al. AIP Advances 7 (2017).
4. Nguyen, C. et al. Chem. Mater. 33 (2021).
5. Spurling, R. et al. J. Mater. Sci 85 (2023).
6. Li, H. et al. Coatings 11 (2021)

Oxygen evolution reaction (OER) is the rate limiting step inhibiting the production of fuel from water.^1,2 Barium nickelate (BaNiO₃) is a potential candidate for OER catalysis because it has an order of magnitude higher catalytic activity for the OER compared to the current benchmark iridium oxide (IrO₂).¹ Understanding the impact of crystallographic orientation and oxygen vacancy concentration on OER catalytic activity is critical to enabling BaNiO₃ catalysts. ^1,2

In this work, we demonstrate the growth of oriented BaNiO₃ films via pulsed laser deposition (PLD). Films were deposited from a BaNiO_2.36 target onto (0001) Al₂O₃ substrates over a range of deposition temperatures, laser fluences, and oxygen partial pressures. X-ray diffraction shows that the orientation of BaNiO₃ is dependent on the deposition temperature. The (11-20) orientation of BaNiO₃ grows at temperatures below 500 °C, whereas increasing the deposition temperature from 400 °C to 500 °C leads to (10-10) BaNiO₃. Further increasing the deposition temperature to 700 °C changes the orientation of the film to predominately (0002) oriented. X-ray photoelectron spectroscopy shows the oxidation state of films deposited under different deposition oxygen partial pressures. Reducing deposition pO₂ from 1.65 x 10^-2 mbar to 1.27 x 10^-2 mbar shows a 4% decrease in oxygen atomic percentage. Atomic force microscopy shows that decreasing the laser fluence and oxygen partial pressure decreases the root mean square surface roughness of the deposited films. Reducing the laser fluence from 3.5 J/cm² to 2.5 J/cm² shows a 3 nm decrease in the root mean square surface roughness, while decreasing the oxygen partial pressure from 9.1 x 10^-2 mbar to 1.65 x 10^-2 mbar shows a 1 nm decrease in root mean square surface roughness. Co-planar top electrodes enabled impedance spectroscopy and dielectric measurements. Impedance data shows the migration of oxygen vacancies which play an important role in OER. Elucidating the impact of crystallographic orientation on oxygen vacancy migration gives insight into how crystallographic orientation affects OER catalytic activity.

References

1. Lee, J. G. et al. A New Family of Perovskite Catalysts for Oxygen-Evolution Reaction in Alkaline Media: BaNiO ₃ and BaNi _0.83 O _2.5. J. Am. Chem. Soc. 138, 3541–3547 (2016).
2. Plevová, M., Hnát, J. & Bouzek, K. Electrocatalysts for the oxygen evolution reaction in alkaline and neutral media. A comparative review. Journal of Power Sources 507, 230072 (2021).