Single Crystalline Films Research Articles

Perspectives for the Growth of Epitaxial 2D van der Waals Layers with an Emphasis on Ferromagnetic Metals for Spintronics

AbstractWafer scale epitaxial growth of 2D van der Waals materials and heterostructures is of ultimate importance for applications. Using synthetic thin film methods is also important to realize 2D materials which do not exist in nature and cannot be grown in equilibrium in bulk form. In this work, the perspectives in wafer scale growth of 2D van der Waals metallic ferromagnets (FMs) and the combination with other structurally and chemically compatible materials are described to propose functional devices. The areas such as 2D van der Waals ferromagnetic π‐junctions for quantum computing, and all 2D magnetic tunnel junctions (MTJs) and spin orbit torque devices which can be used for spintronics are identified. It is foreseen that a new class of Janus materials which can be realized only by epitaxial synthetic methods can enhance the performance of spintronic devices. Finally, this work describes the perspectives to overcome the shortcomings of epitaxial techniques and layer transfer by proposing remote epitaxy for the detachment of 2D materials films from the substrate and the use of liquid substrates or liquid catalysts to achieve single crystalline 2D materials films over large areas on the wafer.

Multimodal analysis of Zr substitution effects on magnetic and crystallographic properties in (Sm1−xZrx)(Fe0.8Co0.2)12 compounds with ThMn12 structure

SmFe12-based compounds containing Zr atoms have been recognized as candidates for novel high-performance permanent magnet materials with a high saturation magnetization. Partial substitution of Sm with Zr in (Sm1−xZrx)(Fe0.8Co0.2)12 has been known to enhance the magnetization of the compound. This study focused on (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films to evaluate the effects of Zr substitution for Sm on their structural and magnetic properties via systematic multimodal analysis. X-ray absorption fine structure (XAFS) and scanning transmission electron microscopy indicated a selective occupation of the Sm site by Zr atoms without disturbing the Fe sublattices. The chemical shift in the Zr-K edge XAFS spectra suggested electron transfer from Zr to the Fe band. Soft X-ray magnetic circular dichroism analysis proved that the magnetic moment of Sm was two orders of magnitude smaller than the magnetic moments of Fe and Co, and its contribution to the total magnetization was negligible. The partial substitution of Sm with Zr resulted in a volumetric expansion and c-axis length elongation despite Zr having a smaller atomic radius, which is unique to (Sm1−xZrx)(Fe0.8Co0.2)12 without structure-stabilizing elements such as Ti, V, and Si. This anisotropic expansion is explained by an increase in the magnetic moment and magnetic interactions in the Fe8f site, which is induced by the electron transfer from Zr to Fe8f atoms. Thus, we conclude that the electron transfer from Zr to the Fe8f atoms principally drives magnetization enhancement in (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films.

Wafer-scale single-crystalline β-Ga2O3 thin film on SiC substrate by ion-cutting technique with hydrophilic wafer bonding at elevated temperatures

Heterogeneous integration of β-Ga2O3 on a highly thermal conductive SiC substrate is an efficient solution to solve its bottleneck of thermal dissipation for high-power electronics. In this work, a 2-inch high-quality \(\left( {\bar 201} \right)\) β-Ga2O3 single-crystalline film was transferred to the 4H-SiC substrate via the ion-cutting technique with hydrophilic bonding at elevated temperatures. The evolution process of the surface blistering on the hydrogen-implanted β-Ga2O3 together with the internal pressure in blisters were investigated systematically to understand the physical mechanisms of the ion-cutting of β-Ga2O3 thin film. As suggested by the finite element simulation, the hydrophilic bonding was carried out at an elevated bonding temperature of 96°C to prevent the debonding of β-Ga2O3/4H-SiC during the ion-cutting process via reducing the thermal stress. The as-transferred β-Ga2O3 thin film exhibited a narrow full width at half maximum of the X-ray diffraction of 79.2 arcsec, and an extremely smooth surface with a root-mean-square roughness of 0.1 nm was achieved after chemical mechanical polishing. It is expected that the β-Ga2O3/4H-SiC heterogeneous integration material obtained by the ion-cutting technique with hydrophilic bonding at elevated temperatures will serve as a practical platform for high-performance β-Ga2O3 power devices.

Composition Engineering of (Lu,Gd,Tb)3(Al,Ga)5O12:Ce Film/Gd3(Al,Ga)5O12:Ce Substrate Scintillators

The paper addresses the development of composite scintillation materials providing simultaneous real-time monitoring of different types of ionizing radiation (α-, β-particles, γ-rays) in mixed fluxes of particles and quanta. The detectors are based on composite heavy oxide scintillators consisting of a thin single-crystalline film and a bulk single-crystal substrate. The film and substrate respond to certain types of ionizing particles, forming together an all-in-one composite scintillator capable of distinguishing the type of radiation through the different time characteristics of the scintillation response. Here, we report the structure, composition, and scintillation properties under different ionizing radiations of (Lu,Gd,Tb)3(Al,Ga)5O12:Ce films deposited using liquid phase epitaxy onto Gd3(Al1−xGax)5O12:Ce (GAGG:Ce) single-crystal substrates. The most promising compositions with the highest light yields and the largest differences in scintillation decay timing under irradiation with α-, β-particles, and γ-rays were selected. Such detectors are promising for environmental security purposes, medical tomography, and other radiation detection applications.

Disentangling ionic and electronic contributions to the switching dynamics of memristive Pr0.7Ca0.3MnO3/Al devices by employing a two-resistor model

Area-dependent memristive devices such as $\mathrm{Al}/{\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ (PCMO) stacks are highly interesting candidates for synapses in neuromorphic circuits due to their gradual switching properties, their reduced variability and the possibility to tune the resistance with the device area. However, due to the complexity of the different processes taking place, the electronic and ionic transport in theses devices is so far only poorly understood and physical compact models to simulate their behavior are missing so far. We developed a mathematical description of the dynamics of theses devices based on a simple two-resistor model that reproduces the device behavior very well. Based on x-ray photoelectron spectroscopy and impedance spectroscopy we assign the two resistors to the ${\mathrm{AlO}}_{x}$ layer and a depletion zone at the ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ layer, respectively. We assign the parameters used within the mathematical model to physical parameters and make use of them in order to explain the dynamics of the switching processes during the SET and RESET process in different voltage regimes. For both poly- and single crystalline PCMO thin film devices, oxygen migration between the ${\mathrm{AlO}}_{x}$ and the PCMO depletion zone is responsible for the resistance change. However, the dynamics differ significantly due to the increased mobility of oxygen vacancies with increasing defect density in the case of the polycrystalline samples. Moreover, we observe volatile subloops in our current-voltage curves, which vanish within millisecond time scale. Based on our two-resistor model and the band diagram derived from spectroscopic measurements, we assign these subloops to the injection of electrons into traps within the ${\mathrm{AlO}}_{x}$ barrier.

Band structures in orientation-controlled CuI thin films under epitaxial strain

Cuprous iodide (CuI) is a wide bandgap (\ensuremath{\sim}3.1 eV) semiconductor of great recent interest because of its high transparency, high hole conductivity, large exciton binding energy, and sizable spin-orbit interaction. However, studies on single-crystalline thin films have been scarcely reported. Here, we report on the epitaxial growth of single-crystalline CuI films and their optical characterization to elucidate the modification of band structure caused by the anisotropic strain. Thin films were grown by molecular beam epitaxy on nearly lattice-matched (only 0.1%-mismatched) InAs substrates with (001), (110), and (111) crystal orientation, yielding pseudomorphic structures with high lattice coherence and atomic-level flatness. Both reflectance and photoluminescence spectra exhibit sharp exciton profiles over a wide temperature range. We assign the exciton transition energies to deduce the band structure modifications associated with the epitaxial strain varying with temperature and growth orientation. Furthermore, we determine the deformation potentials which relate the strain to the band structure modification. The systematic studies of the strain effect for CuI thin films on various substrate planes will pave the way for the future optoelectronic application of CuI-based heterostructures.

Structural, luminescence and photoconversion properties of Lu3Al5O12:Ce single crystalline film phosphors for WLED application

This work focuses on the detailed investigation of structural, luminescence and photoconversion properties of Ce doped Lu3Al5O12:Ce (LuAG:Ce) single crystalline films (SCFs) grown using the Liquid Phase Epitaxy (LPE) method onto Y3Al5O12 (YAG) single crystal substrates as promoting converting film material for high-power WLED. The structure and chemical composition were analyzed using X-ray diffraction (XRD) and High-Resolution Scanning Transmission Electron Microscopy (HR-STEM). HR-STEM analysis and composition analysis revealed an interface with high structural quality including the formation of a transition layer with a 6 nm thickness between LuAG:Ce SCF and YAG substrate. The composition of this layer changes smoothly from YAG substrate towards LuAG:Ce SCF, and after equalizing the SCF/substrate misfit, stable LuAG:Ce film growth is observed. The optical characterization of LuAG:Ce films were performed using absorbance, cathodoluminescence and photoluminescence emission/excitation spectra and photoluminescence decay lifetime, as well as photoconversion measurements under blue, LED excitation. The corresponding color coordinates, color temperatures, color rendering index and luminous efficacy of white light sources based on blue LEDs and LuAG:Ce film converters with thicknesses in the 19–160 nm range were estimated and best characteristics are achieved for the 92 µm thick sample.

Influence of high pressure on Ce3+ luminescence in LuAlO3 and YAlO3 single crystals and single crystalline layers

Results of spectroscopic studies at ambient and high pressures of a LuAlO3:Ce3+ (LuAP:Ce) single crystalline film (SCF) as well as LuAP:Ce and YAlO3:Ce (YAP:Ce) single crystals are reported. Room temperature absorption measurements of the single crystals in the vacuum UV region allowed establishing the bandgap energies of 7.63 eV for YAP and 7.86 eV for LuAP, with an assumption of the direct band-gaps. Luminescence of Ce3+ in LuAP and YAP bulk crystals was measured as a function of temperature from 6 K up to 873 K. Temperature quenching of the Ce3+ luminescence in YAP:Ce was observed above 650 K, which is related to the location of the lowest Ce3+5d level at 1.27 eV below the conduction band minimum. No temperature quenching occurred in LuAP:Ce up to 873 K, mostly due to the lower energy of the 4f levels with respect to the valence band maximum. The barycenter energies and splittings of Ce3+5d states in YAP and LuAP at room temperature were precisely established. Theoretical calculations of the Ce3+5d states energy structure under pressure revealed a discrepancy between the obtained experimental results and the prediction of Dorenbos' theoretical model. The discrepancy can be removed if instead of the 5d state of the free Ce3+ ion the bandgap of the compound is taken as reference energy for the red-shift of the 5d level. This hypothesis also allows us to take into account the pressure-induced increase of the bandgap energy, expected for the studied compounds. Pressure dependences of LuAP:Ce luminescence spectra suggest that a certain type of phase transition occurs above 15 GPa.

Determination of electron and phonon temperatures in gold thin film irradiated with an ultrashort laser pulse

We measured transient reflectivity of single-crystalline and polycrystalline gold thin films in the wavelength region of 450–700 nm pumped by a 400 nm femtosecond pulse. The observed reflectivity changes were reproduced well by a reflectivity model that we developed based on the joint-density-of-states calculated from the band structure of gold and the Drude model. By a combination use of the reflectivity model and the two-temperature model, we evaluated the temporal evolution of the electron and phonon temperatures from the observed reflectivity changes. Based on the resultant electron and phonon temperatures, we derived the electron–phonon coupling factors, which were consistent with the literature values, in the wide range of the absorbed energy density.

Hole doping in a negative charge transfer insulator

RENiO3 is a negative charge transfer energy system and exhibits a temperature-driven metal-insulator transition (MIT), which is also accompanied by a bond disproportionation (BD) transition. In order to explore how hole doping affects the BD transition, we have investigated the electronic structure of single-crystalline thin films of Nd1−xCaxNiO3 by synchrotron based experiments and ab-initio calculations. Here we show that for a small value of x, the doped holes are localized on one or more Ni sites around the dopant Ca2+ ions, while the BD state for the rest of the lattice remains intact. The effective charge transfer energy (Δ) increases with Ca concentration and the formation of BD phase is not favored above a critical x, suppressing the insulating phase. Our present study firmly demonstrates that the appearance of BD mode is essential for the MIT of the RENiO3 series.

Improved frequency response of the 3.5 GHz SMR devices using Ga-doped AlN thin films as the piezoelectric layers

In this paper, the aluminum nitride (AlN) and gallium-doped aluminum nitride (AlGaN) films are adopted to fabricate the 3.5 GHz solidly mounted resonator (SMR) devices. A reactive radio frequency (RF) magnetron sputtering system and a co-puttering subsystem were used to deposit AlN and AlGaN thin films as piezoelectric layers. The sputtering parameters were fixed as sputtering pressure of 35 mTorr and sputtering power of 200 W for AlN piezoelectric layers. For the co-sputtering parameters, the sputtering pressure was set 10 mTorr, and the sputtering powers of Al and GaN targets were 175 W and 25 W, respectively, to deposit AlGaN piezoelectric layers. The X-ray diffraction (XRD) patterns showed that as compared with the 2[Formula: see text] values of the diffraction peaks of AlN thin films, those of AlGaN thin films shifted to lower values. Even the diffraction intensity of (002) peak of AlGaN thin film was higher than that of AlN film, both AlN and AlGaN thin films presented the strongly [Formula: see text]-axis orientated crystallization. The energy dispersive X-ray spectrometer (EDS) analysis showed that the content of Ga element in the AlGaN thin film, ([Formula: see text]), was about 19%. SEM images showed that the obtained AlN and AlGaN thin films exhibited uniform grain sizes and smooth surfaces. Finally, both the frequency responses of the fabricated SMR devices were measured using a network analyzer and the difference in their efficiencies were well compared. The frequency response of the fabricated SMR device with [Formula: see text][Formula: see text]N thin film showed the center frequency of 3.51 GHz, the return loss [Formula: see text] of −10.466 dB, quality factor [Formula: see text] of 170.952, electromechanical coupling coefficient [Formula: see text] of 4.026%, and FoM value of 6.838, respectively. The results show that the frequency response of SMR device has been improved by Ga doped in AlN thin film, although the [Formula: see text] value is still less than that of obtained using single crystalline AlGaN thin film.

Continuous Single-Crystalline GaN Film Grown on WS2 -Glass Wafer.

Use of 2D materials as buffer layers has prospects in nitride epitaxy on symmetry mismatched substrates. However, the control of lattice arrangement via 2D materials at the heterointerface presents certain challenges. In this study, the epitaxy of single-crystalline GaN film on WS2 -glass wafer is successfully performed by using the strong polarity of WS2 buffer layer and its perfectly matching lattice geometry with GaN. Furthermore, this study reveals that the first interfacial nitrogen layer plays a crucial role in the well-constructed interface by sharing electrons with both Ga and S atoms, enabling the single-crystalline stress-free GaN, as well as a violet light-emitting diode. This study paves a way for the heterogeneous integration of semiconductors and creates opportunities to break through the design and performance limitations, which are induced by substrate restriction, of the devices.

Scintillation properties of YAlO3:Ce perovskite co-doped by Mg2+ ions

The effect of Mg2+ co-doping on scintillation, luminescence, and optical properties of YAlO3:Ce perovskite was studied. Herein, a set of single-crystalline films with cerium content of 0.4% and various Mg concentrations ranging from 0 to 0.4% was grown by the liquid phase epitaxy. At increasing Mg content, the scintillation decays were gradually accelerated from 23 ns to less than 4 ns, however, at the expense of the light yield. At high Mg concentrations, the overlap of the 5d-4f(Ce3+) emission and charge-transfer absorption 2p(O2−)-4f(Ce4+) in the ultraviolet range became large and degraded scintillation properties. The low afterglow signal and independence of the light yield on the shaping time confirmed the low intensity of the delayed emission in the studied samples.

Fabrication and characterization of monocrystalline Zn2TiO4 films on MgO (111) substrates by PLD

Cubic Zn2TiO4 films were deposited on MgO (111) single crystal substrates using pulsed laser deposition (PLD). With the assistance of post-annealing process, the best epitaxial Zn2TiO4 film with only (111) single plane was obtained at 950[Formula: see text]C. For the best Zn2TiO4 single crystalline film, the full width at half maximum (FWHM) of the XRD [Formula: see text] rocking curve for the Zn2TiO4 (111) diffraction peak was only 0.12[Formula: see text]. From the TEM spectrum, the film had a regular lattice arrangement and strong periodicity. Structural analysis indicated that the out-plane and in-plane epitaxial arrangement of the film/substrate separately were Zn2TiO4 (111)//MgO (111) and Zn2TiO4 [10[Formula: see text]]//MgO <10[Formula: see text]>. The optical properties of the films were investigated in detail. After removing the influence of the transmittance of the substrate, the relative transmittance of the films exceeded 90% and the optical band gap of the films varied between 3.84 and 4.38 eV.

Epitaxial NiCo2O4 film as an emergent spintronic material: Magnetism and transport properties

The ferrimagnetic inverse spinel NiCo2O4 has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo2O4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo2O4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo2O4 films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo2O4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiCo2O4 thin films for nanoscale spintronic applications.

Structural properties of grain boundary in graphene grown on germanium substrates with different orientations

The direct synthesis of graphene with high-quality on semiconducting germanium (Ge) substrates has been developed recently, which has provided a promising way to integrate graphene with semiconductors for the application of electronic devices. However, the defects such as grain boundaries (GBs) introduced during the growth process have a significant influence on the crystalline quality of graphene and the performance of related electronic devices. Therefore, the investigation of the formation of GBs in graphene grown on a Ge substrate is essential for optimizing the crystalline quality of graphene. Herein, the formation mechanism and microstructure of GBs in graphene grown on Ge (110), Ge (001), and Ge (111) substrates via a chemical vapor deposition method are revealed. Ex situ atomic force microscopy is utilized to monitor the evolution of graphene domains. It is found that a single crystalline graphene film without GBs is formed on Ge (110), while polycrystalline graphene films with GBs are grown on Ge (001) and Ge (111) substrates, as suggested by transmission electron microscopy and x-ray photoelectron spectroscopy measurements. Our work may motivate the future exploration in improving the crystalline quality of graphene grown on a semiconducting substrate and the performance of associated electronic devices.

Epitaxial stabilization of an orthorhombic Mg-Ti-O superconductor

The family of titanium oxide superconductors exhibits many intriguing phenomena comparable to cuprates and iron pnictides/chalcogenides, and thus provides an ideal platform to contrastively study the unconventional pairing mechanism of high-temperature superconductors. Here, we successfully deposit superconducting Mg-Ti-O films on MgAl$_2$O$_4$ substrates with three principal orientations by ablating a MgTi$_2$O$_4$ target. Particularly, it is striking to observed that a single-crystalline film of an unintended structure has been grown on the (011)-oriented substrate, with the highest zero resistance transition temperature ($T_{\mathrm{c}0}$) of 5.0 K among them. The film has a highly reduced Mg/Ti ratio and an orthorhombic Ti$_9$O$_{10}$-like structure (denoted as Mg: Ti$_9$O$_{10}$), demonstrated by further characterizations of chemical composition and structure. Such a structure is unstable in bulk but favorable to be epitaxially stabilized on the (011)-surface of MgAl$_2$O$_4$ due to a relatively small strain at the formed interface. An isotropic upper critical field ($B_{\mathrm{c}2}$) up to 13.7 T that breaks the Pauli limit is observed in the Mg: Ti$_9$O$_{10}$ film, analogous to other superconducting titanium oxides. The similarity points to a common origin for the superconductivity in the family, which will provide valuable opinions for the mechanism of unconventional superconductivity in transition metal compounds.

Measuring and Then Eliminating Twin Domains in SnSe Thin Films Using Fast Optical Metrology and Molecular Beam Epitaxy.

van der Waals (vdW) layered chalcogenides have strongly direction-dependent (i.e., anisotropic) properties that make them interesting for photonic and optoelectronic applications. Orthorhombic tin selenide (α-SnSe) is a triaxial vdW material with strong optical anisotropy within layer planes, which has motivated studies of optical phase and domain switching. As with every vdW material, controlling the orientation of crystal domains during growth is key to reliably making wafer-scale, high-quality thin films, free from twin boundaries. Here, we demonstrate a fast optical method to quantify domain orientation in SnSe thin films made by molecular beam epitaxy (MBE). The in-plane optical anisotropy results in white-light being reflected into distinct colors for certain optical polarization angles and the color depends on domain orientation. We use our method to confirm a high density of twin boundaries in SnSe epitaxial films on MgO substrates, with square symmetry that results in degeneracy between SnSe 90° domain orientations. We then demonstrate that growing on a-plane sapphire, with rectangular lattice-matched symmetry that breaks the SnSe domain degeneracy, results in single-crystalline films with one preferred orientation. Our SnSe bottom-up film synthesis by MBE enables future applications of this vdW material that is particularly difficult to process by top-down methods. Our optical metrology is fast and can apply to all triaxial vdW materials.

Slip-Line-Guided Growth of Graphene.

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Highly heterogeneous epitaxy of flexoelectric BaTiO3-δ membrane on Ge

The integration of complex oxides with a wide spectrum of functionalities on Si, Ge and flexible substrates is highly demanded for functional devices in information technology. We demonstrate the remote epitaxy of BaTiO3 (BTO) on Ge using a graphene intermediate layer, which forms a prototype of highly heterogeneous epitaxial systems. The Ge surface orientation dictates the outcome of remote epitaxy. Single crystalline epitaxial BTO3-δ films were grown on graphene/Ge (011), whereas graphene/Ge (001) led to textured films. The graphene plays an important role in surface passivation. The remote epitaxial deposition of BTO3-δ follows the Volmer-Weber growth mode, with the strain being partially relaxed at the very beginning of the growth. Such BTO3-δ films can be easily exfoliated and transferred to arbitrary substrates like Si and flexible polyimide. The transferred BTO3-δ films possess enhanced flexoelectric properties with a gauge factor of as high as 1127. These results not only expand the understanding of heteroepitaxy, but also open a pathway for the applications of devices based on complex oxides.