C-axis Lattice Constant Research Articles

Structural and optical properties of epitaxial ScxAl1−xN coherently grown on GaN bulk substrates by sputtering method

Three scandium aluminum nitride (ScAlN) thin films with different Sc compositions of 6%, 10%, and 14% were heteroepitaxially grown on n-type GaN bulk substrates by a low-temperature sputtering method. Atomically flat and smooth surfaces were observed by atomic force microscopy. The ScAlN films were coherently grown on GaN, and the c-axis lattice constants increased with increase in the Sc composition, confirmed by x-ray diffraction. The refractive index and the extinction coefficient of ScAlN were extracted by variable angle spectroscopic ellipsometry. The refractive index slightly increased and the extinction coefficient showed red shift with increase in the Sc composition. The optical bandgap of the ScAlN films was also extracted, which slightly shrunk with increase in the Sc composition.

Edge dislocation with [001] [formula omitted] induced positive magnetoresistance in BaTiO3/La0.66Sr0.33MnO3 heterostructure

The magnetoresistance (MR) effect holds immense potential for applications in various fields, including but not limited to storage and sensing. Manganese oxide has garnered significant attention due to its colossal magnetoresistance characteristics. Investigating the effects of the complex magnetic interactions on the magnetism and transport enables a better understanding of magnetic materials. La0.66Sr0.33MnO3 (LSMO) manganese oxides typically exhibiting negative MR as a result of magnetic field-induced spin order, while positive MR typically occurs in systems with spin-orbit coupling (SOC) or P-N junctions. In this work, the magnetic and transport properties of BaTiO3 (BTO)/LSMO heterostructure were studied. By introducing edge dislocation with [001] b→ (burgers vector) in BTO, the c-axis lattice constant is increased, resulting in positive MR in LSMO, which could be modulated by the magnetic field history and the annealing temperature. The nonlinear magnetic order induced by antiferromagnetism at the interface is proposed to cause the positive MR. Finally, the influence of ferroelectric layer on MR could be controlled by the ferroelectric BTO film. The increase in the large lattice constant and the observed variations in MR under magnetic field and annealing temperature provide valuable insights for further studies on the complex oxide.

Asymmetric optical properties and bandgap shift of pre-strained flexible ZnO films

Strain engineering has been extensively explored to modulate the various intrinsic properties of flexible inorganic semiconductor films. However, experimental characterization of tensile and compressive strain-induced modulation of optoelectronic properties and their differences has not been easily implemented in flexible inorganic semiconductor films. Herein, the strain-dependent structural, optical, and optoelectronic properties of flexible ZnO films under pre-tensile and pre-compressive strains are systemically investigated by a Mueller matrix ellipsometry-based quantitative characterization method combined with x-ray diffraction and first-principle calculation. With extended prestress-driven deposition processing under bi-direction bending modes, pre-tensile and pre-compressive strains with symmetric magnitudes can be achieved in flexible ZnO films, which allows precise observation of the strain-driven asymmetric modulation of optoelectronic properties. When the applied prestrain varies approximately equally from 0% (baseline) to −0.99% (compression) and 1.07% (tensility), respectively, the relative changes for the c-axis lattice constant are 0.0133 and 0.0104 Å, respectively. Meanwhile, the dependence factors of the bandgap energy on the pre-compression and pre-tensile strains were determined as −0.0099 and −0.0156 eV/%, respectively, and the complex refractive index also presents an asymmetric varying trend. With the help of the strain–stress analysis and the first-principle calculation, the intriguing asymmetric strain-optical modulation effect could be attributed to the biaxial strain mechanism and the difference in the deformation potential between the two prestrain modes. These systematic investigation consequences are thus promising as a basis for the booming applications of the flexible inorganic semiconductor ensemble.

Investigation of the crystal-oriented behavior of rare earth substituted Sr2NaNb5O15 lead-free piezoelectric materials under a high magnetic field

The crystal-orientation behavior of Sr2NaNb5O15 (SNN) lead-free piezoelectric materials under a magnetic field was investigated by substituting rare-Earth elements (Ln = Nd, Eu, Ho, Yb), which were selected based on their ionic radii and magnetic or non-magnetic ions. The magnetic ions, Nd, Ho, and Yb, did not affect the magnetic anisotropy of the SNN. The nonmagnetic ions (Eu3+) changed from the a, b-axis orientation to the c-axis orientation in the direction of the magnetic field. The Eu-substituted SNN powders revealed that the degree of orientation of the c-axis increased as the Eu content increased. Consequently, c-axis-oriented Eu-substituted SNN ceramics were obtained with the degree of orientation of 0.9. The diamagnetism along the a, b-axis became larger than that along the c-axis owing to a decrease in the c-axis lattice constant. By substituting Eu with SNN, the piezoelectric properties decreased as the hardening of SNN progressed.

Structural characterization of epitaxial ScAlN films grown on GaN by low-temperature sputtering

ScAlN has garnered substantial attention for its robust piezoelectric and ferroelectric properties, holding promise for diverse electronic device applications. However, the interplay between its structural attributes and physical properties remains poorly understood. This study systematically elucidates the structural characteristics of epitaxial ScAlN films grown on GaN by low-temperature sputtering. Correlations between Sc composition, lattice constants, and film strains were revealed utilizing high-resolution X-ray diffraction, reciprocal space mapping, and machine learning analyses. Our machine-learning model predicted c-axis lattice constants of ScAlN grown on GaN under various conditions and suggested that sputtering permits coherent growth over a wide compositional range. These findings advance the understanding of ScAlN and provide valuable insights for the research and development of novel ScAlN-based devices.

Anomalous Diamagnetic Torque Signals in Topological Nodal-Line Semimetal NaAlSi

In topological materials, peculiar surface states protected by crystal symmetries have attracted great interest because of the possibility of unprecedented correlation with superconductivity. A topological nodal-line semimetal with a layered structure, NaAlSi shows moderately anisotropic bulk superconductivity with Tc ≅ 7.0 K. The magnetic torque curves as a function of the field angle in the superconducting state are found to show two distinct diamagnetic signals; a large broad signal arising from bulk superconductivity and a small sharp signal observed only in magnetic fields nearly parallel to the layers. The upper critical field of the bulk superconductivity follows an anisotropic 3D model with a small anisotropic factor of ∼4. A possible scenario is that the sharp signal is due to highly 2D superconductivity with a similar Tc, whose thickness is only several times of the c-axis lattice constant (0.736 nm). The band structure calculations show the presence of large Fermi lines on the (001) plane, mainly formed by the surface Si p bands. The sharp diamagnetic torque signal could arise from the (001) surface superconductivity.

Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals

We fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystals annealed under oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device, we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, the O2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hysteresis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresis loops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygen content along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin films during the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitive than that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.

Effects of Energetic Carbon-Cluster Ion Irradiation on Lattice Structures of EuBa2Cu3O7−x Oxide Superconductor

C-axis-oriented EuBa2Cu3O7−x oxide films that were 100 nm thick were irradiated with 0.5 MeV C monoatomic ions, 2 MeV C4 cluster ions and 4 MeV C8 cluster ions at room temperature. Before and after the irradiation, X-ray diffraction (XRD) measurement was performed using Cu-Ka X-ray. The c-axis lattice constant increased almost linearly as a function of numbers of irradiating carbon ions, but it rarely depended on the cluster size. Cluster size effects were observed in the XRD peak intensity and the XRD peak width. With increasing the cluster size, the decrease in peak intensity becomes more remarkable and the peak width increases. The experimental result implies that the cluster ions with a larger size provide a more localized energy deposition in a sample, and cause larger and more inhomogeneous lattice disordering. As such, local and large lattice disordering acts as a pinning center for quantum vortex; energetic carbon-cluster ion irradiation will be effective for the increment in the critical current of EuBa2Cu3O7−x superconductors.

Investigation of Superconductivity in Ce-Doped (La,Pr)OBiS2 Single Crystals.

Single crystals of Ce-doped (La,Pr)OBiS2 superconductors, the multinary rare-earth elements substituted ROBiS2, were successfully grown. The grown crystals typically had a size of 1–2 mm and a plate-like shape with a well-developed c-plane. The c-axis lattice constants of the obtained (La,Ce,Pr)OBiS2 single crystals were approximately 13.6–13.7 Å, and the superconducting transition temperature was 1.23–2.18 K. Valence fluctuations of Ce and Pr were detected through X-ray absorption spectroscopy analysis. In contrast to (Ce,Pr)OBiS2 and (La,Ce)OBiS2, the superconducting transition temperature of (La,Ce,Pr)OBiS2 increased with the increasing concentrations of the tetravalent state at the R-site.

Superconductivity at 44.4 K achieved by intercalating EMIM+ into FeSe* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12061131001 and 52072170) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000).

Superconductivity with transition temperature T c above 40 K was observed in protonated FeSe (H y -FeSe) previously with the ionic liquid EMIM-BF4 used in the electrochemical process. However, the real superconducting phase is not clear until now. And detailed structural, magnetization, and electrical transport measurements are lacking. By using similar protonating technique on FeSe single crystals, we obtain superconducting samples with T c above 40 K. We show that the obtained superconducting phase is not H y -FeSe but actually an organic-ion (C6H11 referred to as EMIM+)-intercalated phase (EMIM) x FeSe. By using x-ray diffraction technique, two sets of index peaks corresponding to different c-axis lattice constants are detected in the obtained samples, which belong to the newly formed phase of intercalated (EMIM) x FeSe and the residual FeSe, respectively. The superconductivity of (EMIM) x FeSe with T c of 44.4 K is confirmed by resistivity and magnetic susceptibility measurements. Temperature dependence of resistivity with different applied magnetic fields reveals that the upper critical field H c2 is quite high, while the irreversibility field H irr is suppressed quickly with increasing temperature till about 20 K. This indicates that the resultant compound has a high anisotropy with a large spacing between the FeSe layers.

Metallic ground states of undoped Ti2O3 films induced by elongated c-axis lattice constant

Ti2O3 exhibits unique metal–insulator transition (MIT) at ~ 450 K over a wide temperature range of ~ 150 K. The close relationship between MIT and crystal deformation has been proposed. However, as physical properties are governed by the thermodynamic equilibrium in bulk systems, conducting experimental studies under different lattice deformations remains challenging. Epitaxial thin films can offer high flexibility to accommodate adaptive crystal lattices and provide efficient platforms for investigating the MIT. In this study, we report the synthesis of corundum-type Ti2O3 films on various growth temperatures. We found that the metallic ground states appeared in the films grown at low temperatures. The electronic ground states were further investigated by the electronic-structure calculations. Results suggest that the electrical properties of Ti2O3 films were governed by the c/a ratio of the crystal structure, and the absence of the MIT was attributed to the lattice deformation characterized by an elongated c lattice constant.

Growth and Characterization of ROBiS2 High-Entropy Superconducting Single Crystals.

A series of high-entropy superconductors, ROBiS2 (R = La + Ce + Pr + Nd + Sm), have been successfully grown in the form of single crystals using CsCl flux. The obtained single crystals have a platelike shape with a size of 0.5–2.0 mm and a thickness of 70–450 μm, and they are cleavable along the c-plane. The c-axis lattice constants of the obtained ROBiS2 single crystals have similar values of 13.47–13.57 Å. The Ce in the obtained ROBiS2 single crystals was in a mixed-valence state, comprising both Ce3+ and Ce4+. On the other hand, Pr and Sm showed only the trivalent state. The superconducting transition temperatures of ROBiS2 single crystals were approximately 2–4 K. The superconducting transition temperature and superconducting anisotropies of R-site mixed high-entropy samples increased with a decrease in the mean ionic radius of the R-site. Moreover, a deviation in the tendency to exhibit superconducting properties was observed based on the difference in the R-site mixed entropy. R-site mixed entropy in ROBiS2 superconductors may affect their superconducting properties.

Large positive magnetoresistance with high Hall mobility and intercalation of Fe dopants in the quenched Bi2Te3 single crystals

A study of Fe-ion intercalation in Bi2Te3 probed by the x-ray diffraction, magnetization, Hall measurement, and spectra of synchrotron x-ray photoemission spectroscopy is reported, in which a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed in the quenched FexBi2Te3 single crystals with x = 0.01 and 0.15. Large positive MR of ∼470% with a Hall mobility of ∼44000 cm2/Vs at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample with a quenching temperature Tq of 400 °C, where the electrical parameters can be tuned by Tq. Larger c-axis lattice constants and electron-dominant carriers in the quenched samples lead to the inference that the doped Fe atoms are mostly located at the intercalated van der Waals gap positions and exist as a metallic state of Fe0, which is confirmed by the synchrotron x-ray photoemission spectroscopy (XPS) spectra. Combining the x-ray diffraction, magnetization, and Hall-coefficient studies, here we pave a simple way for probing into the location of Fe dopants in Bi2Te3. In addition, novel magnetotransports of Fe-doped Bi2Te3 associated with the absence of ferromagnetism have been presented, and satisfy the scenario that the topological surface state persists without the loss of topological insulating characteristics of the linear band dispersion, and the large MR may originate from the surface magnetism, even in the presence of Fe impurities. This study also discusses the huge enhancement of Hall mobility arising from the surface Dirac electrons that are accompanied with a modified band structure in quenched Fe0.01Bi2Te3.

Multiferroic and photovoltaic current properties of tetragonally strained BiFeO3 thin films

Mn (5%) doped BiFeO3 (MBFO) thin films were epitaxially grown on La2/3Sr1/3MnO3/LaAlO3 substrates via pulsed laser deposition. The MBFO thin film was confirmed to be an epitaxial thin film having a (001) orientation crystal structure by X-ray diffraction experiments. Compared to the BiFeO3 (BFO) thin film, the MBFO thin films have a shrunk a-axis lattice constant and a significantly elongated c-axis lattice constant, showing a c/a ratio of 1.15, and exhibiting high tetragonal strain. Due to the high tetragonal strain, the MBFO thin film exhibited higher polarization, magnetization, and d33 values than the BiFeO3 (BFO) thin film, resulting in enhanced multiferroic properties. From the results of the study on the relationship between the ferroelectric domain structure and the thickness observed by piezoresponse force microscope, it was confirmed that the domain wall energies of the MBFO thin films were larger than those of the BFO thin films. In addition, the I-V curves (photovoltaic current) showed that the MBFO thin films had the open-circuit voltages and photovoltaic currents higher than that of the BFO thin films due to its higher ferroelectric polarization and smaller band gap than those of the BFO thin films.

Linear correlation between the c-axis lattice constant and superconducting critical temperature in FeSe0.5Te0.5 thin films

Using x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and electrical resistance analyses, we investigate structural and superconducting properties of FeSe0.5Te0.5 films deposited by pulsed laser deposition on TiO2-buffered (CeO2-buffered) SrTiO3 substrates with the buffer film thickness varying from 0 to several tens of nanometers. It is found that the SrTiO3/TiO2 (or CeO2)/FeSe0.5Te0.5 film in a proper thickness range of the buffer film shows a higher superconducting transition temperature (Tc) than the SrTiO3/FeSe0.5Te0.5 film without buffer layer, indicating that the buffer layer can enhance Tc. Both Tc and the c-axis lattice constant of FeSe0.5Te0.5 films increase first and then decrease with the buffer film thickness, each exhibiting its maximum at a particular buffer film thickness, and both of them show an almost linear correlation.

Structural and magnetic properties of molecular beam epitaxy grown chromium selenide thin films

Chromium selenide thin films were grown epitaxially on Al${_2}$O${_3}$(0001) and Si(111)-(7${\times}$7) substrates using molecular beam epitaxy (MBE). Sharp streaks in reflection high-energy electron diffraction and triangular structures in scanning tunneling microscopy indicate a flat smooth film growth along the c-axis, and is very similar to that from a hexagonal surface. X-ray diffraction pattern confirms the growth along the c-axis with c-axis lattice constant of 17.39 {\AA}. The grown film is semiconducting, having a small band gap of about 0.034 eV, as calculated from the temperature dependent resistivity. Antiferromagnetic nature of the film with a N\'eel temperature of about 40 K is estimated from the magnetic exchange bias measurements. A larger out-of-plane exchange bias, along with a smaller in-plane exchange bias is observed below 40 K. Exchange bias training effects are analyzed based on different models and are observed to be following a modified power-law decay behavior.

Enhanced superconductivity in a misfit compound (PbSe)1.12 (TaSe2)2 with double TaSe2 layers

We report the discovery of superconductivity with a maximum Tc ≈ 1.85 K in a new misfit layered compound , which consists of alternating layers of a distorted rocksalt layer PbSe and a double-layer dichalcogenide . A large anisotropic parameter of resistivity and that of the upper critical field = are obtained. The estimated c-axis coherence length is comparable to the c-axis lattice constant. Moreover, the estimated upper critical field , significantly exceeding the Pauli paramagnetic limit. All these data indicate a quasi-two-dimensional nature of the superconducting properties in . Compared with the physical properties of the misfit compound superconductor with only the half-layer of the unit, our study suggests that the enhancement of Tc in with double TaSe2 layers is presumably related to the increase in hole carrier density. The reduced charge transfer from the PbSe layer to the conducting TaSe2 layer could account for the variation in the charge carrier density.

Modulation of lattice constants by changing the composition and strain in incommensurate Nowotny chimney-ladder phase FeGeγ epitaxially grown on Si

The single phase FeGeγ nanocrystals (NCs) with incommensurate Nowotny chimney-ladder structure were epitaxially grown on Si substrates by reaction between deposited Fe atoms and Ge nanostructured layers on Si. The Ge nanostructured layers worked as blocking layers of the Fe diffusion to Si substrates. We used the two kinds of the Ge nanostructured layers: Ge wetting layers formed by direct Ge deposition on Si and Ge NCs formed by Ge deposition on ultrathin SiO2 films with ultrasmall nanowindows on Si. In the case of Ge wetting layers, the FeGeγ NCs were directly formed on Si, where there was a-axis strain due to the 2.9% lattice mismatch between FeGeγ and Si. On the other hand, in the case of Ge NCs, formed FeGeγ NCs were elastically-strain-relaxed due to the small contact with Si substrates through nanowindows in the ultrathin SiO2 films. Furthermore, the c-axis lattice constant in the epitaxial FeGeγ NCs changed due to the variation of the Fe-Ge composition. These results demonstrated that a-axis lattice constant was modulated by strain control of FeGeγ NCs due to the change of contact area with Si substrate and that c-axis lattice constant of FeGeγ NCs with incommensurate Nowotny chimney-ladder structure was modulated by manipulating Fe-Ge composition. These modulation of lattice constants will open a road for manipulating phonon group velocity related to the lattice constant and managing the waste heat in various devices implemented on Si substrates.

Chemical tuning of the magnetic properties of epitaxial BaFe12-xScxO19 (0≤x≤2.1) hexaferrite thin films

The composition dependent magnetic structure in epitaxial BaFe12-xScxO19 (0 ≤ x ≤ 2.1, BaScM) thin films have been investigated. Results show that with increasing Sc3+ content, the out-of-plane c-axis lattice constants of BaScM films increase but do not follow the Vegard's law. The changes in hysteresis loops indicate a variation of magneto-crystalline anisotropy from the uniaxial-type for BaFe12O19 to the near-planar type for BaFe10.2Sc1.8O19. The remanent magnetic domains change from a mixture of dots and maze-like structure to more maze-like structure as the Sc3+ content increases. The above magnetic results can be explained by a composition dependent magnetic structure transition from collinear-type ferrimagnetic magnetic structure to the non-collinear conical magnetic structure which appears at Sc3+ content x ≥ 0.6. The ZFC/FC curves further show that the conical magnetic structure transition temperature (Tcone) of the films increases with the Sc3+ content until x ≥ 2.1. The above results may help better apply BaScM films in the electromagnetic devices.

Study the relation between band gap value and lattice constant of MgTi2O4

As an accurate technique, spectroscopic ellipsometry (SE) is utilized to study the dielectric function (ε = εr + iεi) of spinel oxide MgTi2O4 (MTO) thin films with good orientation. C-axis lattice length and Ti ions valence state are characterized by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. Here, we find that c-axis lattice constants have the positive correlation with band gaps which are obtained by fitting the spectra of d2(E2εi)/dE2 via standard critical points model in MTO, and this phenomenon can be revealed by the Ti ions valence state. Moreover, the band gap value of MTO measured by experiment is consistent with the result of first-principles calculations.