Stoichiometric SrVO3 thin films grown by hybrid molecular beam epitaxy are demonstrated, meeting the stringent requirements of an ideal bottom electrode material. They display an order of magnitude ...lower room temperature resistivity and superior chemical stability, compared to the commonly employed SrRuO3, as well as atomically smooth surfaces. Excellent structural compatibility with perovskite and related structures renders SrVO3 a high performance electrode material with the potential to promote the creation of new functional oxide electronic devices.
Driven by an ever‐expanding interest in new material systems with new functionality, the growth of atomic‐scale electronic materials by molecular beam epitaxy (MBE) has evolved continuously since the ...1950s. Here, a new MBE technique called hybrid‐MBE (hMBE) is reviewed that has been proven a powerful approach for tackling the challenge of growing high‐quality, multicomponent complex oxides, specifically the ABO3 perovskites. The goal of this work is to (1) discuss the development of hMBE in a historical context, (2) review the advantageous surface kinetics and chemistry that enable the self‐regulated growth of ABO3 perovskites, (3) layout the key components and technical challenges associated with hMBE, (4) review the status of the field and the materials that have been successfully grown by hMBE which demonstrate its general applicability, and (5) discuss the future of hMBE in regards to technical innovations and expansion into new material classes, which are aimed at expanding into industrial realm and at tackling new scientific endeavors.
The development of hybrid molecular beam epitaxy is described from a historic, scientific, and technical perspective. The rapid progress in this expanding field is comprehensively summarized and emphasis is placed on the growth aspects of ternary and quaternary perovskite oxides with different chemistries, the control over cation stoichiometry and its direct ramification in thin film quality.
The drive toward non‐von Neumann device architectures has led to an intense focus on insulator‐to‐metal (IMT) and the converse metal‐to‐insulator (MIT) transitions. Studies of electric field‐driven ...IMT in the prototypical VO2 thin‐film channel devices are largely focused on the electrical and elastic responses of the films, but the response of the corresponding TiO2 substrate is often overlooked, since it is nominally expected to be electrically passive and elastically rigid. Here, in‐operando spatiotemporal imaging of the coupled elastodynamics using X‐ray diffraction microscopy of a VO2 film channel device on TiO2 substrate reveals two new surprises. First, the film channel bulges during the IMT, the opposite of the expected shrinking in the film undergoing IMT. Second, a microns thick proximal layer in the substrate also coherently bulges accompanying the IMT in the film, which is completely unexpected. Phase‐field simulations of coupled IMT, oxygen vacancy electronic dynamics, and electronic carrier diffusion incorporating thermal and strain effects suggest that the observed elastodynamics can be explained by the known naturally occurring oxygen vacancies that rapidly ionize (and deionize) in concert with the IMT (MIT). Fast electrical‐triggering of the IMT via ionizing defects and an active “IMT‐like” substrate layer are critical aspects to consider in device applications.
Surprising coherently coupled elastodynamics is observed between a VO2 film exhibiting an insulator‐to‐metal transition (IMT) and a TiO2 substrate that also exhibits an IMT‐like response in a micrometers deep surface layer, reveals by spatiotemporal X‐ray diffraction microscopy. Phase‐field model points to native oxygen vacancy defects that respond to electric fields, a promising direction for neuromorphic computing.
Controlling material properties at the nanoscale is a critical enabler of high performance electronic and photonic devices. A prototypical material example is VO2, where a structural phase transition ...in correlation with dramatic changes in resistivity, optical response, and thermal properties demonstrates particular technological importance. While the phase transition in VO2 can be controlled at macroscopic scales, reliable and reversible nanoscale control of the material phases has remained elusive. Here, reconfigurable nanoscale manipulations of VO2 from the pristine monoclinic semiconducting phase to either a stable monoclinic metallic phase, a metastable rutile metallic phase, or a layered insulating phase using an atomic force microscope is demonstrated at room temperature. The capability to directly write and erase arbitrary 2D patterns of different material phases with distinct optical and electrical properties builds a solid foundation for future reprogrammable multifunctional device engineering.
Reconfigurable nanoscale manipulations of VO2 from the pristine monoclinic semiconducting phase to either a stable monoclinic metallic phase, a metastable rutile metallic phase, or a layered insulating phase using an atomic force microscope are demonstrated. The direct writing of different material phases with distinct optical/electrical properties opens up new opportunities for building integrated nanoelectronics on monolithic correlated oxide platforms.
Collective interactions in functional materials can enable novel macroscopic properties like insulator-to-metal transitions. While implementing such materials into field-effect-transistor technology ...can potentially augment current state-of-the-art devices by providing unique routes to overcome their conventional limits, attempts to harness the insulator-to-metal transition for high-performance transistors have experienced little success. Here, we demonstrate a pathway for harnessing the abrupt resistivity transformation across the insulator-to-metal transition in vanadium dioxide (VO2), to design a hybrid-phase-transition field-effect transistor that exhibits gate controlled steep ('sub-kT/q') and reversible switching at room temperature. The transistor design, wherein VO2 is implemented in series with the field-effect transistor's source rather than into the channel, exploits negative differential resistance induced across the VO2 to create an internal amplifier that facilitates enhanced performance over a conventional field-effect transistor. Our approach enables low-voltage complementary n-type and p-type transistor operation as demonstrated here, and is applicable to other insulator-to-metal transition materials, offering tantalizing possibilities for energy-efficient logic and memory applications.
BaTiO3 is a technologically relevant material in the perovskite oxide class with above‐room‐temperature ferroelectricity and a very large electro‐optical coefficient, making it highly suitable for ...emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO3 thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO3 thin films by hybrid molecular beam epitaxy is reported. A self‐regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO3 films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO3 crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO3 thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO3 can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO3 films.
BaTiO3 films are grown by different thin film techniques, however, the lack of stoichiometry control makes it hard to achieve bulk‐like properties. Here, hybrid molecular beam epitaxy is employed to grow BaTiO3 films. Using this approach, a self‐regulated growth window is accessed, making it ideally suited as an easy, robust, straightforward, and scalable growth method to synthesize epitaxial BaTiO3 thin films with sufficient control over the film's stoichiometry and to realize bulk‐like BaTiO3 properties in thin film form.
Transition metal oxides offer functional properties beyond conventional semiconductors. Bridging the gap between the fundamental research frontier in oxide electronics and their realization in ...commercial devices demands a wafer-scale growth approach for high-quality transition metal oxide thin films. Such a method requires excellent control over the transition metal valence state to avoid performance deterioration, which has been proved challenging. Here we present a scalable growth approach that enables a precise valence state control. By creating an oxygen activity gradient across the wafer, a continuous valence state library is established to directly identify the optimal growth condition. Single-crystalline VO2 thin films have been grown on wafer scale, exhibiting more than four orders of magnitude change in resistivity across the metal-to-insulator transition. It is demonstrated that 'electronic grade' transition metal oxide films can be realized on a large scale using a combinatorial growth approach, which can be extended to other multivalent oxide systems.
Strain tuning has emerged as a powerful means to enhance properties and to induce otherwise unattainable phenomena in complex oxide films. However, by employing strain alone, the predicted properties ...sometimes fail to emerge. In this work, the critical role of precise stoichiometry control for realizing strain‐induced ferroelectricity in CaTiO3 films is demonstrated. An adsorption controlled growth window is discovered for CaTiO3 films grown by hybrid molecular beam epitaxy, which ensures an excellent control over the Ti:Ca atomic percent ratio of <0.8% in the films. Superior ferroelectric and dielectric properties are found for films grown inside the stoichiometric growth window, yielding maximum polarization, dielectric constant, and paraelectric‐to‐ferroelectric transition temperatures. Outside this growth window, properties are severely deteriorated and ultimately suppressed by defects in the films. This study exemplifies the important role of precise compositional control for achieving strain‐induced properties. Untangling the effects of strain and stoichiometry on functional properties will accelerate both fundamental discoveries yet to be made in the vast materials design space of strained complex oxide films, as well as utilization of strain‐stabilized phenomena in future devices.
A stoichiometric growth window is discovered for CaTiO3films grown by hybrid molecular beam epitaxy. The structural, chemical, and electrical properties are characterized as a function of stoichiometry, where optimal ferroelectric properties are only found for stoichiometric CaTiO3 films grown inside of the growth window, demonstrating its critical role for realizing theoretically predicted strained properties in perovskite oxide thin films.
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