Oxide Molecular Beam Epitaxy Research Articles

Broadband cavity-enhanced optical flux monitoring

This work describes a new type of sensor for growth process monitoring named broadband cavity-enhanced optical flux monitoring sensor (BBCE-OFM). Like existing optical flux monitoring (OFM) solutions, it relies on absorption spectroscopy. However, the implementation of an optical cavity reduces the measurement uncertainty, enabling efficient operation even at very low growth rates. Using the BBCE-OFM sensor mounted in our solid-source oxide molecular beam epitaxy reactor, we achieved an uncertainty of ±2% on the measurement of Sr and Ti growth rates in SrTiO3 at around 1 Ml/min, to be compared to the ±16% obtained in the same conditions using a conventional OFM setup. Furthermore, our sensor architecture, based on an echelle monochromator and LEDs replacing the hollow cathode lamps used in standard OFM sensors, is more robust against drift.

Enabling 2D Electron Gas with High Room-Temperature Electron Mobility Exceeding 100cm2 Vs-1 at a Perovskite Oxide Interface.

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a 2D electron gas (2DEG) with possible applications in, e.g., high-electron-mobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature (RT) 2DEG performance due to notoriously low electron mobility. In this work, the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO3 and LaInO3 is demonstrated with enhanced RT electron mobility values up to 119 cm2 Vs-1-the highest RT value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, this approach opens up another degree of freedom in optimization and in situ control of the interface between two epitaxial oxide layers away from the substrate interface. Thus this approach is expected to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materialsplatforms.

Alkaline Earth Bismuth Fluorides as Fluoride-Ion Battery Electrolytes.

Fluoride-ion batteries have several potential advantages over lithium-ion batteries. Materials development is still needed, however, to realize electrolytes with sufficiently high anion conductivity and compatibility with anode and cathode layers. Fluoride compounds are difficult to synthesize directly as single crystals but can be realized from oxide film precursors via topotactic chemistry techniques. Here, we create crystalline alkaline earth bismuth fluoride films BaBiF5 and SrBiF5 through oxide molecular beam epitaxy and topotactic fluorination. We characterize their ionic conductivities and demonstrate their potential as electrolytes. Finally, we realize epitaxial synthesis of BaBiF5 on BaF2 substrates, providing a route to thin film fluoride-ion battery devices.

Tuning of optoelectronic performance of SrTiO3 by surface termination and thickness

The outstanding optical and electrical properties make ternary oxides highly suitable for advanced optoelectronic applications comparing to the traditional silicon-based materials and heterostructures. SrTiO3 (STO), a pivotal perovskite oxide with a direct energy band gap of 3.2 eV, exhibits significant promise in ultraviolet (UV) photodetection. However, the photovoltaic performance of homoepitaxial STO has long been neglected. In this study, we investigated the optoelectronic characteristics of STO by manipulating the crucial factors of surface termination and thickness using oxide molecular beam epitaxy (oxide MBE). We achieved an exceptional charge carrier mobility of 56.5975 cm2 V-1s−1, along with an impressive ON/OFF ratio of 6000 % and a specific detectivity (D*) of 2.95 × 109 Jones (cm Hz1/2 W−1) under near-UV irradiation at room temperature. We observed a thickness-dependent semiconductor-to-metal transition with a critical thickness of 3.5 u.c. STO. The metallic STO displayed an extremely high mobility exceeding 105 cm2 V−1 s−1, accompanied by a substantial MR value exceeding 1000 % at 3 K. Furthermore, the thickness-dependent Hall effect and photoluminescence (PL) indicated that oxygen defects may govern the semiconducting/metallic behavior of STO. This study illustrates the optoelectrical capabilities of homoepitaxial STO, paving the way for advanced and high-performance photodetection in intricate optical materials and devices through homoepitaxy.

Etching of elemental layers in oxide molecular beam epitaxy by O2-assisted formation and evaporation of their volatile (sub)oxide: The examples of Ga and Ge

Etching of elemental layers in oxide molecular beam epitaxy by O2-assisted formation and evaporation of their volatile (sub)oxide: The examples of Ga and Ge

Strain-induced modulation of electronic structure in correlated Dirac semimetal Pv-CaIrO3 epitaxial thin films

Perovskite CaIrO3 is theoretically predicted to be a Dirac node semimetal near the Mott transition, which possesses a considerable interplay between electron correlations and spin–orbit coupling. Electron correlations can significantly tune the behavior of relativistic Dirac fermions. Here, we have grown high-quality perovskite CaIrO3 thin films on different substrates using oxide molecular beam epitaxy to modulate both electron correlations and Dirac electron states. Through in situ angle-resolved photoemission spectroscopy, we demonstrate a systematic evolution of the bandwidth and effective mass of Jeff=1/2 band in perovskite CaIrO3 induced by strain. The bandwidth of the Jeff=1/2 band undergoes an evident increase under in-plane compressive strain, which could be attributed to the weakening of electron correlations. The compressive strain can potentially shift the position of the Dirac node relative to the Fermi level and play a vital role in the transition from hole-type to electron-type transport characteristics. Our work provides a feasible approach for manipulating the topological Dirac electron states by engineering the strength of electron correlations.

Role of interdiffusion on the magnetism of ultrathin LaMnO3 films

We present a detailed study of the magnetic, spectroscopic and structural properties of ultrathin LaMnO3 films deposited on SrTiO3(001) substrates by oxide molecular beam epitaxy. We find that the as-grown LaMnO3 films are slightly reduced and present a significant magnetic moment, while annealing to 600 °C fully oxidizes and enhances its magnetic properties. From synchrotron x-ray photoemission spectromicroscopy, we find the presence of Sr, Ca and Si in the LaMnO3 film that diffuse from the SrTiO3 substrate; these impurities act as hole dopants, and can explain the presence of ferromagnetism in LaMnO3. This work highlights the importance of cation mobility at the elevated growth temperature in modifying the magnetic properties of ultrathin LaMnO3 films.

Ultrathin Limit on the Anisotropic Superconductivity of Single-Layered Cuprate Films

Exploring dimensionality effects on cuprates is important for understanding the nature of high-temperature superconductivity. By atomically layer-by-layer growth with oxide molecular beam epitaxy, we demonstrate that La2–x Sr x CuO4 (x = 0.15) thin films remain superconducting down to 2 unit cells of thickness but quickly reach the maximum superconducting transition temperature at and above 4 unit cells. By fitting the critical magnetic field (μ 0 H c2), we show that the anisotropy of the film’s superconductivity increases with decreasing film thickness, indicating that the superconductivity of the film gradually evolves from weak three- to two-dimensional character. These results are helpful to gain more insight into the nature of high-temperature superconductivity with dimensionality.

Why thermal laser epitaxy aluminum sources yield reproducible fluxes in oxidizing environments

Aluminum plays a central role in the world of electronic oxide materials. Yet, aluminum sources are very difficult to handle during oxide molecular-beam epitaxy, the main reason for which is the high oxidization potential of aluminum. In this work, we present a thorough study of the behavior of aluminum sources during oxide thermal laser epitaxy. We identify two distinct operating regimes. At high laser-beam fluences, the source emanates reproducible fluxes independent of an applied oxygen pressure of <10−1 hPa. At lower beam fluences, the flux increases with increasing oxygen pressure (<10−1 hPa) due to suboxide formation. We demonstrate reproducible rate control over a flux range of 5 orders of magnitude, which can be expanded further. These results demonstrate that thermal laser epitaxy does not present the challenges associated with the evaporation of aluminum during oxide molecular-beam epitaxy.

Toward controllable Si-doping in oxide molecular beam epitaxy using a solid SiO source: Application to β -Ga2O3

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux (ΦSiO) from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent ΦSiO was measured in vacuum before and after subjecting the source material to an O2-background of 10−5 mbar (typical oxide MBE regime). The absence of a significant ΦSiO difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The ΦSiO at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., NSi from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided ΦSiO (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced.

(Invited) Integrated Perovskites Oxides on Silicon: From Optical to Quantum Applications

With the slowing down of Moore’s law, related to conventional scaling of integrated circuits, alternative technologies will require research effort for pushing the limits of new generations of electronic or photonic devices. Perovskite oxides with the ABO3 chemical formula have a very wide range of interesting intrinsic properties such as metal-insulator transition, ferroelectricity, pyroelectricity, piezoelectricity, ferromagnetic and superconductivity. For the integration of such oxides, it is of great interest to combine their properties with traditional electronic, memory and optical devices on the same silicon-based platform.In the context of high-speed chip-to-chip optical interconnects, compact high-resolution beam steering and video-rate RGB hologram generation require the integration of fast and efficient optical modulators on top of silicon CMOS devices. For these applications the integration of high quality electro-optical materials A defect-free material-stack deposition on silicon wafers is hence required. Among the possible materials options, barium titanate (BaTiO3) is one promising candidate due to its large intrinsic Pockels coefficients that can be obtained. In a first part of the talk, we will review the different options to integrate BaTiO3 on Silicon substrate though different templates to control the polarization direction and discuss the influence on the physical, electrical and optical properties.Then in the second section we will discuss the use of perovskites oxide in the field of topological based qubits which is one of the promising methods for realizing fault-tolerant computations. It is recognized that superconductor/topological insulator heterostructure interfaces may be a perfect host for the exotic “Majorana” particles. These have relevant topological protection nature as required for processing information. Therefore, the physics at the superconductor/topological insulator heterostructure interface need to be studied further, starting at the material level. In this work, a candidate material Barium Bismuthate (BBO) is studied utilizing the Oxide Molecular Beam Epitaxy (MBE) process. The perovskite structure provides opportunity for easily tailored functionality through substitutional doping. Incorporation of potassium into the lattice of BBO results in a superconducting phase with Curie temperature as high as ~ 30K. In addition, BBO is according to DFT based studies, predicted to form topological surface states when doped with Fluorine. In our work, we integrate BBO perovskite on Si(001) substrate, using an epitaxially grown strontium titanate (STO) single-crystalline buffer layer and discuss the structural and chemical properties of the heterostructure will be established by utilizing physical characterization techniques such as AFM, and TEM in later stages. This will go hand in hand with the understanding of the ARPES studies and related surface reconstruction of BBO observed by RHEED as a criterion for the high-quality films.This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreements No 864483 and 742299)”.

Self-healing Growth of LaNiO3 on a Mixed-Terminated Perovskite Surface.

Developing atomic-scale synthesis control is a prerequisite for understanding and engineering the exotic physics inherent to transition-metal oxide heterostructures. Thus, far, however, the number of materials systems explored has been extremely limited, particularly with regard to the crystalline substrate, which is routinely SrTiO3. Here, we investigate the growth of a rare-earth nickelate─LaNiO3─on (LaAlO3)(Sr2AlTaO6) (LSAT) (001) by oxide molecular beam epitaxy (MBE). Whereas the LSAT substrates are smooth, they do not exhibit the single surface termination usually assumed necessary for control over the interface structure. Performing both nonresonant and resonant anomalous in situ synchrotron surface X-ray scattering during MBE growth, we show that reproducible heterostructures can be achieved regardless of both the mixed surface termination and the layer-by-layer deposition sequence. The rearrangement of the layers occurs dynamically during growth, resulting in the fabrication of high-quality LaNiO3/LSAT heterostructures with a sharp and consistent interfacial structure. This is due to the thermodynamics of the deposition window as well as the nature of the chemical species at interfaces─here, the flexible charge state of nickel at the oxide surface. This has important implications regarding the use of a wider variety of substrates for fundamental studies on complex oxide synthesis.

Canonical approach to cation flux calibration in oxide molecular-beam epitaxy

Molecular-beam epitaxy (MBE) is the gold standard for the epitaxial growth of complex oxides with the best material properties as determined by respective figures of merit. Unfortunately, once more than one cation is involved in the material desired, MBE growth often becomes plagued by difficulties in stoichiometry control. Instead of relying on a quartz crystal microbalance to measure the fluxes of the individual molecular beams, which lacks accuracy, or reflection high-energy electron diffraction oscillations of the targeted multication oxide in layer-by-layer growth, which lacks general applicability, here, we describe a canonical approach based on the growth of films of the constituent binary oxides or metals individually for cation flux calibration. This method can calibrate the flux of each molecular beam with an absolute accuracy of \ifmmode\pm\else\textpm\fi{}1%. After describing the growth parameters of binary oxides or metals enabling the individual fluxes of 39 elements of the periodic table to be determined, we demonstrate the efficacy of this approach by applying it to the growth of the quaternary ferromagnetic metal ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ to achieve films with transport properties rivalling the best reported using thin-film growth techniques providing stoichiometric transfer.

In situ study on the evolution of atomic and electronic structure of LaTiO3/SrTiO3 system

As with other transition-metal oxide interfaces, ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces exhibit interesting electronic properties, such as a two-dimensional electron gas. While the available Mott states and multiple pathways to metallicity in ${\mathrm{LaTiO}}_{3}$ can lead to a variety of applications in oxide electronics, the origin of the different properties remains uncertain. Utilizing real-time, in situ x-ray diffraction at the synchrotron, robust but thin ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures were fabricated using oxide molecular beam epitaxy. A thickness of six unit cells was determined to be critical, at which point the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures begin to sustain sharp, well-defined interfaces. The layers of ${\mathrm{LaTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ were sufficiently thin to benefit from the finite escape length of electrons at resonant soft x-ray photoemission, and the heterostructures were transferred in vacuo after growth to another chamber for synchrotron-based x-ray photoemission spectroscopy studies. With the x-ray energy tuned to the Ti-2p resonance, the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces could be probed with enough penetration depth and selectivity. It is shown that all of the heterointerfaces exhibit the two-dimensional electron gas.

Epitaxial growth of metastable La0.67Sr0.33MnO3−δ films by using Sr3Al2O6 as multifunctional buffer layers

In doped manganites, a substantial tuning of the magnetic and electrical transport properties can be realized by engineering the concentration of oxygen vacancies. To date, most oxygen-deficient La1−xSrxMnO3−δ (0 ≤ x ≤ 1) films are synthesized by after-growth treatments. However, the direct growth of La1−xSrxMnO3−δ films remains challenging due to the metastability of this material. Here, we report the epitaxial growth of high quality single crystalline La0.67Sr0.33MnO3−δ films with an extremely large out-of-plane lattice parameter of 4.26 Å by reactive oxide molecular beam epitaxy. To stabilize this metastable phase, Sr3Al2O6 buffer layers are used to block the oxygen diffusion from the SrTiO3 substrate to the film during the growth process. This work provides an efficient way to obtain metastable La0.67Sr0.33MnO3−δ films.

Engineering ordered arrangements of oxygen vacancies at the surface of superconducting La2CuO4 thin films

We demonstrate how ordered arrangements of oxygen vacancies can be engineered during the growth of superconducting La2CuO4 films by oxide molecular-beam epitaxy. These arrangements are seen using in situ reflection high-energy electron diffraction. Based on qualitative real-time observations, we propose a surface reconstruction mechanism emphasizing the active role of dopants and oxygen vacancies at the film surface. Due to the specific atomic arrangement induced by dopant positions, characteristic surface “stripes” are generated, and they determine the intrinsically heterogeneous structure characterized by distorted checkerboard patterns on the surface. Not only can the surface motif during growth be monitored via characteristic surface reconstructions, but it can also be customized by altering strain, doping, and oxygen activity.

Electrical properties of high permittivity epitaxial SrCaTiO3 grown on AlGaN/GaN heterostructures

Epitaxial integration of perovskite oxide materials with GaN has unlocked the potential to improve functionality and performance in high-power RF and power-switching applications. In this work, we demonstrate structural and electrical properties of high dielectric constant Sr1−xCaxTiO3 epitaxial layers grown on AlGaN/GaN/4H-SiC high-electron-mobility transistor structures with compositions ranging from x = 0 to x = 0.53 and oxide film thicknesses ranging from 7 to 126 nm. We show (111) orientation in the SrCaTiO3 (SCTO) thin films using a 1 nm (100) TiO2 buffer layer grown by RF-plasma-assisted oxide molecular beam epitaxy. Current–voltage measurements show up to 5 orders of magnitude reduced leakage with SCTO films when compared to Schottky contacted samples. Capacitance–voltage measurements show minimal hysteresis, an extracted dielectric constant (κ) as high as 290, and a fixed positive interface charge density of 2.38 × 1013 cm−2 at the SCTO/AlGaN interface. The direct integration of the SCTO layer does not significantly affect the two-dimensional electron gas (2DEG) density or the channel mobility with the 2DEG density as a function of SCTO thickness having good agreement with 1D Poisson–Schrödinger simulations. RF characterization of interdigitated capacitors using the SCTO films on unintentionally doped GaN/SiC shows that the films maintain their high κ into microwave frequencies and only exhibit a slight reduction in κ with increased lateral electric fields. These results demonstrate that the integration of a high-κ oxide with GaN can potentially improve electric field management in RF high-electron-mobility transistors and increase the device breakdown voltage without significant degradation to channel transport properties.

Optical conductivity and superconductivity in highly overdoped La2−xCaxCuO4 thin films

We have used atomic layer-by-layer oxide molecular beam epitaxy to grow epitaxial thin films of [Formula: see text] with x up to 0.5, greatly exceeding the solubility limit of Ca in bulk systems ([Formula: see text]). A comparison of the optical conductivity measured by spectroscopic ellipsometry to prior predictions from dynamical mean-field theory demonstrates that the hole concentration p is approximately equal to x. We find superconductivity with [Formula: see text] of 15 to 20 K up to the highest doping levels and attribute the unusual stability of superconductivity in [Formula: see text] to the nearly identical radii of La and Ca ions, which minimizes the impact of structural disorder. We conclude that careful disorder management can greatly extend the "superconducting dome" in the phase diagram of the cuprates.

Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO3 (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy, and low-energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich–Schwoebel barrier consistent with A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO2 termination or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain the chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

High-resolution ARPES endstation for in situ electronic structure investigations at SSRF

Angle-resolved photoemission spectroscopy (ARPES) is one of the most powerful experimental techniques in condensed matter physics. Synchrotron ARPES, which uses photons with high flux and continuously tunable energy, has become particularly important. However, an excellent synchrotron ARPES system must have features such as a small beam spot, super-high energy resolution, and a user-friendly operation interface. A synchrotron beamline and an endstation (BL03U) were designed and constructed at the Shanghai Synchrotron Radiation Facility. The beam spot size at the sample position is 7.5 (V) μm × 67 (H) μm, and the fundamental photon range is 7–165 eV; the ARPES system enables photoemission with an energy resolution of 2.67 meV at 21.2 eV. In addition, the ARPES system of this endstation is equipped with a six-axis cryogenic sample manipulator (the lowest temperature is 7 K) and is integrated with an oxide molecular beam epitaxy system and a scanning tunneling microscope, which can provide an advanced platform for in situ characterization of the fine electronic structure of condensed matter.