Epitaxial Growth Of Thin Films Research Articles

Real-space characterization of tetragonal CuO epitaxial films

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates.

Epitaxial ferroelectric interfacial devices

Ferroelectric interfacial devices consist of materials systems whose interfacial electronic properties (such as a 2D electron gas or an interfacial magnetic spin configuration) are modulated by a ferroelectric layer set in its immediate vicinity. While the prototypical example of such a system is the ferroelectric field effect transistor first proposed in the 1950s, only with the recent advances in the controlled growth of epitaxial thin films and heterostructures, and the recent physical understanding down to the atomic scale of screening processes at ferroelectric-semiconducting and -metallic interfaces made possible by first principles calculations, have the conditions been met for a full development of the field. In this review, we discuss the recent advances in ferroelectric interfacial systems with emphasis on the ferroelectric control of the electronic properties of interfacial devices with well ordered (epitaxial) interfaces. In particular, we consider the cases of ferroelectric interfacial systems aimed at controlling the correlated state, including superconductivity, Mott metallic-insulator transition, magnetism, charge, and orbital order, and charge and spin transport across ferroelectric tunnel junctions. The focus is on the basic physical mechanisms underlying the emergence of interfacial effects, the nature of the ferroelectric control of the electronic state, and the role of extreme electric field gradients at the interface in giving rise to new physical phenomena. Such understanding is key to the development of ferroelectric interfacial systems with characteristics suitable for next generation electronic devices based on controlling the correlated state of matter.

Surface Pyroelectricity and Piezoelectricity of Centrosymmetric Crystals

AbstractProperties of surfaces often define functionalities and applications of a large variety of materials, e. g., epitaxial growth of thin films, catalytic and optical properties. For this reason, advances in techniques for surface characterization are of great importance. Here we present an overview of the work conducted together with Prof. Meir Lahav over the last decade to detect and quantify near surface polar layers (NSPL). Though thin, these layers may accumulate considerable polarization and, thereby, affect surface properties. We demonstrated that pyroelectric measurement carried out with periodic temperature change protocol (modified Chynoweth) might be a primary tool to study NSPL. Two fundamentally different examples are considered. (i) NSPL in α‐glycine crystals develops as a result of solvent incorporation, e. g., water. This layer is maybe tens or even hundreds of micrometers thick, sometimes, thick enough to allow piezoelectric measurements. (ii) NSPL in SrTiO3 results from surface relaxation and it is only a few Angstroms thick. Nevertheless, NSPL in SrTiO3 has a polarization comparable with strongly ferroelectric materials, tens of μC/cm2.

Epitaxial BaSnO3 thin films with low dislocation density grown on lattice matched LaInO3 substrates

The use of LaInO3 with (110) surface orientation was investigated as a novel orthorhombic substrate for the epitaxial growth of semiconducting BaSnO3 thin films. On the basis of reflection high-energy electron diffraction, energy dispersive x-ray analysis and inductively coupled plasma-optical emission spectrometry measurements, we revealed that slight Ba doping of LaInO3 crystals is beneficial to stabilize the substrate surface, which facilitates the epitaxial growth of well-ordered BaSnO3 thin films by pulsed laser deposition. Fully strained BaSnO3 films without misfit dislocations found by means of transmission electron microscopy were achieved due to the negligible lattice mismatch between BaSnO3 film and Ba-doped LaInO3 substrate. Electric properties of La-doped BaSnO3 films exhibit a Hall-mobility of 69 cm2 V−1 s−1 at room temperature and 99 cm2 V−1 s−1 at 20 K at a constant charge carrier density of 3.8·1019 cm−3.

Giant step bunching on SrTiO3 thin films grown epitaxially on vicinal MgO (1 0 0) surfaces

We report here the controlled formation of giant step bunching in the epitaxial growth of SrTiO3 thin film on vicinal MgO (100) substrates by on-axis radio-frequency (RF)-magnetron sputtering. We have studied the evolution of the surface morphology of such bunched steps by tuning the post-deposition annealing temperature. We demonstrate that the surface periodicity and reorganization of SrTiO3 nanostructures is widely enhanced under controlled post-annealing conditions in addition to Mg content diffusion in the SrTiO3 films. A morphology of facetted islands and bunched steps with extremely smooth terraces were obtained by post-annealing the SrTiO3 thin films at high temperatures. After a double post-annealing treatment at 900 °C and 700 °C, step bunching with a bunched step of fifteen-atomic unit cells and large terraced structures of about 1 µm width has been observed. The formation of these bunches of self-organized SrTiO3 nanostructures is of potential interest as templates for the growth of many functional materials.

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.

Experimental Study of the Effect of Precursor Composition on the Microstructure of Gallium Nitride Thin Films Grown by the MOCVD Process

Abstract Chemical vapor deposition (CVD) is a widely used manufacturing process for obtaining thin films of materials like silicon, silicon carbide, graphene, and gallium nitride that are employed in the fabrication of electronic and optical devices. Gallium nitride (GaN) thin films are attractive materials for manufacturing optoelectronic device applications due to their wide band gap and superb optoelectronic performance. The reliability and durability of the devices depend on the quality of the thin films. The metal-organic chemical vapor deposition (MOCVD) process, which uses compounds that contain metals and organic ligands as precursors in a CVD reactor, is a common technique used to fabricate high-quality GaN thin films. The deposition rate and uniformity of thin films are critical to a successful and useful process. These are determined by the thermal transport processes and chemical reactions occurring in the reactor, and are manipulated by controlling the operating conditions and the reactor geometrical configuration. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) substrates is carried out in two commercial MOCVD systems: a vertical rotating disk MOCVD reactor and a close-coupled showerhead MOCVD reactor. The surface morphology and crystal quality of GaN thin films have been examined using atomic force microscopy (AFM) and scanning electron microscope (SEM). This paper focuses on the composition of the precursor and the carrier gases since earlier studies have shown the importance of precursor composition. The results show that the flow rate of trimethylgallium (TMG), which is the main ingredient in the process, has a significant effect on the deposition rate and uniformity of the films. Also, the carrier gas plays an important role in deposition rate and uniformity. Using hydrogen as a carrier gas enhances the quality of the thin film but a lower deposition rate occurs on the wafer surface. On the other hand, a high flow rate of pure nitrogen gas improves the growth rate of the film. However, it decreases the uniformity of the film and promotes carbon contamination on the wafer surface. Thus, the use of an appropriate mixture of hydrogen and nitrogen as the carrier gas can improve the deposition rate and quality of GaN thin films.

Theoretical Study of Chemical Vapor Deposition Synthesis of Graphene and Beyond: Challenges and Perspectives.

Two-dimensional (2D) materials have attracted great attention in recent years because of their unique dimensionality and related properties. Chemical vapor deposition (CVD), a crucial technique for thin-film epitaxial growth, has become the most promising method of synthesizing 2D materials. Different from traditional thin-film growth, where strong chemical bonds are involved in both thin films and substrates, the interaction in 2D materials and substrates involves the van der Waals force and is highly anisotropic, and therefore, traditional thin-film growth theories cannot be applied to 2D material CVD synthesis. During the last 15 years, extensive theoretical studies were devoted to the CVD synthesis of 2D materials. This Perspective attempts to present a theoretical framework for 2D material CVD synthesis as well as the challenges and opportunities in exploring CVD mechanisms. We hope that this Perspective can provide an in-depth understanding of 2D material CVD synthesis and can further stimulate 2D material synthesis.

Epitaxial film growth by thermal laser evaporation

We demonstrate the epitaxial growth of thin films by thermal laser evaporation. Epitaxial metal oxide films are grown by laser evaporating Ni, V, and Ru elemental sources in a variety of oxygen-ozone atmospheres on laser-heated oxide substrates. This results in NiO (111), VO2 (M1) (020), and RuO2 (110) epitaxial films on Al2O3 (0001) or MgO (100) substrates. The films show well-defined crystallographic orientation relationships with the substrates, as confirmed by in-plane and out-of-plane x-ray measurements. The results reveal the potential of thermal laser epitaxy for the epitaxial growth of ultrahigh-purity oxide heterostructures.

Epitaxial growth of Bi(110) and Bi2Se3 thin films on a ferromagnetic insulator substrate of Cr2Ge2Te6

When a topological insulator (TI) is brought to the proximity of a ferromagnetic insulator (FMI), the breaking of the time-reversal symmetry may give rise to quantum anomalous Hall effect (QAHE). The physical properties of such TI–FMI systems are greatly affected by the interfacial structures of the components. Here, we report the growth and structural properties of Bi(110) and Bi2Se3 thin films on a FMI of Cr2Ge2Te6 (CGT) substrate by scanning tunneling microscopy. We observed various defects and impurities on the CGT surfaces, which serve as the preferential sites for initial nucleation and epitaxial growth of Bi(110) thin films. The exposure of the as-grown Bi(110) thin films to Se vapor leads to the formation of polycrystalline Bi2Se3 thin films with randomly distributed holes. The structure and composition of the as-prepared Bi2Se3 thin films were further confirmed by Raman spectroscopy and x-ray photoelectron spectroscopy. Our work shows that the quality of the CGT crystals is vital for the growth of high-quality TIs on CGT substrates for QAHE.

Extraordinary phase coherence length in epitaxial halide perovskites

SummaryInorganic halide perovskites have emerged as a promising platform in a wide range of applications from solar energy harvesting to computing and light emission. The recent advent of epitaxial thin film growth of halide perovskites has made it possible to investigate low-dimensional quantum electronic devices based on this class of materials. This study leverages advances in vapor-phase epitaxy of halide perovskites to perform low-temperature magnetotransport measurements on single-domain cesium tin iodide (CsSnI3) epitaxial thin films. The low-field magnetoresistance carries signatures of coherent quantum interference effects and spin-orbit coupling. These weak anti-localization measurements reveal a micron-scale low-temperature phase coherence length for charge carriers in this system. The results indicate that epitaxial halide perovskite heterostructures are a promising platform for investigating long coherent quantum electronic effects and potential applications in spintronics and spin-orbitronics.

Orientation-Controlled Large-Area Epitaxial PbI2 Thin Films with Tunable Optical Properties.

Lead iodide (PbI2) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI2 with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI2 remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI2 thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI2 thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI2 on other substrates and heterostructures of PbI2 on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI2 and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

Development of dynamic aurora pulsed laser deposition equipped with reflection high-energy electron diffraction and effects of magnetic fields on room-temperature epitaxial growth of NiO thin film

Pulsed laser deposition (PLD) with an electromagnet (dynamic aurora PLD) and reflection high-energy electron diffraction (RHEED) was developed by installing an 11-tuned solenoid coil in a vacuum chamber. This method achieved thin film deposition under a magnetic field of up to 120 mT using dc current of 1,200 A. The electromagnet is a solenoid coil, which produces a magnetic field that is proportional to an electric field without saturation of the magnetic field. This PLD method enables in situ observation of the crystal structure of the thin film surface after turning the electromagnet off, but without breaking vacuum. Many reports have described studies of PLD in a magnetic field. For most such studies, permanent magnets have been used to apply a magnetic field to the plume. Nevertheless, when doing so, carrying out in-situ observation using RHEED is difficult because the magnetic field deflects the electron beam. This case study examined effects of magnetic field application to the plume on the in-plane orientation and resistivity of a NiO thin film deposited on a MgO(001) single crystal at room temperature (25 °C). Results demonstrate that the in-plane orientation (epitaxial growth) is unchanged, whereas thin film resistivity decreases exponentially with the magnetic field intensity during deposition. These results suggest that the Ni3+ ion concentration can be expected to increase exponentially with the magnetic field during deposition. The room-temperature resistivity of the NiO thin film deposited under a 120 mT magnetic field was 112 Ω cm.

Global well-posedness of solutions for the epitaxy thin film growth model

In this paper, we consider the global well-posedness of solutions for the initial-boundary value problems of the epitaxy growth model. We first construct the local smooth solution, then by combining some a priori estimates, continuity argument, the local smooth solutions are extended step by step to all t > 0, provided that the initial datums sufficiently small and the smooth nonlinear functions satisfy certain local growth conditions.

Temperature dependence of β-Ga2O3 heteroepitaxy on c-plane sapphire using low pressure chemical vapor deposition

β-Ga2O3 has drawn significant attention for power electronics and deep ultraviolet (UV) photodetector applications owing to its wide bandgap of ~4.4–4.9 eV and high electric breakdown strength of ~7–8 MV/cm. Growth of β-Ga2O3 epitaxial thin films with high growth rate has been recently reported using low pressure chemical vapor deposition (LPCVD) technique. In this work, we have investigated the effect of growth temperature on β-Ga2O3 films grown on c-plane sapphire substrates using LPCVD. We performed growths by varying temperatures from 800 °C to 950 °C while keeping all other growth parameters (Ar/O2 gas flow rates, growth pressure, and Gallium precursor to substrate distance) constant. Optical, structural, and surface characterizations are performed to determine the thickness, bandgap, phase purity, crystal orientation, and crystalline quality of the grown thin films. Amorphous islands of Ga2O3 are observed at growth temperature of 800 °C while continuous and crystalline (−201) oriented β-Ga2O3 thin films are achieved for growth temperatures of 850–950 °C. Crystallinity of the films is found to improve with increase in growth temperature with a minimum rocking full width at half maximum of 1.52º in sample grown at 925 °C. For all the samples grown at and above 875 °C, transmittance measurements revealed an optical bandgap of ~4.77–4.80 eV with high growth rate of ~6 µm/h.

Epitaxial growth of BaSi2 thin films by co-sputtering of Ba and Si for solar cell applications

A new growth method for BaSi2 thin film has been developed by co-sputtering Ba and Si to solve the problem that it is difficult to sputter BaSi2 epitaxial films by a single BaSi2 target. A template layer was first optimized for the subsequent BaSi2 deposition. X-ray diffraction results revealed that BaSi2 epitaxial films with high crystalline quality have been achieved under different growth temperatures between 500 °C–600 °C and different growth rates from 9.37–16.7 nm min−1. Compared with molecular beam epitaxy, the growth rate of BaSi2 was increased by more than one order of magnitude. The new growth method provides a high-speed, low-cost way for the growth of high-quality BaSi2 thin films. BaSi2-based devices such as an n-BaSi2/p-Si heterojunction diode and a Ag/n-BaSi2 Schottky junction diode were also fabricated. We demonstrated the obvious rectifying properties in these junctions, which will be a guide to design and fabricate BaSi2 thin-film solar cells.

Epitaxial growth and characterization of Cu thin films deposited on Al2O3(0001) substrates by magnetron sputtering

Epitaxial Cu thin films were grown on Al2O3 (0001) substrate by radio frequency magnetron sputtering. The epitaxial relations were found to be Cu(111)//Al2O3(0001 Cu[1_0]//Al(0001 Cu[1_0]//Al2O3[1_100] in the out-of-plane direction and Cu(1_10)//Al2O3(11_20), Cu[111]//Al2O3[0001] in the in-plane direction. Oxygen adsorbed from the air does not affect the structure of Cu/Al2O3(0001) epitaxial thin films. The evolution of the surface roughness can be described by a dynamic scaling exponent. In these Cu/Al2O3(0001) epitaxial thin films, the electrical resistivity ρ decreased as their thickness tCu increased; the trend was described well using a second-order exponential function; this result suggests that ρ changes rapidly when tCu is thinner than the effective thickness teff, and changes gradually when tCu >teff.

Effect of Ta irradiation on microstructures and current carrying properties of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–</sub><sub><italic>δ</italic></sub>

High-temperature superconducting coated conductors (HTS CCs), which are based on epitaxial growth and biaxial texture technology of REBCO thin films, show excellent physical properties including high transition temperature T c,on, high irreversible field H irr, as well as high critical current density J c. However, YBa2Cu3O7– δ (YBCO) has limited applications in a high magnetic field because of low critical current. Previous studies have shown that building smaller artificial pinning centers (APCs) is an effective way to increase the J c of HTS CCs. Compared with the traditional chemical doping route, the particle irradiation method will result in various types and sizes of defects with uniform distribution by controlling the type, energy and dose of particles and keeping the original chemical ratio of the material. 1.2 µm YBCO superconducting layer was deposited on the LaMnO3 substrate of the samples by metal organic deposition (MOD) method. YBCO thin films were irradiated with 1.9 GeV Ta ions at room temperature and vacuum conditions. The microstructure of irradiated samples was systematically observed using a field emission transmission electron microscope (FETEM). The result shows that Ta ions do produce continuous columnar defects with uniform distribution in the superconducting layer of the YBCO films, and the size of the defects is about 10 nm. The basic superconducting parameters, such as T c,on and J c, were measured using magnetic property measurement system (MPMS). The result shows the T c,on of the pristine sample was 91.3 K. When the irradiation fluence was small, the T c,on of the doped sample was basically unchanged. As the fluence reached 5.0×1010 ions/cm2, the T c,on decreased by 0.6 K. In addition, the superconducting antimagnetic phases did not change much until the fluence exceeded 1.0×108 ions/cm2. After the fluence exceeded 1.0×108 ions/cm2, the superconducting antimagnetic phases increased obviously. They increased with the decrease of temperature as well. The result shows that J c increased obviously with the increase of fluence for the samples. In normal situation, the current carrying properties of the samples irradiated in the low field should decrease compared with that before irradiation, but the J c of the samples increased in the low field because of the increase of superconducting antimagnetic phases in the samples after irradiation. At 30 K and 1 T, the J c increased by 3 times of the original, and at 77 K, the J c also improved by 2.7 times in the same field. To analyze the pinning state in this process, the exponential relationship model of J c∝ H – β was used to fit the data of the samples at 30 K and 77 K. The result shows that the doped samples with 5.0×1010 ions/cm2 had the higher value of H * and β , which suggests the generation of new effective pinning centers due to the Ta ion implantation. The normalized pinning force f p = F p/ F p,max of samples irradiated with different fluence was analyzed as a function of the magnetic field h = H / H max by using Higuchi model. The fitting result shows that Ta ion irradiation does affect the pinning behavior of doped YBCO films. Further study is needed to find out why Ta ions have such effects on the YBCO films.

Influence of Precursor Concentration on Crystalline Quality of GaN Thin Films Grown on a Sapphire Wafer

Abstract Chemical vapor deposition (CVD), which involves chemical reactions in gases for deposition on a heated surface, is an extensively used manufacturing technique for obtaining thin films of materials like silicon, graphene, silicon carbide, aluminum nitride, and gallium nitride (GaN). The process is driven by heat and mass transfer, fluid flow, and chemical reactions in the gases and at the surface. GaN is one of the most promising materials for manufacturing optical and electronic devices. However, the reliability and durability of the GaN-based devices depend on the crystalline quality of the thin films used. In this study, the epitaxial growth of GaN thin films on sapphire (Al2O3) wafers is carried out in a vertical rotating disk metalorganic chemical vapor deposition (MOCVD) system. Epitaxial growth refers to the process of growing a crystal of a particular orientation on the top of another crystal, with the orientation being determined by the underlying crystal. MOCVD reactors are CVD systems that use metalorganic compounds that consist of metal and organic ligands, leading to materials like GaAs, AlN, InN, and GaN. The quality of the thin films is largely determined by the choice of operating conditions such as the flowrate, surface temperature, and concentration of the metalorganic precursors that decompose due to heat in the reactor, react, and deposit the desired material on the surface of a wafer or a heated susceptor. In this experimental study, the crystalline quality and surface morphology of GaN thin films are evaluated using atomic force microscopy (AFM), X-ray diffraction (XRD), and Raman spectroscopy. The correlation between the crystalline quality of GaN thin films and the flowrate of the precursors is examined in detail on the basis of an evaluation of the dislocation density. The results indicate that a low concentration (V/III) ratio, where V and III refer to elements in the fifth and third groups of the periodic table, is beneficial for obtaining a high deposition rate since a low value of this ratio implies a high precursor concentration. However, it negatively affects the crystalline quality of the thin film. Similarly, high V/III ratios lead to low deposition rates and better crystalline quality, indicating the need to optimize the process.

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.