Epitaxial Thin Films Research Articles

Molecular H2 as the Reducing Agent in Low-Temperature Oxide Reduction Using Calcium Hydride.

Low-temperature synthesis is crucial for advancing sustainable manufacturing and accessing novel metastable phases. Metal hydrides have shown great potential in facilitating the reduction of oxides at low temperatures, yet the underlying mechanism─whether driven by H-, H2, or atomic H─remains unclear. In this study, we employ in situ electrical transport measurements and first-principles calculations to investigate the CaH2-driven reduction kinetics in epitaxial α-Fe2O3 thin films. Intriguingly, samples in direct contact with or separated from CaH2 powders exhibit similar apparent activation energies for H2 reduction, although direct contact significantly increases the reduction rate. These findings indicate that molecular H2 is the dominant reducing species in the low-temperature reduction of oxides using CaH2, with a key aspect of the hydrides' superior reducing power attributed to their ability to eliminate residual moisture. This work underscores the critical role of moisture control in enabling effective low-temperature oxide reduction for advanced material synthesis.

Highly Sensitive Solar‐Blind Ultraviolet Photodetectors and 64 × 64 Photodetector Arrays Realized by Molecule Beam Epitaxy Grown Ga2O3 Epitaxial Thin Films

AbstractGallium oxide (Ga2O3) emerges as the most promising semiconductor for next‐generation solar‐blind UV photodetectors (SBPDs) because of its ultra‐wide bandgap of 4.85 eV. However, so far Ga2O3 based SBPDs still suffer from problems such as high dark current and slow response speed. Herein, the fabrication of metal‐semiconductor‐metal (MSM) structures Ga2O3 SBPDs with high performance using high‐quality Ga2O3 epitaxial thin films grown by molecule beam epitaxy (MBE) is reported. The SBPDs are highly sensitive to solar‐blind UV spectrum and achieve a very low dark current of 273 fA at 10 V bias, a high specific detectivity of 1.6 × 1016 Jones with a responsivity of 167 A W−1, and a remarkedly rapid response speed with a decay time of 15 ms. The specific detectivity, dark current, and response speed of the SBPD achieve in this work are among the highest levels in the literature. The enhanced device performance is attributed to high crystalline quality of the film with reduced density of defects and fast photocarrier dynamics. Furthermore, a 64 × 64 photodetector array with high uniformity is fabricated on a 2‐inch Ga2O3 epitaxial wafer, achieving the highest pixel count for Ga2O3 photodetector array to date. This study paves the way for the development of fast‐speed Ga2O3 SBPDs with high sensitivity and large‐scale photodetector arrays.

Highly Thermal-Conductive Cubic Boron Arsenide: Single-Crystal Growth, Properties, and Future Thin-Film Epitaxy.

Heat dissipation has become a critical challenge in modern electronics, driving the need for a revolution in thermal management strategies beyond traditional packaging materials, thermal interface materials, and heat sinks. Cubic boron arsenide (c-BAs) offers a promising solution, thanks to its combination of high thermal conductivity and high ambipolar mobility, making it highly suitable for applications in both electronic devices and thermal management. However, challenges remain, particularly in the large-scale synthesis of a high-quality material and the tuning of its physical properties. This Perspective reviews key research on c-BAs and discusses the future potential of van der Waals (vdW) epitaxy and remote epitaxy for preparing high-quality c-BAs thin-films. Based on superlattice area mismatch calculations, we predict some potential substrates for these epitaxy techniques. Three important design considerations for future vdW or remote epitaxy of c-BAs thin-films are identified: superlattice matching at the heterointerface, the kinetics of B and As adatoms, and the surface modification of vdW or vdW/3D substrates.

Rocksalt-type heavy rare earth monoxides TbO, DyO, and ErO exhibiting metallic electronic states and ferromagnetism.

Solid-phase rare earth monoxides have been recently synthesized via thin film epitaxy. However, it has been difficult to synthesize heavy rare earth monoxides owing to their severe chemical instability. In this study, rocksalt-type heavy rare earth monoxides REOs (RE = Tb, Dy, Er) were synthesized for the first time, as single-phase epitaxial thin films. The REOs were characterized by hard X-ray photoemission spectroscopy, X-ray absorption spectroscopy, resonant photoemission spectroscopy, and magnetization measurements. These REOs showed metallic electronic states with the almost localized 4f states, indicating the 4fn5d1 electronic configurations of Tb, Dy, and Er ions. X-ray magnetic circular dichroism measurements of TbO evidenced the magnetic ordering of the 4f spins below the Curie temperature (TC). The TC was evaluated to be 233 K for TbO, 142 K for DyO, and 88 K for ErO, where the TC decreased with the 4f electron number, approximately proportional to the de Gennes factor.

Influence of 100 MeV Au ion irradiation induced local structure modifications on magnetic properties of epitaxial PrVO3 thin films

Influence of 100 MeV Au ion irradiation induced local structure modifications on magnetic properties of epitaxial PrVO3 thin films

Thickness effect on the evolution of superconducting properties in FeSe0.4Te0.6 thin films

Abstract A series of high-quality FeSe0.4Te0.6 epitaxial thin films with different thicknesses were fabricated on CaF2 substrates by pulsed laser deposition. With increasing the film thickness, the superconducting transition temperature is enhanced rapidly but deteriorates gradually in over thick films. The superconducting transition occurs in all films including the 5 nm one, and Tc above 21 K persists in the films with thicknesses from 45 to 180 nm. We find that Tc and the c-axis lattice constant show a nearly linear correlation, suggesting a strong interdependence between Tc and the lattice strain characteristics. The upper critical field, anisotropy and effective pinning energy are studied in detail for the 15, 60, 90 and 180 nm films. These observations provide key insights in understanding the thickness effect on the superconductivity of the FeSe0.4Te0.6 films.

Ferroelectric Domain Wall Warp Memristor.

Domain walls are quasi-one-dimensional topological defects in ferroic materials, which can harbor emergent functionalities. In the case of ferroelectric domain wall (FEDW) devices, an exciting frontier has emerged: memristor-based information storage and processing approaches. Memristor solid-state FEDW devices presented thus far, however predominantly utilize a complex network of domain walls to achieve the desired regulation of density and charge state. Here, using epitaxial bismuth ferrite thin films as a prototype ferroelectric and advanced scanning probe microscopy and stroboscopic methods, we demonstrate controlled single-wall memristive behavior through deterministic electric field-driven conformal changes. Our design exploits memristive functionality through surface pinning of topological domain walls. That is, although the FEDW is constricted by the surface, it is free to twist in the thickness direction. The resulting vertical variation of FEDW morphology creates a complex interplay between surface-induced pinning and field-induced wall bending. This gives rise to metastable electronic transitions, and hence memristive attributes. Moreover, being a single FEDW memristor, once injected there is no need for repeated injection or erasure. Microscopic insight into device operation from phase field modeling indicates controllable warping of the wall, causing memristive conductivity changes. Our results reaffirm the promise of FEDWS for brain-inspired neuromorphic and in-memory computing applications based on integrated ferroelectric devices.

GaN remote epitaxy on a pristine graphene buffer layer via controlled graphitization of SiC

Freestanding semiconductor membranes hold significant potential for heterogeneous integration technology and flexible electronics. Remote epitaxy, which leverages electrostatic interactions between epilayers and substrates through two-dimensional (2D) materials such as graphene, offers a promising solution for fabricating freestanding single-crystal membranes. Although the thinness, uniformity, and cleanness of 2D materials need to be meticulously controlled to enable the remote epitaxy of high-quality thin films, attaining such ideal growth templates has been challenging thus far. In this study, we demonstrate a controlled graphitization method to form a pristine graphene buffer layer (GBL) directly on SiC substrates and utilize this GBL template for GaN remote epitaxy. The quasi-two-dimensional GBL layer obtained by the method is completely free of damage or contamination, facilitating strong epitaxial interaction between the GaN epilayer and the SiC substrate. Furthermore, we reveal that a two-step growth of GaN on this GBL template enables the formation of single-crystal GaN epilayers and their exfoliation. Thus, this study represents an important step toward developing high-quality, freestanding semiconductor membranes.

Quantum spin-liquid in Ba3CuSb2O9 epitaxial thin films

Hexagonal perovskite materials are emerging quantum spin liquid (QSL) systems providing a fertile ground to realize novel quantum phenomena. The epitaxially grown thin films of such materials offer a compelling approach to utilize exotic quantum phases for device applications with better control over the structure. We fabricate the intriguing QSL triple perovskite Ba3CuSb2O9epitaxially onto a MgO (100) substrate by pulsed laser deposition technique as well as in bulk form for comparison. The presence of only (00l) parallel planes of Ba3CuSb2O9in x-ray diffraction validates the epitaxial growth of the thin film. Temperature-dependent magnetization of thin film reveals no magnetic ordering down to 400 mK, with a large antiferromagnetic Curie-Weiss temperature (θCW≈-11.68 K). This indicates strong magnetic frustration and QSL behaviour, similar to bulk Ba3CuSb2O9. The presence of magnetic correlations at low temperature (in the quantum spin liquid state) is further confirmed by analysing the low temperature magnetic isotherms. These experimental findings underscore the potential of this quantum material for its use in quantum technologies.

High-Temperature Diffusion Enabled Epitaxy of the Ti-O System.

High temperatures promote kinetic processes that can drive crystal synthesis toward ideal thermodynamic conditions, thereby realizing samples of superior quality. While accessing very high temperatures in thin-film epitaxy is becoming increasingly accessible through laser-based heating methods, demonstrations of such utility are still emerging. The study realizes a novel self-regulated growth mode in the Ti-O system by relying on thermally activated diffusion of oxygen from an oxide substrate. The controlled diffusion enables oxidation state selectivity of single-phase films with superior crystallinity to conventional approaches as evidenced by structural and electronic measurements. The diffusion-enabled epitaxy is potentially of wide use in the growth of transition metal oxides, opening up new opportunities for ultra-high purity epitaxial platforms based on d-orbitalsystems.

Study on Influence of AC Poling on Bulk Photovoltaic Effect in Pb(Mg1/3Nb2/3)O3‐PbTiO3 Single Crystals

AbstractThe bulk photovoltaic effect (BPVE) provides a theory of surpassing the Schockley–Queisser limit. However, improving the BPVE efficiency to a level comparable to that of the semiconductor‐based efficiencies is challenging in practice due to conflicting material design requirements. Previous works have shown that a stacked domain structure in a rhombohedral BiFeO3 ferroelectric epitaxial thin film is able to satisfy the criteria of both a small mean free path for an optimized photocurrent and a large distance between electrodes for an optimized photovoltage. Nevertheless, this hypothesis has remained difficult to verify with other materials due to the complication of domain wall manipulation. This work takes advantage of the recent advances in controlling the domain structure in rhombohedral Pb(Mg1/3Nb2/3)O3‐PbTiO3 (PMN‐PT) single crystals via AC poling. By comparing the photovoltage and photocurrent values after domain manipulations in four types of commercial PMN‐PT samples, this work validates the hypothesis of the stacked domain structure enhancing the BPVE by witnessing a simultaneous increase of over 35% for both the open‐circuit voltage and short‐circuit current from DC to the AC‐poled states under a 405 nm laser illumination. This result paves the way for further improving the BPVE efficiency in ferroelectrics.

Transfer of Millimeter‐Scale Strained Multiferroic Epitaxial Thin Films on Rigid Substrates via an Epoxy Method Producing Magnetic Property Enhancement

AbstractThe demonstration of epitaxial thin film transfer has enormous potential for thin film devices free from the traditional substrate epitaxy limitations. However, large‐area continuous film transfer remains a challenge for the commonly reported polymer‐based transfer methods due to bending and cracking during transfer, especially for highly strained epitaxial thin films. In this work, a new epoxy‐based, rigid transfer method is used to transfer films from an SrTiO3 (STO) growth substrate onto various new substrates, including those that will typically pose significant problems for epitaxy. An epitaxial multiferroic Bi3Fe2Mn2Ox (BFMO) layered supercell (LSC) material is selected as the thin film for this demonstration. The results of surface and structure studies show an order of magnitude increase in the continuous area of transferred films when compared to previous transfer methods. The magnetic properties of the BFMO LSC films are shown to be enhanced by the release of strain in this method, and ferromagnetic resonance is found with an exceptionally low Gilbert damping coefficient. The large‐area transfer of this highly strained complex oxide BFMO thin film presents enormous potential for the integration of many other multifunctional oxides onto new substrates for future magnetic sensors and memory devices.

Tuning Self-Polarization of Epitaxial BiFeO3 Thin Films through Interface Effects.

Interface effects and strain engineering have emerged as critical strategies for modulating polarization and internal electric fields in ferroelectric materials, playing a vital role in exploring coupling mechanisms and developing ferroelectric diode devices. In this study, we selected BiFeO3 as a representative ferroelectric material and utilized interface engineering to control its polarization. By precisely manipulating the atomic stacking sequence at the interface, we influenced the electrostatic potential step across the interface, resulting in a bias voltage in the ferroelectric hysteresis loops that defined the ferroelectric state. The introduction of strain and strain gradients through a lattice mismatch between the film and substrate generated a flexoelectric field of approximately 3 MV/m, significantly impacting the internal electric field. Additionally, we successfully modified the Schottky barrier height within BiFeO3 films through the synergy and competition between interfacial and flexoelectric effects. This work expands the potential applications of thin-film flexoelectricity in Schottky diodes, sensors, and memory devices.

Effect of growth temperature on crystalline quality of epitaxial MnSnO3 thin films

Epitaxial single-crystal MnSnO3 thin films were deposited on single-crystal Al2O3 substrates using pulsed laser deposition (PLD) technology, and an analysis was conducted on the impact of the growth temperature on the crystalline quality of the films. The test results show that the growth of MnSnO3 thin films at 900 °C results in sharp diffraction peaks with high intensity in the c-axis direction, better crystalline quality (FWHM of XRD 2θ peak: 0.24°), less roughness (RSM: 0.67 nm) and wider optical band gap (Eg = 2.91 eV) compared with the grown samples at other temperatures. The MnSnO3 thin film deposited at 900 °C exhibits strong photoluminescence at 341.1 and 423.1 nm, as well as high ferroelectric polarization of ∼40 μC/cm2. The fabrication of epitaxial MnSnO3 thin films opens up a new avenue for further research into their ferroelectric photovoltaic properties.

Effect of Growth Induced Residual Stress in Epitaxial AlN thin film on SiC Substrate for MEMS Applications

Effect of Growth Induced Residual Stress in Epitaxial AlN thin film on SiC Substrate for MEMS Applications

Growth and magnetic properties of epitaxial thin films of the [formula omitted]-MAX phase (Mn[formula omitted]Sc[formula omitted])[formula omitted]GaC

i-MAX phases are quaternary variants of the nanolaminated MAX phases, with additional in-plane ordering of the M atoms. The combination of in-plane and out-of-plane ordering potentially gives rise to complex magnetic behaviour. The i-MAX phase (Mn2/3Sc1/3)2GaC has been synthesized in epitaxial thin film form on three different substrates, SiC-4H(001), MgO(111) and Al2O3(0001), by magnetron sputtering using elemental targets. Structural characterization by x-ray scattering and scanning transmission electron microscopy confirms the phase on all three substrates, although the highest crystal quality is obtained on SiC-4H(001). High-resolution images reveal the distinctive i-MAX structure, which is orthorhombic of space group Cmcm. Magnetic characterization reveals that the ground state is most likely antiferromagnetic. This confirms previous theoretical calculations which predicted an antiferromagnetic ground state and establishes the (Mn2/3Sc1/3)2GaC i-MAX phase as a potential candidate for antiferromagnetic spintronic applications.

Microstructure and magnetic properties of epitaxial YIG thin films prepared by RF magnetron sputtering

Microstructure and magnetic properties of epitaxial YIG thin films prepared by RF magnetron sputtering

Noncoplanar magnetic structures in Mn4N epitaxial films evaluated by alternating magnetic force microscopy

Abstract Noncoplanar magnetic structures in the Mn4N epitaxial thin films grown on the 001-oriented MgO and SrTiO3 (STO) substrates were studied, based on the measurements of topological Hall effect (THE) and the observation of magnetic domain nucleation. The typical nucleation diameter of domain was determined using an alternating magnetic force microscope, which proved advantageous for the visualization of the domain with an out-of-plane magnetic component. The nucleation diameter of the domains on the MgO substrate were ∼150 nm for the thickness of 30 nm and ∼110 nm for 10 nm, while ∼130 nm for 30 nm on the STO substrate. The value of THE was one or two orders of magnitude larger than that estimated based on the nucleation diameter, indicating that the existence of a noncoplanar magnetic structure is the primary factor contributing to the THE in the Mn4N films, comparing to the effect from domain nucleation. The noncoplanar magnetic structure was more pronounced with decreasing thickness and substrate-induced strain.

Production and electronic transport in thin films of strontium iridate

The results of the study of epitaxial thin films of SrIrO3 are presented, data on growth technology, crystal structure and electronic transport are presented. In SrIrO3 films received in a mixture of Ar and O2 gases, the dependence of resistance on temperature has a metallic character. For the films deposited in pure argon, the resistance versus temperature curves shows both a metallic and a dielectric behavior. It depends on the deposition pressure and the deposition temperature. The activation energy was calculated for dielectric samples and compared with the activation energy for Sr2IrO4 films.

Nanosecond electric pulse-induced ultrafast piezoelectric responses in Co3+ substituted BiFeO3 epitaxial thin films

Understanding the ultra-fast dynamics of ferroelectric materials is essential for advancing the development of next-generation high speed electronic and photonic devices. Here, the ultrafast piezoelectric response of cobalt-substituted BiFeO3 (BiFe1-xCoxO3) with x = 0.15, consisting of morphotropic phase boundary of monoclinic MC and MA –type phases is investigated. The real-time piezoelectric response in (001)-oriented BiFe0.85Co0.15O3 (BFCO) epitaxial thin film was monitored using the time-resolved X-ray microdiffraction technique under an applied electric field with pulse widths 70 ns and 100 ns. The BFCO thin film yielded a high piezoelectric strain of approximately 0.53 % along [001] direction, with a giant c/a ratio (∼ 1.26) at an electric field of 1.3 MV/cm and a pulse width of 100 ns, with a piezoelectric coefficient of 40 pm/V. This finding is an important step towards the development of a high performance lead-free piezoelectric material for ultrafast operations in advanced technological applications.