High-quality Epitaxial Thin Films Research Articles

CuInO2 epitaxial thin films on epi-GaN wafer: Fabrication and solar-blind photodetector

High-quality p-type CuInO2 epitaxial thin films with a bandgap of ∼3.99 eV were grown on epitaxial GaN (001) (epi-GaN) wafers by using pulsed laser deposition technology. At a constant growth temperature of 750 °C, the quality of the CuInO2 thin film increases with the decrease in oxygen pressure within the range of 0.09–1 Pa. For the single crystal film obtained at 0.09 Pa, the epitaxial relationship between the film and the substrate is determined to be CuInO2 (001) || GaN (001) with CuInO2 〈100〉 || GaN [100]. The self-powered ultraviolet photodetector prepared based on the highest quality film shows high photoresponsivity 0.31 mA/W and fast response speed (rise time: 0.34 s, decay time: 0.34 s) at zero voltage under 254-nm UV-light, meaning good solar-blind photoresponsivity.

Solid-phase epitaxy of a CuAlO2 template on c-Al2O3 for delafossite growth

Thin-film growth of ABO2 delafossites has recently attracted significant attention due to its attractive transport properties and potential applications. A fundamental requirement for achieving high-quality thin films is the availability of lattice matching substrates and chemical compatibility. However, there are still many obstacles to achieving high-quality thin films. Here, we report a process to further engineer a template ABO2 delafossite structure by solid-phase epitaxy of CuAlO2 on the surface of a commercial sapphire substrate, which offers a promising route to growing high-quality epitaxial thin films. The starting reagents involve a layer of polycrystalline Cu2O deposited on a c-Al2O3 substrate by pulsed laser deposition (PLD). Subsequent thermal treatment activates a solid-state interface reaction between the film and substrate, producing a CuAlO2 thin film. The reaction temperature and dwell time parameters were optimized in this study to prepare a phase diagram for CuAlO2 samples without phase impurities. This method provides an essential stepping-stone toward the approachability of a lattice matching template (i.e., substrate-buffer layer) for ABO2 heterostructures. An example of successful epitaxial growth of highly conducting PdCrO2 is also demonstrated by using a CuAlO2 buffer layer.

Optimization of the Growth Process of Double Perovskite Pr2-δNi1-xMn1+xO6-y Epitaxial Thin Films by RF Sputtering.

Epitaxial thin films of Pr2−δNi1−xMn1+xO6−y (PNMO) double perovskite were grown on (001)-oriented SrTiO3 substrates by RF magnetron sputtering. The influence of the growth parameters (oxygen pressure, substrate temperature, and annealing treatments) on the structural, magnetic and transport properties, and stoichiometry of the films was thoroughly investigated. It is found that high-quality epitaxial, insulating, and ferromagnetic PNMO thin films can only be obtained in a narrow deposition parameter window. It is shown that a careful selection of the growth conditions allows for obtaining a high degree of Ni/Mn cation ordering, which is reflected in the values of the Curie temperature, TC, and saturation magnetization, MS, which are very close to those of bulk material.

Co[formula omitted]Fe[formula omitted]Ge[formula omitted]: A single-phase full Heusler alloy with highest magnetic moment and Curie temperature

Elemental substitution by a different element is a well utilized technique of stabilizing a single-phase compound, in case the parent alloy is multi-phase, and this has been demonstrated specifically in a number of Heusler systems. This can, however, give an increased propensity for chemical disorder as well as adding complexity to synthesis and characterization.In this paper, we present the successful synthesis of a single-phase compound, Co2Fe1.25Ge0.75, by tuning the parent Co2FeGe stoichiometry (which exhibits multi-phase structure) rather than introducing a fourth element. The compound is found to crystallize in L21 structure (space group # 225). Magnetization measurements reveal Co2Fe1.25Ge0.75 has a saturation magnetization as high as 6.7±0.1μB/f.u. at 5 K and a Curie temperature of 1135 ± 5 K – both being the highest reported to date for cubic full Heusler alloys to our knowledge. Thin films of Co2Fe1.25Ge0.75 deposited on Al2O3(110) and MgAl2O4(100) substrates show excellent expitaxial quality, among the reported for Heusler films to date, and exhibit magnetic properties comparable to bulk samples. First principle calculations suggest the system exhibits total energy minimum at the experimentally-observed lattice parameter. Furthermore, the calculations corroborate the observed enhancement in magnetization and point to the importance of on-site Coulomb interactions. While our novel approach of substitution led to the discovery of stable Co2Fe1.25Ge0.75 alloy with very high moment and Curie temperature that can be readily grown as a high-quality epitaxial thin film, making it a candidate for device applications, this approach can be taken as a new paradigm for the discovery of novel single-phase Heusler compounds with enhanced magnetic properties.

Anomalous Hall effect and anisotropic magnetoresistance of molecular beam epitaxy grown Cr2Te3 thin films

High-quality Cr2Te3 epitaxial thin films with thicknesses ranging from 2.5 to 30 nm have been epitaxially grown on SrTiO3(111) single-crystal substrates using molecular beam epitaxy. The optimal growth temperature of the films was found to be 275 °C. All Cr2Te3 films show semiconductor-to-metal transition which is coupled with the paramagnetic-to-ferromagnetic transition at TC = 175 K. Consequently, all Cr2Te3 films show negative magnetoresistance effect which is strongest near TC and can be understood within the framework of electronic phase separation scenario. Both anomalous Hall effect and out-of-plane magnetic hysteresis loops show that all Cr2Te3 films have perpendicular magnetic anisotropy with the easy magnetization axis along the c-axis of the films. With decreasing film thickness the coercivity increases and reaches 10 kOe for the thinnest 2.5-nm film. Magnetoresistance measurements at different angles between the direction of the magnetic field and the easy magnetization axis reveal that Cr2Te3 films show anisotropic magnetoresistance whose out-of-plane values are larger than those of in-plane ones.

Epitaxial integration of a perpendicularly magnetized ferrimagnetic metal on a ferroelectric oxide for electric-field control

Ferrimagnets, which contain the advantages of both ferromagnets (detectable moments) and antiferromagnets (ultrafast spin dynamics), have recently attracted great attention. Here we report the optimization of epitaxial growth of a tetragonal perpendicularly magnetized ferrimagnet Mn2Ga on MgO. Electrical transport, magnetic properties and the anomalous Hall effect (AHE) were systematically studied. Furthermore, we successfully integrated high-quality epitaxial ferrimagnetic Mn2Ga thin films onto ferroelectric PMN-PT single crystals with a MgO buffer layer. It was found that the AHE of such a ferrimagnet can be effectively modulated by a small electric field over a large temperature range in a nonvolatile manner. This work thus demonstrates the great potential of ferrimagnets for developing high-density and low-power spintronic devices.

Modulation of the Bi3+ 6s2 Lone Pair State in Perovskites for High-Mobility p-Type Oxide Semiconductors.

Oxide semiconductors are key materials in many technologies from flat‐panel displays，solar cells to transparent electronics. However, many potential applications are hindered by the lack of high mobility p‐type oxide semiconductors due to the localized O‐2p derived valence band (VB) structure. In this work, the VB structure modulation is reported for perovskite Ba2BiMO6 (M = Bi, Nb, Ta) via the Bi 6s2 lone pair state to achieve p‐type oxide semiconductors with high hole mobility up to 21 cm2 V−1 s−1, and optical bandgaps widely varying from 1.5 to 3.2 eV. Pulsed laser deposition is used to grow high quality epitaxial thin films. Synergistic combination of hard x‐ray photoemission, x‐ray absorption spectroscopies, and density functional theory calculations are used to gain insight into the electronic structure of Ba2BiMO6. The high mobility is attributed to the highly dispersive VB edges contributed from the strong coupling of Bi 6s with O 2p at the top of VB that lead to low hole effective masses (0.4–0.7 m e). Large variation in bandgaps results from the change in the energy positions of unoccupied Bi 6s orbital or Nb/Ta d orbitals that form the bottom of conduction band. P–N junction diode constructed with p‐type Ba2BiTaO6 and n‐type Nb doped SrTiO3 exhibits high rectifying ratio of 1.3 × 104 at ±3 V, showing great potential in fabricating high‐quality devices. This work provides deep insight into the electronic structure of Bi3+ based perovskites and guides the development of new p‐type oxide semiconductors.

Effect of lattice mismatch on film morphology of the quasi-one dimensional conductor K0.3MoO3.

High quality epitaxial thin films of the quasi-one dimensional conductor K0.3MoO3 have been successfully grown on SrTiO3(100), SrTiO3(110), and SrTiO3(510) substrates via pulsed laser deposition. Scanning electron microscopy revealed quasi-one dimensional rod-shaped structures parallel to the substrate surface, and the crystal structure was verified by using X-ray diffraction. The temperature dependence of the resistivity for the K0.3MoO3 thin films demonstrates a metal-to-semiconductor transition at about 180 K. Highly anisotropic resistivity was also observed for films grown on SrTiO3(510).

Magnetic Weyl Semimetallic Phase in Thin Films of Eu_{2}Ir_{2}O_{7}.

The interplay between electronic interactions and strong spin-orbit coupling is expected to create a plethora of fascinating correlated topological states of quantum matter. Of particular interest are magnetic Weyl semimetals originally proposed in the pyrochlore iridates, which are only expected to reveal their topological nature in thin film form. To date, however, direct experimental demonstrations of these exotic phases remain elusive, due to the lack of usable single crystals and the insufficient quality of available films. Here, we report on the discovery of signatures for the long-sought magnetic Weyl semimetallic phase in (111)-oriented Eu_{2}Ir_{2}O_{7} high-quality epitaxial thin films. We observed an intrinsic anomalous Hall effect with colossal coercivity but vanishing net magnetization, which emerges right below the onset of a peculiar magnetic phase with all-in-all-out (AIAO) antiferromagnetic ordering. The anomalous Hall conductivity obtained experimentally is consistent with the theoretical prediction, likely arising from the nonzero Berry curvature emanated by Weyl node pairs near the Fermi level that act as sources and sinks of Berry flux, activated by broken cubic crystal symmetry at the top and bottom terminations of the thin film.

Nanoscale Design of High-Quality Epitaxial Aurivillius Thin Films

Efforts for the integration of ferroelectric materials in nonvolatile, low energy consuming memories have so far been focused on perovskite oxide materials. Their down-scaling for nanodevices is, h...

High-quality epitaxial thin films of topological kagome metal CoSn by magnetron sputtering

The topological kagome metal CoSn hosts orbital-selective Dirac bands and very flat bands near the Fermi energy that lead to a range of exotic phenomena, such as fractional quantum Hall states. In this work, we report the synthesis of high-quality epitaxial (0001) CoSn films by magnetron sputtering. Comprehensive structural characterizations demonstrate high crystalline quality with low disorder, sharp interfaces, and a smooth surface. Complementary magnetic and transport properties show a paramagnetic, metallic ground state as seen in bulk. Our work creates a synthetic foundation to investigate and utilize rich topological physics in CoSn thin films and heterostructures.

Strain relaxation and dislocation annihilation in compositionally graded α-(AlxGa1-x)2O3 layer for high voltage α-Ga2O3 power devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) and the different thermal expansion coefficients between α-Ga2O3 and the sapphire substrate induce crystalline defects and thermal strain, leading to a high density of threading dislocations and degraded electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvents these issues, compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations, and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. Moreover, owing to dislocations annihilation in the graded α-(AlxGa1-x)2O3 layers, the calculated density of threading dislocations in α-Ga2O3 films with a graded α-(AlxGa1-x)2O3 layer is reduced by 64.9% compared with that of α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diodes reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality using the graded α-(AlxGa1-x)2O3 layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Self-regulated growth of candidate topological superconducting parkerite by molecular beam epitaxy

Ternary chalcogenides, such as parkerites and shandites, are a broad class of materials exhibiting a rich diversity of transport and magnetic behavior and an array of topological phases, including Weyl and Dirac nodes. However, they remain largely unexplored as high-quality epitaxial thin films. Here, we report the self-regulated growth of thin films of the strong spin–orbit coupled superconductor Pd3Bi2Se2 on SrTiO3 by molecular beam epitaxy. Films are found to grow in a self-regulated fashion, where, in excess Se, the temperature and relative flux ratio of Pd to Bi control the formation of Pd3Bi2Se2 due to the combined volatility of Bi, Se, and Bi–Se bonded phases. The resulting films are shown to be of high structural quality, and the stoichiometry is independent of the Pd:Bi and Se flux ratio and exhibits a superconducting transition temperature of 800 mK and a critical field of 17.7 ± 0.5 mT, as probed by transport and magnetometry. Understanding and navigating the growth of the chemically and structurally diverse classes of ternary chalcogenides open a vast space for discovering new phenomena and enabling new applications.

Epitaxial Thin Films of a Chalcogenide Perovskite

Chalcogenide perovskites have emerged as a new class of electronic materials, but fundamental properties and applications of chalcogenide perovskites remain limited by the lack of high quality epitaxial thin films. We report epitaxial thin film growth of BaZrS3, a prototypical chalcogenide, by pulsed laser deposition. X-ray diffraction studies show that the films are strongly textured out of plane and have a clear in-plane epitaxial relationship with the substrate. Electron microscopy studies confirm the presence of epitaxy for the first few layers of the film at the interface, even though away from the interface the films are polycrystalline with a large number of extended defects suggesting the potential for further improvement in growth. X-Ray reflectivity and atomic force microscopy show smooth film surfaces and interfaces between the substrate and the film. The films show strong light absorption near the band edge and photoluminescence in the visible region. The photodetector devices show fast and efficient photo response with the highest ON/OFF ratio reported for BaZrS3 films thus far. Our study opens up opportunities to realize epitaxial thin films, heterostructures, and superlattices of chalcogenide perovskites to probe fundamental physical phenomena and the resultant electronic and photonic device technologies.

Gallium-gallium weak bond that incorporates nitrogen at atomic steps during GaN epitaxial growth

Growth of high-quality epitaxial thin films of GaN is indispensable for the energy-saving power electronics and the clarification of the mechanism of the epitaxial growth, which is unachieved yet, is essential to advance nanoscience and technology. We here report quantum-theory-based first-principles calculations that elucidate atomic structures and formation energies of the surface steps of GaN (0001) surfaces, unveil atom-scale elementary processes of N incorporation at the step edges, and then provide a microscopic picture of the step-flow epitaxial growth of GaN for the first time. We find that the NH unit produced after the decomposition of NH3 on the surface terrace diffuses to the stable step edges and is incorporated in the GaN thin film, thus clarifying an elementary process of the step-flow epitaxial growth. Our calculations indicate that the essential factor to promote the epitaxial growth is the presence of the Ga-Ga weak bond on the growing surface.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Interface control of tetragonal ferroelectric phase in ultrathin Si-doped HfO2 epitaxial films

Nanoscaled HfO2-based ferroelectric thin films are a favored candidate for the integration of next-generation memory and logic devices. The unique advantage is that the ferroelectric polarization becomes more robust than the traditional perovskite ferroelectrics when the size is reduced. Understanding and controlling the ferroelectricity requires high-quality epitaxial thin films to explore intrinsic ferroelectric mechanism and evaluate device applications. Here, we report a semicoherent growth of ITO as a bottom electrode that enables genuine ultrathin epitaxial films of Si-doped HfO2 on YSZ[001]/[110]/[111] substrates. The deposited films, which are under epitaxial compressive strain, display large ferroelectric polarization values up to 42 μC/cm2 and do not need wake-up cycling. Structural characterization reveals the presence of crystalline domains with short axes of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and can be reversibly switched with a phase change of 180o. Ferroelectric polarization can be controlled by ITO surface polarity which can easily exploit the interfacial valance mismatch to influence the electrostatic potential across the interface. These findings have implications for our understanding of ferroelectric switching and offer easy method to manipulate domain reversal state in HfO2-based ferroelectric materials.

Strain Relaxation and Dislocation Annihilation in Compositionally Graded α-(Al xGa 1-x) 2O 3 Layer for High Voltage α-Ga 2O 3 Power Devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) between α-Ga2O3 and the sapphire substrate induces a high density of threading dislocations, which act as breakdown leakage paths and lead to deterioration of the crystallinity and electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvent this issue compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. The calculated density of threading dislocations in an α-Ga2O33 films with a graded α-(AlxGa1-x)2O3 buffer layer is reduced by 64.9% compared with that of an α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diode reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality by using the graded α-(AlxGa1-x)2O3 buffer layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Ferroelectric domain structure of Bi2FeCrO6 multiferroic thin films

Bi2FeCrO6 (BFCO) multiferroic thin films were deposited on SrRuO3(SRO)/SrTiO3(STO) (001) and STO (001) substrates by a pulsed laser deposition method. High-quality epitaxial BFCO thin films with a typical step-flow surface morphology were successfully prepared. X-ray diffraction reveals that the high phase-purity epitaxial BFCO films with good crystallinity are obtained. Using both piezoelectric force microscopy (PFM) and x-ray reciprocal space mapping, we studied the ferroelectric domain structure and ferroelectricity characterization of BFCO thin films with and without an SRO buffer layer. The ferroelectric domain structure of an epitaxial BFCO film deposited on SRO/STO (001) is characterized by eight polarization variants. However, the ferroelectric domain structure of the epitaxial BFCO film directly grown on the STO (001) substrate is degenerate from eight polarization variants to four or even two. The feasibility of ferroelectric domain engineering is demonstrated by growing BFCO films on STO (001) with and without SRO buffer layers.

Thin-film growth of MAX phases as functional materials

Abstract Layered nanolaminate ternary carbides, nitrides and carbonitrides with general formula Mn+1AXn or MAX (n = 1, 2, or 3, M is an early transition metal, A is mostly group 13 or 14 element and X is C and/or N) has revolutionized the world of nanomaterials, due to the coexistence of both ceramic and metallic nature, giving rise to exceptional mechanical, thermal, electrical, chemical properties and wide range of applications. Although several solid-state bulk synthesis methods have been developed to produce a variety of MAX phases, however, for certain applications, the growth of MAX phases, especially in its high-quality epitaxial thin-films form is of increasing interest. Here, we summarize the progress made thus far in epitaxial growth and property evaluation of MAX phase thin films grown by various deposition techniques. We also address the important future research directions to be made in terms of thin-film growth. Overall, in the future, high-quality single-phase epitaxial thin-film growth and engineering of chemically diverse MAX phases may open up interesting new avenues for next-generation technology.