Epitaxial Films Research Articles

Improvement of Ga2O3 vertical Schottky barrier diode by constructing NiO/Ga2O3 heterojunction

The high critical electric field strength of Ga2O3 enables higher operating voltages and reduced switching losses in power electronic devices. Suitable Schottky metals and epitaxial films are essential for further enhancing device performance. In this work, the fabrication of vertical Ga2O3 barrier diodes with three different barrier metals was carried out on an n–-Ga2O3 homogeneous epitaxial film deposited on an n+-β-Ga2O3 substrate by metal−organic chemical vapor deposition, excluding the use of edge terminals. The ideal factor, barrier height, specific on-resistance, and breakdown voltage characteristics of all devices were investigated at room temperature. In addition, the vertical Ga2O3 barrier diodes achieve a higher breakdown voltage and exhibit a reverse leakage as low as 4.82 ×10−8 A/cm2 by constructing a NiO/Ga2O3 heterojunction. Therefore, Ga2O3 power detailed investigations into Schottky barrier metal and NiO/Ga2O3 heterojunction of Ga2O3 homogeneous epitaxial films are of great research potential in high-efficiency, high-power, and high-reliability applications.

Highly Polarization-Deep-Ultraviolet-Sensitive β-Ga2O3 Epitaxial Films by Disrupting Rotational Symmetry and Encrypted Solar-Blind Optical Communication Application.

Ultrawide bandgap semiconductor β-Ga2O3 (4.9 eV), with its monoclinic crystal structure, exhibits distinct anisotropic characteristics both optically and electrically, making it an ideal material for solar-blind polarization photodetectors. In this work, β-Ga2O3 epitaxial films were deposited on sapphire substrates with different orientations, and the mechanisms underlying the anisotropy of these epitaxial films were investigated. Compared to c-plane sapphire, the lattice mismatch between m- or r-plane sapphire and β-Ga2O3 is more pronounced, disrupting the rotational symmetry of the films and rendering them anisotropic. Thanks to the improved anisotropy, the polarization ratio of the photodetector based on β-Ga2O3 films grown on r-plane substrates is 0.24, nearly ten times higher than that on c-plane substrates. Finally, by utilizing these polarization-sensitive photodetectors, we developed an encrypted solar-blind ultraviolet optical communication system. Our work provides a new approach to facilitate the fabrication and application of high-performance polarization-sensitive solar-blind photodetectors.

High-performance orthorhombic correlated transparent conducting epilayer integrated on a flexible mica substrate: The case of CaMoO3

Recently, cubic perovskite SrTMO3 (TM = V, Nb, and Mo) compounds have attracted great interest as correlated transparent conducting oxides (CTCOs). However, in theory, none of these three oxides is capable of completely covering the entire visible (Vis) light wavelength range with transparency. Herein, epitaxial orthorhombic CaMoO3 thin films were grown on flexible mica substrates using a pulsed laser deposition (PLD) method. By exploiting the tilting of MoO6 octahedra, an optimal balance between plasma energy (ћωp) and electrical conductivity (σ) was realized in CaMoO3. Electronic transport characterizations indicated that the σ value of CaMoO3 is comparable to that of SrTMO3. The optical transmittance spectra demonstrated that the CaMoO3 film possesses superior Vis transparency compared to SrTMO3. From the ellipsometry analysis, it was found that the O 2p – V 3d interband transition occurs at around 4.16 eV while the screened plasma energy (ћωp) is approximately 1.37 eV. The favorable absorption edges for both the long and short wavelength regions ensure the high transparency of CaMoO3 across the entire Vis spectrum. Additionally, bending cyclic tests illustrated the excellent mechanical durability and endurance of the CaMoO3/mica films. The present work is instructive for exploring alternative CTCOs beyond SrTMO3, demonstrating that the optimal balance between ћωp and σ can be realized by manipulating the TMO6 octahedral tilting to improve the transparent conducting performance of correlated perovskite-type oxides.

In-Situ Atomic-Scale Observation of Brownmillerite to Ruddlesden–Popper Phase Transition Tuned by Epitaxial Strain in Cobaltites

Phase transitions involving oxygen ion extraction within the framework of the crystallographic relevance have been widely exploited for sake of superconductivity, ferromagnetism, and ion conductivity in perovskite-related oxides. However, atomic-scale pathways of phase transitions and ion extraction threshold are inadequately understood. Here we investigate the atomic structure evolution of LaCoO3 films upon oxygen extraction and subsequent Co migration, focusing on the key role of epitaxial strain. The brownmillerite to Ruddlesden–Popper phase transitions are discovered to stabilize at distinct crystal orientations in compressive- and tensile-strained cobaltites, which could be attributed to in-plane and out-of-plane Ruddlesden–Popper stacking faults, respectively. A two-stage process from exterior to interior phase transition is evidenced in compressive-strained LaCoO2.5, while a single-step nucleation process leaving bottom layer unchanged in tensile-strained situation. Strain analyses reveal that the former process is initiated by an expansion in Co layer at boundary, whereas the latter one is associated with an edge dislocation combined with antiphase boundary. These findings provide a chemo-mechanical perspective on the structure regulation of perovskite oxides and enrich insights into strain-dependent phase diagram in epitaxial oxides films.

The effect of substrate-induced defects on structural and resistive switching properties in Gd0.2Ca0.8MnO3 thin films

Gd0.2Ca0.8MnO3 thin films were deposited on various substrate materials and their structural and resistive switching (RS) properties were investigated. The deposition resulted in epitaxial and polycrystalline films, with the latter also exhibiting distorted film surfaces. Both epitaxial and a part of polycrystalline films used as RS devices showed consistent RS performance in which an order of magnitude, or higher, switching ratios were achieved between high and low resistance states. The devices showed strong endurance during repeated switching cycles. However, under retention characterization, the resistance states did not remain distinguishable in devices constructed on polycrystalline films, while other devices maintained separable resistance states. The RS results are discussed in relation to the structural characteristics of the films, and this work helps us understand the RS mechanisms that still remain elusive in manganite-based devices.

State of the art, trends, and opportunities for oxide epitaxy

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.

Dielectric properties of low-temperature-grown homoepitaxial (−201) β-Ga2O3 thin film by atmospheric pressure plasma-assisted CVD

The atmospheric pressure plasma-assisted chemical vapor deposition technique has successfully demonstrated unintentionally doped (UID) Ga2O3 growth at 350 °C. This technique allows independent and homogeneous multiple nuclei growth of Ga2O3, leading to three-dimensional grain growth at a rate of ⁓0.17 μm/h. In the study of Schottky barrier diodes, the Schottky-like current (I)–voltage (V) response shows typical behavior on Ga2O3. This is a good sign at an early stage of device development on the grown sample. The extracted barrier height of ⁓2.20 eV was higher, which may be due to unintentional PtOx formation on the Ga2O3 surface. Furthermore, the extracted capacitance (C)–voltage (V) depth profiling of the effective impurity concentration was nearly flat, ⁓1.5 × 1017 cm−3, in the unintentionally doped grown film. The effective impurity concentration is comparable to the UID carrier concentration of epitaxial films fabricated using the high temperature growth technique. Therefore, low-temperature-grown homoepitaxial Ga2O3 thin films grown by atmospheric pressure-plasma-assisted chemical vapor deposition can be used in future Ga2O3-based power device applications.

Tuning the metal-insulator transition temperature by the controlled generation of oxygen vacancies on La0.7Ca0.3MnO3-x epitaxial thin films

This report presents a study on the effect of the controlled generation of oxygen vacancies on the electronic transport properties of La0.7Ca0.3MnO3-x epitaxial thin films. Oxygen defects cause significant changes in the crystalline structure of mixed-valence manganites and lower the Mn valence, resulting in a shift of the metal-insulator transition to lower temperatures due to modifications in the angles and lengths of Mn-O bonds. Experiments using Polarized Extended X-Ray Absorption Fine Structure Spectroscopy have provided strong evidence that oxygen vacancies are mainly formed in the basal plane of the MnO6 octahedra. The accurate control of oxygen vacancies generation opens possibilities for managing and enhancing the magneto-electronic properties of epitaxial manganite thin films with potential in industrial applications.

Low‐Pressure Chemical Vapor Deposition of Hexagonal Boron Nitride on a‐Plane Sapphire Using BCl3 as a Boron Source

Hexagonal boron nitride (h‐BN) thin films are grown on a‐plane sapphire substrates at different temperatures, Tg, by low‐pressure chemical vapor deposition with BCl3 and NH3 as boron and nitrogen sources, respectively, and their crystallographic and luminescence properties are compared with those grown on c‐plane sapphire. A notable difference from the c‐plane sapphire is that the substrate surface is significantly nitrided during film growth at Tg higher than 1200 °C, whereas no such nitridation is observed up to 1500 °C for the growth on c‐plane sapphire. The nitridation of the substrate surface is found to cause the degradation of the film quality. However, the properties of the films grown on a‐plane sapphire at Tg lower than 1200 °C are comparable to or better than those grown on c‐plane sapphire. The h‐BN epitaxial film is grown at 1150 °C epitaxially on the a‐plane sapphire substrate with the out‐of‐plane and in‐plane relationships, h‐BN//sapphire andh‐BN//sapphire, respectively. This study suggests that a‐plane sapphire is suitable for growing high‐quality h‐BN films at relatively low Tg.

Highly oriented epitaxial Cu2O (011) thin film grown on MgO (001) substrate by dynamic aurora PLD method

The epitaxial Cu2O films are grown on MgO (001) substrates by the dynamic aurora pulsed laser deposition (PLD) method. The structure and epitaxial growth of the as-prepared thin films at different substrate temperatures (25 °C to 600 °C) were investigated. The thin film prepared at room temperature (RT, 25 °C) exhibits (111) plane of monoclinic CuO phase with poor crystallinity. The thin films deposited at 200 °C to 600 °C show single-phase cubic Cu2O with good crystallinity. The highly oriented epitaxial Cu2O thin film on the MgO substrate is achieved at low temperatures of 200 °C by applying an induced electromagnetic field during thin film growth. The quality and crystallinity of epitaxial Cu2O thin films remarkably improved with increasing temperature from 200 °C to 600 °C and all these thin films exhibited an out-of-plane epitaxial relationship of Cu2O (011) ‖ MgO (001). The CuO thin film grown on MgO substrate at RT shows a single domain structure, as observed by RHEED. In constrast, Cu2O films grown at 200 °C to 600 °C exhibit two different kinds of domain structure with corresponding in-plane epitaxial relationship of Cu2O [100] ‖ MgO [110] and Cu2O [100] ‖ MgO [11¯0]. This report reveals, for the first time, the feasibility of epitaxial growth of highly oriented Cu2O (011) thin film achieved at the low temperature of 200 °C by the dynamic aurora PLD method. The quality of epitaxial Cu2O (011) is significantly enhanced with increasing substrate temperature from 200 °C to 600 °C.

Temperature-dependent morphology and composition of ultra-thin GeSn epilayers prepared by remote plasma enhanced chemical vapor deposition

Two distinct ultra-thin Ge1−xSnx (x ≤ 0.1) epilayers were deposited on (001) Si substrates at 457 and 313 °C through remote plasma-enhanced chemical vapor deposition. These films are considered potential initiation layers for synthesizing thick epitaxial GeSn films. The GeSn film deposited at 313 °C has a thickness of 10 nm and exhibits a highly epitaxial continuous structure with its lattice being compressed along the interface plane to coherently match Si without mismatch dislocations. The GeSn film deposited at 457 °C exhibits a discrete epitaxial island-like morphology with a peak height of ∼30 nm and full-width half maximum (FWHM) varying from 20 to 100 nm. GeSn islands with an FWHM smaller than 20 nm are defect free, whereas those exceeding 25 nm encompass nanotwins and/or stacking faults. The GeSn islands form two-dimensional modulated superlattice structures at the interface with Si. The GeSn film deposited at 457 °C possesses a lower Sn content compared to the one deposited at lower temperature. The potential impact of using these two distinct ultra-thin layers as initiation layers for the direct growth of thicker GeSn epitaxial films on (001) Si substrates is discussed.

Broadband Photodetection of Centimeter-Scale T-Phase Gallium Telluride Grown by Molecular Beam Epitaxy.

Two-dimensional (2D) semiconductors have recently attracted considerable attention due to their promising applications in future integrated electronic and optoelectronic devices. Large-scale synthesis of high-quality 2D semiconductors is an increasingly essential requirement for practical applications, such as sensing, imaging, and communications. In this work, homogeneous 2D GaTe films on a centimeter scale are epitaxially grown on fluorphlogopite mica substrates by molecular beam epitaxy (MBE). The epitaxial GaTe thin films showed an atomically 2D layered lattice structure with a T phase, which has not been discovered in the GaTe geometric isomer. Furthermore, semiconducting behavior and high mobility above room temperature were found in T-GaTe epitaxial films, which are essential for application in semiconducting devices. The T-GaTe-based photodetectors demonstrated respectable photodetection performance with a responsivity of 13 mA/W and a fast response speed. By introducing monolayer graphene as the substrate, we successfully realized high-quality GaTe/graphene heterostructures. The performance has been significantly improved, such as the responsivity was enhanced more than 20 times. These results highlight a feasible scheme for exploring the crystal phase of 2D GaTe and realizing the controlled growth of GaTe films on large substrates, which could promote the development of broadband, high-performance, and large-scale photodetection applications.

Effect of template on the photoresponsivity of BaSi2 films grown on Ge(111) substrates by molecular beam epitaxy

Epitaxial growth of BaSi2 films on Si(111) has demonstrated that the BaSi2 template can serve as a seed crystal for BaSi2 overlayers by molecular beam epitaxy (MBE) and shows high photoresponsivity, but not yet on Ge(111) substrates. We have investigated the effect of various templates on the photoresponsivity of BaSi2 films grown on Ge(111) substrates. Samples with MBE-grown templates exhibited high a-axis orientation, but the surface was uneven, leading to partial oxidation of the BaSi2 film. On the other hand, the sample without such templates showed a smoother surface but was confirmed to be polycrystalline. When a template formed by the combination of solid phase epitaxy (SPE) and subsequent annealing for 30 min was used, a-axis-oriented BaSi2 together with 111-oriented Si appeared in the θ–2θ x-ray diffraction (XRD) patterns. The fact that no crystalline Si was detected by the surface-sensitive Raman spectroscopy and that the full width at half maximum of the XRD BaSi2 600 peak was increased to 2.686° in such samples suggests that Si aggregates were at the BaSi2/Ge interface. The photoresponsivity of the epitaxial BaSi2 film with MBE-grown templates reached 0.14 A W−1 at a wavelength of 790 nm. This is the highest photoresponsivity reported so far for BaSi2 films on Ge substrates. Even for samples without the MBE-grown templates, SPE-grown templates significantly increased the photoresponsivity up to 0.10 A W−1. These results confirm that the template has a significant impact on the photoresponsivity of BaSi2 films on Ge(111) substrates.

The Effect of Sputtering Target Density on the Crystal and Electronic Structure of Epitaxial BaTiO3 Thin Films

Barium titanate (BaTiO3) is a promising material for silicon-integrated photonics due to its large electro-optical coefficients, low loss, high refractive index, and fast response speed. Several deposition methods have been employed to synthesize BaTiO3 films. Magnetron sputtering is one of these methods, which offers specific advantages for growing large-scale films. However, there is a scarcity of studies investigating the effect of sputtering target density on the quality of BaTiO3 films. Therefore, this study aims to uncover the effect of sputtering targets on the crystal and electronic structures of epitaxial BaTiO3 thin films. Two BaTiO3 ceramic targets were sintered at different densities by altering the sintering temperatures. The crystal structure and chemical composition of the targets were then characterized using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Subsequently, BaTiO3 epitaxial films were grown by magnetron sputtering using these two targets. The crystal and electronic structures of the BaTiO3 films were analyzed using high-resolution X-ray diffraction, X-ray photoemission spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Notably, the BaTiO3 films grown with high-density targets show superior quality but contain oxygen vacancies, whereas those films synthesized with low-density targets display high surface roughness. These findings provide insights into the effect of sputtering target density on the crystal and electronic structures of epitaxial BaTiO3 thin films.

Planar defects in α-Ga2O3 thin films produced by HVPE

The defect structure of α-phase gallium oxide thin films was investigated using transmission electron microscopy (TEM). Epitaxial Ga2O3 films were grown via halide vapor-phase epitaxy on c-plane sapphire substrates. TEM analysis revealed a high density of extended planar defects within the films, primarily located along prismatic planes of {112¯0} type. Displacement vectors were determined using the invisibility criterion for stacking faults. The study encompassed both planar and cross-sectional views of the films. It is hypothesized that these defects form due to the motion of edge partial dislocations with the 13⟨11¯00⟩ Burgers vector. Various mechanisms of their formation have been explored.

High-temperature annealing effects on epitaxial TiN films on 4H-SiC

To gain deeper insights into the impact of very high-temperature annealing on TiN film quality and thermal diffusion phenomena, the deposited TiN films underwent annealing in an induction heating furnace across a temperature range from 1150 °C to 1700 °C. Our findings indicate that TiN films remain stable, and the full-width at half-maximum (FWHM) of TiN (111) noticeably reduced by about 30 % when annealed at 1250 °C. However, as the annealing temperature rises to 1400 °C, the film composition gradually enriches with carbon as TiCxN1-x with x up to 0.5. After annealing at 1700 °C, the film is further enriched with x larger than 0.5. For a long time annealing at 1700 °C, the decomposition of SiC substrate with void formation is verified from observations by cross-sectional transmission electron microscopy.

Continuous strain-mediated perpendicular magnetic anisotropy in epitaxial (111) CoFe2O4 thin films grown on LiTaO3 substrates

Perpendicular magnetic anisotropy (PMA) in magnetic thin films has attracted much attention due to its potential applications in spintronics devices. Here, we report the continuous strain-mediated PMA in epitaxial (111) CoFe2O4 (CFO) thin films grown on (0001) LiTaO3 substrates. A large variation in lattice strain (∼0.9%) in a continuous way is realized in the CFO thin films by changing substrate temperature during deposition due to the difference in the thermal expansion coefficient between CFO and LiTaO3. As a result, the PMA of the (111) CFO thin films can be continuously mediated by the strain with uniaxial magnetic anisotropy energy in the range of 0.12-14.69×106 erg/cm3. Furthermore, the strain as well as the consequent PMA in the (111) CFO thin films can be maintained within the thickness of 25–205 nm, which is consistent with the scenario of the magnetoelastic effect. Our results reveal that the CFO/LiTaO3 system can be regarded as an ideal platform to realize robust PMA and its continuous strain tuning in the (111) CFO thin films by virtue of strain-induced magnetic anisotropy.

Tuning the Electro‐Optic Properties of BaTiO3 Epitaxial Thin Films via Buffer Layer‐Controlled Polarization Rotation Paths

AbstractBaTiO3 thin films emerge as a highly promising material platform for integrated photonics due to their outstanding electro‐optic properties and diverse functionalities. Despite extensive studies, there remains a notable gap in the understanding of the intricate relationship between the phase structure, domain switching kinetics and electro‐optic performance of these materials. By controlling the structural phase evolution, it is possible to gain deep insights into the physical mechanisms underlying the electro‐optic performance. Here, the phase constitution of BaTiO3 epitaxial thin films is successfully tuned by the insertion of a GdScO3 buffer layer. Continuous polarization rotation paths are observed from an out‐of‐plane tetragonal‐like phase, to an intermediate rhombohedral‐like phase, and finally an in‐plane tetragonal‐like phase, achieving an enhanced effective electro‐optic coefficient of 175 pm V−1 compared to unbuffered films. Furthermore, by in situ second harmonic generation microscopy and electro‐optic measurements, the domain switching behaviors in both statistical and spatially resolved manners are examined, resulting in a clear delineation of the key domain nucleation processes. The in‐depth explorations of these structural mechanisms and kinetics inform the rational design of strong electro‐optic thin films and help in the realization of high‐performance integrated photonic devices such as electro‐optic modulators and multilevel phase shifters.

Electron-irradiation induced unconventional phase transition of β-Ga2O3 epitaxial single-crystal thin film observed by in-situ TEM

In this work, the phase transition process of β-Ga2O3 thin films under heating and (or) electron irradiation condition was investigated by in-situ transmission electron microscopy. It is found that only under the high temperature, β-Ga2O3 did not undergo phase transition even when the temperature was up to 1000 °C. When under electron irradiation condition, the unconventional phase transition from β-Ga2O3 to δ-Ga2O3 occurred with a slow rate. When electron irradiation was coupled with a thermal field, the initial temperature of phase transition decreased, and the speed of phase transition also greatly accelerated. The new phase maintains crystallography relationship with the β-Ga2O3 parent phase as following: [010]β//[011‾]δ, (200)β//(2‾11)δ, and (402‾)β//(411)δ. The results reveal that electron irradiation can trigger the phase transition process from β-Ga2O3 to δ-Ga2O3, and the high temperature obviously accelerates the rate of phase transition process.

Novel Route for Enhancing Piezoelectricity of Ferroelectric Films: Controlling Nontrivial Polarization States in Pb(Zr, Ti)O3 Monodomain Superlattice Structure.

Artificial superlattice films made of Pb(Zr0.4Ti0.6)O3 and Pb(Zr0.6Ti0.4)O3 were investigated for their polarization states and piezoelectric properties theoretically and experimentally in this study. The developed theory predicts nontrivial polarization along neither [001] nor [111] directions in (111)-epitaxial monodomain superlattice films with uniform compressive strain. Such films were achieved via pulsed laser deposition. When the layer thickness is reduced to 3 nm, d33 becomes 128 ± 3.8 pm/V at 100 kV/cm and 71.3 ± 2.83 pm/V at 600 kV/cm, comparable to that of (111)-oriented Pb(Zr0.4Ti0.6)O3 or Pb(Zr0.6Ti0.4)O3 bulks and clearly exceeding that of the typical clamped films. The measurement agrees with the theoretical analysis, which reveals that the enhanced piezoelectricity is due to rotation of the nontrivial polarization. Furthermore, the theoretical study predicts an even larger d33 exceeding 300 pm/V for specific parameters in superlattice films with uniform tensile strain, which is promising for applications of microelectromechanical systems.