18 Search Results

Abstract Molecular beam epitaxy (MBE), a workhorse of the semiconductor industry, has progressed rapidly in the last few decades in the development of novel materials. Recent developments in condensed matter and materials physics have seen the rise of many novel quantum materials that require ultra-clean and high-quality samples for fundamental studies and applications. Novel oxide-based quantum materials synthesized using MBE have advanced the development of the field and materials. In this review, we discuss the recent progress in new MBE techniques that have enabled synthesis of complex oxides that exhibit ‘quantum’ phenomena, including superconductivity and topological electronic states. We showmore » how these techniques have produced breakthroughs in the synthesis of 4d and 5d oxide films and heterostructures that are of particular interest as quantum materials. These new techniques in MBE offer a bright future for the synthesis of ultra-high quality oxide quantum materials.« less

Transition metal spinel oxides comprised of earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely controlled materials to explore the variations reported in the literature. In this work, Mn_xCo_3–xO₄ samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Here, films weremore » characterized via in situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and polarized K-edge x-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn³⁺ and Mn⁴⁺ character with evidence of phase segregation tendencies with the increasing Mn content and increasing Mn³⁺ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn⁴⁺ and Jahn–Teller active Mn³⁺ octahedra, respectively. Our results help in explaining the reported differences across samples in these promising materials for renewable energy technologies.« less

4d transition metal oxides have emerged as promising materials for numerous applications including high mobility electronics. SrNbO₃ is one such candidate material, serving as a good donor material in interfacial oxide systems and exhibiting high electron mobility in ultrathin films. However, its synthesis is challenging due to the metastable nature of the d¹ Nb⁴⁺ cation and the limitations in the delivery of refractory Nb. To date, films have been grown primarily by pulsed laser deposition (PLD), but development of a means to grow and stabilize the material via molecular beam epitaxy (MBE) would enable studies of interfacial phenomena and multilayermore » structures that may be challenging by PLD. To that end, SrNbO₃ thin films were grown using hybrid MBE for the first time using a tris(diethylamido)(tert-butylimido) niobium precursor for Nb and an elemental Sr source on GdScO₃ substrates. Varying thicknesses of insulating SrHfO₃ capping layers were deposited using a hafnium tert-butoxide precursor for Hf on top of SrNbO₃ films to preserve the metastable surface. Grown films were transferred in vacuo for x-ray photoelectron spectroscopy to quantify elemental composition, density of states at the Fermi energy, and Nb oxidation state. Ex situ studies by x-ray absorption near edge spectroscopy and scanning transmission electron microscopy illustrate that the SrHfO₃ capping plays an important role in preserving the crystalline quality of the material and the Nb 4d¹ metastable charge state under atmospheric conditions.« less

Ternary chalcogenides, such as parkerites and shandites, are a broad class of materials exhibiting a rich diversity of transport and magnetic behavior and an array of topological phases, including Weyl and Dirac nodes. However, they remain largely unexplored as high-quality epitaxial thin films. Here, we report the self-regulated growth of thin films of the strong spin–orbit coupled superconductor Pd₃Bi₂Se₂ on SrTiO₃ by molecular beam epitaxy. Films are found to grow in a self-regulated fashion, where, in excess Se, the temperature and relative flux ratio of Pd to Bi control the formation of Pd₃Bi₂Se₂ due to the combined volatility of Bi,more » Se, and Bi–Se bonded phases. The resulting films are shown to be of high structural quality, and the stoichiometry is independent of the Pd:Bi and Se flux ratio and exhibits a superconducting transition temperature of 800 mK and a critical field of 17.7 ± 0.5 mT, as probed by transport and magnetometry. Understanding and navigating the growth of the chemically and structurally diverse classes of ternary chalcogenides open a vast space for discovering new phenomena and enabling new applications.« less

The titanomagnetites (Fe_2-xTi_xO₄, x ≤ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe_2-xTi_xO₄ were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250 – 375°C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe₃O₄ and rutile TiO₂. Less oxidizing conditions resulted inmore » polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe³⁺, Fe²⁺, Fe⁰). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe²⁺. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe²⁺ was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe²⁺ was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. Furthermore, this work suggests a path towards obtaining stoichiometric, well-crystallized Fe_2-xTi_xO₄ by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.« less

Recent investigations on spinel CoMn2O4 have shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline Co1+xMn2-xO4 thin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional formore » octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMn2O4 to understand how the Jahn-Teller distortion and hybridization in Mn-O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMn2O4 and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.« less

Reflection high-energy electron diffraction (RHEED) is a ubiquitous in situ molecular beam epitaxial (MBE) characterization tool. Although RHEED can be a powerful means for crystal surface structure determination, it is often used as a static qualitative surface characterization method at discrete intervals during a growth. A full analysis of RHEED data collected during the entirety of MBE growths is made possible using principle component analysis (PCA) and $$\textit{k}$$-means clustering to examine significant boundaries that occur in the temporal clusters grouped from RHEED data and identify statistically significant patterns. This process is applied to data from homoepitaxial SrTiO₃ growths, heteroepitaxial SrTiO₃more » grown on scandate substrates, BaSnO₃ films grown on SrTiO₃ substrates, and LaNiO₃ films grown on SrTiO₃ substrates. We report this analysis may provide additional insights into the surface evolution and transitions in growth modes at precise times and depths during growth, and that video archival of an entire RHEED image sequence may be able to provide more insight and control overgrowth processes and film quality.« less

The behavior of polar LaMnO₃ (LMO) thin films deposited epitaxially on nonpolar SrTiO₃(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X-ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built-in electric field in LMO films of any thickness is found. Measurements of the Mn valence by Mn L-edge X-ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanningmore » transmission electron microscopy images reveal that Mn²⁺ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface.« less

The behavior of polar LaMnO3 (LMO) thin films deposited epitaxially on non-polar SrTiO3(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ x-ray photoelectron spectros-copy (XPS), we directly measure the valence band offset (VBO) of LMO/STO heterojunctions as a function of thickness, and find that the VBO is 2.5 eV for thicker (= 3 u.c.) films. We find no evidence of a built-in potential in LMO films of any thickness. Measurements of the Mn valence by Mn L-edge x-ray absorption spectroscopy (XAS) and by spatially-resolved electron energy loss spectra inmore » scanning transmission electron microscopy images (STEM-EELS) reveal that Mn2+ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by DFT simula-tions that confirm a VBO of approximately 2.5 eV for both ideal and intermixed interfaces. We propose a model for the behavior of polar / non-polar LMO/STO heterojunctions in which the built-in potential is alleviated by the formation of oxygen vacancies at the LMO surface.« less

The polarity of oxide surfaces can dramatically impact their surface reactivity, in particular with polar molecules such as water. The surface species that result from this interaction change the oxide electronic structure and chemical reactivity in applications such as photoelectrochemistry, but are challenging to probe experimentally with atomic-scale understanding. Here we report a detailed study of the surface chemistry and electronic structure of the perovskite LaFeO3 in humid conditions using ambient pressure X-ray photoelectron spectroscopy. Comparing the two possible terminations of the polar (001)-oriented surface, we find that the LaO surface is more reactive toward water, forming hydroxyl species andmore » adsorbing molecular water at lower relative humidity than its FeO2-terminated counterpart. Our results demonstrate how the termination of a complex oxide can dramatically impact its reactivity, providing insight into the design of catalyst materials.« less