Oxygen vacancy control of electrical,optical,and magnetic properties of Fe_(0.05)Ti_(0.95)O_(2) epitaxial films

High-quality Fe-doped TiO_(2) films are epitaxially grown on MgF_(2) substrates by pulsed laser deposition.The x-ray diffraction and Raman spectra prove that they are of pure rutile phase.High-resolution transmission electron microscopy(TEM)further demonstrates that the epitaxial relationship between rutile-phased TiO_(2) and MgF_(2) substrates is 110 TiO_(2)||110 MgF_(2).The room temperature ferromagnetism is detected by alternative gradient magnetometer.By increasing the ambient oxygen pressure,magnetization shows that it decreases monotonically while absorption edge shows a red shift.The transport property measurement demonstrates a strong correlation between magnetization and carrier concentration.The influence of ambient oxygen pressure on magnetization can be well explained by a modified bound magnetization polarization model.

Anomalous Hall effect of facing-target sputtered ferrimagnetic Mn_(4)N epitaxial films with perpendicular magnetic anisotropy

Epitaxial Mn_(4)N films with different thicknesses were fabricated by facing-target reactive sputtering and their anomalous Hall effect(AHE)is investigated systematically.The Hall resistivity shows a reversed magnetic hysteresis loop with the magnetic field.The magnitude of the anomalous Hall resistivity sharply decreases with decreasing temperature from300 K to 150 K.The AHE scaling law in Mn_(4)N films is influenced by the temperature-dependent magnetization,carrier concentration and interfacial scattering.Different scaling laws are used to distinguish the various contributions of AHE mechanisms.The scaling exponentγ>2 for the conventional scaling in Mn_(4)N films could be attributed to the residual resistivityρ_(xx0).The longitudinal conductivityσ_(xx)falls into the dirty regime.The scaling ofρ_(AH)=αρ_(xx0)+bρ_(xx)~nis used to separate out the temperature-independentρ_(xx0)from extrinsic contribution.Moreover,the relationship betweenρ_(AH)and pxx is fitted by the proper scaling to clarify the contributions from extrinsic and intrinsic mechanisms of AHE,which demonstrates that the dominant mechanism of AHE in the Mn4 N films can be ascribed to the competition between skew scattering,side jump and the intrinsic mechanisms.

First-principles study of FeSe epitaxial films on SrTiO_3

The discovery of high temperature superconductivity in FeSe films on SrTiO3 substrate has inspired great experimen- tal and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of FeSe epitaxial films, emphasiz- ing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between FeSe monolayer and bilayer films on SrTiO3 is discussed.

Exploring heteroepitaxial growth and electrical properties of α-Ga_(2)O_(3) films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga_(2)O_(3) thin films deposited on a-plane, mplane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga_(2)O_(3) thin films on m-plane and r-plane sapphire substrates are higher than α-Ga_(2)O_(3) thin film on a-plane sapphire substrates. All three thin films have a high transmission of over 80% in the visible and near-ultraviolet regions, and their optical bandgaps stay around 5.02–5.16 eV. Hall measurements show that the α-Ga_(2)O_(3) thin film grown on r-plane sapphire has the highest conductivity of 2.71 S/cm, which is at least 90 times higher than the film on a-plane sapphire. A similar orientation-dependence is seen in their activation energy as revealed by temperature-dependent conductivity measurements, with 0.266, 0.079, and 0.075eV for the film on a-, m-, r-plane, respectively. The origin of the distinct transport behavior of films on differently oriented substrates is suggested to relate with the distinct evolution of oxygen vacancies at differently oriented substrates. This study provides insights for the substrate selection when growing α-Ga_(2)O_(3) films with tunable transport properties.

Fe-doped epitaxial YBCO films prepared by chemical solution deposition

YBa2Cu3O7-d （YBCO）-coated conductors havewide-ranging potential in large-scale applications such assuperconducting maglev trains and superconducting electriccables, but low current carrying capability restrains thepractical application of YBCO-coated conductors at hightemperatures and high magnetic fields. It is crucial todevelop YBCO-coated conductors with high critical currentdensity. In this paper, epitaxial, dense, smooth, andcrack-free Fe-doped YBCO films were prepared on aLaAlO3 single crystal substrate via a fluorine-free polymerassistedmetal organic deposition method. The effects ofthe dilute Fe doping on microstructure and superconductingcharacter of YBCO films were investigated. The criticaltemperature for superconducting of the Fe-dopedYBCO films decreases slightly. However, the in-fieldcritical current density of YBCO films improves with diluteFe doping of amounts less than x = 0.005, compared to thepure YBCO film. Therefore, the current carrying capabilityof YBCO film can improve by doping with appropriateamounts of Fe. This means that dilute Fe doping in YBCOfilms may be a feasible way to prepare high-performancecoated conductors.

Magnetic and Transport Properties of Mn_(0.98)Cr_(0.02)Te Epitaxial Films Grown on Al_2O_3 Substrates

The epitaxial Mno0.98Cr0.02Te films on single crystal Al2O3 （0001） substrates were prepared by pulsed laser deposition. The X-ray diffraction and scanning electron microscopy results showed that the good continuous epitaxial film was obtained with substrate temperature of 500 ℃. When the substrate temperature reached 700 ℃, the film was island growth and the manganese oxides phase appeared. The temperature dependence of both the magnetization and electrical resistance showed a sharp rise at around 60 K due to the magnetoelastic coupling. The temperature dependence of the electrical resistance of Mno0.98Cr0.02Te provided evidence for a transition from the metallic to semiconducting state at 305 K due to the spin disorder scattering with a large contribution from the influence of magnon drag.

Observation of Tunneling Gap in Epitaxial Ultrathin Films of Pyrite-Type Copper Disulfide

We report scanning tunneling microscopy investigation on epitaxial ultrathin films of pyrite-type copper disulfide.Layer-by-layer growth of CuS_（2 ）films with a preferential orientation of（111）on SrTiO_3（001）and Bi_2Sr_2Ca Cu_2O_（8＋）substrates is achieved by molecular beam epitaxy growth.For ultrathin films on both kinds of substrates,we observe symmetric tunneling gap around the Fermi level that persists up to^15 K.The tunneling gap degrades with either increasing temperature or increasing thickness,suggesting new matter states at the extreme twodimensional limit.

Bipolar Resistive Switching in Epitaxial Mn_3O_4 Thin Films on Nb-Doped SrTiO_3 Substrates

Spinel （O01）-orientated Mn304 thin films on Nb-doped SrTi03 （001） substrates are fabricated via the pulsed laser deposition method. X-ray diffraction and high-resolution transmission electron microscopy indicate that the as-prepared epitaxial fihn is well crystaiHzed. In the film plane the orientation relationship between the film and the substrate is [lOOjMn3 04 ｜｜[110] Nb-doped SrTiO3. After an electroforming process, the film shows bipolar nonvolatile resistance switching behavior. The positive voltage bias drives the sample into a low resistance state, while the negative voltage switches it back to a high resistance state. The switching polarity is different from the previous studies. The complex impedance measurement suggests that the resistance switching behavior is of filament type. Due to the performance reproducibility and state stability, Mn3O4 might be a promising candidate for the resistive random access memory devices.

Effect of Oxygen Partial Pressure on Epitaxial Growth and Properties of Laser-Ablated AZO Thin Films

Al-doped ZnO（AZO） thin films were grown on c-sapphire substrates by laser ablation under different oxygen partial pressures（P_（O2））.The effect of P_（O2） on the crystal structure,preferred orientation as well as the electrical and optical properties of the films was investigated.The structure characterizations indicated that the as-grown films were single-phased with a wurtzite ZnO structure,showing a significant c-axis orientation.The films were well crystallized and exhibited better crystallinity and denser texture when deposited at higher P_（O2）.At the optimum oxygen partial pressures of 10- 15 Pa,the AZO thin films were epitaxially grown on c-sapphire substrates with the（0001） plane parallel to the substrate surface,i e,the epitaxial relationship was AZO（000 1） // A1_2O_3（000 1）.With increasing P_（O2）,the value of Hall carrier mobility was increased remarkably while that of carrier concentration was decreased slightly,which led to an enhancement in electrical conductivity of the AZO thin films.All the films were highly transparent with an optical transmittance higher than 85%.

Growth of β-Ga_2O_3 Films on Sapphire by Hydride Vapor Phase Epitaxy

Two-inch Ga_2O_3 films with（ˉ201）-orientation are grown on c-sapphire at 850–1050°C by hydride vapor phase epitaxy. High-resolution x-ray diffraction shows that pure β-Ga_2O_3 with a smooth surface has a higher crystal quality, and the Raman spectra reveal a very small residual strain in β-Ga_2O_3 grown by hydride vapor phase epitaxy compared with bulk single crystal. The optical transmittance is higher than 80% in the visible and near-UV regions, and the optical bandgap energy is calculated to be 4.9 e V.

Substrate Effect on the Structural and Electrical Properties of LaNiO_(3) Thin Films

Epitaxial LaNiO_(3)(LNO)thin films prepared from the sols modified with polyethyleneimine(PEI)were grown on single-crystal LaAlO_(3),(LaAlO_(3))_(0.3)(SrAlTaO_(6))_(0.7),and SrTiO_(3) substrates,respectively,using a simple polymer assisted deposition(PAD).The epitaxial structure,surface morphologies and transport of the LNO films were studied by X-ray diffraction(θ/2θ symmetric scan,ω-scan,and in-planeφ-scan),the field emission scanning electron microscopy,and a standard dc four-probe method.It is found that,compared with that of LNO bulk,the c-axis parameter of the LNO film increases under compressive strain and decreases under tensile strain.All the LNO films exhibit metal properties in the temperature-dependent resistivity.The resistivity of the LNO films shows an increasing trend with the lattice mismatch strain changing from compressive to tensile.It is suggested that the oxygen vacancy compensated by more Ni^(2+)changed from Ni^(3+)in the film increases with the strain changing from compressive to tensile,which results in the increase of the resistivity.

Molecular Beam Epitaxy Growth of Tetragonal FeS Films on SrTiO3(001) Substrates

We report the successful growth of the tetragonal FeS film with one or two unit-cell （UC） thickness on SrTiO33（001） substrates by molecular beam epitaxy. Large lattice constant mismatch with the substrate leads to high density of defects in single-UC FeS, while it has been significantly reduced in the double-UC thick film due to the lattice relaxation. The scanning tunneling spectra on the surface of the FeS thin film reveal the electronic doping effect of single-UC FeS from the substrate. In addition, at the Fermi level, the energy gaps of approximately 1.5？meV are observed in the films of both thicknesses at 4.6？K and below. The absence of coherence peaks of gap spectra may be related to the preformed Cooper-pairs without phase coherence.

Tuning the Electronic Structure of Sr2IrO4 Thin Films by Bulk Electronic Doping Using Molecular Beam Epitaxy

By means of oxide molecular beam epitaxy with shutter-growth mode, we fabricate a series of electron-doped （Sr1-xLax）2IrO4 （001） （x=0, 0.05, 0.1 and 0.15） single crystalline thin films and then investigate the doping dependence of the electronic structure utilizing in-situ angle-resolved photoemission spectroscopy. It is found that with the increasing doping content, the Fermi levels of samples progressively shift upward. Prominently, an extra electron pocket crossing the Fermi level around the M point is evidently observed in the 15% nominal doping sample. Moreover, bulk-sensitive transport measurements confirm that the doping effectively suppresses the insulating state with respect to the as-grown Sr2IrO4, though the doped samples still remain insulating at low temperatures due to the localization effect possibly stemming from disorders including oxygen deficiencies. Our work provides another feasible doping method to tune electronic structure of Sr2 IrO4.

Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of Pyrite CuSe2 Films on SrTiO3

We perform molecular beam epitaxy growth and scanning tunneling microscopy study of copper diselenide （CuSe2 ） films on SrTiO3 （001）. Using a Se-rich condition, the single-phase pyrite CuSe2 grows in the Stranski-Krastanov （layer-plus-island） mode with a preferential orientation of （111）. Our careful inspection of both the as-grown and post-annealed CuSe2 films at various temperatures invariably shows a Cu-terminated surface, which, depending on the annealing temperature, reconstructs into two distinct structures 2 ×√3 and √x ×√3-R30°. The Cu termi- nation is supported by the depressed density of states near the Fermi level, measured by in-situ low temperature scanning tunneling spectroscopy. Our study helps understand the preparation and surface chemistry of transition metal pyrite dichalcogenides thin films.

Effect of elastic domains on electromechanical response of epitaxial ferroelectric films with a three-domain architecture

Thermodynamics of(001)epitaxial ferroelectric films completely relaxed due to the formation of elastic domains with a three-domain architecture is presented.The polydomain structure and electromechanical response of such films are analyzed for two cases corresponding to immobile and mobile elastic domain walls.It is shown that immobile elastic domains provide additional constraint which increases the mechanical and electrical clamping,thereby significantly reducing the piezoelectric and dielectric responses.On the other hand,a polydomain ferroelectric film adapts to the variations in the applied electric field by reversible domain wall displacements in the case of mobile domain walls.The comparison of the theory with experiments shows that the elastic domain walls are mobile in the fully relaxed films of~1μm thickness.In addition,if the substrate constraint is reduced via decreasing lateral size of a polydomain ferroelectric film,its piezoresponse will increase dramatically,as is experimentally verified on small islands of polydomain ferroelectric films.The general conclusions can be readily applied to other constrained polydomain films.

Flexible oxide epitaxial thin films for wearable electronics:Fabrication,physical properties,and applications

Recently,flexible oxide epitaxial thin films are of increasing interests owing to their excellent physical properties and wide applications.The oxide epitaxial thin films with flexible,lightweight and wearable are promising for the applications in flexible and wearable devices,such as flexible sensors,flexible detectors,flexible oscillators,flexible spintronics,wearable displays and electronic skin,etc.This review aims to summarize the fabrication,physical properties and applications of the flexible oxide epitaxial thin films for wearable electronics in most recent few years.The fabrication of flexible oxide epitaxial thin films reviewed here mainly includes the deposition on flexible substrates at high temperature and epitaxial lift-off(ELO)from rigid substrates.The physical properties and applications of flexible oxide epitaxial thin films reviewed here chiefly focus on the area of electricity and magnetism,including stable and tunable physical properties in the flexible oxide epitaxial thin films.In final,the perspectives and challenges of flexible oxide thin films for wearable electronics have been also addressed.

Giant electrocaloric effect in BiFeO_(3) and La codoped PbZr_(0.7)Ti_(0.3)O_(3) epitaxial thin films in a broad temperature range

Ferroelectric thin/thick films with large electrocaloric(EC)effect are critical for solid state cooling technologies.Here,large positive EC effects with two EC peaks in a broad temperature range(~100 K)were obtained in 0.95Pb_(0.92)La_(0.08)(Zr_(0.70)Ti_(0.30))_(0.98)O_(3)-0.05BiFeO_(3)(BFO-La-codoped PZT)epitaxial thin films deposited on the(100),(110)and(111)oriented SrTiO_(3)(STO)substrates by a sol-gel method.The thin film deposited on the(111)oriented STO substrate exhibited a stronger EC effect(~20.6 K at 1956 kV/cm)near room temperature.However,the thin films deposited on the(100)and(110)oriented STO substrates exhibited a stronger EC effect(~18.8 K at 1852 kV/cm and~20.8 K at 1230 kV/cm,respectively)around the peak of the dielectric permittivity(T_(m),~375 K).Particularly,as the direction of the applied electric field was switched(E<0),the DT of the(100)-oriented thin films around T_(m) was enhanced significantly from 18.8 K to 38.1 K.The self-induced-poling during the preparing process maybe plays a key role on the magic phenomenon.It can be concluded that the BFO-La-codoped PZT epitaxial thin films are promising candidates for application in the next solid-state cooling devices.

Microstructure and secondary phases in epitaxial LaBaCo_2O_(5.5 + δ) thin films

Aberration-corrected scanning transmission electron microscopy was employed to investigate the microstructures and secondary phases in LaBaCo2O5.5＋δ（LBCO） thin films grown on SrTiO3 （STO） substrates. The as-grown films showed an epitaxial growth on the substrates with atomically sharp interfaces and orientation relationships of [100]LBCO//[100]STO and （001）LBCO//（001）STO. Secondary phases were observed in the films, which strongly depended on the sample fabrication conditions. In the film prepared at a temperature of 900 ℃, nano-scale CoO pillars nucleated on the substrate, and grew along the [001] direction of the film. In the film grown at a temperature of 1000 ℃, isolated nano-scale C0304 particles appeared, which promoted the growth of {111 } twinning structures in the film. The orientation relationships and the interfaces between the secondary phases and the films were illustrated, and the growth mechanism of the film was discussed.

Grain boundary boosting the thermal stability of Pt/CeO_(2)thin films

Understanding how defect chemistry of oxide material influences the thermal stability of noble metal dopant ions plays an important role in designing high-performance heterogeneous catalytic systems.Here we use in-situ ambient-pressure X-ray photoemission spectroscopy(APXPS)to experimentally determine the role of grain boundary in the thermal stability of platinum doped cerium oxide(Pt/CeO_(2)).The grain boundaries were introduced in Pt/CeO_(2)thin films by pulsed laser deposition without significantly change of the surface microstructure.The defect level was tuned by the strain field obtained using a highly/low mismatched substrate.The Pt/CeO_(2)thin film models having well defined crystallographic properties but different grain boundary structural defect levels provide an ideal platform for exploring the evolution of Pt–O–Ce bond with changing the temperature in reducing conditions.We have direct demonstration and explanation of the role of Ce^(3+)induced by grain boundaries in enhancing Pt2+stability.We observe that the Pt^(2+)–O–Ce^(3+)bond provides an ideal coordinated site for anchoring of Pt^(2+)ions and limits the further formation of oxygen vacancies during the reduction with H_(2).Our findings demonstrate the importance of grain boundary in the atomic-scale design of thermally stable catalytic active sites.

Error Estimate of a Second Order Accurate Scalar Auxiliary Variable (SAV) Numerical Method for the Epitaxial Thin Film Equation

A second order accurate(in time)numerical scheme is analyzed for the slope-selection(SS)equation of the epitaxial thin film growth model,with Fourier pseudo-spectral discretization in space.To make the numerical scheme linear while preserving the nonlinear energy stability,we make use of the scalar auxiliary variable(SAV)approach,in which a modified Crank-Nicolson is applied for the surface diffusion part.The energy stability could be derived a modified form,in comparison with the standard Crank-Nicolson approximation to the surface diffusion term.Such an energy stability leads to an H2 bound for the numerical solution.In addition,this H2 bound is not sufficient for the optimal rate convergence analysis,and we establish a uniform-in-time H3 bound for the numerical solution,based on the higher order Sobolev norm estimate,combined with repeated applications of discrete H¨older inequality and nonlinear embeddings in the Fourier pseudo-spectral space.This discrete H3 bound for the numerical solution enables us to derive the optimal rate error estimate for this alternate SAV method.A few numerical experiments are also presented,which confirm the efficiency and accuracy of the proposed scheme.