Session F16: Physics Enabled by Film Growth

Superconducting Sr2RuO4 thin films with record transition temperature up to 2 K by molecular-beam epitaxy

Clarifying the superconducting order parameter of Sr₂RuO₄ has remained an enigma despite more than two decades of research on high-quality Sr₂RuO₄ single crystals.^[1] High-quality Sr₂RuO₄ thin films with robust superconductivity enable an alternative approach toto clarifying its superconducting through phase-sensitive experiments, as well as fabricating quantum circuits. Unfortunately, the growth of reproducible high-quality superconducting Sr₂RuO₄ thin films has been limited due to its extreme sensitivity to structural disorder and narrow growth window.^[2]  

In this work, we grew superconducting Sr₂RuO₄ thin films on NdGaO₃ (110) substrates by molecular-beam epitaxy (MBE) in the adsorption-controlled regime.^[2] We characterized the Sr₂RuO₄ thin films by x-ray diffraction (XRD), atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), and low-temperature transport. Our optimized Sr₂RuO₄ thin films show a superconducting transition temperature (midpoint) as high as 2.1 K and residual resistivity ratio as 122; both are the best values ever reported. We also clarified that the quality of superconductivity in Sr₂RuO₄ thin film was highly related to the microstructure measured by STEM.

 

References

[1] Y. Maeno et al., Nature 372, 532 (1994)

[2] H. P. Nair et al., APL Materials 6, 101108 (2018)   

Growth of Superconducting Sr2RuO4 Thin Films via Thermal Laser Epitaxy

Thermal laser epitaxy (TLE) is a novel technique for thin film deposition which employs continuous wave lasers to simultaneously heat both the substrate and elemental sources. This laser heating approach allows for evaporation or sublimation of nearly all elements from the periodic table, ultrahigh substrate temperatures exceeding 2000 C, and broad compatibility with process gases at a wide range of pressures from UHV up to 1 Torr, among other benefits. As a result, TLE dramatically expands the parameter space available for thin film synthesis compared to existing epitaxy techniques. However, to date it has proven experimentally challenging to achieve simultaneous control of multiple laser based elemental sources with the flux stability and systematic fidelity necessary for the growth of ternary or multernary systems of interest such as complex oxides.

In order to establish the capabilities of TLE for the growth of such complex materials, we demonstrate here the successful epitaxial synthesis of several Ruddlesden-Popper phases of the Sr-Ru-O ternary oxide system via TLE.  We find that the “n=1” phase Sr₂RuO₄ can be reliably synthesized at substrate temperatures in excess of 1200 C and in a background environment of pure molecular oxygen, within an adsorption-controlled growth window that is inaccessible to conventional MBE approaches. We show that Sr₂RuO₄ films grown under these conditions demonstrate extremely high structural, electronic, and chemical quality, as evidenced by the appearance of superconductivity at relatively high critical temperatures. In particular, the higher growth temperatures and elemental source fluxes afforded by laser heating allow us to achieve phase pure 214 without higher-N intergrowths typically observed in MBE-grown films. A detailed accounting of the experimental approach, growth thermodynamics and film characterization will be discussed.

This work not only demonstrates the feasibility of TLE for the synthesis of high-quality complex oxide thin films, but also suggests new routes to achieving thin film growth in other materials systems that remain as-yet inaccessible to conventional epitaxy techniques.   

Epitaxial growth of superconducting Fe(Te, Se) films on a threefold magnetic material with Hybrid symmetry.

Fe(Te, Se), FTS, thin films have received significant attention since superconductivity was observed in bulk FTS crystals. Typically, because of the fourfold inherent to FTS, it is required for substrates to possess fourfold in-plane symmetry to facilitate the epitaxial growth of FTS films. However, a recent discovery highlights the potential for the epitaxial growth of superconducting FTS films on materials with a threefold symmetry. This unique occurrence is attributed to the uncommon uniaxial lattice match between FTS and an underneath material, a phenomenon referred to as hybrid symmetry epitaxy.  

Here, we present our findings on the structural characterization of FTS thin films grown epitaxially on a magnetic material with threefold symmetry and transport properties of heterostructure comprising a fourfold superconductor and a threefold magnetic material. We believe that this study provides an exciting avenue for investigating the interplay between superconductivity and magnetism.    

Epitaxial Growth of La-doped BiFeO3 by Digital RF Sputtering

BiFeO₃ (BFO) has been investigated extensively due to its attractive ferroelectric properties and ability to incorporate in various heterostructures. However, BFO films suffer from high leakage currents, hindering its applicability and performance in devices. One solution to this drawback is the substitution of La at the A-site of BFO, which has been shown to reduce the leakage current densities while preserving its ferroelectric properties. In this work, we present the growth of epitaxial La-doped BFO by digital sputtering. This method allows control of the dopant by altercation of the sputtering cycle, in which the doping concentration is set by the sputtering cycle time of the LaFeO₃ target. High quality epitaxial films were grown by off-axis co-sputtering of the BFO and LaFeO₃ targets. Film quality was characterized by XRD, AFM, and TEM showing epitaxial growth on SrTiO₃(001) and DyScO₃(110), where a SrRuO₃ buffer layer was used as a bottom electrode for ferroelectric characterization. Piezoresponse Force Microscopy was used to image the ferroelectric domains and capacitance hysteresis measurements show the leakage currents for the La-doped BFO films.   

Integration of Electronically Reconfigurable Complex Oxide Membranes with Si

Recent reports of electronically reconfigurable complex oxide heterointerfaces show the feasibility of its integration with other materials in the form of free-standing membranes [1]. A novel method for creating reconfigurable silicon nanodevices capable of storing and gating single electrons and their spins is proposed. The approach takes advantage of a reprogrammable LaAlO3/SrTiO3 (LAO/STO) heterostructure which can be fabricated as a free-standing membrane and can be switched between a conductive and insulating phase using ultra-low voltage electron-beam lithography (ULV-EBL) [1][2]. The ULV-EBL technique enables patterning of buried layers, and therefore enables functional integration of LAO/STO with silicon. Here we describe efforts to create Si-based field effect transistor devices gated by an overlying LAO/STO membrane. The principal advantage of this technique is the potential for high-resolution gate patterning of silicon quantum devices and in turn a potential to advance silicon quantum computing.

[1] Eom, K. et al. Science advances 7, 33 (2021)

[2] Yang, D. et al., Appl. Phys. Lett. 117, 253103 (2020)   

Epitaxial SrTaO3 and Heterostructures Grown by Hybrid Molecular Beam Epitaxy

SrTaO3, a metallic perovskite oxide, possesses invaluable properties such as large SOC, high mobility, large magnetoresistance, and transparency. Integration into heterostructures can induce distinctive states and properties, including 2DEGs and topological states. While SrTaO3 has been grown previously by methods such as solid-state reactions and PLD, these methods can lead to various defects (e.g., chemical impurities or dislocations), which can hinder optimization or realization of the intended phenomena. MBE offers a route to synthesis of thin films with precise control of stoichiometry, lattice structure, and defect mitigation. Due to the high melting point of elemental tantalum and its propensity to overoxidize from the desired Ta4+ to Ta5+, the material was grown by hybrid MBE using a nitrogenous organometallic precursor. Characterization by in-situ reflection high energy electron diffraction and x-ray photoelectron spectroscopy, as well as ex-situ high resolution XRD, electrical transport, and STEM was undertaken to confirm the formation of epitaxial SrTaO3. Following successful implementation of single-layer SrTaO3, various (SrCoO3)m/( SrTaO3)n heterostructures were grown to probe the emergent interfacial and superlattice states.   

Investigation of interfacial spin transport in chiral RhSi epitaxial thin films

The rise of nonmagnetic chiral topological semimetals as a uniquely attractive playground for the observation and control of various spin-orbit effects has ushered in the promising field of topological spintronics. RhSi, a spin-1 chiral semimetal with a noncentrosymmetric cubic B20 structure, has attracted attention as it accommodates unconventional multifold fermions that extends our understanding beyond common Dirac and Weyl fermions. In this work, we have investigated the spin-to-charge interconversion through spin pumping and inverse-spin Hall effect(ISHE) in sputter-grown epitaxial RhSi thin films and its subsequent transport across RhSi/Py interface. From the observed modulation of Gilbert damping parameter and ISHE voltage with RhSi thickness, the spin-Hall angle of RhSi and interfacial spin transparency of RhSi/Permalloy interface is determined. A stark variation of spin Hall angle and interfacial spin transparency is observed with ambient temperature in this heterostructure. The spin Hall angle and spin Hall conductivity is found to be maximum of 1.42% and 251.64, respectively. A spin-mixing conductance and interfacial spin transparency as high as 34.7 nm^-2 and 88% is attainable in these heterostructures. This study expands the horizon of topological spintronics and highlights the controlled spin-charge interconversion and interfacial spin-transport process in a chiral semimetal/ferromagnet heterostructure which can be beneficial for the development of chiraltronics and spin-orbitronics devices.   

Logarithmic Temperature-Dependent Resistance of an indium tin oxide thin film from 300 K down to dilution refrigerator temperatures

Indium tin oxide (ITO) is a technologically relevant transparent conducting oxide, with a wide range of applications such as electrodes in organic light emitting diodes, solar cells, and transparent gates for photonic devices. We measured the resistance of a 20 nm thick ITO film over four orders of magnitude in temperature from 300 K down to dilution refrigerator temperatures. Three regions are distinguishable in the resistance as a function of temperature: (i) 300 K – 140 K, where the resistance decreases with decreasing temperature consistent with electron-phonon scattering, (ii) 140 K – 0.1 K where the resistance increases with rising temperature, and (iii) less than 0.1 K, where the temperature dependence weakens, possibly due to thermal decoupling of the charge carriers. In region (ii) we observe a logarithmic rise of the resistance with decreasing temperature with a significant contribution from weak localization (WL), consistent with previous reports. We also measured the temperature evolution of the magnetoresistance and Hall resistance. We observed a strong WL peak at lower temperatures, which broadens and decreases with increasing temperatures until it disappears at about 120 K, while the carrier concentration remains constant. Our results, together with microstructural analysis, give us insight into the quantum-coherent transport in these (possibly granular) materials.   

Intercalation of Cr or Mn in between two VSe2 van der Waals layers

The insertion of metal layers between layered transition metal dichalcogenides (TMDs) allows the design of pseudo-2D nanomaterials; compared to the host TMDs, these new materials may possess different properties, such as magnetism. In this study, VSe₂ films are grown by van der Waals epitaxy and post-growth metal intercalation is accomplished by vacuum deposition of transition metals (TM), Cr or Mn. These TM atoms intercalate between the VSe₂ layers, and the initial 1×1 structure of VSe₂ systematically changes to a 2×2 and then 2×1 superstructures. DFT calculation predicts that the intercalated TM atoms prefer to occupy the octahedral sites. The energy benefits of inserting TM atoms into the van der Waals (vdW) gap become less with increasing TM concentration. It is observed experimentally that this saturation limit occurs after the formation of 2×1 superstructure when 50% of the intercalation sites are occupied. Further addition of TM atoms results in TM adsorption on the surface. Inserted Cr and Mn atoms have charge states of 3+ and 2+, respectively. Also, magnetic moments for intercalated Cr are high for the dilute Cr regime. A decrease in average magnetic moment is observed with increasing Cr concentration, suggesting antiferromagnetic ordering between Cr ions.   

Tuning transport properties via rare-earth doping and epitaxial strain in Sr2IrO4 thin films

Sr₂IrO₄ is predicted to be a candidate for high-temperature superconductivity upon carrier doping, whereas extensive research has proved it challenging to obtain a metallic phase in this compound, especially in thin films. Here, we explore the impact of stoichiometry on the crystallinity, as well as carrier doping and epitaxial strain on the transport properties of Sr_2−xNd_xIrO₄ thin films. Via fine tuning the stoichiometry, the crystallinity of Sr₂IrO₄ films can be greatly enhanced, as indicated by the minimized systematic deviations of the x-ray diffraction peak positions. As the cation doping level increases, the resistivity of Sr_2−xNd_xIrO₄ films decreases, but it remains semiconducting even at a high level of x = 0.4 where the resistivity has dropped by three orders of magnitude. By further applying compressive epitaxial strain, the Sr_1.8Nd_0.2IrO₄ films exhibit a metallic-like behavior with an upturn at low temperature. Our finding reveals a promising combination of electron doping and compressive strain to narrow the band gap and tune the transport properties of iridate films for potential superconductivity.

Impact of Off-Stoichiometry on Defect Structure and Magnetism in LaCoO3 Thin Films

By growing LaCoO3 (LCO) in a thin film geometry and imposing tensile strain via the substrate, LCO at low temperatures is transformed from a low-spin diamagnet to a ferromagnet. The origin of this magnetic state has been intensely studied, but no consensus has been achieved. One striking feature observed in most TEM studies of this material is a periodic array of dark stripes. The composition of the dark stripes and their relationship to magnetism has been investigated, but no clear connection has been established. By using low-pressure, off-axis rf sputtering, we are able to grow highly stoichiometric LCO thin films that are still ferromagnetic despite possessing no dark stripes. The magnetism in our films is weaker than what has been previously reported, suggesting that defect structure has a strong impact on the magnetization. We also fabricate La deficient samples that show an enhancement of magnetic properties after annealing in air, further demonstrating the effect of stoichiometry and defect structure on the magnetism.   

X-ray photoelectron spectroscopy of some inverse spinel oxide thin films and nanoparticles

Despite the numerous X-ray photoelectron spectroscopy (XPS) studies that have been carried out on inverse spinel oxide materials, there has been a lack of agreement on the physical interpretation of the components of the 2p_3/2 core level XPS spectra of the cations in inverse spinel oxides of type AB₂O₄. In this study, we have been able to establish that the 2p_3/2 core level XPS spectra of AB₂O₄ materials represent surface and bulk weighted components, implying surface-to-bulk core level shifts in the binding energies of the core levels [1]. The ratio of surface weighted components to bulk weighted components of the Ni and Fe core levels in NiFe₂O₄ thin films significantly depend on the emission angle, with respect to the surface normal, in angle resolved X-ray photoelectron spectroscopy (ARXPS). For nanoparticles representing a range of alloys between Ni_xCo_1-xFe₂O₄ (x=0.2, 0.5, 0.8, 1), the behavior observed in X-ray photoelectron spectroscopy (XPS) resembles the surface of the NiFe₂O₄ thin film regardless of composition. Surface-to-bulk core level shifts observed in some inverse spinel thin films are difficult to capture in the XPS of the nanoparticles. Moreover, our study shows that the nature of surface contamination could affect the degree to which the ratio of surface weighted to bulk weighted components depends on the photoemission angle.

[1] A. Subedi et al., J. Vac. Sci. Technol. A 40, 023201 (2022).   

Homoepitaxial sapphire for superconducting qubits

Superconducting qubits are fundamentally limited by the material loss at the interface between superconductor and dielectrics, and bulk substrate. Semiconductor industry has circumvented these kinds of losses by using a nucleation layer grown above the substrate to suppress the bulk and interface defects. One of the most promising methods of achieving this is by molecular beam epitaxy which promises defect-free material growth and atomically-abrupt heterojunction. As Sapphire is one of the widely used substrates owing to its low dielectric loss, we demonstrate the homoepitaxial MBE growth of the sapphire nucleation layer on M- plane sapphire substrates. Temperature and flux study was performed and an Oxygen-rich growth regime found to be beneficial for the growth of atomically smooth homoepitaxial layer. A growth rate of 45nm/hr hour was observed at 600C substrate temperature. Atomically smooth surface with terrace width of 35nm (~ 30 unit cells) was found using Atomic Force Microscopy. We observed that a high temperature anneal (1050C) in a furnace and an in-situ Oxygen polishing prior to growth were essential for observing atomic steps. XRD and XRR analysis revealed phase pure films. Comparative studies are being performed between superconductors (NbN, TiN etc) growth epitaxially on substrates with and without the sapphire nucleation layer by fabrication of Coplanar- Waveguide resonators to correlate material properties with the quality factor and eventually to qubit lifetimes.   

Surface reconstructions and electronic structure of metallic delafossite thin films

Motivated by the potential to create electronic and magnetic characteristics that are not accessible in bulk systems, the study of low-dimensional metallic delafossites with the general chemical formula ABO2 is gaining popularity. Understanding the surface behavior of the delafossite compounds is crucial due to their distinct layer structure, which gives rise to diverse surface states, electronic reconstructions, and surface reconstructions. We study epitaxial PtCoO₂, PdCoO₂, and PdCrO₂ films by combining molecular-beam epitaxy and angle-resolved photoemission spectroscopy. Through precise control of surface termination and treatment, we observed distinct surface reconstructions on the PtCoO₂ films, PdCoO₂ films, and PdCrO₂ films. Notably, these reconstructions have not been reported in prior studies of delafossites. Furthermore, our computational analysis reveals the relative stability of the BO₂ surface and the significant reduction in surface formation energy achievable through reconstruction on the A-terminated surface. These experimental and theoretical insights shed light on the surface phenomena occurring in metallic delafossites, paving the way to exploring their distinctive properties in low-dimensional studies.   

Cation-vacancy defects and interfacial reconstruction in hexagonal ferrite heterostructures

Polar narrow bandgap oxides, such as the ferroelectric h-LuFeO₃, are driving interest due to their enhanced optical responsivity in the visible range. The presence of polarization charges combined with the existence of bulk photovoltaic effect lead to a complex interplay between photoresponse and polar domain structure, which itself is critically modulated by the microstructure of samples. Here we present a comprehensive study of h-LuFeO₃ thin films’ microstructure grown on different substrates and buffers: α-Al2O3, α-Al2O3/Pt, YSZ/Pt and YSZ/ITO. Combining the atomic-resolution imaging and spectroscopic capability of scanning transmission electron microscopy with density functional theory calculations, first, we unveil the presence of cation-vacancy defects within the ferroelectric matrix. We show the nature of point defects and how the substrate determines the concentration of cation vacancies in the ferroelectric layer. Second, we identify a structural, chemical and electronic reconstruction at the interface between the h-LuFeO₃ films and the substrate/buffer. This interface reconstruction consists in double Fe-O atomic layers appearing independently from the conductivity of the adjacent film, suggesting that the Fe-O layers can modulate their valence state to compensate the polarity of the interface and contribute to the screening. Finally, we scrutinize the role of point defects as source of uncompensated moments, that may mask or overrule any intrinsic multiferroicity.