In-plane Lattice Constant Research Articles

The mechanism of layer number and strain dependent bandgap of 2D crystal PtSe2

Besides its promising high electron mobilities at room temperature, PtSe2 has a finite bandgap sensitively dependent on the number of monolayers combined by the van der Waals interaction according to our calculations based on the density functional theory. It was found that the frontier orbitals of the valence band maximum and the conduction band minimum are mainly contributed by pz and px+y orbitals of Se, which are sensitive to the out-of-plane and the in-plane lattice constants, respectively. The van der Waals force enhances the bonding out-of-plane, which in turn influences the bonding in-plane. We explain that the layer number dependent bandgap has the same electronic reason as the strain dependent bandgap based on the scenario above. This work shows the flexibilities of tuning the electronic and optical properties of PtSe2 in a wide range, which provides an advantage for applications of PtSe2 in sensors.

Valence band splitting in bulk dilute bismides

The electronic structure of bulk GaAs1−xBix systems for different atomic configurations and Bi concentrations is calculated using density functional theory. The results show a Bi-induced splitting between the light-hole and heavy-hole bands at the Γ-point. We find a good agreement between our calculated splittings and experimental data. The magnitude of the splitting strongly depends on the local arrangement of the Bi atoms but not on the uni-directional lattice constant of the supercell. The additional influence of external strain due to epitaxial growth on GaAs substrates is studied by fixing the in-plane lattice constants.

Comparative study of the phase stability in SrTaO2N

Recently, ferroelectric behavior was observed in compressed SrTaO2N thin films epitaxially grown on SrTiO3 substrates. Piezoresponse force microscopy measurements revealed small domains (101–102nm) that exhibited classical ferroelectricity, a behavior not previously observed in perovskite oxynitrides. The surrounding matrix region exhibited relaxor ferroelectric-like behavior. Bulk SrTaO2N samples do not show ferroelectricity, thus suggesting that the origin of it may be related with the strain induced by the substrate. Ab-initio calculations reported that the small domains and the surrounding matrix had trans-type and a cis-type anion arrangements, respectively, but do not describe the experimentally observed equilibrium phase, nor the strain dependent polarization. In this work, we present high accurate all-electron first-principles calculations on the different possible local structures that can explain the ferroelectric-like properties of the strained material. The determined local structure and oxygen/nitrogen ordering has been related with polarization and epitaxial strain. The potential energies and polarization as functions of the in-plane lattice constant are reported.

Chirped Superlattices as Adjustable Strain Platforms for Metamorphic Semiconductor Devices

Chirped superlattices are of interest as buffer layers in metamorphic semiconductor device structures, because they can combine the mismatch accommodating properties of compositionally-graded layers with the dislocation filtering properties of superlattices. Important practical aspects of the chirped superlattice as a buffer layer are the surface strain and surface in-plane lattice constant. In this work we studied two basic types of InGaAs/GaAs chirped superlattice buffers; type I is compositionally modulated while type II is thicknesses modulated. For both types, we have investigated the equilibrium surface strain and surface in-plane lattice constant, and have compared the behavior to linearly-graded buffers having the same top target composition of indium.

Digital growth of thick N-polar InGaN films on relaxed InGaN pseudosubstrates

Smooth relaxed N-polar InGaN films were grown by metal–organic CVD (MOCVD) on N-polar InGaN pseudosubstrates (PSs) using a novel digital approach consisting of a constant In precursor flow with the pulsed injection of H2 carrier gas. InGaN layers grown on PSs exhibited an In composition of about 50% higher than those of the layers grown on N-polar GaN templates, assuming the in-plane lattice constant of the relaxed PSs, corresponding to In0.11Ga0.89N. Additionally, the luminescence recorded from InGaN layers grown on PSs at 490 nm was twice as intense as that obtained from the layers deposited on coloaded GaN-on-sapphire templates, which emitted at 430 nm.

Large lattice mismatch effects on the epitaxial growth and magnetic properties of FePt films

Heteroepitaxial film growth is crucial for magnetic and electronic devices. In this work, we reported the effects of the large lattice mismatch and film thickness on the epitaxial growth and magnetic properties of FePt films on ZrxTi1−xN (001) intermediate layer. FePt films with different thickness were deposited on ZrTiN intermediate layers with various doping concentration of TiN in ZrN. The increase in doping concentration of TiN caused a decrease in the lattice parameters of ZrTiN intermediate layer. It was found that (001) epitaxy of FePt 10nm films was only achieved on ZrTiN intermediate layer when the TiN composition was ≥25vol%, while (001) texture of 5nm films was achieved on ZrTiN intermediate layer with a minimum of 50vol% TiN composition. The in-plane lattice constants of FePt and Zr0.70Ti0.30N (25vol% TiN) were 3.870Å and 4.476Å, respectively, which resulted in a lattice mismatch as large as 15.7%. These large lattice mismatch heterostructures adopted 7/6 domain matching epitaxy. The magneto-crystalline anisotropy of FePt films was improved with the increase in lattice mismatch. Intrinsic magnetic properties were extrapolated for FePt (30nm)/Zr0.70Ti0.30N (30nm)/TaN (30nm)/MgO, and the Ms(0K) and K1(0K) were 1042emu/cc and 5.10×107erg/cc, respectively, which is comparable to that of bulk L10 FePt.

Comparison of Chirped Superlattices and Linearly-Graded Buffer Layers As Adjustable-Strain Platforms for Metamorphic InAaAs/GaAs (001) Devices

Chirped superlattices are of interest as buffer layers in metamorphic semiconductor device structures, because they can combine the mismatch accommodating properties of compositionally-graded layers with the dislocation filtering properties of superlattices. Important practical aspects of the chirped superlattice as a buffer layer are the surface strain and surface in-plane lattice constant. In this work we study two basic types of InGaAs/GaAs chirped superlattice buffers; type I is compositionally modulated while type II is thickness modulated. For both types, we have studied the equilibrium surface strain and surface in-plane lattice constant, and we have compared the surface strain behavior to linearly-graded buffers having the same top target composition of indium.

Bias voltage effects on tunneling magnetoresistance in Fe/ MgAl2O4/Fe(001) junctions: Comparative study with Fe/MgO/Fe(001) junctions

We investigate bias voltage effects on the spin-dependent transport properties of Fe/MgAl${}_2$O${}_4$/Fe(001) magnetic tunneling junctions (MTJs) by comparing them with those of Fe/MgO/Fe(001) MTJs. By means of the nonequilibrium Green's function method and the density functional theory, we calculate bias voltage dependences of magnetoresistance (MR) ratios in both the MTJs. We find that in both the MTJs, the MR ratio decreases as the bias voltage increases and finally vanishes at a critical bias voltage $V_{\rm c}$. We also find that the critical bias voltage $V_{\rm c}$ of the MgAl${}_2$O${}_4$-based MTJ is clearly larger than that of the MgO-based MTJ. Since the in-plane lattice constant of the Fe/MgAl${}_2$O${}_4$/Fe(001) supercell is twice that of the Fe/MgO/Fe(001) one, the Fe electrodes in the MgAl${}_2$O${}_4$-based MTJs have an identical band structure to that obtained by folding the Fe band structure of the MgO-based MTJs in the Brillouin zone of the in-plane wave vector. We show that such a difference in the Fe band structure is the origin of the difference in the critical bias voltage $V_{\rm c}$ between the MgAl${}_2$O${}_4$- and MgO-based MTJs.

Design and simulation to improve the structural efficiency of green light emission of GaN/InGaN/AlGaN light emitting diode

This study considered the design of an efficient, high brightness polar InGaN/GaN light emitting diode (LED) structure with AlGaN capping layer for green light emission. The deposition of high In (>15%) composition within InGaN quantum well (QW) has limitations when providing intense green light. To design an effective model for a highly efficient InGaN green LEDs, this study considered the compositions of indium and aluminum for In x Ga1–xN QW and Al y Ga1–yN cap layers, along with different layer thicknesses of well, barrier and cap. These structural properties significantly affect different properties. For example, these properties affect electric fields of layers, polarization, overall elastic stress energy and lattice parameter of the structure, emission wavelength, and intensity of the emitted light. Three models with different composition and layer thicknesses are simulated and analyzed to obtain green light with in-plane equilibrium lattice parameter close to GaN (3.189 A ) with the highest oscillator strength values. A structure model is obtained with an oscillator strength value of 1.18 10–1 and least in-plane equilibrium lattice constant of 3.218 A. This emitter can emit at a wavelength of 540 nm, which is the expected design for the fabrication of highly efficient, bright green LEDs.

Structural and electrical analysis of epitaxial 2D/3D vertical heterojunctions of monolayer MoS2 on GaN

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which corresponds well to the expected mismatch between bulk MoS2 and GaN. The vdW gap between MoS2 and GaN, extracted from the XTEM measurements, is consistent with the vdW gap of 3.1 Å predicted by our first principles calculations. The effect of monolayer (1L) MoS2 on the electrical characteristics of 2D/3D semiconductor heterojunctions was studied using conductive atomic force microscopy (CAFM). The electrical current across the CAFM-tip/1L-MoS2/GaN vertical junctions is dominated by the tip/GaN interface of both n- and p-doped GaN. This electronic transparency of 1L-MoS2 tells us that a 2D crystal component has to be above a certain thickness before it can serve as an independent semiconductor element in 2D/3D heterojunctions.

Size-dependent structural and electronic properties of Bi(111) ultrathin nanofilms from first principles

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm $(\ensuremath{\sim}80\phantom{\rule{0.16em}{0ex}}\AA{})$ to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements.

The influence of the in-plane lattice constant on the superconducting transition temperature of FeSe0.7Te0.3 thin films

Epitaxial Fe(Se,Te) thin films were prepared by pulsed laser deposition on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT), CaF2-buffered LSAT and bare CaF2 substrates, which exhibit an almost identical in-plane lattice parameter. The composition of all Fe(Se,Te) films were determined to be FeSe0.7Te0.3 by energy dispersive X-ray spectroscopy, irrespective of the substrate. Albeit the lattice parameters of all templates have comparable values, the in-plane lattice parameter of the FeSe0.7Te0.3 films varies significantly. We found that the superconducting transition temperature (Tc) of FeSe0.7Te0.3 thin films is strongly correlated with their a-axis lattice parameter. The highest Tc of over 19 K was observed for the film on bare CaF2 substrate, which is related to unexpectedly large in-plane compressive strain originating mostly from the thermal expansion mismatch between the FeSe0.7Te0.3 film and the substrate.

Thermal stability of Fe16N2 thin film on GaAs (0 0 1) substrate

There is a major need for a high performance magnetic system for high temperature applications. One propitious low cost permanent magnet candidate is Fe16N2, with its giant magnetic moment predicted to be above other materials from conventional first principles calculations. Here we report on a comprehensive study of the thermal stability of Fe16N2 thin films on GaAs (0 0 1) substrate. Using polarized neutron reflectometry (PNR), the saturation magnetization depth profile (Ms) of the films and its modification with temperature is directly measured at various thermal conditions. The structural modifications probed by x-ray diffraction (XRD) unravel that above 250 °C the Fe16N2 thin films decompose into α-Fe and γ′-Fe4N phases. An influence of the strain effect is investigated by grazing incidence x-ray diffraction (GIXRD). We reveal that despite a large Fe16N2 in-plane lattice constant of ~5.88 Å and different strains from substrate or seed layer (up to 2.8% tensile strain), Fe16N2 thin films have the same thermal stability. Our results demonstrate that the high thermal stability of partially order Fe16N2 thin films makes them very promising candidates for spintronics and permanent magnet applications.

Significant modification of perpendicular magnetic anisotropy of W/Fe(001) multilayer by controlling in-plane lattice constant

First-principles calculation predicts a large negative to positive change in the intrinsic perpendicular magnetic anisotropy (PMA) on the order of 107 erg/cm3 of a W/Fe(001) multilayer upon reducing the in-plane lattice constant. This PMA arises at the W/Fe interface, and the second interfacial W site plays an important role. To experimentally verify this theoretical prediction, we have grown W/Fe/W(001) epitaxial trilayers on Cr(001) underlayers. By varying the W layer thickness, the in-plane lattice constant of W can be widely controlled, and the large change in PMA from negative to positive is successfully demonstrated.

First-principles prediction of a rising star of solar energy material: SrTcO3.

SrTcO3 as a new star of solar energy material is investigated in terms of its band gap evolution with biaxial strain from first-principles calculations. Compared to the theoretical equilibrium lattice constant a(b) of bulk SrTcO3, a set of lattice constants with a deviation of -8.75% to +3.35% are considered to include the strain effect. Since the in-plane lattice constant of SrTcO3 is larger than that of the commonly used substrate SrTiO3(STO)/La0.3Sr0.7Al0.35Ta0.35O9 (LSAT)/NdGaO3(NGO)/LaAlO3(LAO), we mainly focus on the modulation of compressive strain. It is found that the band gap decreases with increasing compressive/tensile strain. When the compressive strain reaches 8.75%, the band gap drops to zero and an insulator-metal phase transition appears. Particularly, upon a compressive strain of 1.3%/2.2%/2.4%/4.1%, which can be realized by growing SrTcO3 on substrate STO/LSAT/NGO/LAO, the band gap becomes 1.56/1.47/1.43/1.12 eV, which falls in the range for efficient solar cell materials. Our work suggests that SrTcO3 is a good candidate for a new solar energy material.

Cerium ion doping into self-assembled Ge using three-dimensional dot structure

Rare earth element, Ce, doped Ge (Ge:Ce) three-dimensional crystal is fabricated on Si (001) via a solid source low-temperature molecular beam epitaxy. Ce affects the formation of the self-assembled Ge dot, including its structure, initial growth and the density because of the self-limited process to suppress the total energy contained in the surface, the interface and bulk stress. Owing to the compressive strain from Si substrate, the out-of-plane lattice parameters of Ge:Ce dots are larger than that of bulk Ge, while in-plane lattice constants are smaller. Based on the lattice constant change and the full width at half maximum of the Raman peaks found as a function of Ce concentration, the solid solubility limit of Ce in the substitutional sites of Ge is as high as 0.7at%. The magnitude of Raman shift does not depend upon Ce concentration, which suggests the growth or incorporation of Ge:Ce dots on Si (001) is limited by the stress.

Highly oriented epitaxial (α′′+α′)-Fe16N2 films on α-Fe(001) buffered MgAl2O4(001) substrates and their magnetization

We grew epitaxial films of α′-Fe8N by molecular beam epitaxy on α-Fe buffered MgO(001) and MgAl2O4(MAO)(001) substrates. The lattice mismatches of these substrates with α′-Fe8N are −4.0% and 0.08%, respectively. We discuss effects on the crystal orientation of the grown layers in terms of the substrate material and the thickness of an α-Fe buffer layer. The 3-nm-thick α-Fe buffer layer on MAO was compressively strained and provided a more suitable in-plane lattice constant for growth of α′-Fe8N. The full width at half maximum of the x-ray ω-scan rocking curve peak intensity of the α′-Fe8N(002) plane was 0.09° when it was formed on α-Fe(3nm)/MAO. This value was much smaller than that obtained for α-Fe(3nm)/MgO samples (1.85°). (α′′+α′)-Fe16N2 films, which had partly ordered N-sites, were formed by post-annealing of the α′-Fe8N films on α-Fe/MgO(001) and α-Fe/MAO(001) at 165°C. The degree of N-site ordering reached 0.08 after 20h for (α′′+α′)-Fe16N2 grown on α-Fe(3nm)/MAO(001). The saturation magnetizations were approximately 1650emu/cm3, regardless of the annealing time and substrate used. The saturation field of the (α′′+α′)-Fe16N2 films increased by the post-annealing.

Layer-dependent properties ofSnS2andSnSe2two-dimensional materials

The layer dependent structural, electronic and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg = 2.18 eV and 1.07 eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 eV and 1.69 eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, non-local 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex~0.9 eV, is substantially reduced to Ex = 0.14 eV in bulk SnS2 and Ex = 0.09 eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2 which can be used to identify number of layers in a 2D sample.

Metallization of Epitaxial VO2 Films by Ionic Liquid Gating through Initially Insulating TiO2 Layers.

Ionic liquid gating has been shown to metallize initially insulating layers formed from several different oxide materials. Of these vanadium dioxide (VO2) is of especial interest because it itself is metallic at temperatures above its metal-insulator transition. Recent studies have shown that the mechanism of ionic liquid gated induced metallization is entirely distinct from that of the thermally driven metal-insulator transition and is derived from oxygen migration through volume channels along the (001) direction of the rutile structure of VO2. Here we show that it is possible to metallize the entire volume of 10 nm thick layers of VO2 buried under layers of rutile titanium dioxide (TiO2) up to 10 nm thick. Key to this process is the alignment of volume channels in the respective oxide layers, which have the same rutile structure with clamped in-plane lattice constants. The metallization of the VO2 layers is accompanied by large structural expansions of up to ∼6.5% in the out-of-plane direction, but the structure of the TiO2 layer is hardly affected by gating. The TiO2 layers become weakly conducting during the gating process, but in contrast to the VO2 layers, the conductivity disappears on exposure to air. Indeed, even after air exposure, X-ray photoelectron spectroscopy studies show that the VO2 films have a reduced oxygen content after metallization. Ionic liquid gating of the VO2 films through initially insulating TiO2 layers is not consistent with conventional models that have assumed the gate induced carriers are of electrostatic origin.

Structural and Magnetic Properties of LaCoO3/SrTiO3 Multilayers.

Structural and magnetic properties of the LaCoO3/SrTiO3 (LCO/STO) multilayers (MLs) with a fixed STO layer of 4 nm but varied LCO layer thicknesses have been systematically studied. The MLs grown on Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) and SrTiO3 (STO) exhibit the in-plane lattice constant of the substrates, but those on LaAlO3 (LAO) show the in-plane lattice constant between those of the first two kinds of MLs. Compared with the LCO single layer (SL), the magnetic order of the MLs is significantly enhanced, as demonstrated by a very slow decrease, which is fast for the SL, of the Curie temperature and the saturation magnetization as the LCO layer thickness decreases. For example, clear ferromagnetic order is observed in the ML with the LCO layer of ∼1.5 nm, whereas it vanishes below ∼6 nm for the LCO SL. This result is consistent with the observation that the dark stripes, which are believed to be closely related to the magnetic order, remain clear in the MLs while they are vague in the corresponding LCO SL. The present work suggests a novel route to tune the magnetism of perovskite oxide films.