Epitaxial Variants Research Articles

Shear strain stabilized high-temperature metallic monoclinic VO2 variants with symmetry permission

In this work, temperature-dependent in situ x-ray diffraction reciprocal space mappings are employed to reveal three-dimensional lattice shear deformation in epitaxial VO2 variants, and linear Lagrangian strain tensors of epitaxial lattices are deduced by metric tensors of the reciprocal space at various temperatures. An equilibrium modulated-monoclinic phase is identified above the critical temperature, which is permitted in a translationengleiche subgroup from high symmetry with an index of 4. Being different from the conventional low-temperature monoclinic phase, the high-temperature monoclinic phase presents a specific metallic feature, which is ascribed to the Mott–Hubbard and charge density wave mechanisms. Under the proposed general procedure, which precisely identifies the normal and shear strain status of deformed lattices, it is demonstrated that the structural symmetry reduction under shear deformation unambiguously acts as the origin of unexpected metallic modulated-monoclinic VO2 at high temperatures. Our results highlight the significance of precise detection and contribution of shear deformation in various fields of physics.

First-principles study of twin grain boundaries in epitaxial BaSi2 on Si(111)

Epitaxial films of BaSi2 on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi2 epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi2 revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba(II) atoms form hexagons containing central Ba(I) atoms in both the bulk and DB regions. Four possible interface models containing Ba(I)-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba(I) atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba(II) atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m2. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi2.

Epitaxial growth of β-FeSi2 thin films on Si(111) substrates by radio frequency magnetron sputtering and their application to near-infrared photodetection

β-FeSi2 thin films were epitaxially grown on p-type Si(111) substrates at a substrate temperature of 560 °C and Ar pressure of 2.66 × 10−1 Pa by radio-frequency magnetron sputtering (RFMS) using a sintered FeSi2 target, without postannealing. The resultant n-type β-FeSi2/p-type Si heterojunctions were evaluated as near-infrared photodiodes. Three epitaxial variants of β-FeSi2 were confirmed by X-ray diffraction analysis. The heterojunctions exhibited typical rectifying action at room temperature. At 300 K, the heterojunctions showed a substantial leakage current and minimal response for irradiation of near-infrared light. At 50 K, the leakage current was markedly reduced and the ratio of the photocurrent to dark current was considerably enhanced. The detectivity at 50 K was estimated to be 3.0 × 1011 cm Hz1/2/W at a zero bias voltage. Their photodetection was inferior to those of similar heterojunctions prepared using facing-target direct-current sputtering (FTDCS) in our previous study. This inferiority is likely because β-FeSi2 films prepared using RFMS are located in plasma and are damaged by it.

Thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/semiconductor superlattices

We report on the thermal stability of epitaxial cubic-TiN/(Al,Sc)N metal/semiconductor superlattices with the rocksalt crystal structure for potential plasmonic, thermoelectric, and hard coating applications. TiN/Al0.72Sc0.28N superlattices were annealed at 950 and 1050 °C for 4, 24, and 120 h, and the thermal stability was characterized by high-energy synchrotron-radiation-based 2D X-ray diffraction, high-resolution (scanning) transmission electron microscopy [HR(S)/TEM], and energy dispersive X-ray spectroscopy (EDX) mapping. The TiN/Al0.72Sc0.28N superlattices were nominally stable for up to 4 h at both 950 and 1050 °C. Further annealing treatments for 24 and 120 h at 950 °C led to severe interdiffusion between the layers and the metastable cubic-Al0.72Sc0.28N layers partially transformed into Al-deficient cubic-(Al,Sc)N and the thermodynamically stable hexagonal wurtzite phase with a nominal composition of AlN (h-AlN). The h-AlN grains displayed two epitaxial variants with respect to c-TiN and cubic-(Al,Sc)N. EDX mapping suggests that scandium has a higher tendency for diffusion in TiN/(Al,Sc)N than titanium or aluminum. Our results indicate that the kinetics of interdiffusion and the cubic-to-hexagonal phase transformation place constraints on the design and implementation of TiN/(Al,Sc)N superlattices for high-temperature applications.

Nonisostructural complex oxide heteroepitaxy

The authors present an overview of the fundamentals and representative examples of the growth of epitaxial complex oxide thin films on structurally dissimilar substrates. The authors will delineate how the details of particular crystal structures and symmetry of different oxide surfaces can be employed for a rational approach to the synthesis of nonisostructural epitaxial heterostructures. The concept of oxygen eutaxy can be widely applied. Materials combinations will be split into three categories, and in all cases the films and substrates occur in different crystal structures: (1) common translational and rotational symmetry between the film and substrate planes; (2) translational symmetry mismatch between the substrates and films that is distinct from a simple mismatch in lattice parameters; and (3) rotational symmetry mismatch. In case (1), in principle single-crystalline thin films can be attained despite the films and substrates possessing different crystal structures. In case (2), antiphase boundaries will be prevalent in the thin films. In case (3), thin-film rotational variants that are joined by tilt boundaries will be present. Diffraction techniques to determine crystallographic alignment and epitaxial variants are discussed, and transmission electron microscopy studies to investigate extended defects in the thin films will also be reviewed. The authors end with open problems in this field regarding the structure of oxide interfaces that can be topics for future research.

Fabrication and characterization of BaSi2 epitaxial films over 1 µm in thickness on Si(111)

We attempted to fabricate a-axis-oriented BaSi2 epitaxial films up to 2180 nm in thickness. First, we investigated the influence of growth temperature and growth rate on the crystalline quality of approximately 400-nm-thick BaSi2 layers, and then optimized the above two growth conditions based on X-ray diffraction measurements. We next grew BaSi2 films with various layer thicknesses at 580 °C in the range between 100 and 2180 nm, and characterized their properties. The a-axis-oriented BaSi2 thick epitaxial films had three epitaxial variants rotating 120° with each other around the surface normal. The microwave photoconductive decay measurements for the 1640-nm-thick BaSi2 epitaxial film showed that the minority-carrier lifetime was approximately 8 µs at room temperature. These achievements open up the possibilities of thin-film solar cell applications of BaSi2.

Heteroepitaxy of distorted rutile-structure WO2 and NbO2 thin films

Abstract

Epitaxial variants of VO2 thin films on complex oxide single crystal substrates with 3m surface symmetry

Rutile-based monoclinic VO2 films were grown on (111) LaAlO3, (0001) Al2O3, (111) MgAl2O4, (111) MgO, and Zn-faced (0001) and O-faced (0001̄) ZnO substrates. The out-of-plane alignment of the two-fold screw [010] axis of monoclinic VO2 with three-fold rotational axes of the substrates is made possible by the formation of epitaxial domains. We find that the in-plane [100] axis of VO2 is always oriented with one of the three in-plane eutactic axes of the all of the substrates, despite significant differences in lattice mismatch from substrate to substrate. From diffractometry analysis and upon considering the in-plane symmetry of the substrates with respect to the (010) VO2 film, we conclude the presence of 120° rotational variants and 60° twins. The electrical transport characteristics of all of the films except for those on the ZnO substrates are comparable, showing metal–insulator transitions with bulk-like transition temperatures. The growth of different stoichiometric phases and film orientations on ZnO is attributed to large lattice mismatch, and also leads to the apparent in-plane conductance transition being smeared out in temperature. We find that it is more challenging to stabilize (010) VO2 on O-faced ZnO compared to Zn-faced ZnO and discuss the differences in tetrahedra stacking between the two polarities.

Realization of Large-Domain Barium Disilicide Epitaxial Thin Film by Introduction of Miscut to Si(111) Substrate

The domain structure of BaSi2 epitaxial films grown on vicinal Si(111) substrates has been studied in order to fabricate high-quality BaSi2 crystals with large domains. The X-ray pole figure measurement shows that the BaSi2 films grown on vicinal substrates as well as the on-axis substrate consist of three epitaxial variants which are equivalent in terms of 60° in-plane rotations, and that one of the variants is dominant in the film grown on the 2°-inclined substrate. The orientation maps produced by electron backscatter diffraction show that the domains with the b axis parallel to the miscut direction are larger than the others in the film grown on the 2°-inclined substrate, while the domain sizes of three variants are found similar in the films grown on the on-axis and 4°-inclined substrates. The possible origin of the large domain formation is discussed with the focus on the initial growth stage observed by atomic force microscopy. Nucleation from the step edge is proposed as the mechanism of the large-domain formation considering the lattice matching to the step edge, while nucleation is suggested to occur at the terrace edges on the 4°-inclined substrate.

Fabrication of BaSi2 films on transparent CaF2 (111) substrates by molecular beam epitaxy for optical characterization

We have grown BaSi2 films on transparent CaF2(111) substrates by molecular beam epitaxy using Si template layers. The crystalline quality of the Si layers on the CaF2(111) was improved by electron stimulated desorption (ESD) technique prior to the deposition of Si. We successfully formed highly-oriented Si films on the ESD-treated CaF2(111) surface at 650 °C. The RHEED pattern suggested that the two kinds of 〈111〉-oriented Si epitaxial variants known as type A and type B were formed on the CaF2(111) substrates. The BaSi2 films were grown on the Si layers formed on CaF2(111) substrates, but they were not a-axis oriented, probably due to the rough surface of the Si layers.

Superparamagnetism of ultrathin Co film on antiferromagnetic Cr2O3 layer

The effect of antiferromagnetic Cr2O3 thin film on the magnetic properties of ultrathin Co film has been investigated. To achieve the investigation, we have also investigated the fabrication of Cr2O3 thin film of high quality using MBE. The crystalline quality of Cr oxide film strongly depends on the in-plane epitaxial variants of Cr(110) before the oxidation. We have successfully fabricated Cr2O3(0001) film by oxidizing three fold-symmetric Cr(110) film. On Cr2O3(0001) thin film, the magnetization of Co is stabilized parallel to the Cr spin direction below the Néel temperature of Cr2O3.

Epitaxial SmCo5 thin films with perpendicular anisotropy

Epitaxial SmCo5 thin films with strong perpendicular magnetic anisotropy have been developed by pulsed laser deposition on heated Ru buffered Al2O3(0001) substrates. Pole figure measurements on thicker films reveal the existence of two epitaxial variants, 30° in-plane rotated to each other, which reduces to a single epitaxial relation for smaller film thickness. The presence of a single SmCo5 phase with perpendicular direction of the c-axis is established by multiple peak analysis of x-ray data in pole figure geometry and is confirmed by a numerical fit of hysteresis measurements, which results in an anisotropy constant of Ku=7.6 MJ/m3.

Epitaxial variant structures of the perovskite manganites with the high tunnel magnetoresistance

We have found that single-crystal substrates of ZrO 2(Y 2O 3) with (0 0 1) and (1 1 1) orientation are suitable for the growth of (1 1 0) perovskite films of La 0.7Ca 0.3MnO 3, La 0.7Sr 0.3MnO 3 and La 1− x MnO 3 ( x=0.1–0.25) with the in-plane variant structure. The orientation types and crystallographic domains were determined by X-ray diffraction and high-resolution TEM. Special high-angle boundaries formed between the nanodomains of the epitaxial variants result in the high tunnel magnetoresistance (above 15% at 77 K and 0.1 T). The magnetic reversal transition was also detected in the field below 0.01 T, the sharpness of the transition was found to be very sensitive to the film microstructure. Further enhancement of the magnetoresistance was achieved in the heterostructures including perovskite manganite and ferrimagnetic insulating oxide layers.

Effect of substrate temperature on the epitaxial growth of Au on TiO 2(1 1 0)

We have studied the effects of substrate temperature on the epitaxial growth of Au on TiO 2(1 1 0) surface by combined high resolution scanning electron microscopy and electron backscatter diffraction. The Au was evaporated under UHV conditions onto stoichiometric and unreconstructed (1×1) TiO 2(1 1 0) surfaces. Two deposition conditions were used consisting of RT deposition followed by annealing at 775 K and direct deposition at 775 K. Two different epitaxial orientations have been observed for the two conditions. After room temperature deposition and annealing, the epitaxial orientation relationship is (1 1 1)Au//(1 1 0)TiO 2 with 〈1 1 0〉 Au//[0 0 1]TiO 2. While for direct deposition at high temperature the epitaxial orientation relationship is (1 1 2)Au//(1 1 0)TiO 2 with 〈1 1 0〉 Au//(0 0 1)TiO 2. For both orientations, two epitaxial variants are observed which are rotated from each other by 180° around the TiO 2(1 1 0) surface normal. These two variants are in a twin orientation relationship. For the high temperature deposition, extensive (1 1 1) twinning of the Au islands is observed which are the boundaries separating two orientation variants.

Crystallographic characterization of sputter-deposited epitaxial Nb-Cu-Co and Nb-Cu-Permalloy multilayers using electron back-scatter diffraction patterns

Abstract Epitaxial Nb-Cu-Co and Nb-Cu-Permalloy™ (Py) multilayers have been grown on (1120) sapphire substrates by sputter deposition. Electron backscatter diffraction patterns (EBSPs) have been used to characterize individual layers. while atomic force microscopy was used for studying the surface topography. EBSPs have been demonstrated to be a viable technique for characterization of these thin sputtered films. EBSP results have shown that the Nb films grow as high quality epitaxial single crystals with (110)bcc orientations. Cu films grown on the epitaxial Nb display two in-plane epitaxial variants corresponding to two stacking sequences of {111}fcc planes. These Cu variants take up the Nishiyama-Wasserman orientation relationship with the underlying Nb. Subsequent sputtering of Co or Py on the epitaxial Cu films results in the growth of two variants of 111fcc layers. Orientation maps of the Cu, Co and Py films, which illustrate the size and distribution of the growth variants, are presented.

Real Time Observations of Dislocation-Mediated Plasticity in the Epitaxial AI (011)/Si(100) Thin Film System

ABSTRACTDespite numerous theoretical and experimental studies of strain relaxation in metal films on silicon substrates, the exact mechanisms by which dislocations mediate plasticity in these structures are not well understood. To elucidate these mechanisms, we present results from in-situ transmission electron microscopy annealing of thin aluminum films grown on Si (100). As a model system, we have chosen to focus on aluminum films which contain two (011) epitaxial variants with respect to the silicon substrate. In this paper we discuss our observations of the glide and climb behavior of dislocations in these structures during thermal cycling. These observations give qualitative insight into the mechanisms by which dislocation motion accommodates thermally induced strains in thin metal films.

Epitaxy and Atomic Structure Determination of Au/TiO2Interfaces by Combined EBSD and HRTEM

AbstractWe have studied the effects of deposition conditions on the epitaxial orientation of Au or TiO2(110) and on the atomic structure of Au/TiO2interfaces by combined EBSD and HRTEM. Two experimental conditions were explored consisting of deposition of a 12 nim Au film at 300K followed by annealing at 770K and direct deposition of a 12 nm Au film at 770K. Deposition at 300K followed by annealing at 770K give rise to a (111)Au//(110)TiO2epitaxial orientation relationship, while direct deposition at 700K temperature give rise to an epitaxial orientation relationship given by (112)Au//(110)TiO2. For both orientations, two epitaxial variants are observed which are twin related. The (112)Au//(110)TiO2orientation has been found to minimize the interfacial lattice misfit while maximizing the number of Au-Ti bonds across the interface.

Epitaxial Variants and Grain Boundary Structures in Heteroepitaxial Lithium Tantalate on Basal Sapphire

AbstractSingle crystal heteroepitaxial ferroelectric films are desired for non-linear optical applications to maximize the electro-optic coefficient and minimize waveguide losses. In this study, lithium tantalate films were deposited on (0001) sapphire from lithium hexaethoxytantalate by chemical beam epitaxy. Characterization showed that films had nearly stoichiometric composition, epitaxial orientation, and a high degree of crystalline perfection. However, the films exhibited high optical waveguide losses. Additional characterization by TEM revealed that the films had a two dimensional grain structure with epitaxial variants related by translation and a twin orientation to the substrate. To better understand the nature of the heteroepitaxial growth of lithium tantalate on (0001) sapphire, a model was developed to explain the observed epitaxial orientations, misfit dislocation networks, and grain boundary structures of lithium tantalate on (0001) sapphire.

Simulation of Multicomponent Thin Film Deposition and Growth

ABSTRACTResults from a multicomponent Monte Carlo simulation of the deposition and growth of YBa2Cu3O7 are presented and discussed. In particular, a detailed examination of the growth modes active during different morphological growth conditions is performed. At higher deposition rates, both [001] and [100] epitaxial variants (‘c’ and ‘a’ type growth, respectively) are observed to grow by modes attributed to the classic Volmer-Weber mechanism. At very low deposition rates, the film is observed to grow in a distinct, cyclic, multi-stage process. Small islands of [001] epitaxy nucleate and grow to one unit cell height followed by primarily horizontal growth or “ledge extension” until one unit cell layer has formed. This process then repeats. Simulated RHEED amplitude data from this growth process compares favorably to experimentally obtained data.

Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon

Semiconducting β-FeSi2 is drawing much current research interest because of hoped-for silicon-based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron-diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron-diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high-energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible with epitaxial systems such as this one, which exhibit a tendency toward islanding. The possibilities for experimentally distinguishing, with this technique, the competing epitaxial relationships on Si(111) are clarified; it is found that the β-FeSi2(110) matching face is certainly present in these samples and the (101) may be also. An experimental determination of the two-dimensional space groups of the (100), (110), and (101) faces is also presented—in the first and third cases the surface unit meshes are different from the simple projections of the bulk crystalline unit cell.