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Hysteretic magnetoresistance (MR) is often used as a signature of ferromagnetism in conducting oxide films and heterostructures. Here, magnetotransport is investigated in a nonmagnetic La-doped SrSnO3 film. A 12 nm La:SrSnO3/2 nm SrSnO3/GdScO3 (110) film with insulating behavior exhibited a robust hysteresis loop in the MR at T < 5 K accompanied by an anomaly at ∼±3 T at T < 2.5 K. Furthermore, MR with the field in-plane yielded a value exceeding 100% at 1.8 K. Using detailed temperature-, angle- and magnetic field-dependent resistance measurements, we illustrate the origin of hysteresis is not due to magnetism in the film but rather is associated with the magnetocaloric effect of the substrate. Given GdScO3 and similar substrates are commonly used, this work highlights the importance of thermal coupling to processes in the substrates which must be carefully accounted for in the data interpretation for heterostructures utilizing these substrates.

The discovery and development of ultra-wide bandgap (UWBG) semiconductors is crucial to accelerate the adoption of renewable power sources. This necessitates an UWBG semiconductor that exhibits robust doping with high carrier mobility over a wide range of carrier concentrations. Here we demonstrate that epitaxial thin films of the perovskite oxide NdxSr1−xSnO3 (SSO) do exactly this. Nd is used as a donor to successfully modulate the carrier concentration over nearly two orders of magnitude, from 3.7 × 1018 cm−3 to 2.0 × 1020 cm−3. Despite being grown on lattice-mismatched substrates and thus having relatively high structural disorder, SSO films exhibited the highest room-temperature mobility, ~70 cm2 V−1 s−1, among all known UWBG semiconductors in the range of carrier concentrations studied. The phonon-limited mobility is calculated from first principles and supplemented with a model to treat ionized impurity and Kondo scattering. This produces excellent agreement with experiment over a wide range of temperatures and carrier concentrations, and predicts the room-temperature phonon-limited mobility to be 76–99 cm2 V−1 s−1 depending on carrier concentration. This work establishes a perovskite oxide as an emerging UWBG semiconductor candidate with potential for applications in power electronics.

The epitaxial growth of functional oxides using a substrate with a graphene layer is a highly desirable method for improving structural quality and obtaining freestanding epitaxial nanomembranes for scientific study, applications, and economical reuse of substrates. However, the aggressive oxidizing conditions typically used in growing epitaxial oxides can damage graphene. Here, we demonstrate the successful use of hybrid molecular beam epitaxy for SrTiO

The epitaxial growth of functional oxides using a substrate with a graphene layer is a highly desirable method for improving structural quality and obtaining freestanding epitaxial nanomembranes for scientific study, applications, and economical reuse of substrates. However, the aggressive oxidizing conditions typically used in growing epitaxial oxides can damage graphene. Here, we demonstrate the successful use of hybrid molecular beam epitaxy for SrTiO 3 growth that does not require an independent oxygen source, thus avoiding graphene damage. This approach produces epitaxial films with self-regulating cation stoichiometry. Furthermore, the film (46-nm-thick SrTiO 3 ) can be exfoliated and transferred to foreign substrates. These results open the door to future studies of previously unattainable freestanding oxide nanomembranes grown in an adsorption-controlled manner by hybrid molecular beam epitaxy. This approach has potentially important implications for the commercial application of perovskite oxides in flexible electronics and as a dielectric in van der Waals thin-film electronics.

Much progress has been made in the area of wide bandgap semiconductors for applications in electronics and optoelectronics such as displays, power electronics, and solar cells. New materials are being sought after and considerable attention has been given to complex oxides, specifically those with the perovskite crystal structure. Molecular-beam epitaxy (MBE) has come to the forefront of this field for the thin film synthesis of these materials in a high-quality manner and achieves some of their best figures of merit. Here, we discuss the development of MBE from its beginnings as a method for III–V semiconductor growth to today for the growth of many contenders for next-generation electronics. Comparing MBE with other physical vapor deposition techniques, we identify the advantages of MBE as well as many of the challenges that still must be overcome should this technique be applied to other up-and-coming wide bandgap complex oxide semiconductors.

Distinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO3 with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.

SignificanceSemiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO

Significance Semiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO 3 films with record high electron mobilities. We show that the junction between doped and undoped SrTiO 3 required to make dielectric constant measurements masks the dielectric properties of the undoped film. Through modeling, we extract the latter and show that it is much higher than previously measured.

The study of subtle effects on transport in semiconductors requires high-quality epitaxial structures with low defect density. Using hybrid molecular beam epitaxy (MBE), SrTiO 3 films with a low-temperature mobility exceeding 42,000 cm 2 V −1 s −1 at a low carrier density of 3 × 10 17 cm −3 were achieved. A sudden and sharp decrease in residual resistivity accompanied by an enhancement in the superconducting transition temperature were observed across the second Lifshitz transition where the third band becomes occupied, revealing dominant intraband scattering. These films further revealed an anomalous behavior in the Hall carrier density as a consequence of the antiferrodistortive (AFD) transition and the temperature dependence of the Hall scattering factor. Using hybrid MBE growth, phenomenological modeling, temperature-dependent transport measurements, and scanning superconducting quantum interference device imaging, we provide critical insights into the important role of inter- versus intraband scattering and of AFD domain walls on normal-state and superconducting properties of SrTiO 3 .

SrSnO3 (SSO) is an emerging ultrawide band gap (UWBG) semiconductor with potential in high-power applications. In-plane compressive strain was recently shown to stabilize the high-temperature tetra...

Hybrid molecular beam epitaxy (MBE) growth of Sn-modified BaTiO3 films was realized with varying domain structures and crystal symmetries across the entire composition space. Macroscopic and microscopic structures and the crystal symmetry of these thin films were determined using a combination of optical second harmonic generation (SHG) polarimetry and scanning transmission electron microscopy (STEM). SHG polarimetry revealed a variation in the global crystal symmetry of the films from tetragonal ( P4 mm) to cubic [Formula: see text] across the composition range, x = 0 to 1 in BaTi1− xSn xO3 (BTSO). STEM imaging shows that the long-range polar order observed when the Sn content is low ( x = 0.09) transformed to a short-range polar order as the Sn content increased ( x = 0.48). Consistent with atomic displacement measurements from STEM, the largest polarization was obtained at the lowest Sn content of x = 0.09 in Sn-modified BaTiO3 as determined by SHG. These results agree with recent bulk ceramic reports and further identify this material system as a potential replacement for Pb-containing relaxor-based thin film devices.

A SrSnO3 high-frequency field-effect transistor (FET) is demonstrated. The device structure consists of a recessed Schottky-gate FET with a heavily doped cap layer. DC measurements on devices with 0.5- $\mu \text{m}$ gate length and 4- $\mu \text{m}$ source/drain spacing show a maximum drain current of 53 mA/mm and a maximum transconductance of 43.2 mS/mm. Radio frequency (RF) characterization reveals a cut-off frequency, ${f} _{\text {T}}$ , of 1.31 GHz (0.97 GHz) and a maximum oscillation frequency, ${f} _{\text {max}}$ , of 3.25 (3.25) GHz, after (before) de-embedding. These results represent an important advancement in developing perovskite materials for RF applications.

Fabrication of gate-recessed SrSnO3 (SSO) metal-semiconductor field-effect transistors (MESFETs) with Ni Schottky gates is reported on bi-layer epitaxial SSO films with a thin heavily-doped cap layer. Devices with 0.5- $\mu \text{m}$ gate length showed enhancement-mode behavior with a saturation drain current, ${I}_{\text {DSAT}}$ , of 33 mA/mm and peak transconductance, ${g}_{\text {m,max}}$ , of 65 mS/mm. The ${g}_{\text {m,max}}$ value is a roughly $2\times $ improvement over control devices fabricated on single-layer films. Gate-recessed SSO MESFETs with Pt Schottky gates were also explored on the bi-layer films. Devices with 1- $\mu \text{m}$ gate length displayed ${I}_{\text {DSAT}} =133$ mA/mm and $g_{\text {m,max}} =73$ mS/mm, after thermal annealing.

The discovery of alternative methods of producing electrical energy that avoid the generation of greenhouse gases and do not contribute to global warming is a compelling problem of our time. Ubiquitous, but often highly distributed, sources of energy on earth exist in the small-temperature-difference regime, 10–250°C. In this review, we discuss a family of methods that can potentially recover this energy based on the use of first-order phase transformations in crystalline materials combined with ferromagnetism or ferroelectricity. The development of this technology will require a better understanding of these phase transformations, especially ferroelectric/ferromagnetic properties, hysteresis, and reversibility, as well as strategies for discovering improved materials.

Advanced functional materials 2113023 (2022). doi:10.1002/adfm.202113023, Published by Wiley-VCH, Weinheim

Through a combination of thin film growth, hard X-ray photoelectron spectroscopy (HAXPES), scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS), magneto-transport measurements, and transport modeling, we report on the demonstration of modulation-doping of BaSnO3 (BSO) using a wider bandgap La-doped SrSnO3 (LSSO) layer. Hard X-ray photoelectron spectroscopy (HAXPES) revealed a valence band offset of 0.71 +/- 0.02 eV between LSSO and BSO resulting in a favorable conduction band offset for remote doping of BSO using LSSO. Nonlinear Hall effect of LSSO/BSO heterostructure confirmed two-channel conduction owing to electron transfer from LSSO to BSO and remained in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schr\"odinger equations. Angle-dependent HAXPES measurements revealed a spatial distribution of electrons over 2-3 unit cells in BSO. These results bring perovskite oxides a step closer to room-temperature oxide electronics by establishing modulation-doping approaches in non-SrTiO3-based oxide heterostructure.

Hysteretic magnetoresistance (MR) is often used as a signature of ferromagnetism in conducting oxide films and heterostructures. Here, magnetotransport is investigated in a nonmagnetic La-doped SrSnO

Advances in physical vapor deposition techniques have led to a myriad of quantum materials and technological breakthroughs, affecting all areas of nanoscience and nanotechnology which rely on the innovation in synthesis. Despite this, one area that remains challenging is the synthesis of atomically precise complex metal oxide thin films and heterostructures containing “stubborn” elements that are not only nontrivial to evaporate/sublimate but also hard to oxidize. Here, we report a simple yet atomically controlled synthesis approach that bridges this gap. Using platinum and ruthenium as examples, we show that both the low vapor pressure and the difficulty in oxidizing a “stubborn” element can be addressed by using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a preoxidized state along with excellent thermal and air stability. We demonstrate the synthesis of high-quality single crystalline, epitaxial Pt, and RuO(2) films, resulting in a record high residual resistivity ratio (=27) in Pt films and low residual resistivity, ∼6 μΩ·cm, in RuO(2) films. We further demonstrate, using SrRuO(3) as an example, the viability of this approach for more complex materials with the same ease and control that has been largely responsible for the success of the molecular beam epitaxy of III-V semiconductors. Our approach is a major step forward in the synthesis science of “stubborn” materials, which have been of significant interest to the materials science and the condensed matter physics community.

This work reports the quantification of rise in channel temperature due to self-heating in two-terminal SrSnO3 thin film devices under electrical bias. Using pulsed current–voltage (I–V) measurements, thermal resistances of the thin films were determined by extracting the relationship between the channel temperature and the dissipated power. For a 26-nm-thick n-doped SrSnO3 channel with an area of 200 μm2, a thermal resistance of 260.1 ± 24.5 K mm/W was obtained. For a modest dissipated power of 0.5 W/mm, the channel temperature rose to ∼176 °C, a value which increases further at higher power levels. Electro-thermal simulations were performed which showed close agreement between the simulated and experimental I–V characteristics both in the absence and presence of self-heating. The work presented is critical for the development of perovskite-based high-power electronic devices.

We investigate the ion gel gating of wide bandgap oxide, La-doped SrSnO3 films grown using radical-based molecular beam epitaxy. An applied positive bias resulted in a reversible electrostatic control of sheet resistance over 3 orders of magnitude at low temperature driving sample from Mott variable range hopping to a weakly localized transport. Analysis of low temperature transport behavior revealed electron-electron interaction and weak localization effects to be the dominant scattering mechanisms. A large voltage window (-4 V ≤ Vg ≤ +4 V) was obtained for reversible electrostatic doping of SrSnO3 films showing robustness of stannate with regards to redox chemistry with electrolyte gating irrespective of the bias type.

High-speed electronics require epitaxial films with exceptionally high carrier mobility at room temperature (RT). Alkaline-earth stannates with high RT mobility show outstanding prospects for oxide electronics operating at ambient temperatures. However, despite significant progress over the last few years, mobility in stannate films has been limited by dislocations because of the inability to grow fully coherent films. Here, we demonstrate the growth of coherent, strain-engineered phases of epitaxial SrSnO3 (SSO) films using a radical-based molecular beam epitaxy approach. Compressive strain stabilized the high-symmetry tetragonal phase of SSO at RT, which, in bulk, exists only at temperatures between 1062 and 1295 K. We achieved a mobility enhancement of over 300% in doped films compared with the low-temperature orthorhombic polymorph. Using comprehensive temperature-dependent synchrotron-based X-ray measurements, electronic transport, and first principles calculations, crystal and electronic structures ...

The wide band gap semiconducting perovskite $\mathrm{BaSn}{\mathrm{O}}_{3}$ is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of ${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$ films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of $96\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ is achieved in vacuum-annealed $\mathrm{GdSc}{\mathrm{O}}_{3}$(110)/$\mathrm{BaSn}{\mathrm{O}}_{3}$(120 nm)/${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$(200 nm) heterostructures, as well as $92\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ on unbuffered substrates and $87\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped $\mathrm{BaSn}{\mathrm{O}}_{3}$ films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

A metallic line defect is found in wide-bandgap perovskite oxide thin films., A line defect with metallic characteristics has been found in optically transparent BaSnO3 perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level–crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO3 films.

As a prototypical Mott insulator with ferromagnetic ordering, ${\mathrm{YTiO}}_{3}$ (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterizations of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed. DC transport and Seebeck measurements on thin films and bulk single crystals identify $p$-type Arrhenius transport behavior with an activation energy of $\ensuremath{\sim}0.17\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory calculations. Hard x-ray photoelectron spectroscopy measurements show the lower Hubbard band at 1.1 eV below the Fermi level, whereas a Mott-Hubbard band gap of $\ensuremath{\sim}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is determined from photoluminescence measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.

We investigate the surface electronic structure of SrTiO3 (STO) films grown by a hybrid molecular beam epitaxy that are both stoichiometric and nonstoichiometric by means of x-ray photoelectron spectroscopy and electron energy loss spectroscopy. Increasing the fraction of the surface that is terminated with an SrO layer is correlated with a decrease in the chemical potential whereby the valence band maximum moves closer to the Fermi level, but without a significant change in the bandgap. Inasmuch as SrO-terminated STO (001) has previously been shown to act as an electron scavenger in which carriers from the bulk are trapped, we argue that the high fraction of SrO in the terminal layer is what lowers the chemical potential in Sr-rich STO. Our experimental results provide important insights into various physical phenomena that can occur on STO (001) surfaces and their effect on bulk electronic properties.

Quantum corrections to electrical resistance can serve as sensitive probes of the magnetic landscape of a material. For example, interference between time-reversed electron paths gives rise to weak localization effects, which can provide information about the coupling between spins and orbital motion, while the Kondo effect is sensitive to the presence of spin impurities. Here we use low-temperature magnetotransport measurements to reveal a transition from weak antilocalization (WAL) to Kondo scattering in the quasi-two-dimensional electron gas formed at the interface between SrTiO$_3$ and the Mott insulator NdTiO$_3$. This transition occurs as the thickness of the NdTiO$_3$ layer is increased. Analysis of the Kondo scattering and WAL points to the presence of atomic-scale magnetic impurities coexisting with extended magnetic regions that affect transport via a strong magnetic exchange interaction. This leads to distinct magnetoresistance behaviors that can serve as a sensitive probe of magnetic properties in two dimensions., 5 pages, 3 figures

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 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 SrTiO$_{3}$ growths, heteroepitaxial SrTiO$_{3}$ grown on scandate substrates, BaSnO$_{3}$ films grown on SrTiO$_{3}$ substrates, and LaNiO$_{3}$ films grown on LaAlO$_{3}$ substrates. 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 over growth processes and film quality., 12 pages, 12 figures

We have explored the effect of magnetic rare-earth dopants substitutionally incorporated on the Ba sites of ${\mathrm{BaSnO}}_{3}$ in terms of electronic transport, magnetism, and optical properties. We show that for ${\mathrm{Ba}}_{0.92}{R}_{0.08}{\mathrm{SnO}}_{3}$ thin films (where $R=\text{La,}\phantom{\rule{4.pt}{0ex}}\text{Pr,}\phantom{\rule{4.pt}{0ex}}\text{Nd,}\phantom{\rule{4.pt}{0ex}}\text{Gd}$), there is a linear increase of mobility with carrier concentration across all doping schemes. La-doped films have the highest mobilities, followed by Pr- and Nd-doped films. Gd-doped samples have the largest ionic size mismatch with the Ba site and correspondingly the lowest carrier concentrations and electron mobilities. However, crystallinity does not appear to be a strong predictor of transport phenomena; our results suggest that point defects more than grain boundaries are key ingredients in tuning the conduction of ${\mathrm{BaSnO}}_{3}$ films grown by pulsed laser deposition. Pronounced, nonhysteretic x-ray magnetic dichroism signals are observed for Pr-, Nd-, and Gd-doped samples, indicating paramagnetism. Finally, we probe the optical constants for each of the ${\mathrm{BaSnO}}_{3}$ doping schemes and note that there is little change in the transmittance across all samples. Together these results shed light on conduction mechanisms in ${\mathrm{BaSnO}}_{3}$ doped with rare-earth cations.

By combining temperature-dependent resistivity and Hall effect measurements, we investigate donor state energy in Si-doped \b{eta}-Ga2O3 films grown using metal-organic vapor phase epitaxy (MOVPE). High magnetic field Hall effect measurements (H = +/-90 kOe) showed non-linear Hall resistance for T < 150 K revealing two-band conduction. Further analyses revealed carrier freeze-out characteristics in both bands yielding donor state energies of ~ 33.7 and ~ 45.6 meV. The former is consistent with the donor energy of Si in \b{eta}-Ga2O3 whereas the latter suggests a residual donor state, likely associated with a DX center. This study provides a critical insight into the impurity band conduction and the defect energy states in \b{eta}-Ga2O3 using high-field magnetotransport measurements., 14 pages