In-plane Crystallographic Orientation Research Articles

Experimental study of thermal transport across metal/h-BN interfaces: Comparison with metal/graphite interfaces

The continuing miniaturization of layered material electronics highlights the increasingly significant role of interfacial thermal conductance (G) in the efficient nanoscale heat dissipation in these electronics. Despite its importance, the mechanisms of thermal transport across the interfaces of layered materials and the underlying substrate remain unclear. In this work, we comprehensively investigate the interfacial thermal and phonon transport across layered-material interfaces through our ultrafast pump-probe measurements and detailed calculations on G for the interfaces of hexagonal boron nitride (h-BN), one of the most promising layered materials. We first experimentally measure Gc for interfaces between four different metals (Al, Pd, Au and Ti) and h-BN along c-axis using time-domain thermoreflectance (TDTR) from 80 to 300 K. We successfully achieve high accuracy of our measurements with uncertainty <10 % through using the carefully chosen high modulation frequency (15.6 MHz) and relatively large laser spot size (22.5 μm). We find that the measured Gc for interfaces between metals (except Al) and (100) plane of h-BN is lower than that of metal/graphite interfaces, while Gc of Al/h-BN is comparable to that of Al/graphite interfaces. Furthermore, together with our proposed anisotropic model and the diffuse mismatch model (DMM), we determine the transmission probability for the various metal/h-BN interfaces and find that the strong bonding strength enhances the phonon transmission across Ti/h-BN or Ti/graphite interfaces. Finally, we predict the orientation-dependent thermal conductance (i.e., the different interfacial thermal transport along c-axis (Gc) and in-plane (Gab) crystallographic orientations) for both metal/h-BN and metal/graphite. We find Gab between metals and (001) plane is up to 2–3 times higher than Gc. This can be attributed to the highly anisotropic elastic properties in the h-BN and graphite. This work provides important insight into the phonon transmission mechanisms across metal/h-BN interfaces, and thus contributes significantly to the thermal management of layered material electronics.

Experimental and ab initio derivation of interface stress in nanomultilayered coatings: Application to immiscible Cu/W system with variable in-plane stress

Interface stress is a fundamental descriptor for interphase boundaries and is defined in strict relation to the interface energy. In nanomultilayers with their intrinsically high interface density, the functional properties are dictated by the interface structure, which is governed by the delicate interaction of residual interface and volume stresses. In the present work, experimental estimations of the interface stress in Cu/W NMLs were compared with corresponding theoretical values as calculated using DFT (adopting an incoherent bcc W{110}/fcc Cu{111} interface with variable in-plane strain). The Cu/W interface stress was experimentally tuned monotonically from positive to negative values by changing the residual stress in the W nanolayers. Qualitative agreement between experiment and simulation was achieved, confirming a decrease of the interface stress from the compressive to the tensile regime. The DFT simulations showed that Cu atoms at the strained Cu/W interfaces are displaced along the in-plane and out-of-plane directions in response to the interface stress. Using a preliminary neural network potential for the Cu–W system, specific in-plane crystallographic orientation relationships for the Cu/W interfaces were also tested, which improved quantitative agreement between experiment and theory. Assumptions and limitations in experiments and theory for deriving the interface stress are critically discussed.

Three‐dimensional mesostructured single crystalline Fe 3 O 4 for ultrafast electrochemical capacitor electrode with AC line filtering performance

Epitaxial growth of three-dimensional (3D) architectures has led to new opportunities in the optoelectronic devices and nanoscale thermal engineering. The feature that makes these materials distinct is to form a complex geometry while preserving crystal epitaxy. This enables to exploit functionalities given by 3D interconnected pores and suppress charge trapping at defects. One technology that finds it useful is electrochemical energy storage in response to ultrafast alternate current (AC) switch. Here, we extend the concept of 3D mesostructured single crystals to electrochemical capacitors (ECs) for AC line filtering. We present bottom-up epitaxial infilling of Fe3O4 into the self-assembled 300 nm diameter colloid templates using electrodeposition. The resulting 3D mesostructured single crystalline Fe3-δO4 (111) film (δ ≈ −0.06) is highly oriented along both out-of-plane and in-plane crystallographic orientations. The mesostructured epitaxial electrode responds as an EC capacitor to ultrafast voltage scans at a rate of 50 V s−1 and exhibits excellent AC line filtering properties at 120 Hz including the small resistor–capacitor time constant (0.86 ms), large areal capacitance (601 μF cm−2), and modest phase angle (−60°). We attribute this to combined effects of efficient charge transport in epitaxial matrix with low defect density and rapid ion diffusion through 3D interconnected periodic pores. The present study demonstrates the power of the 3D structured epitaxy as a tool of advancing the broader area of technology where fast delivery of both charge and mass is important.

In-plane crystallographic orientations related spin-orbit torque in epitaxial Pt(111)/Co/Ta heterostructures

Current induced spin–orbit torque (SOT) in heavy metals with strong spin–orbit coupling strength has attracted considerable attention due to its potential applications in spintronic technology. Pt, as one of the mostly used heavy metals in SOT-based spintronic devices, shows large spin Hall angle (θSH) with its single phase and alloy counterparts. In this work, the in-plane crystallographic orientations related θSH of epitaxial Pt(111) layer is reported in MgO(111)/Pt(111)/Co/Ta heterostructures with strong perpendicular magnetic anisotropy. The θSH shows a quite large difference with values, respectively, around 0.083 and 0.057 when the current applied along the [11¯0] and [112¯] crystallographic directions of Pt(111) by the damping-like SOT efficiency using the harmonic Hall voltage measurement technique. The critical switching current densities also show large difference between these two orthogonal crystallographic orientations with the trend of that the larger SOT efficiency leads to the smaller critical switching current density. It independently confirms the generation of different damping-like SOT efficiency when current along [11¯0] and [112¯] directions of Pt(111). Moreover, a perpendicularly magnetized Pt/Co/Ta reference heterostructures with Pt having polycrystalline phase shows tiny variation of damping-like SOT efficiency in in-plane two orthogonal directions, which also indirectly indicates the crystallographic orientations related θSH in epitaxial Pt(111) layers. This study indicates that the θSH of epitaxial Pt is a crystallographic orientations related parameter.

Plasmon-Mediated Synthesis of Periodic Arrays of Gold Nanoplates Using Substrate-Immobilized Seeds Lined with Planar Defects.

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

The hexaferrite Sr3Co2Fe24O41 thin films by chemical solution deposition method: Synthesis and characterization

Thin films of Sr3Co2Fe24O41 hexaferrite were prepared through the chemical solution deposition method using SrTiO3(1 1 1) single crystal substrates. The influence of the annealing temperature and annealing duration time on the phase composition, structural, microstructural and magnetic properties was investigated by examining the out- and in-plane crystallographic orientation, temperature and magnetic-field dependence of the magnetization, and surface topography. The Rietveld analysis performed on the XRD patterns indicates Sr3Co2Fe24O41 being the dominant phase, according to the XRD texture analysis the orientation relationship between Z ferrite and substrate can be expressed as (0 0 l)Z || (1 1 1)STO || and [1 1 0]Z || [1 0 0]STO. The magnetization data show anomalies in the magnetic behavior occurring at temperatures close to the room temperature that are characteristic for collinear to non-collinear spin structure transitions. The magnetic-field dependence of the magnetization proves soft magnetic character of ferrite film together with pronounced magnetic anisotropy, and shows a two-step increase of magnetization at lower fields (≤1 T) before it becomes almost saturated at around 2 T.

Characterization and optimization of sputtered AlN buffer layer on r-plane sapphire substrate to improve the crystalline quality of nonpolar a-plane GaN

Here we examined the use of AlN buffer layers of various thicknesses to improve the crystalline quality of nonpolar a-plane GaN (a-GaN) grown on an r-plane sapphire substrate. Three types of AlN buffer layers were used: sputtered AlN buffer layers (sp-AlNs) with or without annealing, and epitaxially grown AlN buffer layers (ep-AlNs). Buffer layer thicknesses of 30, 90, and 180 nm were used. We found that the surface morphological transitions with increasing thickness were different between the sp-AlNs and the ep-AlNs, and that the sp-AlNs had poorer crystallographic orientations than did the ep-AlNs. Annealing caused marked changes to occur in the surface morphologies and crystallographic orientations of the sp-AlNs; however, the positive effect of annealing was limited because the in-plane crystallographic orientation degraded with increasing layer thickness. The optimal buffer layer was found to be the 30-nm-thick annealed sp-AlN, which was composed of uniformly arranged oval grains with better crystallographic orientation than the other sp-AlNs and annealed sp-AlNs. The crystalline quality of the a-GaN epilayer grown on 30-nm-thick annealed sp-AlN had a narrower X-ray rocking curve–full width at half maximum for both the on- and off-axis planes compared with that grown on any other AlN buffer layers.

Quantitative analysis of anisotropic edge retraction by solid-state dewetting of thin single crystal films

When single crystal thin films undergo solid state dewetting, film edges retract at a rate that is strongly affected by their crystallographic orientations. Lithographically patterned macroscopic edges with a limited number of specific in-plane crystallographic orientations remain straight as they retract. Macroscopic edges with other crystallographic orientations develop in-plane facets, whose in-plane normals are the same as those of kinetically stable edges. Therefore, a quantitative understanding of the retraction of kinetically stable edges can serve as the basis for understanding the retraction of edges with all other in-plane orientations. Measurements of the rates of retraction of kinetically stable edges for single crystal (100) and (110) Ni films on MgO are reported. Retracting edges develop out-of-plane facets that are generally consistent with the facets expected from the equilibrium Wulff shape. To capture the observed anisotropic character of the edge retraction rate, edge retraction through surface diffusion driven by the surface Laplacian of the weighted mean curvature of fully faceted edges has been modeled. The 2-dimensional model and experiments show a similar time scaling for the edge retraction distance (∼tn, with n ∼ 0.4) and the rim height and width (n ∼ 0.2). Also, they are consistent with the specific observed retraction rate anisotropy, within the range of known error of the surface energies and diffusivities used in the model. However, formation of valleys ahead of the rims is observed in the experiments on (110) films but not in the simulation.

Epitaxy of Orthorhombic BaSi2 with Preferential In-Plane Crystal Orientation on Si(001): Effects of Vicinal Substrate and Annealing Temperature

We attempted to grow orthorhombic BaSi2 epitaxial films having preferential in-plane crystallographic orientation on both exact and vicinal Si(001) substrates with a miscut angle of 2° toward Si[1̄10] by reactive deposition epitaxy (RDE) and subsequent molecular beam epitaxy (MBE). On the vicinal Si(001) substrate, the initial BaSi2 nuclei formed by RDE tended to grow unidirectionally with the [010] direction parallel to Si[110] when the annealing temperature of the Si substrate before the growth was increased from 830 to 1000 °C. Transmission electron microscopy showed that the grain size of the BaSi2 films grown by MBE increased up to approximately 9 µm on the vicinal Si(001) substrate when the substrate annealing temperature was 1000 °C. This is the largest grain size ever obtained for BaSi2. Even in the case of the exact Si(001) substrate, the MBE-grown BaSi2 grains preferentially grew with the [010] direction along Si[110] when the annealing temperature was 1000 °C.

Tailoring in-plane crystallographic orientation of epitaxial YBa2Cu3O7 − x thin films having perpendicular c-axis texture

Control of the in-plane crystallographic orientation of YBa2Cu3O7−x (YBCO) films on (100) MgO substrates is of significant application value due to the selective enhancement of superconducting properties. In the present work, the preparation, crystallographic and superconducting properties of YBCO films deposited on MgO substrates are reported. Crystallographic in-plane orientation was realized by means of tailoring the pulsed laser deposition conditions and the use of interfacial buffering structures. Superconductiong properties were measured for films having different in-plane orienations. The results indicate that the 0° in-plane oriented films showed the highest current density of 1.62MA/cm2 that was attributed to the elimination of high-angle grain boudaries. Additionally, the growth mechanism of YBCO films was discussed in terms of crystallographic and thermodynamic theory.

Morphology-Controlled Three-Dimensional Nanoarchitectures Produced by Exploiting Vertical and In-Plane Crystallographic Orientations in Hydrothermal ZnO Crystals

Herein we describe the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoarchitectures via a facile hydrothermal route. In this approach, vertical and in-plane crystallographic orientations of ZnO crystals were tuned by appropriate patterning of growth masks and seed layer control, which enabled the formation of three types of ZnO nanoarchitectures. If polycrystalline ZnO layers with poor c-axis orientation were used as a seed, flower-like ZnO nanoarchitectures composed of radially oriented ZnO nanorods were achieved. In the case of c-axis oriented ZnO seeds, polygonal ZnO pillars grew vertically at the center of the growth holes. Even in the latter case, multidomain columnar–joint structures or single domain crystal structures with well-defined hexagonal facets were achieved based on the existence of six-fold in-plane symmetry of the ZnO seed layers. These morphology-controlled ZnO nanoarchitectures exhibited clear differences in light propagation characteristics, which could be ascribed ...

High-Resolution TEM Analysis of Perpendicular CoCrPt-SiO$_{2}$ Media

We investigated the relationship among the grain size, the grain boundary width, and the in-plane crystallographic orientation of CoCrPt-SiO2 perpendicular media using high-resolution transmission electron microscope (HR-TEM). The average grain size in a CoCrPt-SiO2 magnetic layer and its dispersion were 7.5 nm and 18%, respectively. The width of the grain boundary was 0.68 nm in average and increased with grain size. In addition, we found a group of neighboring grains with parallel a-axes of h.c.p.-structure, which is defined as a crystal grain cluster. The grain boundary width tended to decrease as the cluster size increased. The results suggested that the size of the crystal grain cluster should be reduced for controlling the intergranular exchange coupling that presumably depends on the grain boundary width.

Facet and in-plane crystallographic orientations of GaN nanowires grown on Si(111)

We have determined the in-plane orientation of GaN nanowires relative to the Si (111)substrate on which they were grown. We used x-ray diffraction pole figure measurements toevidence two types of crystallographic orientation, all the nanowires having lateral facets. The proportion of these two orientations was determined and shown to beinfluenced by the pre-deposition of Al(Ga)N intermediate layers. In the main orientation,the GaN basal directions are aligned with the directions. This orientation corresponds to an in-plane coincidence of GaN and Silattices.

Aligned-Crystalline Si Films on Non-Single-Crystalline Substrates

AbstractWe summarize recent progress in growth and characterization of aligned-crystalline silicon (ACSi) films on polycrystalline metal and amorphous glass substrates. The ACSi deposition process uses, as a key technique, ion-beam-assisted deposition (IBAD) texturing on a non-single-crystalline substrate to achieve a biaxially-oriented (i.e., with preferred out-of-plane and in-plane crystallographic orientations) IBAD seed layer, upon which homo- and hetero-epitaxial buffer layers and hetero-epitaxial silicon (i.e., ACSi) films with good electronic properties can be grown. We have demonstrated the versatility of our approach by preparing ACSi films on customized architectures, including fully insulating and transparent IBAD layer and buffer layers based on oxides on glass and flexible metal tape, and conducting and reflective IBAD layer and buffer layers based on nitrides on flexible metal tape. Optimized 0.4-μm-thick ACSi films demonstrate out-of-plane and in-plane mosaic spreads of 0.8° and 1.3°, respectively, and a room-temperature Hall mobility of ∼90 cm2/V.s (∼50% of what is achievable with epitaxial single-crystalline Si films, and ∼1000 times that of amorphous Si films) for a p-type doping concentration of ∼4×1016 cm−3. By using various experimental techniques, we have confirmed the underlying crystalline order and the superior electrical characteristics of low-angle (&lt;5°) grain boundaries in ACSi films. Forming gas anneal experiments indicate that Si films with low-angle grain boundaries do not need to be passivated to demonstrate improved majority carrier transport properties. Measurements on metal-insulator-semiconductor structures using ACSi films yield near-electronic-grade surface properties and low surface defect densities in the ACSi films. A prototype n+/p/p+–type diode fabricated using a 4.2-μm-thick ACSi film shows minority carrier lifetime of ∼3 μs, an estimated diffusion length of ∼30 μm in the p-Si layer with a doping concentration of 5×1016 cm−3, and external quantum efficiency of ∼80% at 450 nm with the addition of an MgO film anti-reflector.

Epitaxial Growth of Wurtzite ZnO Crystals in an Aqueous Solution System

Abstract Vertical and in-plane crystallographic orientation of ZnO rods was preferentially achieved through epitaxial growth on a Zn-face single-crystalline ZnO(001) substrate in an aqueous solution of ZnSO4 under basic conditions at 60 °C.

Structure evolution of the biaxial alignment in sputter-deposited MgO and Cr

The growth of biaxially aligned layers, i.e. layers with both a preferential out-of-plane and an in-plane crystallographic orientation, on non-aligned metallic substrates is investigated. Unbalanced magnetron sputtering on an inclined substrate is used to deposit the layers. This method enables us to grow biaxially aligned layers for different classes of materials with different physical and chemical properties. The results for biaxially aligned MgO which is a cubic metal oxide (FCC rocksalt structure), and pure metallic chromium films (BCC) are presented. A comparison between biaxially aligned MgO and Cr concerning the microstructure and crystallographic texture is discussed. A correlation between the sputter deposition parameters on the biaxial alignment of both materials is observed. Both materials have a columnar V-shape structure with a faceted surface, corresponding to zone T of the well known structure zone model of Thornton. The MgO layers exhibit a [111] out-of-plane orientation, while Cr layers have an [100] preferential orientation. MgO as well as Cr show a strong in-plane alignment.

ナノコーティング 電子ビーム物理蒸着法によるジルコニア皮膜の柱状組織成長過程における結晶配向性の発達

Crystallographic orientation of columnar structure in yttria-stabilized zirconia (YSZ) layers produced by electron beam physical vapor deposition (EB-PVD) is an important aspect to develop nanostructure-controlled thermal barrier coatings (TBCs). Several specimens were produced by EB-PVD on a rotating sample holder whose horizontal axis is normal to the vapor plume axis. The developments of the out-of-plane and in-plane crystallographic orientations in the YSZ layers were investigated by using XRD pole figure analysis. -oriented columns were developed perpendicular to the YSZ layer/bond layer interface. This out-of-plane orientation rapidly grew and was completed in the early stage of the deposition. On the other hand, a twisted zone in which in-plane direction differed at about 10° from the direction of the substrate rotation axis during deposition was formed up to 60 μm from the interface. Well-aligned in-plane orientation along the substrate rotation axis was observed directly above the twisted zone. Processing parameters, such as substrate rotation speeds or deposition temperatures, affected the growth of the in-plane twisted zone. Raising the substrate rotation speed promoted the growth of the twisted zone and delayed the development of the in-plane orientation. In contrast, high deposition temperature accelerated the development of the in-plane orientation. Strain energy in the coating layer during deposition probably dominates the formation of the twisted zone.

Characterization of surface deformation around vickers indents in monocrystalline materials

The deformation surrounding Vickers indents on copper and MoSi2 monocrystals has been characterized for two orientations of the indenter diagonals using optical, electron, and scanning probe microscopy. The surface topography around the indents was characterized using atomic force microscopy (AFM), whereas orientation imaging microscopy (OIM) was used to map the local crystallographic orientation of the surface surrounding one indent on copper and measure lattice rotations due to plastic strain. The results indicate that sink-in and pile-up behavior depend on in-plane crystallographic orientations rather than the orientation of the indenter. Regions with multiple slip show larger lattice rotations and sink-in, whereas regions with lower slip density have smaller lattice rotations and show pile-ups. Strains outside one indent were obtained from OIM data using kinematical relations for single-crystal plasticity. A von Mises strain of 29 pct next to the middle of indent edge on the surface of the sample was deduced from the analysis. This agrees well with the characteristic strain reported for Vickers indents.

Simultaneous Effects on Topography, Composition and Texture in Ion Assisted Deposition of Thin Films

ABSTRACTIon bombardment during deposition may simultaneously affect thin film topography, composition and crystallographic texture. Ion etching can produce periodic ripples that depend on the angle of ion incidence and surface temperature. When applied during deposition, ion bombardment can produce in-plane crystallographic orientation in polycrystalline materials for specific angles of incidence. In addition, ion bombardment changes the composition of multicomponent thin films according to the local angles of ion incidence and ion/atom ratios. Therefore, these three mechanisms may be linked under certain deposition conditions to generate novel topographically patterned materials with locally controlled composition and texture. Examples include metal alloys, oxides and nitrides, and recommendations for specific nanoscale structures are given.

Origins of substrate-topography-induced magnetic anisotropy in sputtered cobalt-alloy films

Cr(001)/CoCrPt (112̄0) films, used in magnetic hard disks, exhibit in-plane magnetic anisotropy when grown over a substrate patterned with shallow grooves. In this experiment, Cr/CoCrPt films are grown over substrates that have been patterned using lithographic techniques to produce well-controlled topography. The in-plane anisotropy, which is of the order of 104 J m−3, increases with the groove height and frequency. This magnetic anisotropy is a result of anisotropic in-plane crystallographic orientation and anisotropic stress. Stress-induced anisotropy accounts for up to 25% of the total in-plane anisotropy, but the major contribution to the anisotropy is a preferential Co c-axis alignment parallel to the groove direction.