The growth of a wide range of binary nitride films with excellent properties is reported. These films are deposited by thermal laser epitaxy (TLE). If equipped with CO2–laser substrate heating, TLE allows growth temperatures ≫ 1000 °C combined with strong nitriding conditions, which are established by using ammonia (NH3) gas as a nitrogen precursor. Films deposited on c-plane sapphire substrates include semiconducting nitrides (BN, AlN, ScN, and YN), superconducting nitrides (TiN, VN, ZrN, and NbN), and magnetic nitrides (CrN and GdN). The superconducting transition temperatures of TiN, VN, ZrN, and NbN films are 5.7, 8.9, 8.9, and 16.9 K, respectively. The Néel temperature of CrN films is ≃280 K, and the Curie temperature of GdN films is 66 K; both lie within the reported ranges. The controlled fabrication of different phases of individual nitride compounds is explored, revealing that subnitride phases can be grown at lower NH3 pressures. This study also demonstrates the heteroepitaxial growth of multilayer nitride films on c-plane sapphire.

We have developed and optimized a method to grow ruthenium films of unprecedented quality. Our three-step process is reminiscent of solid-phase epitaxy. First, c-cut sapphire substrates are terminated at their Al-rich √31 × √31R ± 9° reconstruction by in situ annealing. Second, 3D structured epitaxial Ru films are deposited at low temperatures by laser evaporation of Ru rods. Third, the films, thus, obtained are epitaxially transformed by high-temperature annealing. X-ray diffraction studies reveal good crystallinity of the obtained 15–60 nm-thick films: peak widths of the rocking curve are one order of magnitude smaller than those of the best published films. Scanning transmission electron microscopy and electron energy loss studies show that the interface between the sapphire substrates and the flat Ru films is atomically sharp with very limited intermixing. These results demonstrate the usefulness of postanneal processes for producing high-quality epitaxial films of elemental metals on insulating substrates.

Substrates are essential in thin‐film deposition, yet they do not always meet the requirements of a given application. For example, they might be prohibitively expensive or not even available with the necessary crystal lattice structures. This paper introduces the innovative concept of “vector substrates,” where the template layer for thin‐film growth, referred to as vector, is chemically and crystallographically independent of the bulk of the substrate. This approach reduces material costs and offers unparalleled flexibility. The fabrication of vector substrates leverages thin‐film membrane technology. The process begins by growing a template layer on a parent substrate. The template layer is then transferred onto a carrier substrate, thereby generating the vector substrate. Vector substrate technology enables researchers and manufacturers with thin‐film deposition facilities to fabricate optimized substrates that would otherwise be difficult to obtain using bulk single‐crystal growth technology. An in‐depth discussion of the benefits and limitations of vector substrates compared with traditional substrates is provided. Furthermore, the practical viability of the concept is demonstrated by fabricating sets of vector substrates. Although this concept is still in its infancy, it has a significant potential to complement conventional substrates for future advancements in substrate technology and thin‐film deposition.

Abstract Transparent oxide thin film transistors (TFTs) are an important ingredient of transparent electronics. Their fabrication at the back‐end‐of‐line (BEOL) opens the door to novel strategies to more closely integrate logic with memory for data‐intensive computing architectures that overcome the scaling challenges of today's integrated circuits. A recently developed variant of molecular‐beam epitaxy (MBE) called suboxide MBE (S‐MBE) is demonstrated to be capable of growing epitaxial In2O3 at BEOL temperatures with unmatched crystal quality. The fullwidth at halfmaximum of the rocking curve is 0.015° and, thus, ≈5x narrower than any reports at any temperature to date and limited by the substrate quality. The key to achieving these results is the provision of an In2O beam by S‐MBE, which enables growth in adsorption control and is kinetically favorable. To benchmark this deposition method for TFTs, rudimentary devices were fabricated.

Superconducting films of α-Ta are promising candidates for the fabrication of advanced superconducting qubits. However, α-Ta films suffer from many growth-induced structural inadequacies that negatively affect their performance. We have therefore explored a new synthesis method for α-Ta films, which allows for the growth of these films with an unprecedented quality. Using this method, high quality α-Ta films are deposited at a comparably high substrate temperature of 1150 °C. They are single-phase α-Ta and have a single out-of-plane (110) orientation. They consist of grains ≥2 μm that have one of three possible in-plane orientations. As shown by scanning transmission electron microscopy and electron energy loss studies, the substrate–film interfaces are sharp with no observable intermixing. The obtained insights into the epitaxial growth of body-centered-cubic films on quasi-hexagonal substrates lay the basis for harnessing the high structural coherence of such films in various applications.

It has been suggested that Ba3In2O6might be a high-Tcsuperconductor. Experimental investigation of the properties of Ba3In2O6was long inhibited by its instability in air. Recently epitaxial Ba3In2O6with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6is determined to be 2.99 eV in-the (001) plane and 2.83 eV along thec-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. NeitherA- norB-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6was successfully grown epitaxially on an epitaxial SrRuO3bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6,leaving the answer to the initial question of whether Ba3In2O6is a high-Tcsuperconductor to be 'no' thus far.

Oxides have attracted enormous attention for both application-driven and fundamental solid-state research owing to their broad range of unusual and unique properties. Oxides play an important role in countless applications, e.g., as catalysts or functional materials in electronic devices. The ability to produce high-quality epitaxial films is often crucial for these purposes. Various approaches to oxide epitaxy have been evolving for many years, each of which has its own features and strengths. However, oxide epitaxy also poses numerous challenges, the main ones being (1) the difficulty of finding a universal, versatile, and clean way to transfer an element from a source onto a substrate and (2) the ability to control the phase formation in a growing film. For oxides, this is an especially relevant topic due to the high oxidization potentials needed to achieve many desired compounds, the high temperatures required for numerous oxide phases to form, and the high temperatures necessary to grow films in adsorption-controlled growth modes. We provide a non-exhaustive overview of the state-of-the-art of oxide epitaxy within the context of these challenges. We also examine exciting advances and recent trends to overcome those challenges. Concluding, we discuss the implications of ongoing developments and the future of oxide epitaxy. An emphasis is put on thermal laser epitaxy and CO2 laser heaters, which we deem especially promising.

β -Ga2O3 is actively touted as the next ultrawide bandgap material for power electronics. To fully utilize its high intrinsic critical electric field, development of high-quality robust large-barrier height junctions is essential. To this end, various high-work function metals, metal oxides, and hole-conducting oxides have been deposited on Ga2O3, primarily formed by sputter deposition. Unfortunately, reports to date indicate that measured barrier heights often deviate from the Schottky–Mott model as well as x-ray photoelectron spectroscopy (XPS) extractions of conduction band offsets, suggesting significant densities of electrically active defects at these junctions. We report Schottky diodes made from noble metal oxides, IrO2 and RuO2, deposited by ozone molecular beam epitaxy (ozone MBE) with barrier heights near 1.8 eV. These barriers show close agreement across extraction methods and robust to high surface electric fields upward of 6 MV/cm and 60 A/cm2 reverse current without degradation.

Sapphire is a technologically highly relevant material, but it poses many challenges when performing epitaxial thin-film deposition. We have identified and applied the conditions for adsorption-controlled homoepitaxial growth of c-plane sapphire. The films thus grown are atomically smooth, have a controlled termination, and are of outstanding crystallinity. Their chemical purity exceeds that of the substrates. The films exhibit exceptional optical properties, such as a single-crystal-like bandgap and a low density of F+ centers.

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 1016 to 1019 cm−3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature (Tsub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 1017 cm−3 reveal a room-temperature mobility of 124 cm2 V−1 s−1 that increases to 627 cm2 V−1 s−1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga2O3 films grown by S-MBE at growth rates of ∼1 µm/h.

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.

We observe a catalytic mechanism during the growth of III-O and IV-O materials by suboxide molecular-beam epitaxy ($S$-MBE). By supplying the molecular catalysts In$_2$O and SnO we increase the growth rates of Ga$_2$O$_3$ and In$_2$O$_3$. This catalytic action is explained by a metastable adlayer $A$, which increases the reaction probability of the reactants Ga$_2$O and In$_2$O with active atomic oxygen, leading to an increase of the growth rates of Ga$_2$O$_3$ and In$_2$O$_3$. We derive a model for the growth of binary III-O and IV-O materials by $S$-MBE and apply these findings to a generalized catalytic description for metal-oxide catalyzed epitaxy (MOCATAXY), applicable to elemental and molecular catalysts. We derive a mathematical description of $S$-MBE and MOCATAXY providing a computational framework to set growth parameters in previously inaccessible kinetic and thermodynamic growth regimes when using the aforementioned catalysis. Our results indicate MOCATAXY takes place with a suboxide catalyst rather than with an elemental catalyst. As a result of the growth regimes achieved, we demonstrate a Ga$_2$O$_3$/Al$_2$O$_3$ heterostructure with unrivaled crystalline quality, paving the way to the preparation of oxide device structures with unprecedented perfection.

Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1−xO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.

We demonstrate the epitaxial growth of the first two members, and the n=∞ member of the homologous Ruddlesden–Popper series of Ban+1InnO2.5n+1 of which the n=1 member was previously unknown. The films were grown by suboxide molecular-beam epitaxy where the indium is provided by a molecular beam of indium-suboxide [In2O (g)]. To facilitate ex situ characterization of the highly hygroscopic barium indate films, a capping layer of amorphous SiO2 was deposited prior to air exposure. The structural quality of the films was assessed by x-ray diffraction, reflective high-energy electron diffraction, and scanning transmission electron microscopy.

This paper introduces a growth method---suboxide molecular-beam epitaxy (S-MBE)---which enables the growth of Ga2O3 and related materials at growth rates exceeding 1 micrometer per hours with excellent crystallinity in an adsorptioncontrolled regime. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 micrometer per hour for Ga2O3--Al2O3 heterostructures with unprecedented crystalline quality and also at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular-beams of targeted suboxides with a kinetic model developed for the S-MBE of III-VI compounds to identify appropriate growth conditions. Using S-MBE we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4 micrometer. With the high growth rate of S-MBE we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide-range of oxides. S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices.

Lateral inhomogeneities in the formation of two-dimensional electron gases (2DEG) directly influence their electronic properties. Understanding their origin is an important factor for fundamental interpretations, as well as high quality devices. Here, we studied the local formation of the buried 2DEG at LaAlO3/SrTiO3 (LAO/STO) interfaces grown on STO (100) single crystals with partial TiO2 termination, utilizing in situ conductive atomic force microscopy (c-AFM) and scattering-type scanning near-field optical microscopy (s-SNOM). Using substrates with different degrees of chemical surface termination, we can link the resulting interface chemistry to an inhomogeneous 2DEG formation. In conductivity maps recorded by c-AFM, a significant lack of conductivity is observed at topographic features, indicative of a local SrO/AlO2 interface stacking order, while significant local conductivity can be probed in regions showing TiO2/LaO interface stacking order. These results could be corroborated by s-SNOM, showing a similar contrast distribution in the optical signal which can be linked to the local electronic properties of the material. The results are further complemented by low-temperature conductivity measurements, which show an increasing residual resistance at 5 K with increasing portion of insulating SrO-terminated areas. Therefore, we can correlate the macroscopic electrical behavior of our samples to their nanoscopic structure. Using proper parameters, 2DEG mapping can be carried out without any visible alteration of sample properties, proving c-AFM and s-SNOM to be viable and destruction-free techniques for the identification of local 2DEG formation. Furthermore, applying c-AFM and s-SNOM in this manner opens the exciting prospect to link macroscopic low-temperature transport to its nanoscopic origin.

We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A2Sn2O7 (A = La and Y) and find that it is possible to incorporate ∼25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry, allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by a shift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increase La2Sn2O7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good film quality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y2Sn2O7. An alternative n-type dopant, fluorine on oxygen, is shallow according to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La2Sn2O7 and Y2Sn2O7 films were determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively.

We present a study of the trade-off between the retention and variability of SrTiO3-based memristive devices. We identified the applied switching current and the device stoichiometry as main influence factors. We show that the SrO formation at the electrode interface, which has been revealed to improve the device retention significantly, is associated with an increased cycle-to-cycle and device-to-device variability. On the other hand, devices with homogeneous, Ti-terminated SrTiO3–Pt interfaces exhibit poor retention but the smallest variability. These results give valuable insights for the application of memristive SrTiO3 devices as non-volatile memory or in neural networks, where the control of variability is of key relevance.