Single Crystalline Films Research Articles

Effect of f–c hybridization on the γ → α phase transition of cerium studied by lanthanum doping

The hybridization between the localized 4f level (f) with conduction (c) electrons in γ-Ce upon cooling has been previously revealed in single crystalline thin films experimentally and theoretically, whereas its influence on the γ → α phase transition was not explicitly verified, due to the fact that the phase transition happened in the bulk-layer, leaving the surface in the γ phase. Here in our work, we circumvent this issue by investigating the effect of alloying addition of La on Ce, by means of crystal structure, electronic transport and angle resolved photoemission spectroscopy measurements, together with a phenomenological periodic Anderson model and a modified Anderson impurity model. Our current researches indicate that the weakening of f–c hybridization is the major factor in the suppression of γ → α phase transition by La doping. The consistency of our results with the effects of other rare earth and actinide alloying additions on the γ → α phase transition of Ce is also discussed. Our work demonstrates the importance of the interaction between f and c electrons in understanding the unconventional phase transition in Ce, which is intuitive for further researches on other rare earth and actinide metals and alloys with similar phase transition behaviors.

Maximizing intrinsic anomalous Hall effect by controlling the Fermi level in simple Weyl semimetal films

A large intrinsic anomalous Hall effect (AHE) originating in the Berry curvature has attracted growing attention for potential applications. The recently proposed magnetic Weyl semimetal ${\mathrm{EuCd}}_{2}{\mathrm{Sb}}_{2}$ provides an excellent platform for controlling the intrinsic AHE because it only hosts a Weyl-point-related band structure near the Fermi energy. Here, we report the fabrication of ${\mathrm{EuCd}}_{2}{\mathrm{Sb}}_{2}$ single-crystalline films and control of their anomalous Hall effect by a film technique. As also analyzed by first-principles calculations of energy-dependent intrinsic anomalous Hall conductivity, the obtained anomalous Hall effect shows a sharp peak as a function of carrier density, demonstrating a clear energy dependence of the intrinsic AHE.

The modulated oxygen evolution reaction performance in La2/3Sr1/3CoO3 by a design of stoichiometry offset

The accurate stoichiometry control of single-crystalline transition metal oxide thin film and the electrochemical performance such as in oxygen evolution reaction (OER) offer a useful pathway towards the mechanism understanding and the strategy of improving electrochemical activity at the atomic level. As a stable, low cost, and high active OER catalyst, La2/3Sr1/3CoO3 (LSCO) has drawn substantial attention recently. Here we report a modulated OER performance of LSCO by different stoichiometry offsets, an overall ∼2074.63% tunability can be seen. Meanwhile, the overpotential of LSCO exhibits a volcano trend as a function of oxygen concentration. These phenomena can be understood on the competition of double- and super-exchanges in Co3+/Co4+ with the help of the overlap of O 2p and Co 3d orbits. Our investigation provides an effective platform for obtaining a narrow window of improved electrochemical activity in correlated transition metal oxides.

Crystal structure and electronic property modification of Ca2RuO4 thin films via fluorine doping

Layered ruthenium oxyfluorides have various crystal structures and Ru oxidation states and exhibit unique physical properties. While various layered ruthenates have reportedly been topochemically fluorinated with Sr as $A$ sites, investigation of the fluorination of layered ruthenates containing smaller Ca ions is lacking. In this paper, we fabricated phase-pure and single-crystalline thin films of ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{2.5}{\mathrm{F}}_{2}$ on ${\mathrm{LaSrAlO}}_{4}$ (001) substrates via topochemical fluorination of the ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ precursor using polyvinylidene fluoride. The obtained fluorinated thin films had a chemical composition of ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{2.5}{\mathrm{F}}_{2}$ with the ${\mathrm{Ru}}^{3+}$ state, as determined by energy-dispersive x-ray spectroscopy and x-ray photoemission spectroscopy, whereas the film prepared via ${\mathrm{Sr}}_{2}{\mathrm{Ru}}^{4+}{\mathrm{O}}_{4}$ fluorination had a composition of ${\mathrm{Sr}}_{2}{\mathrm{Ru}}^{4+}{\mathrm{O}}_{3}{\mathrm{F}}_{2}$. Scanning transmission electron microscopy revealed that ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{2.5}{\mathrm{F}}_{2}$ has only 1 ${\mathrm{F}}^{\ensuremath{-}}$ site in CaO rock-salt blocks, whereas ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{3}{\mathrm{F}}_{2}$ has two inequivalent ${\mathrm{F}}^{\ensuremath{-}}$ sites in the SrO layers. The ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{2.5}{\mathrm{F}}_{2}$ film was insulating, with a resistivity (\ensuremath{\rho}) of $8.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ at 300 K. Moreover, the temperature behavior of \ensuremath{\rho} was well described by the two-dimensional variable range hopping model. These results demonstrate that local distortion is an important factor that governs the topochemical fluorination of ruthenates and affects the crystal and electronic structures of the reactants.

The electrical conductivity of cubic (In1−x Ga x )2O3 films (x ≤ 0.18): native bulk point defects, Sn-doping, and the surface electron accumulation layer

The alloying of the group-III transparent semiconducting sesquioxides In2O3 and Ga2O3 can lead to a modulation of the properties of the parent compounds, e.g. the shallow- and deep-donor character of the oxygen vacancy or the presence and absence of a surface electron accumulation layer, respectively. In this work, we investigate the effect of alloying on the electron transport properties of unintentionally-doped single-crystalline and textured bixbyite (In1−x Ga x )2O3 thin films annealed in oxygen and vacuum with Ga contents up to x = 0.18. Hall effect measurements demonstrate a surprising increase in electron density due to native defects with added Ga, possibly induced by Ga-related unit-cell distortions. Based on the measured electron mobility we assign this increase to oxygen vacancies rather than Ga-interstitials. The large electron density of >7 × 1019 cm−3 in vacuum-annealed, Ga-containing films would place the 2+/0 transition level of the O-vacancy in our films at least 340 meV above the conduction band minimum. A combined investigation based on hard and soft X-ray photoelectron spectroscopy measurements demonstrates the existence of the surface electron accumulation layer for all alloy films irrespective of annealing and, hence, no depletion up to x = 0.18. Comparison of Hall and Seebeck measurements confirms negligible contribution of the SEAL to the electron transport in the few-100 nm thick, unintentionally-doped films. Finally, we additionally demonstrate a single-crystalline (In0.92Ga0.08)2O3:Sn film, as a possible transparent conductive oxide with a wider band gap than that of (Sn-doped) In2O3, with transport properties similar to those of ITO.

The In-Plane-Two-Folders Symmetric a-Plane AlN Epitaxy on r-Plane Sapphire Substrate

In the present work, a single-crystalline epitaxial nonpolar a-plane AlN film with in-plane two-folder symmetries was successfully achieved on an r-plane sapphire substrate, by combining physical vapor deposition and a high-temperature annealing technique. Moreover, by varying the AlN thickness, the evolution of crystalline quality and structure were systematically investigated using X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The crystalline quality was much improved by the annealing treatment. Most importantly, when the thickness of AlN was increased up to 1000 nm, the AlN lattice was found to endure strong distortion along the out-of-plane direction, and the lattice showed an obvious expansion. The change of the surface morphology induced by high-temperature annealing was also tracked, and the morphology displayed structural anisotropy along the [11¯00] direction. Our results act as a crucial platform to better understand and employ the nonpolar AlN template; in particular, it is of importance for subsequent device fabrication.

Single-crystalline LiNbO3 film based wideband SAW devices with spurious-free responses for future RF front-ends

This work demonstrates super-high frequency wave-guided surface acoustic wave (SAW) devices based on LiNbO3/SiO2/Si layered substrates. SAW resonators with fundamental operating frequency to the 7-GHz range were developed, and wave modes like the Rayleigh, Shear-horizontal (SH), Pseudo-Bulk, etc., were observed. Furthermore, effective coupling coefficients (k2eff) and quality factor (Q) of the devices have been investigated systematically. Specifically, spurious-free SAW resonators with the SH wave frequency over 7.40 GHz and the high k2eff value of ∼7.8% were obtained at a nanoscale wavelength of 480 nm. Finally, high-performance filters with a bandwidth over 300 MHz were achieved. The demonstrated filters show sharp roll-off and spurious-free responses within the passband with insertion loss <−5 dB and a small temperature coefficient of frequency of ∼−45.8 ppm/K at a super-high frequency of 7.17 GHz. Upon further optimizations, high-performance SAW devices built on a single crystalline LiNbO3 film can potentially enable the low-loss and wideband signal processing functions, promising for the next-generation radio frequency front-end system applications.

Composite Detectors Based on Single-Crystalline Films and Single Crystals of Garnet Compounds.

This manuscript summarizes recent results on the development of composite luminescent materials based on the single-crystalline films and single crystals of simple and mixed garnet compounds obtained by the liquid-phase epitaxy growth method. Such composite materials can be applied as scintillating and thermoluminescent (TL) detectors for radiation monitoring of mixed ionization fluxes, as well as scintillation screens in the microimaging techniques. The film and crystal parts of composite detectors were fabricated from efficient scintillation/TL materials based on Ce3+-, Pr3+-, and Sc3+-doped Lu3Al5O12 garnets, as well as Ce3+-doped Gd3−xAxAl5−yGayO12 mixed garnets, where A = Lu or Tb; x = 0–1; y = 2–3 with significantly different scintillation decay or positions of the main peaks in their TL glow curves. This work also summarizes the results of optical study of films, crystals, and epitaxial structures of these garnet compounds using absorption, cathodoluminescence, and photoluminescence. The scintillation and TL properties of the developed materials under α- and β-particles and γ-quanta excitations were studied as well. The most efficient variants of the composite scintillation and TL detectors for monitoring of composition of mixed beams of ionizing radiation were selected based on the results of this complex study.

An in-situ TEM characterization of electron beam induced dislocation motion in a single-crystalline gold thin film

Dislocation motions driven by electron-beam illumination were in-situ observed in a single-crystalline gold thin-foil specimen, under a medium-voltage transmission electron microscope operated at 200 kV. The dislocation motion characteristics were in-situ observed under the bright-field and weak-beam dark-field conditions. The structures formed through the dislocation motion were also characterized using bright−/dark-field imaging and high-resolution techniques. Two types of dislocation motions are concluded, where one type of motion left no apparent traces, and the other type left a clear trace of passage. The dislocation motions have jerky behaviors, and the motions are accelerated when the beam intensity is increased. The immobile trace structures left by the dislocation motions are identified to be twin structures oriented along the 〈111〉 crystal directions. The dislocation motion mechanism is discussed and the motion might be caused by the electron beam heating effect or the electron beam induced phonon effects. The dislocation disassociation, planar defect formation in the Au crystal, and the beam heating impacts are also discussed.

Fabrication of Wafer-Scale Organic Single-Crystal Films with Uniform In-Plane Orientation via Wetting-Assisted In-Air Sublimation for High-Performance Transistor Arrays

The production of single-crystalline organic thin films on a large scale is challenging but imperative for industrial applications of organic electrics. This work improves a vapor-based method on the basis of our recently invented microspacing in-air sublimation (MAS) to achieve wafer-scale uniform single-crystalline organic films. The method alters the wettability of both the bottom and top substrates in MAS. By virtue of the vapor-to-melt-to-crystal process and the unique genetic relationship between the morphology of source materials and that of the grown crystals, wettability control on the one hand promotes the dispersion of the molten source materials and on the other hand drives the liquid crystal phase on the top substrate to form uniform single-crystalline films over the whole substrate. Using 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) as an example, centimeter-sized crystalline films with pure orientations are achieved. Transistor arrays fabricated on the crystalline films exhibit high yield and uniform performance with an average mobility of 3.91 cm2 V–1 s–1. This method provides an opportunity to realize vapor-grown single-crystalline films but not random and discrete single crystals via physical vapor transport nor the amorphous/polycrystalline films via vacuum deposition, which may be a step toward future commercialization of organic electronics.

Epitaxial lift-off of single crystalline CuI thin films

Single crystalline thin films of the transparent, p-type semiconductor copper iodide (CuI) were grown by pulsed laser deposition on SrF2(111) and sodium bromide (NaBr) sacrificial layers to create free-standing CuI films.

Showcasing the optical properties of monocrystalline zinc phosphide thin films as an earth-abundant photovoltaic absorber.

Zinc phosphide, Zn3P2, is a semiconductor with a high absorption coefficient in the spectral range relevant for single junction photovoltaic applications. It is made of elements abundant in the Earth's crust, opening up a pathway for large deployment of solar cell alternatives to the silicon market. Here we provide a thorough study of the optical properties of single crystalline Zn3P2 thin films grown on (100) InP by molecular beam epitaxy. The films are slightly phosphorus-rich as determined by Rutherford backscattering. We elucidate two main radiative recombination pathways: one transition at approximately 1.52 eV attributed to zone-center band-to-band electronic transitions; and a lower-energy transition observed at 1.3 eV to 1.4 eV attributed to a defect band or band tail related recombination mechanisms. We believe phosphorus interstitials are likely at the origin of this band.

Novel composite color converters based on Tb1.5Gd1.5Al5O12:Ce single crystalline films and Y3Al5O12:Ce crystal substrates

The structural, luminescence and photoconversion properties of composite color converters based on Tb1.5Gd1.5AG:Ce single crystalline films, grown by the liquid phase epitaxy method onto YAG:Ce single crystal substrates, are investigated.

Spin‐Torque Switching in Rare‐Earth‐Free Compensated Ferrimagnet Mn4N Films

AbstractIn this paper, the growth of rare‐earth‐free compensated ferrimagnet (C‐FIM) Mn4N single crystalline films is reported, with perpendicular magnetic anisotropy (PMA) on the MgO(001) substrates by reactive magnetron sputtering. Subsequently, the current‐induced spin‐orbit torque (SOT) switching is experimentally demonstrated through passing currents in the Hall bar device made of Mn4N/Pt bilayer, with the thickness of Mn4N approaching 60 nm. Furthermore, the accompanied SOT efficiencies are characterized by employing the harmonic Hall voltage measurements. In particular, an enhanced effective SOT efficiency with the increased thickness of Mn4N films is observed, suggesting the existence of a bulk‐like spin torque in the present material system. The results demonstrate that the perpendicularly magnetized C‐FIM Mn4N films can provide a promising material platform for developing the rare‐earth‐free FIM spintronics devices.

Wafer-Scale Epitaxy of Flexible Nitride Films with Superior Plasmonic and Superconducting Performance.

Transition-metal nitrides (e.g., TiN, ZrN, TaN) are incredible materials with excellent complementary metal-oxide semiconductor compatibility and remarkable performance in refractory plasmonics and superconducting quantum electronics. Epitaxial growth of flexible transition-metal nitride films, especially at the wafer scale, is fundamentally important for developing high-performance flexible photonics and superconducting electronics, but the study is rare thus far. This work reports the high-quality epitaxy of 2-in. titanium nitride (TiN) films on flexible fluorophlogopite-mica (F-mica) substrates via reactive magnetron sputtering. Combined measurements of spectroscopic ellipsometry and electrical transport reveal the superior plasmonic and superconducting performance of TiN/F-mica films owing to the high single crystallinity. More interestingly, the superconductivity of these flexible TiN films can be manipulated by the bending states, and enhanced superconducting critical temperature TC is observed in convex TiN films with in-plane tensile strain. Density functional theory calculations reveal that the strain can tune the electron-phonon interaction strength and the resultant superconductivity of TiN films. This study provides a promising route toward integrating scalable single-crystalline transition-metal nitride films with flexible electronics for high-performance plasmonics and superconducting electronics.

Promoting the Oxygen Evolution Activity of Perovskite Nickelates through Phase Engineering.

Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO3 by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO3 film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO3 single crystalline film. The obtained current density of 1038 μΑ cm-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.

Van der Waals coherent epitaxy of GaN and InGaN/GaN multi-quantum-well via a graphene inserted layer

GaN-based films grown on sp3-bonded single-crystalline substrates can maintain the coherent growth character. However, exfoliating III-nitride films from these substrates proves difficult because of the strong sp3-type covalent bonds between the substrates and epilayers. The sp2-bonded two-dimensional (2D) materials exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, so the GaN epilayer grown on 2D materials can be transferred onto foreign substrates with ease. In this paper, graphene is used as the inserted layer (IL) on freestanding GaN substrate and the van der Waals coherent epitaxy of GaN-based single-crystalline films on such graphene/GaN templates is investigated. Density functional theory computations are performed to probe the transmission of crystallographic information of wurtzite GaN through the graphene IL. The appropriate layer numbers of graphene IL and GaN growth temperature are optimized to demonstrate the coherent epitaxy character. Both theoretical and experimental results support that the coherent epitaxy of GaN can only be achieved by using a monolayer graphene IL, and the crystalline quality of optimized GaN film can reach the same level of that grown directly on GaN freestanding substrates.

Growing 3D-nanostructured carbon allotropes from CO2 at room temperature under the dynamic CO2 electrochemical reduction environment

Synthesis of nanostructured carbon allotropes using CO2 as a cheap carbon source is challenging and usually limited by high temperature activation. Herein, formation of 3D nanostructured carbon allotropes including nano-graphene and single crystalline nanodiamond films is feasible at room temperature on various single crystal metals (e.g., Bi, Ag, Zn, and Co) that were formed under the dynamic CO2 electrochemical reduction reaction environment at relatively low applied potential (−1.1 to −1.6 V vs. Ag/AgCl). The nanocrystalline carbon was obtained as the major products from CO2 (≥96% selectivity, ∼1 μm thickness) without any liquid products. Upon applying the negative potential, nanoclustering of the self-limiting ultrathin metal oxide layers of metal particles on the highly conductive substrate could lead to formation of negatively charged metal clusters, which were well stabilized by the ternary electrolyte system containing [BMIm]+[BF4]-/propylene carbonate/water. This system allows the reduction of CO2 into single atoms C∗ and the subsequently electrocrystallization of C∗ into carbon allotropes on the crystallographic planes of the single crystal metals formed as the building blocks. The CO2-derived Ag–C/epoxy composites show promising thermal conductivity. The results present a breakthrough advancement in the growth of nanostructured carbon allotropes from CO2 by the viable negative CO2 emission approach.

Study of optical and luminescent properties of the epitaxial garnet films doped with Ce3+

Ce-doped (Pb, Gd)3(Al, Ga)5O12 and Ce-doped (Bi, Gd)3(Al, Ga)5O12 single crystalline garnet films were grown using liquid-phase epitaxy method from supercooled PbO–B2O3- and Bi2O3–B2O3-based melt solutions on substrates from Gd3Ga5O12, Gd3(Al, Ga)5O12 and Y3Ga5O12 single crystals. Optical absorption, photo- and cathodoluminescent and scintillation properties of the films were studied. Ce-doped (Pb,Gd)3(Al, Ga)5O12 and Ce-doped (Bi, Gd)3(Al, Ga)5O12 garnet films can be used as a fast phosphor and scintillation screens.

Role of transferred graphene on atomic interaction of GaAs for remote epitaxy

Remote epitaxy is a recently discovered type of epitaxy, wherein single-crystalline thin films can be grown on graphene-coated substrates following the crystallinity of the substrate via remote interaction through graphene. Although remote epitaxy provides a pathway to form freestanding membranes by controlled exfoliation of grown film at the graphene interface, implementing remote epitaxy is not straightforward because atomically precise control of interface is required. Here, we unveil the role of the graphene–substrate interface on the remote epitaxy of GaAs by investigating the interface at the atomic scale. By comparing remote epitaxy on wet-transferred and dry-transferred graphene, we show that interfacial oxide layer formed at the graphene–substrate interface hinders remote interaction through graphene when wet-transferred graphene is employed, which is confirmed by an increase of interatomic distance through graphene and also by the formation of polycrystalline films on graphene. On the other hand, when dry-transferred graphene is employed, the interface is free of native oxide, and single-crystalline remote epitaxial films are formed on graphene, with the interatomic distance between the epilayer and the substrate matching with the theoretically predicted value. The first atomic layer of the grown film on graphene is vertically aligned with the top layer of the substrate with these atoms having different polarities, substantiating the remote interaction of adatoms with the substrate through graphene. These results directly show the impact of interface properties formed by different graphene transfer methods on remote epitaxy.