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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. Graphic abstract

Rich electron-matter interactions fundamentally enable electron probe studies of materials such as scanning transmission electron microscopy (STEM). Inelastic interactions often result in structural modifications of the material, ultimately limiting the quality of electron probe measurements. However, atomistic mechanisms of inelastic-scattering-driven transformations are difficult to characterize. Here, we report direct visualization of radiolysis-driven restructuring of rutile TiO 2 under electron beam irradiation. Using annular dark field imaging and electron energy-loss spectroscopy signals, STEM probes revealed the progressive filling of atomically sharp nanometer-wide cracks with striking atomic resolution detail. STEM probes of varying beam energy and precisely controlled electron dose were found to constructively restructure rutile TiO 2 according to a quantified radiolytic mechanism. Based on direct experimental observation, a “two-step rolling” model of mobile octahedral building blocks enabling radiolysis-driven atomic migration is introduced. Such controlled electron beam-induced radiolytic restructuring can be used to engineer novel nanostructures atom-by-atom. Radiolysis is known for damaging crystals. Here, using STEM, researchers observed radiolysis-driven bond-breakage, atomic movements, & crystal restructuring in rutile TiO2, and proposed a “2-step rolling” model of building blocks. These results open possibilities for constructive use of radiolysis.

Incipient ferroelectricity bridges traditional dielectrics and true ferroelectrics, enabling advanced electronic and memory devices. Firstly, we report incipient ferroelectricity in freestanding SrTiO 3 nanomembranes integrated with monolayer MoS 2 to create multifunctional devices, demonstrating stable ferroelectric order at low temperatures for cryogenic memory devices. Our observation includes ultra-fast polarization switching (~10 ns), low switching voltage (<6 V), over 10 years of nonvolatile retention, 100,000 endurance cycles, and 32 conductance states (5-bit memory) in SrTiO 3 -gated MoS 2 transistors at 15 K and up to 100 K. Additionally, we exploit room-temperature weak polarization switching, a feature of incipient ferroelectricity, to construct a physical reservoir for pattern recognition. Our results showcase the potential of utilizing perovskite material properties enabled by advancements in freestanding film growth and heterogeneous integration, for diverse functional applications. Notably, the low 180 °C thermal budget for fabricating the 3D-SrTiO 3 /2D-MoS 2 device stack enables the integration of diverse materials into silicon complementary metal-oxide-semiconductor technology, addressing challenges in compute-in-memory and neuromorphic applications. Sen et al. report the stacking of a perovskite incipient ferroelectric nanomembrane with atomically thin 2D material for a back-end-of-line compatible ferroelectric-like field effect transistors, functioning as a cryogenic memory at 15 K and as an inference engine at room temperature.

Creating 1D or 2D extended defects in thin films that propagate throughout the film thickness enables engineering nanoscale materials with anisotropic properties governed by these defects. Performing defect engineering of thin films with location specificity facilitates new nanoscale device architectures that harness the unique properties of these anisotropic extended defects. Here we demonstrate that, by combining Ga focused ion-beam (FIB) exposure and subsequent heat treatment, it is possible to pattern nanoscale structural perturbations on the substrate surface that promote nucleation and propagation of extended defects in thin films epitaxially grown on these substrates. Using SrTiO 3 as a substrate for growing perovskite BaSnO 3 and SrSnO 3 thin films, we demonstrate engineering ultra-high densities of threading 1D dislocations and 2D Ruddlesden-Popper faults with nanometer-level location specificity limited only by the resolution of the patterning Ga ion-beam of the FIB. Given the versatility of this method, it can be applied to different substrates and films, serving as a flexible means of defect-driven material engineering. A method of nanometre-level patterning of high-density extended 1D and 2D defects in thin films of perovskite oxides is developed.

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 Nd x Sr 1 − x SnO 3 (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 × 10 18  cm −3 to 2.0 × 10 20  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 cm 2  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 cm 2  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. Semiconductor research is undergoing transformative changes thanks to the discovery and development of novel materials. The authors disclose the role of electron-phonon interaction and impurity scattering in a novel ultrawide bandgap perovskite oxide, paving the way for new applications in high-power electronics.

We report on terahertz characterization of La-doped BaSnO 3 (BSO) thin-films. BSO is a transparent complex oxide material, which has attracted substantial interest due to its large electrical conductivity and wide bandgap. The complex refractive index of these films is extracted in the 0.3 to 1.5 THz frequency range, which shows a metal-like response across this broad frequency window. The large optical conductivity found in these films at terahertz wavelengths makes this material an interesting platform for developing electromagnetic structures having a strong response at terahertz wavelengths, i.e. terahertz-functional, while being transparent at visible and near-IR wavelengths. As an example of such application, we demonstrate a visible-transparent terahertz polarizer.

Germanium-based oxides such as rutile GeO 2 are garnering attention owing to their wide band gaps and the prospects of ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP), a metal-organic chemical precursor, as a source of germanium for the demonstration of hybrid molecular beam epitaxy for germanium-containing compounds. We use Sn 1- x Ge x O 2 and SrSn 1- x Ge x O 3 as model systems to demonstrate our synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy confirms the successful growth of epitaxial rutile Sn 1- x Ge x O 2 on TiO 2 (001) substrates up to x  = 0.54 and coherent perovskite SrSn 1- x Ge x O 3 on GdScO 3 (110) substrates up to x  = 0.16. Characterization and first-principles calculations corroborate that germanium occupies the tin site, as opposed to the strontium site. These findings confirm the viability of the GTIP precursor for the growth of germanium-containing oxides by hybrid molecular beam epitaxy, thus providing a promising route to high-quality perovskite germanate films. Germanium-based oxides are wide bandgap semiconductors with the prospects of ambipolar doping. Here, a hybrid molecular beam epitaxy is demonstrated for the growth of both rutile Sn 1- x Ge x O 2 and perovskite SrSn 1- x Ge x O 3 films.

Wide bandgap perovskite oxides with high room temperature conductivities and structural compatibility with a diverse family of organic/inorganic perovskite materials are of significant interest as transparent conductors and as active components in power electronics. Such materials must also possess high room temperature mobility to minimize power consumption and to enable high-frequency applications. Here, we report n-type BaSnO 3 films grown using hybrid molecular beam epitaxy with room temperature conductivity exceeding 10 4  S cm −1 . Significantly, these films show room temperature mobilities up to 120 cm 2  V −1  s −1 even at carrier concentrations above 3 × 10 20  cm −3 together with a wide bandgap (3 eV). We examine the mobility-limiting scattering mechanisms by calculating temperature-dependent mobility, and Seebeck coefficient using the Boltzmann transport framework and ab-initio calculations. These results place perovskite oxide semiconductors for the first time on par with the highly successful III–N system, thereby bringing all-transparent, high-power oxide electronics operating at room temperature a step closer to reality. With impressive electronic transport properties, wide bandgap perovskite oxides are promising transparent conductors. Prakash et al . report n-type BaSnO 3 films with room temperature conductivity exceeding 10 4 S cm −1 and investigate factors limiting carrier mobility.