In-plane Lattice Constant Research Articles

Giant Self-Polarization in FeRAM Element Based on Sol-Gel PZT Films

ABSTRACTIntegrated ferroelectric capacitors Pt/PZT/Pt/Ti/SiO2/Si with sol-gel deposited PZT films are studied. The (111) textured polycrystalline films are shown to have nonconductive PZT grain boundaries. The short-circuited photocurrents measured under illumination of the films by light with the quantum energy of 2.7 eV indicate the polarization inside the film directed from the top to the bottom electrode. Using the modified method of depolarization hysteresis loops, we found a non-switchable part of polarization which was measured to be -16 μC/cm2 and directed from the top to the bottom electrode. We consider this result to be a giant self-polarization and explain it in terms of flexoelectricity caused by lattice mismatch between the PZT and bottom Pt layers. The strain gradient across the PZT film thickness is estimated from the in-plane lattice constants measured in Pt and PZT films to be ∼103cm-1, which can produce the downward flexoelectric polarization of ∼14 μC/cm2, coinciding well with the measured one. Nonsymmetrical depolarization loops are found in the films when the polarization switching itself becomes more difficult under the negative or positive driving voltage. We show experimentally how depolarization with compensating bias or film illumination can affect the film polarization switching.

Polarization properties of wurtzite structure Zn1-xMgxO and band offset at Zn0.75Mg0.25O/ZnO interfaces: A GGA+U investigation

Two-dimensional (2D) electron gas with high-mobility is found in wurtzite ZnO/Zn(Mg)O heterostructure, which probably arises from the polarization discontinuity at the ZnO/Zn(Mg)O interface, and the 2D electron gas in the heterostructure is usually also regarded as resulting from polarization-induced charge. In order to explore both the formation mechanism and the origin of the 2D electron gas in ZnMgO/ZnO heterostructure, it is necessary to study the polarization properties of Zn1-xMgxO alloy and energy band alignment of ZnO/Zn1-xMgxO super-lattice. In this paper, we study the polarization properties of Zn1-xMgxO alloy with different Mg compositions by using first-principles calculations with GGA+U method, and the polarization properties are calculated according to Berry-phase method. Owing to the excellent match between the in-plane lattice constants of ZnO and Zn1-xMgxO, the lattice constants of the ZnO and Zn1-xMgxO interface are similar, ZnO/Zn1-xMgxO super-lattice could be constructed easily. The planar-averaged electrostatic potential for the Mg0.25Zn0.75O/ZnO super-lattice and the macroscopically averaged potential along Z(0001) direction are calculated. The large size of (5+3) Mg0.25Zn0.75O/ZnO super-lattice ensures the convergence of potential to its bulk value in the region of the ZnO layer and Mg0.25Zn0.75O layer far from ZnO/Zn1-xMgxO interface. Besides, the valence band offset at the Mg0.25Zn0.75O/ZnO interface is calculated to be 0.26~eV based on the macroscopically averaged potential mentioned above, and the ratio of conduction band offset (EC) to valence band offset (EV) is in a reasonable range, and this is in substantial agreement with the values reported in recent experimental results. Because strain induces additional piezoelectric polarization in MgxZn1-xO, which is introduced by Mg dopant, the lack of inversion symmetry and the bulk ZnO induce its spontaneous polarization in the [0001] direction. The polarization discontinuity at the Mg0.25Zn0.75O/ZnO interface leads to the charge accumulation in the form of interface monopoles, giving rise to built-in electric fields in the super-lattice. In addition, energy alignment determination of the Mg0.25Zn0.75O/ZnO super-lattice is performed, which shows a type-I band alignment with EV=0.26 eV and EC=0.33 eV. The determination of the band alignment indicates that the Mg0.25Zn0.75O/ZnO super-lattice is competent to the confining of both electron and hole. These findings will be useful for designing and optimizing the 2D electron gas at Mg0.25Zn0.75O/ZnO interface, which can be regarded as an important reference for studying the 2D electron gas at MgxZn1-xO/ZnO super-lattices for electronics and optoelectronics applications.

Epitaxial strain and composition-dependent magnetic properties ofMnxGa1−xalloys

Using first-principles calculation, we have investigated the structural and magnetic properties of ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$ alloys with Mn content ($x$) changing from 0.5 to 0.75 in step of $x=0.125$. The ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$ systems crystallize in tetragonal structures, and the tetragonal ratio ($c/a$) is strongly influenced by chemical composition. The in-plane lattice constants ($a$) show very weak concentration dependence, but the lattice parameters along the $z$ axis ($c$) are significantly modified by $x$. We have found that the magnetic properties of ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$ alloys are strongly affected by the Mn content and the epitaxial strain. Very interestingly, the magnetocrystalline anisotropy (MCA) of ${\mathrm{Mn}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$ alloys follows totally different behaviors with $x$ and epitaxial strain. An especially wide range of MCA values are obtained by straining the ${\mathrm{Mn}}_{0.5}{\mathrm{Ga}}_{0.5}$ alloy. The spin polarization (at the Fermi level) of ${\mathrm{Mn}}_{0.75}{\mathrm{Ga}}_{0.25}$ has been calculated from 44% to 57% with epitaxial strain.

Structural and transport properties of epitaxial Ba(Fe1−xCox)2As2 thin films on various substrates

A comprehensive microstructural study was conducted on optimally-doped epitaxial Ba(Fe1−xCox)2As2 thin films grown by pulsed laser deposition on various substrates of a wide range of lattice constants: SrTiO3, LaAlO3, (La,Sr)(Al,Ta)O3, MgO, CaF2, and BaF2. We found that epitaxial strain directly affects the superconductivity in the film, with the transition temperature decreasing linearly with increasing in-plane lattice constant of the film. However, the strain is not determined by the lattice mismatch between the film and substrate. Instead, the mosaic spread of the grain orientation in the film and the thermal expansion coefficient of the substrate were found to correlate well with the in-plane lattice constant of the film. The result confirms the importance of structural distortions to the superconductivity in the Ba(Fe1−xCox)2As2 films.

Topotactic reductive fluorination of strontium cobalt oxide epitaxial thin films

Since the discovery of oxyfluoride cuprate superconductors, many efforts have been made to search for new transition-metal oxyfluoride compounds. Recently, the topotactic fluorination reaction using polyvinylidene fluoride (PVDF) has gained attention because of the low-temperature synthesis of oxyfluorides. In this study, we report the fabrication of SrCoO x F y epitaxial thin films via topotactic fluorination of SrCoO2.5 precursor films with PVDF. X-ray diffraction analysis showed that the SrCoO x F y film, with an anion-vacant perovskite structure, was obtained by fluorination at 150 °C and that the in-plane lattice constant was completely dependent on the substrate. Energy dispersive X-ray spectrometry revealed that the chemical composition of the fluorinated film was SrCoO1.9±0.4F0.5±0.1 and X-ray photoemission spectroscopy showed that the Co ions had a mixed valence state of 2+ and 3+. This valence state was smaller than that in the SrCoO2.5 precursor film, indicating that PVDF acted as a reductive fluorinating agent for the SrCoO2.5 film. Moreover, the SrCoO1.9±0.4F0.5±0.1 film did not exhibit ferromagnetism even at 10 K, suggesting the presence of an antiferromagnetic interaction between the Co ions.

Band structure of silicene on zirconium diboride (0001) thin-film surface: Convergence of experiment and calculations in the one-Si-atom Brillouin zone

So far, it represents a challenging task to reproduce angle-resolved photoelectron (ARPES) spectra of epitaxial silicene by first-principles calculations. Here, we report on the resolution of the previously controversial issue related to the structural configuration of silicene on the ZrB$_2$(0001) surface and its band structure. In particular, by representing the band structure in a large Brillouin zone associated with a single Si atom, it is found that the imaginary part of the one-particle Green's function follows the spectral weight observed in ARPES spectra. By additionally varying the in-plane lattice constant, the results of density functional theory calculations and ARPES data obtained in a wide energy range converge into the "planar-like" phase and provide the orbital character of electronic states in the vicinity of the Fermi level. It is anticipated that the choice of a smaller commensurate unit cell for the representation of the electronic structure will be useful for the study of epitaxial two-dimensional materials on various substrates in general.

Metallic transport and large anomalous Hall effect at room temperature in ferrimagnetic Mn4N epitaxial thin film

We report the electrical transport properties of ferrimagnetic Mn4N (001) epitaxial thin films grown by pulsed laser deposition on MgO (001) substrates. The Mn4N thin films were tetragonally distorted with a ratio of out-of-plane to in-plane lattice constants of 0.987 and showed perpendicular magnetic anisotropy with an effective magnetic anisotropy constant of 0.16 MJ/m3, which is comparable with that of a recently reported molecular-beam-epitaxy-grown film. The thin films exhibited metallic transport with a room temperature resistivity of 125 μΩ cm in addition to a large anomalous Hall effect with a Hall angle tangent of 0.023.

GeSn/Ge multiquantum well photodetectors on Si substrates.

Vertical incidence GeSn/Ge multiquantum well (MQW) pin photodetectors on Si substrates were fabricated with a Sn concentration of 7%. The epitaxial structure was grown with a special low temperature molecular beam epitaxy process. The Ge barrier in the GeSn/Ge MQW was kept constant at 10 nm. The well width was varied between 6 and 12 nm. The GeSn/Ge MQW structures were grown pseudomorphically with the in-plane lattice constant of the Ge virtual substrate. The absorption edge shifts to longer wavelengths with thicker QWs in agreement with expectations from smaller quantization energies for the thicker QWs.

Surface polarization, rumpling, and domain ordering of strained ultrathinBaTiO3(001) films with in-plane and out-of-plane polarization

${\mathrm{BaTiO}}_{3}$ ultrathin films (thickness $\ensuremath{\approx}1.6$ nm) with in- and out-of-plane polarization are studied by first-principles calculations. Out-of-plane polarization is simulated using the method proposed by Shimada et al. [Phys. Rev. B 81, 144116 (2010)], which consists in building a supercell containing small domains with alternating up and down polarization. This allows one to investigate the properties of defect free ${\mathrm{BaTiO}}_{3}$ ultrathin films with polarization perpendicular to the surface, as a function of in-plane lattice constant, i.e., epitaxial strain. The configurations with polarization perpendicular to the surface $(c$ phase) are found stable under compressive strain, while under tensile strain, the polarization tends to lie in-plane ($aa$ phase), along [110]. In the $c$ phase, the most stable domain width is predicted to be 1 to 2 lattice constants, and the magnitude of the surface rumpling varies according to the direction of the polarization (upwards versus downwards), though its sign is unchanged, the oxygen anions pointing in all cases outwards. Finally, all the surfaces studied are found to be insulating. Analysis of the atom-projected electronic density of states gives insight into the surface contributions to the electronic structure. An important reduction of the Kohn-Sham band gap is predicted at ${\mathrm{TiO}}_{2}$ terminations in the $c$ phase ($\ensuremath{\approx}1$ eV with respect to the $aa$ phase). The Madelung potential at the surface plays the dominant role in modifications of the surface electronic structure.

Characterization of solution-bonded GaAs/InGaAs/GaAs features on GaAs

Engineered or ‘virtual’ substrates are of interest to extend the range of epitaxially-grown semiconductor heterostructures available for device applications. To this end, elastically strain-relaxed square features up to 30 µm in size and having an in-plane lattice constant as much as 0.49% larger than the lattice constant of GaAs were fabricated from MOCVD-grown GaAs/In0.08Ga0.92As/GaAs heterostructures by the in-place bonding method, using either AlAs or Al0.7Ga0.3As as the sacrificial layer. TEM images show that the solution-bonded interface is flat with a network of sessile edge dislocations that accommodates the different in-plane lattice constants of the feature and the GaAs substrate and a small rotation of the bonded features. Micro-Raman spectroscopy, which has a spatial resolution of ∼1 µm, was shown to be useful for characterizing lattice mismatch strain ≥ 0.0023, i.e. with an order of magnitude lower sensitivity than high-resolution XRD.

Thermal expansion of free-standing graphene: benchmarking semi-empirical potentials

The thermodynamical properties of free-standing graphene have been investigated under constant zero pressure as a function of temperature using Monte Carlo simulations. A variety of atomistic models have been used, including the simple three-body Stillinger potential and a series of bond-order many-body potentials based on the Tersoff–Brenner seminal models, with recent reparametrizations dedicated to graphene, extensions to medium-range or long-range dispersion corrections. In addition, we have also tested a tight-binding potential in the fourth-moment approximation. The simulations reveal significant discrepancies in the in-plane lattice parameter and the thermal expansion coefficient, which despite showing monotonically increasing variations with temperature, can be positive, negative or change sign at moderate temperature depending on the potential. Comparison with existing experimental and theoretical data obtained from complementary approaches indicates that empirical potentials limited to nearest-neighbour interactions give rather dispersed results, and that van der Waals corrections generally tend to flatten the variations of the in-plane lattice constant, in contradiction with experiment. Only the medium-range corrected potentials of Los and Fasolino, as well as the tight-binding model in the fourth-moment approximation, are reasonably close to the reference results near room temperature. Our results suggest that classical potentials should be used with caution for thermal properties.

Indirect-to-direct gap transition in strained and unstrainedSnxGe1−xalloys

The transition from an indirect to a direct gap semiconductor in unstrained as well as compressively and tensile strained ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys is investigated as a function of the Sn content 0 \ensuremath{\le} $x$ \ensuremath{\le} 1 by means of both a very accurate supercell approach and the more approximate virtual crystal approximation (VCA). In the local density approximation we calculate the bowing parameter of the lattice constant of unstrained ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys. Provided that pseudopotentials suitable for the VCA are used, the random supercell and VCA approaches yield consistent bowing parameters for the lattice constant of \ensuremath{-}0.21 and \ensuremath{-}0.28 \AA{}, respectively, in the entire Sn concentration range. The band structures and energy gaps are calculated using the modified Becke-Johnson potential, which, for Ge, yields a one-electron band gap in very good agreement with experimental data. The crossover from an indirect to a direct gap semiconducting alloy is determined at about 4.5% Sn in unstrained ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$. When ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ is grown commensurately and thus strained on Ge(100), a transition to a direct gap is also observed but at Sn concentrations of about 10%. We finally predict the direct and indirect band gaps as a function of the in-plane lattice constant and Sn concentration for ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloys grown on (100) substrates.

Enhancement of perpendicular magnetic anisotropy by compressive strain in alternately layered FeNi thin films

The effect of the lattice strain on magnetic anisotropy of alternately layered FeNi ultrathin films grown on a substrate, Cu(tCu = 0–70 ML)/Ni48Cu52(124 ML)/Cu(0 0 1) single crystal, is systematically studied by means of in situ x-ray magnetic circular dichroism (XMCD) and reflection high-energy electron diffraction (RHEED) analyses. To investigate the magnetic anisotropy of the FeNi layer itself, a non-magnetic substrate is adopted. From the RHEED analysis, the in-plane lattice constant, ain, of the substrate is found to shrink by 0.8% and 0.5% at tCu = 0 and 10 ML as compared to that of bulk Cu, respectively. Fe L-edge XMCD analysis is performed for n ML FeNi films grown on various ain, and perpendicular magnetic anisotropy (PMA) is observed at n = 3 and 5, whereas the film with n = 7 shows in-plane magnetic anisotropy. Moreover, it is found that PMA is enhanced with decreasing ain, in the case where a Cu spacer layer is inserted. We suppose that magnetic anisotropy in the FeNi films is mainly carried by Fe, and the delocalization of the in-plane orbitals near the Fermi level increases the perpendicular orbital magnetic moment, which leads to the enhancement of PMA.

Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer inFeSe/Nb:SrTiO3/KTaO3Heterostructures Using Photoemission Spectroscopy

Single-layer FeSe films with an extremely expanded in-plane lattice constant of 3.99±0.02 Å are fabricated by epitaxially growing FeSe/Nb:SrTiO3/KTaO3 heterostructures and studied by insitu angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillouin zone corner are resolved with negligible hybridization between them, indicating that the symmetry of the low-energy electronic structure remains intact as a freestanding single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70K for the iron-based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results place strong constraints on current theories.

Lattice-engineered Si1-xGex-buffer on Si(001) for GaP integration

We report a detailed structure and defect characterization study on gallium phosphide (GaP) layers integrated on silicon (Si) (001) via silicon-germanium (SiGe) buffer layers. The presented approach uses an almost fully relaxed SiGe buffer heterostructure of only 400 nm thickness whose in-plane lattice constant is matched to GaP—not at room but at GaP deposition temperature. Single crystalline, pseudomorphic 270 nm thick GaP is successfully grown by metalorganic chemical vapour deposition on a 400 nm Si0.85Ge0.15/Si(001) heterosystem, but carries a 0.08% tensile strain after cooling down to room temperature due to the bigger thermal expansion coefficient of GaP with respect to Si. Transmission electron microscopy (TEM) studies confirm the absence of misfit dislocations in the pseudomorphic GaP film but growth defects (e.g., stacking faults, microtwins, etc.) especially at the GaP/SiGe interface region are detected. We interpret these growth defects as a residue of the initial 3D island coalescence phase of the GaP film on the SiGe buffer. TEM-energy-dispersive x-ray spectroscopy studies reveal that these defects are often correlated with stoichiometric inhomogeneities in the GaP film. Time-of-flight Secondary ion mass spectrometry detects sharp heterointerfaces between GaP and SiGe films with a minor level of Ga diffusion into the SiGe buffer.

Interface magnetic moments enhancement ofFePt−L10/MgO(001): Anab initiostudy

The interface between FePt--L1$_0$ and MgO(001) alloys has been studied using density functional calculations. Because the stacking of the face-centered tetragonal L1$_0$ phase is formed by alternating Fe and Pt planes, both the Fe- and Pt-terminated contact layers were studied. Furthermore, due to the large mismatch between the in-plane lattice constants of both systems, we have chosen some common $a$ values for both alloys in order to explore in detail the adsorption energy, the electronic structure and the interface magnetism. The adsorption energy has been calculated by subtracting the energy of clean FePt and MgO alloys from the total energy. The preferred adsorbed geometric sites for Fe/Pt atoms are when they lie on $top$ of the O species, having a smaller adsorption energy for the remaining positions. We found that expanding the MgO lattice enhances the magnetic moment of the Fe species but the Pt moments remain almost constant.

Effect of phosphorus doping concentration on n-type Ge layer growth

The n-type Ge layers with various phosphorus doping concentrations were grown on p-type Si (100) wafers by using Rapid Thermal Chemical Vapor Deposition (RTCVD). The root-mean-square (RMS) surface roughness of the n-type Ge layer increased from 0.167 to 14.131 nm when the phosphorus doping concentration was increased from 3 × 1016 to 3 × 1019 cm−3. High-Resolution X-ray Diffraction was used to evaluate the in-plane lattice constants and the tensile strain of the n-type Ge layer as functions of the phosphorus doping concentration. The presence of tensile strain was further confirmed from the Raman measurements, which revealed a strain reduction from 0.071 to 0.065% when the phosphorus doping concentration was increased from 3 × 1016 to 3 × 1019 cm−3. Moreover, a considerable increase in the photocurrent peak energy was observed.

Doping-concentration dependence of a boron-doped p-type Ge layer grown on a Si (100) substrates by using RTCVD

Boron-doped p-type Ge layers were grown on n-type Si (100) wafers at various boron doping concentrations by using Rapid Thermal Chemical Vapor Deposition. The root-mean-square was surface roughness of p-type Ge layer increase from 0.708 nm to 19.980 nm as the doping concentrations increase from 3 × 1016 cm−3 to 4 × 1022 cm−3. From High Resolution X-ray diffraction, the in-plane lattice constants and tensile strains of the p-type Ge layer were evaluated as function of boron doping concentration the Raman shift of the for each location indicate a tensile strain from the p-type Ge layer. The tensile strain of the p-type Ge layer decreases from 0.071% to 0.032% as the boron doping concentrations increases from 3 × 1016 cm−3 to 4 × 1022 cm−3. Moreover, the photocurrent peak energies increased. The temperature dependence of the direct band-gap energy, E, of the p-type Ge layer could be described by using Varshnis empirical expression: EΓ(T) = 0.86 − 5.82 × 10−4 T2/(T + 296).

Magnetic domain observation of FeCo thin films fabricated by alternate monoatomic layer deposition

FeCo thin films are fabricated by alternate monoatomic layer deposition method on a Cu3Au buffer layer, which in-plane lattice constant is very close to the predicted value to obtain a large magnetic anisotropy constant. The variation of the in-plane lattice constant during the deposition process is investigated by reflection high-energy electron diffraction. The magnetic domain images are also observed by a photoelectron emission microscope in order to microscopically understand the magnetic structure. As a result, element-specific magnetic domain images show that Fe and Co magnetic moments align parallel. A series of images obtained with various azimuth reveal that the FeCo thin films show fourfold in-plane magnetic anisotropy along ⟨110⟩ direction, and that the magnetic domain structure is composed only of 90∘ wall.

Effects of Pseudopotentials on the Phonon Dispersion of Graphene

The dependence of the various curves in the phonon dispersion of graphene on the particular pseudopotential used is analysed. By using three different pseudopotentials, we did a first principle calculation using the Quantum-ESPRESSO. It was found that the phonon frequencies of the various branches of graphene and some few layers graphene (FLGs) in addition to the crossings at some special symmetry points were dependent on the pseudopotential used. Keyword: Graphene, phonon dispersion, pseudopotential, first principle, force constant -1 , leading to differences in the slopes and even in the general shapes of the calculated phonon dispersions. The contradiction in most existing calculations as spelt out above is the motivation behind this study. We calculate the phonon dispersion of graphene and some FLGs using the density functional theory (DFT) performed using Quantum ESPRESSO (QE) codes (27) with the local density approximation (LDA). We used the Trouller Martin (TM) norm-conserving pseudopotentials and Rappe Rabe Kaxiras Joannopoulos ultra-soft (RRKJUS) pseudopotential, which is a Perdew-Zunger (LDA) exchange correlation functional. The wave functions were expanded using energy cut-off between 25 50 Ryd depending on the pseudopotential. We used Methfessel Paxton smearing (28) in the calculations with an energy cut off of 0.02 Ryd. A Monkhorst-Pack grid of 16 × 16 × 1was used for the Brillouin zone sampling. An in-plane lattice constant of 4.6595 a.u. (or 2.466 A) and an interlayer distance of 10 were used in the calculation. The results are fitted to the experimental data of Refs. (9, 10). We also compared the correlations and discrepancies between the experimental data and the calculated ones.