Session A38: Novel Synthesis Approaches for Complex Oxide Films and Heterostructures

Prediction and synthesis of novel ultra-wide-band-gap semiconducting oxides

Ultra-wide-band-gap (UWBG) semiconductors with band gaps ranging from 3.5 eV to 6.2 eV have emerged as the next frontier in power electronics and UV optoelectronics research. However, state-of-the-art materials such as AlGaN/AlN, diamond, β-Ga₂O₃, and c-BN, suffer from doping asymmetry and/or thermal management. Here I will discuss the theoretical framework that we developed to understand the chemical and structural origins of semiconducting behavior in UWBG semiconductors, and to uncover new UWBG semiconductors that can alleviate the challenges of established materials. Among the UWBG materials surveyed, we identified rutile GeO₂ as an UWBG (4.68 eV) semiconductor with ambipolar dopability and high thermal conductivity. To synthesize the UWBG materials identified by theory, we employed molecular-beam epitaxy using molecular beams from novel suboxide precursors to navigate kinetic pathways and synthesize new phases with narrow phase stability windows. I will present the details of single-crystalline rutile GeO₂ film growth that navigates competing amorphous and hexagonal phases, as well as potential high-mobility p-type oxides with oxidation states that are difficult to synthesize such as SnO and PbO. The findings provide opportunities to realize new materials for UWBG semiconductor research. 

    

Adsorption-controlled growth and electronic transport properties of La-doped CaSnO3 films

The alkaline earth stannates are touted for their wide band gaps and the highest room-temperature electron mobilities among all perovskite oxides. CaSnO₃ has the highest measured band gap in this family and is thus a particularly promising ultra-wide band gap semiconductor. Here we demonstrate the first molecular beam epitaxy (MBE) growth study of epitaxial CaSnO₃ films. By using hybrid MBE, which provides an adsorption-controlled growth window, and a GdScO₃ substrate, which provide a template that closely matched the lattice parameters, we demonstrate the coherent growth of epitaxial CaSnO₃ films. By introducing lanthanum as a dopant, we demonstrate robust and predictable doping of CaSnO₃. Finally, we show the first ever field-effect transistor that use CaSnO₃ as a channel material. This work opens the door to many future studies on the semiconducting properties of CaSnO₃ and the many devices that could benefit from CaSnO₃’s ultra-wide band gap.   

SrHfO3 Films and Heterostructures Grown by Hybrid Molecular Beam Epitaxy

SrHfO₃ is a band insulating dielectric perovskite oxide with an extremely large bandgap (E_g = 6.2 eV). According to density functional theory (DFT) modeling, SrHfO₃ will not accept electrons from any other transition metal perovskite oxide and thus will act as a good dielectric barrier and atmospheric capping layer in oxide heterostructures. Some works have predicted and observed that epitaxial strain can stabilize the octahedral tilting of SfHfO₃ and produce a metastable ferroelectric phase. Epitaxial films of SrHfO₃ were grown on GdScO₃ and SrTiO₃ using hybrid molecular beam epitaxy, where Hf was introduced using  Tetrakis(dimethylamino)hafnium [Hf(N(CH3)2)4] and growth was monitored via Reflected High Energy Electron Diffraction. Stoichiometry and surface termination were characterized by in situ X-ray Photoelectron Spectroscopy and Rutherford Back Scattering to examine the growth window for the material. X-ray diffraction, scanning transmission electron microscopy, and second harmonic generation studies were performed to examine the possibility of ferroic distortions in the films due to strain or formation of epitaxial heterostructures with SrTiO₃. We also examine the functionality of SrHfO₃ as an atmospheric capping layer in oxide heterostructures.

    

Investigation of structure transformation in ultra-thin SrRuO3 films driven by SrTiO3 capping layer

In a cubic perovskite oxide of ABO₃, the BO₆ octahedral rotation and tilting result in a structural distortion and transform the cubic structure into tetragonal or orthorhombic phases, where the lowering of the system symmetry may result in changes in electronic and physical properties. In this presentation, we demonstrated that a transformation from tetragonal (T-) phase to orthorhombic (O-) phase in ultra-thin films of SrRuO₃ (SRO) can be achieved by an additional SrTiO₃ (STO) capping layer. A series of nominal STO (t nm)/SRO (~3 nm) films with t ranging from 0 to 3 nm were grown on STO (001) substrates using an oxide molecular beam system with adsorption controlled growth technique. From rigorous high precision X-ray measurements, a clear transition from T-phase to O-phase was observed when the STO capping layer thickness increases from 0 to 3 nm, inferring a favorable multiple RuO₆ rotation and tilting with an additional STO capping layer. In addition, detailed analysis also revealed the structural orientations of SRO [001]_o along STO [010]_c for O-phase and SRO [001]_T along STO [001]_c for T-phase. The corresponding anomalous Hall measurements showed minor variations, suggesting the robustness of the ferromagnetism in SRO thin films that appear to be insensitive to the orthorhombic-phase to tetragonal-phase transition.

    

Giant enhancement of the electron-phonon coupling in dimensionality-controlled SrRuO3 heterostructures

Electrons in the crystals closely interact with quantized lattice degree of freedom, determining fundamental electrodynamic behaviors and versatile correlated functionalities. Meanwhile, the strength of the electron-phonon interaction is usually determined as an intrinsic value for a given material. Here we demonstrate that the electron-phonon coupling in SrRuO₃ can be largely controlled by multiple knobs available in synthetic crystal structure. The coupling strength of quasi-2D SrRuO₃ is enhanced by about three-hundred-fold compared to that of bulk SrRuO₃. The giant enhancement of the electron-phonon coupling strength is attributed to a non-local nature of the coupling within the well-defined synthetic atomic network, which becomes predominant in the limit of two-dimensional electronic state. Our results provide valuable opportunities to engineer the electron-phonon coupling, leading to a deeper understanding of various novel phenomena observed in quantum materials   

Towards all-epitaxial lead-free metal-ferroelectric-metal heterostructures

Realizing metal-ferroelectric-metal heterostructures with atomically sharp and clean interfaces is a prerequisite for examining the full potential of thin film ferroelectric materials. Additionally, atomic layer control over the interfaces is crucial for tuning its ferroelectric properties with epitaxial strain. However, the premature loss of ferroelectricity due to inadequate stoichiometry control, the presence of dielectric dead layers, and dimensionality effects at the metal-ferroelectric interface is a long-standing challenge. We address the defect-controlled growth of the metal (SrRuO₃) and ferroelectric (BaTiO₃) layers with a metal-organic precursor-based hybrid molecular beam epitaxy (MBE) technique for both B-site cations. We observe an adsorption-controlled growth window for self-regulating stoichiometry in SrRuO₃ and BaTiO₃ films. We describe the effect of cationic flux ratios on SrRuO₃ properties such as surface morphology, electronic transport, and residual resistivity ratio (RRR). With this technique, we report a RRR = 87, the highest for coherently strained SrRuO₃ films grown on SrTiO₃ (001). Using all-epitaxial SrRuO₃/BaTiO₃/SrRuO₃ heterostructures, we will discuss the effect of stoichiometry, strain, and dimensionality on the dielectric properties of BaTiO₃ films.   

Structure characterization and magneto-transport property of untwinned SrRuO3 thin films with different thicknesses

The growth of SrRuO₃ (SRO) film with a low defect level is challenging due to the volatility of ruthenium oxide, where significant Ru vacancies can appear in SRO films and thus give rise to a high residual resistivity at low temperatures. To overcome this issue, excess Ru flux was supplied and followed by an adsorption-controlled growth of SRO on a substrate, where the film’s stoichiometry can be thermodynamically self-regulated. We found that the growth condition of initial SrO layer on a TiO₂ terminated SrTiO₃ (STO) (001) substrate is crucial for achieving a low residual resistivity in SRO films grown by adsorption-controlled growth technique, where a c(2x2) superstructure for the optimum initial SrO layer was revealed by the low energy electron diffractions. By using a miscut STO substrate of about 0.1 degrees for the growth of SRO (t-nm)/STO (001) thin films, the volume fraction for the dominant orthorhombic domain can be increased significantly to achieve a nearly single-domain and thus untwinned structure, and it equals ~ 92% for t = 13.7 nm, which was determined from the X-ray azimuthal scan across SRO (02±1)_o reflection. A series of untwinned films with the thickness (t) ranging from 7.7 to 35.3 nm were then grown with a low residual resistivity going from 12.3 to 3.2 μΩcm at 2.5 K, where pronounced quantum oscillations in resistivity can be observed at low temperatures. On the other hand, a crossover from positive magnetoresistance to negative magnetoresistance was found when the magnetic field is gradually tilting along the current direction. All those results support for SRO thin films as an ideal platform for the study of thickness-dependent transport signatures in a topological Weyl metal system.   

Engineering Metal Oxidation Towards Epitaxial Growth of Complex Iridates

Oxides of the platinum group metals like Ir and Ru have received increasing attention due to the delicate interplay of electron correlations, crystal field and spin-orbit coupling energies. However, their low vapor pressures and oxidation potentials make the synthesis of their oxide thin films challenging in an ultra-high vacuum system such as Molecular Beam Epitaxy (MBE). We address these challenges using a novel solid-source metal-organic MBE approach [1,2] and epitaxial strain. We demonstrate atomically precise synthesis of binary and complex iridates. Combining detailed growth, x-ray diffraction, transmission and scanning electron microscopy, spectroscopy techniques, and DFT calculations, we demonstrate a vital role of epitaxial strain on Ir oxidation. We will present a detailed growth study, structural characterization and electrical transport in high quality, epitaxial IrO₂ and Sr₂IrO₄ films.

References:

[1] W. Nunn, S. Nair, H. Yun, A. K. Manjeshwar, A. Rajapitamahuni, D. Lee, K. A. Mkhoyan, and B. Jalan, "Solid source metal-organic molecular beam epitaxy of epitaxial RuO₂" APL Mater. 9, 091112 (2021) 

[2] W. Nunn, A. K. Manjeshwar, J. Yue, A. Rajapitamahuni, T. K. Truttmann and B. Jalan, "Novel synthesis approach for “stubborn” metals and metal oxides", Proc. Natl. Acad. Sciences 118, e2105713118 (2021) 

    

Semi-metallic SrIrO3 films using solid-source metal-organic molecular beam epitaxy

SrIrO₃ and its heterostructures show many interesting properties such as topological Hall effect and persistent metallicity in ultrathin films. Molecular beam epitaxy (MBE) is one of the commonly used methods for making high quality thin films because of its precise control on stoichiometry and defect density. However, MBE synthesis of iridium based thin films is extremely challenging due to high temperature (>2000°C) required to evaporate iridium and high oxygen pressure required to oxidize the iridium metal precursor. In this talk, we illustrate these synthesis challenges using a thermodynamics of the MBE phase diagram. We demonstrate our novel synthesis method that utilizes iridium precursor chemistry to solve these issues. We grew 35 nm thick (001)_pseudo-cubic SrIrO₃ films on (001) SrTiO₃ substrate at 650°C substrate temperature using co-deposition of Sr, iridium acetylacetonate, and oxygen plasma. Reflection high-energy electron diffraction confirms our films are epitaxial with (1x2) surface reconstruction. High-resolution X-ray diffraction scans indicate our films are single-crystalline and show Laue oscillations. Reciprocal space map reveals the films are fully strained to the substrate. Using electrical transport measurements, we show that the films are semi-metallic from 300 K to 1.8 K and exhibit Kondo-like resistivity behavior. Magnetotransport studies reveal positive magnetoresistance and weak antilocalization [1].

    

Epitaxial SrTiO3 Films with Dielectric Constants Exceeding 25,000

SrTiO₃ (STO) is an incipient ferroelectric perovskite oxide for which the onset of ferroelectric order is suppressed by quantum fluctuations. This property results in a very large increase in static dielectric constant from ~300 at room temperature to ~20,000 at liquid He temperature in bulk single crystals. However, the low-temperature dielectric constant of epitaxial STO films is typically a few hundred to a few thousand. In this talk, we will resolve this long-standing issue. Using all-epitaxial capacitor structure of the form n-STO/undoped STO/n-STO (001) prepared by hybrid molecular beam epitaxy, we demonstrate intrinsic dielectric constants of an unstrained STO (001) film exceeding 25,000 at low temperature. A careful analysis of the temperature-dependent dielectric constants reveals that the n-STO/undoped STO interface plays a vital role in determining the dielectric properties. Furthermore, using different dopant-types in n-STO, we reveal that it is the depleted side of the interface, i.e. n-STO that governs the overall measured dielectric constant [1]. A detailed growth study combined with the temperature- and frequency-dependent dielectric measurement will be presented.

References

[1] Z. Yang, D. Lee, J. Yue, J. Gabel, T.-L Lee, R. D. James, S. A. Chambers, B. Jalan, Proc. Natl. Acad. Sci. 119 (2022) e2202189119.   

Voltage control of patterned materials in lateral SrFeO3-d/SrFeO2F heterostructures via ionic gating

We demonstrate reversible voltage control between metal/insulator and insulator/insulator lateral patterns of epitaxial perovskite oxides and oxyfluorides via electrolytic gating, leading to electric field modulation of anisotropic transport and optical responses. Lateral stripes of SrFeO_2.5/SrFeO₂F were patterned on conducting SrTiO₃:Nb substrates through lithographically-defined topochemical fluorination reactions. Using a back-contact geometry, biasing across an ionic gel results in a conversion of the SrFeO_2.5 brownmillerite regions to SrFeO₃ perovskite, while SrFeO₂F remains unchanged after biasing. The application of an oxidizing bias (V_G = -3 V) converts the all-insulating SrFeO_2.5/SrFeO₂F pattern into SrFeO₃/SrFeO₂F metal/insulator lateral structures with highly anisotropic in-plane transport and strong optical contrast between stripes. The application of a positive bias (V_G = 1 V) results in reduction of the SrFeO₃ phase, while the SrFeO₂F regions again remain unchanged. The brownmillerite oxide regions can be reversibly reverted to the SrFeO₃ perovskite phase with a negative gate bias. [Adv. Funct. Mater. 2208434 (2022)]   

Synthesis and stability of thin films of purported skyrmion lattice metal SrFeO3

Solid state systems possessing non-trivial topological properties are of interest in the context of spintronic applications as well as the fundamental study of quantum materials with a Berry phase. A particularly fascinating example of such a system is the correlated oxide strontium ferrite, SrFeO₃, due to its persistent metallicity and plethora of magnetic phases that arise from the high symmetry of its crystal structure ¹. One type of multi-q spin structure is suggested to host a skyrmion lattice, an appealing prospect for future integration into oxide electronic devices ². With this in mind, there is a clear need for high quality SrFeO₃ thin films, typically a challenge due to an unfavorable iron oxidation state and materials degradation ³. We synthesize and characterize SrFeO₃ thin films grown by pulsed laser deposition, addressing these challenges by varying growth conditions and encapsulating layers.

 

1.        Ishiwata, S. et al. Versatile helimagnetic phases under magnetic fields in cubic perovskite SrFeO₃. Phys. Rev. B  84, 1–5 (2011).

2.        Ishiwata, S. et al. Emergent topological spin structures in the centrosymmetric cubic perovskite SrFeO₃. Phys. Rev. B 101, 134406 (2020).

3.        Wang, L. et al. Time- and strain-dependent nanoscale structural degradation in phase change epitaxial strontium ferrite films. npj Mater. Degrad. 4, 3–8 (2020).   

Rotating lattice microstructure in solid-phase epitaxial lateral crystallization of complex oxides

Lateral crystallization of the model perovskite oxide SrTiO₃ from an amorphous precursor can be seeded by selected areas of the surface of a patterned SrTiO₃ (001) single-crystal substrate. The crystallized SrTiO₃ exhibits a rotating lattice microstructure in which the crystallographic orientation evolves continuously as a function of the lateral crystallization distance. The rotation rate resulting from crystallization at 550 °C was 50° per μm of lateral crystallization. Key features, including the rotation rate and the direction of the rotation, were reproduced in each of the many lithographically patterned seeds. The results of structural characterization studies are consistent with a model in which the rotation results from a high concentration of dislocations, on the order of 1 per 10 nm of crystallization distance, with a non-random population of Burgers vectors. The nucleation of dislocations with the preferred Burgers vector occurs due to stress near the amorphous-crystalline interface arising from the large volume difference (16%) between amorphous and crystalline SrTiO₃. The development of the rotating lattice microstructure has the potential to enable the creation of complex oxides in stress and orientation configurations not available through other epitaxial growth methods.