In-plane Alignment Research Articles

Mitigating the geometrical limitations of conventional sputtering by controlling the ion-to-neutral ratio during high power pulsed magnetron sputtering

High power pulsed magnetron sputtering has been used to grow thin chromium layers on substrates facing and orthogonal to the target. It is demonstrated that at low peak target current density, j T < 0.6 A/cm 2 corresponding to a low ion-to-neutral flux ratio, films grown on substrates facing the target exhibit in-plane alignment. This is due to the rectangular shape of the target that yields an asymmetry in the off-normal flux of sputtered species. With increasing j T the biaxial alignment degrades, as the major portion of the incoming flux (ions) can be effectively steered by the electric field of the substrate to remove asymmetry imposed by geometrical restrictions. Eventually, at j T = 1.7 A/cm 2 a fiber texture is obtained. For films grown on substrates orthogonal to the target, the large column tilt characteristic for growth at low j T , decreases with increasing ion content in the flux and almost disappears at the highest value of j T . The latter indicates that material flux to the substrate is highly ionized so that deposition takes place along substrate normal despite the high nominal inclination angle. Thus, in the limit of high j T the artifacts of conventional physical vapor deposition, resulting from the line-of-sight deposition, are effectively eliminated and the film growth proceeds more or less unaffected by the substrate orientation. Samples mounted orthogonally thus possess a similar texture, morphology, and topography as those facing the target.

Epitaxial growth of (100)-oriented ceria film on c-plane GaN/Al2O3 using YSZ/TiO2 buffer layers by pulse laser molecular beam epitaxy

Ceria (CeO2) films with fluorite structures were grown on c-axial-oriented GaN/Al2O3 substrates with and without YSZ/TiO2 double-bridge layer using pulse laser molecular beam epitaxy, respectively. The growth behavior was in situ monitored by reflection high-energy electron diffraction (RHEED). The epitaxial orientation relationship was confirmed by the x-ray diffraction (XRD) technique. With the introduction of the YSZ/TiO2 double-buffer layer, high-quality a-axial-oriented CeO2 films were successfully grown on GaN substrate. The epitaxial relation-ships of this heterostructure were CeO2(200)∥YSZ(200)∥TiO2(200)∥GaN(0002) and CeO2[010]∥YSZ[010]//TiO2[001]//GaN[112¯0]. XRD and RHEED analyses reveal in-plane tensile strain in CeO2 film, which is mainly caused by lattice mismatch. The in-plane alignment of CeO2 film on YSZ/TiO2 bridge layer is attributed to the interface stress between the film and substrate. Furthermore, without the YSZ/TiO2 buffer layer, CeO2 film directly grown on GaN was oriented along the [111] direction. The different out-of-plane orientations of CeO2 films on GaN substrate could be explained by the different in-plane crystallographic symmetries of templates.

Effect of deposition angle on the structure and properties of pulsed-DC magnetron sputtered TiAlN thin films

This article reports the comparison of structure and properties of titanium aluminum nitride (TiAlN) films deposited onto Si(100) substrates under normal and oblique angle depositions using pulsed-DC magnetron sputtering. The substrate temperature was set at room temperature, 400 °C and 650 °C, and the bias was kept at 0, − 25, − 50, and − 80 V for both deposition angles. The surface and cross-section of the films were observed by scanning electron microscopy. It was found that as the deposition temperature increases, films deposited under normal incidence exhibit distinct faceted crystallites, whereas oblique angle deposited (OAD) films develop a kind of “tiles of a roof” or “stepwise structure”, with no facetted crystallites. The OAD films showed an inclined columnar structure, with columns tilting in the direction of the incident flux. As the substrate temperature was increased, the tilting of columns nearly approached the substrate normal. Both hardness and Young's modulus decreases when the flux angle was changed from α = 0° to 45° as measured by nanoindentation. This was attributed to the voids formed due to the shadowing effect. The crystallographic properties of these coatings were studied by θ–2 θ scan and pole figure X-ray diffraction. Films deposited at α = 0° showed a mixed (111) and (200) out-of-plane orientation with random in-plane alignment. On the other hand, films deposited at α = 45° revealed an inclined texture with (111) orientation moving towards the incident flux direction and the (200) orientation approaching the substrate normal, showing substantial in-plane alignment.

Lipid-Controlled Peptide Topology and Interactions in Bilayers: Structural Insights into the Synergistic Enhancement of the Antimicrobial Activities of PGLa and Magainin 2

To gain further insight into the antimicrobial activities of cationic linear peptides, we investigated the topology of each of two peptides, PGLa and magainin 2, in oriented phospholipid bilayers in the presence and absence of the other peptide and as a function of the membrane lipid composition. Whereas proton-decoupled 15N solid-state NMR spectroscopy indicates that magainin 2 exhibits stable in-plane alignments under all conditions investigated, PGLa adopts a number of different membrane topologies with considerable variations in tilt angle. Hydrophobic thickness is an important parameter that modulates the alignment of PGLa. In equimolar mixtures of PGLa and magainin 2, the former adopts transmembrane orientations in dimyristoyl-, but not 1-palmitoyl-2-oleoyl-, phospholipid bilayers, whereas magainin 2 remains associated with the surface in all cases. These results have important consequences for the mechanistic models explaining synergistic activities of the peptide mixtures and will be discussed. The ensemble of data suggests that the thinning of the dimyristoyl membranes caused by magainin 2 tips the topological equilibrium of PGLa toward a membrane-inserted configuration. Therefore, lipid-mediated interactions play a fundamental role in determining the topology of membrane peptides and proteins and thereby, possibly, in regulating their activities as well.

The rapid prototyping of textured amorphous surfaces for the graphoepitaxial deposition of CdTe films using focused ion beam lithography

Cadmium telluride films deposited on amorphous substrates exhibit a grain structure characterized by [111]-oriented grains, but where the in-plane grain orientation is randomized due to the absence of epitaxy. Here, we explore the viability of promoting an in-plane grain alignment through graphoepitaxy. Fifteen different substrate surface textures were fabricated using focused ion beam lithography. This approach allows for the side-by-side deposition of surface textures where both the areal extent and depth of the surface features are varied in a systematic manner. CdTe films deposited overtop these textures show grain structures with dramatic variations, revealing that particular length scales have the most pronounced effect on the grain structure.

Homeotropic Alignment and Director Structures in Thin Films of Triphenylamine-Based Discotic Liquid Crystals Controlled by Supporting Nanostructured Substrates and Surface Confinement

We explore the effects of nanoscale morphology of supporting solid substrates on alignment, defects, and director structures exhibited by thin films of triphenylamine-based discotic liquid crystals. Fluorescence confocal polarizing microscopy and intrinsic polarized fluorescence properties of studied molecules are used to visualize three-dimensional director fields in the liquid crystal films. We demonstrate that, by controlling surface anchoring on supporting or confining solid substrates such as those of carbon nanotube electrodes on glass plates, both uniform homeotropic and in-plane (edge-on) alignment and nonuniform structures with developable domains can be achieved for the same discotic liquid crystal material.

Effect of Bi deficiency on grain alignment of BiPb-2223 thin film fabricated using rf sputtering process and on critical current density properties

In the present study, thin films prepared as a function of the Bi concentration in the BiPbSrCaCuO system were synthesised. Thin films were fabricated using radio frequency sputtering method. Crystal structure of the films fabricated was determined from X-ray diffraction measurements. The crystal orientation was analysed by X-ray pole figure and in-plane alignment. Both X-ray diffraction and pole figure analysis revealed that crystallinity in the films decreased significantly with decreasing Bi concentration in the system. A systematic decrease in the superconducting transition temperature and hole concentration per CuO plane was obtained with decreasing Bi concentration. The Jc values of the films were calculated using Bean formula. It exhibits a significant dependence on the magnetic field and temperature. It was found that Jc decreased sharply with increasing applied magnetic field. The highest Jc value was found to be 1·06×106 A cm−2 at 10 K, which corresponded to the best flux pinning among the films fabricated.

Post-deposition annealing control of phase and texture for the sputtered MoO3 films

Thin films were sputtered on Si substrates from a metallic Mo target at room temperature and then annealed in air at the temperatures between 200–550 °C to form MoO3 of various phases and textures. The films annealed at 450 °C for 1 h showed a pure α phase and their texture was dependent on the ambient pressure during the initial sputtering deposition. The films sputtered under 3 mTorr were b-axis oriented with a broad in-plane alignment after annealing, while the films sputtered under higher pressures showed a texture with the Mo-O6 double-layers upright to the substrate. The films annealed between 350–400 °C were composed of both α and β phases. Below 350 °C, nearly a pure β phase was obtained, which showed a preferred a-axis orientation if the annealing temperature was in the range of 350–300 °C. Ammonia gas sensing properties were tested for the films. The β phase showed best sensitivity, while the α phase with the planar texture showed shortest recovery time. MoO3 films have wide-ranging applications, each of which prefers a specific phase and texture. This simple but productive method is therefore of practical importance.

Distribution of structural domains in MnAs layers grown on GaAs substrates

Electron backscatter diffraction is utilized to determine the distribution of coexisting structural domains in MnAs layers prepared on GaAs substrates. In a layer grown on a GaAs(111)B substrate using solid phase epitaxy, the structural domains roughly correspond to the morphological features of the surface. The domains are, in contrast, considerably larger in size than the surface roughness when the substrate is GaAs(113)A. We examine the role of the freedom in in-plane crystalline alignment for causing such a difference using a numerical model. In a layer grown at an extraordinary high temperature (600 °C) on a GaAs(111)B substrate, (0001) and (11¯0l) orientations are interwoven. While the (0001) component is present mainly as thin flat films, a mixture of all the components forms thick elongated islands in the surrounding of the films. Consequences of such an inhomogeneity on the magnetic and electrical properties of the layer are discussed.

Magnetic domain crossover in FePt thin films

We have investigated the crossover in the magnetic domain structure of FePt thin films as a function of film thickness. We have directly observed by magnetic force microscopy (MFM) that at a critical thickness ${d}_{cr}\ensuremath{\sim}30\text{ }\text{nm}$ the orientation of the magnetization in the magnetic domains changes from in-plane alignment to a system of stripes in which a component perpendicular to the film plane points alternately in opposite directions. The same critical thickness was also estimated from in-plane magnetization vs field measurements. From the MFM images we have also found that the stripe period is an increasing function of the film thickness following a square root law. These data were interpreted with two different models that yield parameters (magnetization, anisotropy, and exchange stiffness) compatible with those determined from magnetization measurements. Films with thicknesses above ${d}_{cr}$ show a strong dependence of the domain configuration on the magnetic history. Rotatable anisotropy was found in these samples, with a rotational anisotropy field that became stronger with the increase in film thickness. Bubblelike domains could be also observed when the sample is saturated perpendicular to the film plane. All magnetic measurements as a function of film thickness can be interpreted using the same values of magnetization, anisotropy, and exchange stiffness.

Orientation Dependence of Transverse Piezoelectric Properties of Epitaxial BaTiO3 Films

The orientation dependence of the piezoelectric properties of epitaxial BaTiO3 (BTO) films was investigated. The (001), (101), and (111)-oriented BTO films of 2–2.5 µm thickness were deposited on SrRuO3/Pt/MgO or SrRuO3/SrTiO3 substrates by rf magnetron sputtering. X-ray diffraction measurements showed that (001), (101), and (111)BTO films were epitaxially grown on the substrates. The lattice parameters of each BTO film were different from those of the bulk single crystal, and the unit cell volume of the BTO films was larger than that of bulk BTO. The transverse piezoelectric coefficients e31*=d31/s11 of (001)BTO films was almost independent of applied electric field, whereas e31* of (111)BTO increased with voltage owing to the domain motion. The piezoelectric properties of (101)BTO films strongly depended on the in-plane alignment of the crystal structure, and a relatively large e31* of -1.3 C/m2 could be achieved by enhancement of the domain motion. Although the absolute values of e31* are smaller than theoretical calculation values, we could demonstrate that the optimization of crystal orientation is effective for enhancing BTO-based lead-free piezoelectric films.

Anisotropy easy axes alignment of deposited Co nanoclusters

We report on a method to orient the easy axes of ferromagnetic nanoclusters during their deposition, with the use of a magnetic field supplied by a set of permanent magnets. This field is applied close to the substrate where the preformed clusters are deposited. The efficiency is demonstrated on an assembly of 4 nm Co clusters produced by a sputtering and gas-condensation technique. The magnetization of the deposited clusters measured at 6 K exhibits an overall in-plane alignment of the easy axes in the direction of the field applied during deposition, H D . Using a Stoner–Wohlfarth model, it is estimated that half of the clusters easy axes lie within 35 ∘ from H D .

Crystal Grain Orientation in Organic Homo- and Heteroepitaxy of Pentacene and Perfluoropentacene Studied with X-ray Spectromicroscopy

We show that the prototypical p- and n-conducting molecular semiconductors pentacene (PEN) and perfluoropentacene (PFP) exhibit correlated crystal orientation in neighboring grains within a thin film. We use scanning transmission X-ray microscopy (STXM) to measure the film topography in PEN and PFP, and importantly X-ray/optical dichroism also makes it possible to map the grain orientation. PEN exhibits an average grain size of 0.46 ± 0.05 μm2, but clusters of aligned grains are measurably larger at >1.9 μm2. This finding is rationalized through nucleation of small grains that maintain the epitaxial relation with an underlying larger grain during homoepitaxy. The orientation of PEN grains in (buried) layers of PEN/PFP heterostructures is also assessed with STXM, but no orientational in-plane alignment is found between layers of the two different materials. The findings are important to quantify the number and type of (orientational) grain boundaries for an understanding of charge carrier mobility and exci...

Biaxially textured LuNi 2B 2C thin films on MgO single crystals

Biaxially textured LuNi 2B 2C thin films with high T c up to 15.8 K and a residual resistivity ratio up to 15 have been prepared on MgO(1 1 0) substrates using pulsed laser deposition from a stoichiometric target. Via the precise control of the deposition rate, a biaxial texture is favored within a broad range of deposition temperatures, with an epitaxial relationship between the borocarbide film, the Lu 2O 3 interface layer and the substrate of (0 1 1)[0 0 1]MgO || (0 1 1)[0 0 1]Lu 2O 3 || (0 0 1)[1 0 0]LuNi 2B 2C. A very sharp in-plane alignment of about 1 ° and an out-of-plane order of about 2.5 ° was achieved in the LuNi 2B 2C layer. It is shown that the temperature is crucial for phase formation and superconducting properties. Applying optimized deposition conditions, critical temperatures of up to 15.8 K and steep superconducting transitions of about 0.3 K are reproducibly obtained. The upper critical field H c2 of the optimized films was resistively measured along the [0 0 1] direction and fitted with a power law. The resulting value of H c2 (0) = 9.82 T is in very good agreement with single crystal data, whereas the power exponent describing the positive curvature for small external magnetic fields α = 0.19 indicates a relatively low intraband scattering in the films.

Synergistic Ag (111) and Cu (111) texture evolution in phase-segregated Cu1−xAgx magnetron sputtered composite thin films

The authors investigated the texture and microstructure evolution of Cu1−xAgx composite thin films through x-ray diffraction pole figures as a function of composition for x≤0.5. As codeposited at room temperature by magnetron sputtering, the fcc nanocomposite Cu1−xAgx forms a phase-segregated thin film, when the Ag volume fraction represents more than 15 at. % up to 50 at. %, and a single phase thin film when the Ag volume fraction represents less than 15 at. %. Comparison between texture evolutions of Cu (111) and Ag (111) in phase-segregated Cu–Ag thin films and Cu (111) and Ag (111) in the same thickness pure copper or silver thin films shows that texture selection in each phase evolves in two different modes during microstructure evolution. First, a stronger perpendicular (111) fiber texture is obtained in the phase-segregated films than in the pure Cu or Ag films. Second, biaxial crystallographic alignment is observed for the Ag component that develops an in-plane alignment of Ag (200) related to the deposition direction. By the use of transmission electron microscopy we found that development of Cu (111) and Ag (111) textures does not correlate to the in-plane grain size differences observed for the different compositions of Cu1−xAgx thin films.

Epitaxial-like Growth of Anisotropic Mesostructure on an Anisotropic Surface of an Oblique Nanocolumnar Structure

Tetrahedral amorphous carbon (ta-C) films with nanoscale structural anisotropy, which are obliquely deposited on a substrate by a filtered cathodic vacuum arc deposition (FAD) technique, allow anisotropic growth of mesostructured silica films thereon. The ta-C films have a uniformly tilted nanoscale columnar structure, which is caused by the self-shadowing effect during the oblique deposition, and consequently, the surface of the film can be morphologically anisotropic when the deposition angle is large enough. When silica films with a two-dimensional hexagonal mesostructure are grown under hydrothermal conditions on these ta-C films, the cylindrical mesochannels are aligned perpendicularly to the deposition direction of ta-C. The distribution of the in-plane alignment direction of the mesochannels can be controlled by the deposition angle of ta-C; it becomes narrower with the increase of the deposition angle and the consequent increase of the surface roughness. The observed alignment of the mesochannels is caused by the anisotropic accommodation of the surfactant molecules on the structurally anisotropic surface of the ta-C films, which is consistent with the fact that the ta-C films prepared at small deposition angles with smoother surface morphology have little alignment controllability. The ta-C film can be removed with the surfactant by calcination, allowing the formation of an aligned mesoporous silica film directly on a substrate. In contrast to this, obliquely evaporated SiO(2) films with a distinct tilted columnar structure and an anisotropic surface morphology provide neither continuous film formation nor controlled alignment of mesochannels even after providing hydrophobicity by a silylation process. This suggests the specificity, in particular, intrinsic strong hydrophobicity, of the ta-C films for the aligned mesostructured silica film formation.

Poly(ε-caprolactone) diol functionalized with a cinnamoyl group and its UV-triggered in-plane alignment

Stimuli-sensitive biomaterials can present variable and tailored functions under specific external stimuli, thus showing potential for application in various biosciences. Significant efforts have focused on the utilization of light as a stimulus in biomedical applications because of its unique advantages, especially the precise control over its position, movement, and non-contact irradiation. This article highlights the preparation of a photo-reactive oligomer composed of biodegradable moieties and its UV-triggered in-plane molecular orientation, implicating its surface as a potential molecular alignment layer. We focus especially on the alignment of nematic liquid crystalline (LC) molecules on the biodegradable layer with molecular orientation, because they are promising candidates for sensing and interfacial applications.

Unique behaviour of RE1Ba2Cu3O7−δ superconducting tapes producing drastic reduction of pinning loss

The reduction of flux-pinning loss is a paramount concern for power applications ofRE1Ba2Cu3O7−δ (RE: rare earth, Y, Gd, etc; REBCO) superconducting tapes. We have discovered a uniqueelectromagnetic behaviour of such tapes where the magnetization loops in an obliquemagnetic field showed a drastic deviation from the conventional theoretical prediction. Itbrought about a drastic reduction of the pinning loss contrary to common knowledge thatpinning loss is proportional to a critical current density. The deviation was enhanced atlower temperatures, at higher magnetic fields, when the applied magnetic field angleto the tape face was smaller, in the tapes with higher critical current densities,and with a higher in-plane alignment of REBCO grains. We also verified thisunusual behaviour is intrinsic to REBCO superconducting thin films and inferredthe origin lies in the fact that magnetic flux has a tendency to penetrate into,and exude out of, REBCO superconducting films in a direction parallel to theCuO2 planes in its crystal structure. The results suggest the possibility of reducing the pinningloss by improving the in-plane alignment of REBCO grains instead of reducing thegeometrical width of superconductors as suggested by the conventional theory.

Optimal growth conditions for GdBa2Cu3O7 thin-film coated conductors characterized by polarized Raman scattering spectroscopy

High temperature superconducting GdBa2Cu3O7 (GdBCO) thin films were grown by pulsed laser ablation. Textured MgO on metal substrates was used as a template for second generation wire applications. Growth conditions of GdBCO thin films were investigated for substrate temperature (Ts) and oxygen partial pressure (PO2) during deposition. Superconducting critical currents of the films were obtained in the films grown at 790–810°C of Ts and at 100–700mTorr of PO2. Scanning electron micrographs of the films revealed uniform and well-connected grains with some outgrown structures. X-ray θ–2θ scans of the films grown at 810°C and 300–500mTorr exhibited c-axis oriented texture. In-plane alignment and c-axis mosaic spread of the films were determined from X-ray Φ scans and rocking curves, respectively. Polarized Raman scattering spectroscopy was used to characterize optical phonon modes, oxygen content, cation disorder, and some possible second phases of the films. The Raman spectra of the films with large critical current density showed modes at 326–329cm−1, 444–447cm−1, 500–503cm−1 related to vibration of oxygen atoms. Origin of small peaks near 600cm−1 will be discussed as well. The information obtained from Raman scattering measurements will be useful for quality control of the conductors as well as optimization of the process conditions.

Control of epitaxial growth orientation in ZnO nanorods on c-plane sapphire substrates

This paper presents a study on low temperature hydrothermal growth of ZnO nanorods (NRs) on pre-seeded (0001) sapphire substrates. Prior to hydrothermal growth of ZnO NRs, epitaxial ZnO seeds were grown by metal-organic chemical vapour deposition under various process conditions. Findings show that the majority of ZnO NRs inclined at a specific angle of about 38° to the direction perpendicular to the substrate surface and exhibited a preferential in-plane alignment, besides other NRs growing vertically from the sapphire surface. X-ray diffraction φ-scan measurements reveal that the ZnO nanorods displayed two distinct epitaxial relationships with sapphire which were (0001) ZnO//(0001) sapphire and (0001) ZnO//(101̅4) sapphire, respectively. Reduced lattice mismatch between ZnO and sapphire is responsible for the inclined ZnO NRs growth. The growth direction of ZnO NRs is remarkably dependent on the growth conditions of ZnO seeds and sapphire substrate pre-treatment. The epitaxial orientations of ZnO seeds grown on the sapphire substrate dominate the subsequent ZnO NRs growth and can be controlled through adjusting growth conditions.