C-axis Grain Research Articles

Enhancing sputtered GaN/Si film quality by adding AlGaN buffer layer in a continuous deposition process

In this work we demonstrate the achievement of significative improvements in the structural and morphological quality of wurtzite GaN films (approximately 250 nm thick) by introducing an initial growth step involving an AlGaN buffer layer on p-type Si (100) substrates through a continuous reactive sputtering process. We investigated the influence of using single and multiple AlxGa1-xN buffer layers with varying Al content (x ranging from 0.07 to 0.37) and different thicknesses (from 166 nm to 1 µm). The obtained samples underwent characterization through X-ray diffraction and scanning electron microscopy. The results reveal a slight increase in the c-axis preferred growth direction and grain sizes even with a single and thin (250 nm) Al0.07Ga0.93N buffer layer. Conversely, the use of a single Al0.37Ga0.63N buffer layer led to significant morphological changes and a remarkable improvement in the c-axis preferred orientation. The most favorable outcomes were observed with the implementation of a triple AlGaN buffer layer, featuring decreasing Al content from the substrate, indicating the attainment of a high-quality GaN top layer comparable to epitaxial GaN.

Sputtering Process of ScxAl1−xN Thin Films for Ferroelectric Applications

Several key sputtering parameters for the deposition of ScxAl1−xN such as target design, sputtering atmosphere, sputtering power, and substrate temperature are reviewed in detail. These parameters serve a crucial role in the ability to deposit satisfactory films, achieve the desired stoichiometry, and meet the required film thickness. Additionally, these qualities directly impact the degree of c-axis orientation, grain size, and surface roughness of the deposited films. It is systematically shown that the electric properties of ScxAl1−xN are dependent on the crystal quality of the film. Although it is not possible to conclusively say what the ideal target design, sputtering atmosphere, sputtering power, and substrate temperature should be for all sputtering processes, the goal of this paper is to analyze the impacts of the various sputtering parameters in detail and provide some overarching themes that arise to assist future researchers in the field in quickly tuning their sputtering processes to achieve optimum results.

Transition of vortex pinning behaviour induced by an artificial microstructure design in Ba(Fe0.94Co0.06)2As2 pnictide superconductor

Among various systems of iron-based superconductors, the doped Ba122 polycrystals have shown the largest application value because of their brilliant properties and chemical stability. Since there have been few reports on the vortex dynamics in Ba122 polycrystal, we firstly conduct a comparative investigation on the differences of vortex dynamics in Ba(Fe0.94Co0.06)2As2 single crystal, grain oriented polycrystal and random oriented powder in this paper. Herein, the polycrystal, which has c-axis grain texture, is prepared by the compaction of designed single crystal powder. The second magnetization peak (SMP) effects are observed in the hysteresis loops of single crystal and totally disappear in those of polycrystal and powder. Conventional magnetization relaxation and Maley's method are applied to analyse the vortex dynamics. In single crystal, the SMP effect is primarily related to the crossover of elastic-plastic vortex creep. The weak pinning in single vortex regime results in large magnetization relaxation rates (S) in low magnetic field regime. In polycrystal for H//AD (axial direction), the introduction of strong pins is effective to single vortex pinning, which brings about much lower S values in low magnetic field regime. Furthermore, the results of single crystal for H//ab, polycrystal for H//RD (radial direction) and random oriented powder are analyzed. Synergistic effects of strong pins and microscopic anisotropy on the vortex dynamics are discussed. These results will provide new knowledge for the design of high-performance Ba122 superconductors.

Fabrication of high Jc Bi2223 thick films through grain alignment technique using a permanent magnet

In order to realize practical superconducting joints between Bi2223 tapes towards persistent-current magnets, Bi2223 thick films with high critical current density (Jc) suitable for joint intermediate layers are needed. On the other hand, the methodology of c-axis grain alignment for improving the critical current properties in the polycrystalline materials with a relatively large thickness has not been established. Although the magnetic grain alignment technique is known to be effective for Bi2223 materials, there have not been any successful studies achieving high intergrain Jc of Bi2223 polycrystalline materials prepared thorough the magneto-scientific method thus far. In the present study, fabrication and evaluation of the physical properties of the c-axis-aligned thick films obtained through grain alignment technique using a permanent magnet were performed. Improvement of the intergrain Jc according to the magnetic grain alignment was confirmed in the thick film samples. Control of the microstructure demonstrated in the present study is one of the promising way for realization of the practical superconducting joints between the Bi2223 commercial tapes.

Removal of Heavy Metals (Lead, Cadmium and Iron) from Low-Grade Nanoscale Zinc Oxide Using Ammonium Carbonate Solution as a Leaching Agent

This study investigated the purification and refining method for producing a nanometer size zinc oxide (ZnO) from the low-grade ZnO commercial powder using low cost ammonium carbonate solution as a leaching agent. The atomic absorption spectroscopy results show that the concentration of iron, lead and cadmium can be dramatically reduced by ammonium carbonate leaching and washing. X-ray diffraction (XRD) and scanning electron microscope (SEM) results show that structural properties can improve the degree of the preferential c-axis orientation, grain size, and surface morphology of ZnO by solvent evaporation. All physical and chemical results are of particular significance for the preparation of purified ZnO for device fabrication in photovoltaic industry, functional ZnO coatings, and polymer nanocomposite applications.

Fabrication and characteristic of force sensor based on piezoelectric effect of Li-doped ZnO thin films

In this paper, a force sensor based on piezoelectric effect of Li-doped ZnO (LZO) thin films was presented, which constituted by Pt/LZO/Pt/Ti functional layers and Si cantilever beam. The chips were designed and fabricated by micro electro-mechanical system (MEMS) technology on silicon wafer with [Formula: see text] orientation. In this sandwich structure, the top electrode (TE) was Pt and bottom electrode (BE) was Pt/Ti, LZO piezoelectric thin films were prepared by radio frequency (RF) magnetron sputtering method. The microstructure and morphology were analyzed through X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), analysis results shows that the LZO thin films with highly c-axis orientation and uniform grain size distribution under sputtering power of 220 W. The experimental results show, when external force loaded on the tip of the beam, the output voltage [Formula: see text] was 280.3 mV at external force of 5 N, the sensitivity of the proposed sensor was 46.1 mV/N in the range of 1–5 N.

Using Mosaicity to Tune Thermal Transport in Polycrystalline Aluminum Nitride Thin Films.

The effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport in textured polycrystalline aluminum nitride (AlN) thin films is experimentally and theoretically investigated. We show that by controlling the sputtering conditions we are able to deposit AlN thin films with varying c-axis grain tilt (mosaicity) from 10° to 0°. Microstructural characterization shows that the films are nearly identical in thickness and grain size, and the difference in mosaicity alters the grain interface quality. This has a significant effect to thermal transport where a thermal conductivity of 4.22 vs 8.09 W/mK are measured for samples with tilt angles of 10° versus 0° respectively. The modified Callaway model was used to fit the theoretical curves to the experimental results using various phonon scattering mechanisms at the grain interface. It was found that using a non-gray model gives an overview of the phonon scattering at the grain boundaries, whereas treating the grain boundary as an array of dislocation lines with varying angle relative to the heat flow, best describes the mechanism of the thermal transport. Lastly, our results show that controlling the quality of the grain interface provides a tuning knob to control thermal transport in polycrystalline materials.

C-Oriented YBa2Cu3O7−δ film with embedded a-oriented grains grown by liquid phase epitaxy under fine-tuning supersaturation

The performance of superconductor films is related to their crystallographic orientations, which are strongly dependent on the supersaturation (σ) in the solution used for liquid phase epitaxy (LPE). To date, except for two extreme states, low and high σ for the preparation of a- and c-axis-oriented YBa2Cu3O7−δ films, respectively, little attention has been directed toward intermediate σ, which is of great importance for achieving a variety of artificial microstructures that are in principle difficult to obtain by existing crystal growth methods. Here, a further step is taken towards the comprehension of how the crystallographic orientations and microstructure are correlated with supersaturation. Fine-tuning of σ to an intermediate state is realized by introducing an additional factor, namely the holding time, that adjusts the initially uncertain state to a certain and stable one. This factor is controlled along with the commonly used variables of the amount of fresh solvent material and the melting time. Consequently, for the first time, a composite epitaxial microstructure of a c-axis-oriented YBa2Cu3O7−δ film with embedded a-axis grains on a (110) NdGaO3 substrate was successfully grown by LPE. The epitaxial interface between a- and c-axis grains potentially serves as a flux pinning site. This work provides further insights into how control of artificial microstructures can be used to enhance superconducting properties.

Gravimetric sensors operating at 1.1\u2009GHz based on inclined c-axis ZnO grown on textured Al electrodes

Shear mode solidly mounted resonators (SMRs) are fabricated using an inclined c-axis ZnO grown on a rough Al electrode. The roughness of the Al surface is controlled by changing the substrate temperature during the deposition process to promote the growth of inclined ZnO microcrystals. The optimum substrate temperature to obtain homogeneously inclined c-axis grains in ZnO films is achieved by depositing Al at 100 °C with a surface roughness ~9.2 nm, which caused an inclination angle of ~25° of the ZnO c-axis with respect to the surface normal. Shear mode devices with quality-factors at resonance, Qr and effective electromechanical coupling factors, {{boldsymbol{k}}}_{{bf{eff}}}^{{bf{2}}}, as high as 180 and 3.4% are respectively measured. Mass sensitivities, Sm of (4.9 ± 0.1) kHz · cm2/ng and temperature coefficient of frequency (TCF) of ~−67 ppm/K are obtained using this shear mode. The performance of the devices as viscosity sensors and biosensors is demonstrated by determining the frequency shifts of water-ethanol mixtures and detection of Rabbit immunoglobin G (IgG) whole molecule (H&L) respectively.

Magnetic properties and millimeter wave loss of highly oriented scandium substituted barium hexaferrite thin films for millimeter wave applications

Scandium substituted barium hexaferrite (BaFe12−xScxO19, BaM) were grown on platinum-coated Si substrates using sol–gel method. It is found that scandium pay an important role in the structure, magnetic and millimeter wave properties of BaM system. The number of hexagonal grains decreased with increasing the substituted contents x of Sc. XRD results confirmed that BaM films have crystal texture of c-axis grains perpendicular to film plane, and the degree of texture for all the films are as high as 92%. The anisotropy field decreased from 16.2 kOe (x = 0) to 2.5 kOe (x = 2), meanwhile, the saturated magnetization decreased from 4.1 to 1.8 kG. The millimeter wave loss was studied by ferromagnetic resonance (FMR) system from 35 to 60 GHz, it is found that it has lowest FMR linewith of 129 Oe at 45 GHz for x = 0 sample, and the FMR field shift to higher for substituted thin films, together with the FMR linewith became broader. These results mean that the anisotropy field and FMR of BaM thin films were tunable and these thin films can be used in millimeter wave devices.

パウダー・イン・チューブ (PIT) 法による (Ba(Sr),K)Fe<sub>2</sub>As<sub>2 </sub>超電導線材の開発

The iron-based superconductors of K-doped Ba(Sr)Fe2As2(Ba(Sr)-122) are most potentially useful for high-field applications as a result of their high upper critical fields of over 50T and their small anisotropies. For Ba(Sr)-122, a powder-intube(PIT) method using an Ag sheath has been developed for tape and wire fabrications. Two important factors that greatly influence the transport Jc of PIT Ba(Sr)-122 tape are the density and c-axis grain orientation of the Ba(Sr)-122 core. The Jc of our PIT Ba-122/Ag tapes approach 4×104 A/cm2 at 4.2 K and 10 T for flat-rolled thin tapes. Alloying Ag with Sn is effective for improving the density and grain orientation of the Ba-122 core, and hence the Jc values. The application of uniaxial pressing in the final stage of deformation significantly enhanced Jc values up to ~7×104 A/cm2. This is also due to improving the density and grain orientation of the Ba-122 core. Another effective method to increase the density and grain orientation is double sheathing with an Ag or Ag-Sn inner sheath and stainless steel (SUS) outer sheath. This double-sheath architecture brings about much improved packing density and c-axis grain orientation in the Ba-122 core. The flat-rolled SUS/(Ag-Sn) sheathed Ba-122 tapes show Jc values of 1×105 A/cm2 at 4.2 K and 10 T with high homogeneity. The application of uniaxial pressing to this doublesheathed tape further increased Jc up to 1.4×105 A/cm2. The Jc values of both rolled and pressed Ba-122 tapes show extremely small magnetic field dependence up to 28 T, indicating that Ba-122 is a promising candidate as a conductor for high-field magnets.

Change of scattering mechanism and annealing out of defects on Ga-doped ZnO films deposited by radio-frequency magnetron sputtering

This study examines the change of carrier scattering mechanism and defects states in Ga-doped ZnO (GZO) thin films deposited by radio-frequency magnetron sputtering as a function of the substrate temperature (Ts) during deposition. The GZO films deposited at room temperature exhibited a high defect density that resulted in a lower carrier concentration, lower Hall mobility, and optical absorption in visible wavelength range. Such defects were created by ion bombardment and were eliminated by increasing the Ts. The defects related to the optical absorption disappeared at a Ts of 125 °C. The defects responsible for the suppression of the carrier concentration gradually decreased with increasing Ts up to 200 °C. As a result, the carrier concentration and in-grain carrier mobility gradually increased. The Hall mobility was also influenced by film structural properties depending on the Ts. In addition to the c-axis preferred orientation, other oriented grains such as the (101¯1) plane parallel to the substrate surface appeared below 150 °C. This orientation of the (101¯1) plane significantly reduced the Hall mobility via grain boundary scattering. The films deposited at a Ts higher than 175 °C exhibited perfect c-axis orientation and grain boundary scattering was thus negligible in these films. The appearance of the 101¯1 peak in x-ray diffraction profile was correlated with the contribution of grain boundary scattering in heavily doped GZO films.

Sol ageing effect on the structural, optical and electrical properties of Ga-doped ZnO thin films

Gallium-doped zinc oxide thin films (GZO) were grown on a glass substrate by a simple sol–gel process and spin coating technique. The effects of sol ageing time on the structural, optical and electrical properties of the GZO films were investigated. Experimental results showed that the microstructure, optical transmittance and electrical resistivity of the GZO films depend directly on the sol ageing time. XRD studies revealed the polycrystalline nature of the films with the c-axis grain orientation. Optical transmittance of the films was higher than about 82% within the visible region and the optical direct band gap was increased with increasing sol ageing time. A minimum resistivity of 3.16 × 10−5 Ω m was observed in the film deposited by a sol aged for 48 h. These results indicate that an appropriate ageing time of the sol is important for the improvement of the properties of GZO films derived from sol–gel method.

Electromigration mechanisms in Sn-0.7Cu/Cu couples by four dimensional (4D) X-ray microtomography and electron backscatter diffraction (EBSD)

The microstructure evolution of surface-encapsulated Sn-0.7Cu solder volumes of approximately 230 μm diameter is observed from electromigration tests at 150 °C and with current densities between 4.0·104 and 1.6·105 A/cm2. X-ray computed tomography (XCT) is implemented to characterize the volumetric evolution of the microstructural features, and electron backscatter diffraction (EBSD) is used to characterize the grain structures. The growth of Cu–Sn intermetallics is quantified using XCT in order measure the effective diffusivity and effective charge product, D˜Z*, of Cu in the solder. The D˜Z* parameter for Cu has been measured as roughly 8·10−7 atoms/cm2s. The measured D˜Z* parameter increases with greater c-axis grain alignment with the electrical current pathway. Literature reports for Z* of Cu are polled in order to decouple the effective diffusivity from D˜Z*. The D˜ of Cu in the tests is compared with literature reports for diffusivity along the a-axis and c-axis of Sn, and the comparison is interpreted using EBSD orientation image microscopy (OIM) results of the tested specimens.

Physical and Electrical Characterization of ZnO Thin Films Prepared by Sol-Gel Method

In this paper, Zinc Oxide (ZnO) thin films are deposited on glass substrate by sol-gel method. The ZnO thin films with well defined orientation were acquired by spin coating of zinc acetate dehydrate monoethanolamine (MEA), de-ionized water and isopropanol alcohol. These films were pre-heated at 275°C for 10 min and then annealed at 350°C, 450°C and 550°C for 80 min. The effect of annealing temperature and different thickness on structure and surface morphology of the thin films were verified by Atomic Force Microscopy (AFM). It was found that there was a significant effect of annealing temperature on the structural parameters of the films such as roughness exponent, fractal dimension and interface width. Thin films also were characterizied by X-ray Diffractometery (XRD) method. XRD analysis revealed that the annealed ZnO thin films consist of single phase ZnO with wurtzite structure and show the c-axis grain orientation. Increasing annealing temperature increased the crystallite size and the c-axis orientation of the film after 450°C. Also In this study, ZnO thin films in different thickness have been prepared by sol-gel method on the glass substrate at room temperature. The thicknesses of films are 100, 150 and 250 nm. Using fractal analysis, morphological characteristics of surface films thickness in amorphous state were investigated. The results show that with increasing thickness, surface roughness (RMS) and lateral correlation length (ξ) are decreased. Also, the roughness exponent (α) and growth exponent (β) were determined to be 0.74±0.02 and 0.11±0.02, respectively. Keywords—ZnO, Thin film, Fractal analysis, Morphology, AFM, annealing temperature, different thickness, XRD.

The impact of argon admixture on the c-axis oriented growth of direct current magnetron sputtered ScxAl1−xN thin films

The piezoelectric properties of wurtzite aluminium nitride (w-AlN) are enhanced by alloying with scandium (Sc), thus offering superior properties for applications in micro electro-mechanical systems devices. ScxAl1−xN thin films have been prepared by DC reactive magnetron sputtering on Si (100) substrates from a single target. When targeting a concentration range from x = 0 up to x = 0.15, the preparation conditions have been optimized by varying the Ar/N2 ratio in the sputtering gas. To incorporate an increasing Sc concentration, a higher Ar/N2 ratio has to be applied during the deposition process. Hence, the argon concentration in the sputtering gas becomes a crucial parameter for microstructure-related parameters. To determine phase purity, degree of c-axis orientation, lattice parameter, and grain size, the ScxAl1−xN thin films were investigated by techniques, such as scanning electron microscopy, transmission electron microscopy, and X-ray diffraction.

Novel shearing apparatuses in confined flow for investigating recrystallization and fabric evolution processes in mono- and polycrystalline ice

AbstractThe orientations of individual crystals within a polycrystalline aggregate subjected to stress have a strong influence on its bulk strain rate and flow behavior. The ability to include the effect of crystal fabric and recrystallization processes in an ice flow law, especially at the bottom of glaciers and ice sheets where temperature is close to the pressure-melting point, is important because the stratigraphy of the ice body may be affected and the paleoclimate reconstruction hampered. We present herein three newly developed deformation apparatuses offering the possibility, from single experiments, of investigating different finite strain stages and their corresponding c-axis fabric and grain texture patterns in various confined, shear flow configurations (simple shear, pure shear and compression/extension bending). The technical set-ups and major advantages compared to classical methods are explained, and results from experiments are discussed in order to illustrate the functioning and purposes of the methods. In all experiments, significant variations in the microstructural development have been observed that reflect the varying orientations of the anisotropy and its relationship to the stress pattern. In monocrystalline ice-bending experiments, the pre-existing c-axis fabric is shown to have a profound influence on the response to stress and possibly to the type of slip system activated.

Magnetic and self-biased properties of highly oriented M-type barium ferrite films on Pt-coated Si substrate by magnetron sputtering

Highly oriented M-type barium ferrite (BaM) thin films were deposited by sputtering on Pt-coated Si Substrate with different substrate temperatures. X-ray diffraction and atomic force microscopy results show that BaM films have crystal texture of c-axis grains perpendicular to film plane with apparent hexagonal morphology. On the other hand, it is clearly observed that the remanent magnetization (Mr) in out-plane is higher than those in in-plane, and the Mr in out-plane increases with increasing the substrate temperature, reaching maximum when substrate temperature (Ts) is 600 °C. The hysteresis curves also indicate that the BaM thin film exhibits nice self-biased property with 4πMr of 3803.04 Gs, squareness ratio (Mr/Ms) of 0.96, and coercivity of 1767.3 Oe. These results make sure that these BaM films have potential for use in self-biasing microwave/millimeter wave magnetic devices such as circulators and isolators.

Polarization Properties of Bismuth Strontium Tantalate Ceramic Films Deposited by Aerosol Deposition Method

We report aerosol deposition (AD) method for preparation of bismuth layer-structured ferroelectric thick films with enhanced ferroelectric and insulating properties. Constitution phase, crystal structure, polarization and leakage current properties of SrBi2Ta2O9 (SBTa) thick films with thickness of 2–4 µm were investigated. While obtained as-deposited SBTa films have slight c-axis grain orientation, the degree of orientation for the films annealed at higher than 1000 °C was the same as that for sintered bulks. Fracture cross-sectional scanning electron microscopy (SEM) images revealed that the as-deposited SBTa films on glass substrates had a fully dense microstructure. The low leakage current density (J) of less than 10-7 A/cm2 at 1 MV/cm was observed for SBTa films annealed at less than 900 °C, and the SBTa films annealed at 900 °C indicated a remanent polarization (Pr) of 5.7 µC/cm2, which is larger than that of sintered bulks (Pr of 4.7 µC/cm2) fabricated by conventional solid-state reaction using the same starting raw powder used for the AD method.

Texture and self-biased property of an oriented M-type barium ferrite thick film by tape casting

In this paper, the oriented M-type barium ferrite (BaM) thick films with different thicknesses are prepared by tape casting. It is found that the crystallographic alignment degree (f), the pore and the squareness ratio (Mr/Ms) are not affected by the thickness of the film. XRD and SEM results show that the thick film has hexagonal morphology with a crystal texture of c-axis grains perpendicular to film plane. The hysteresis curve indicates that the BaM thick film exhibits a self-biased property with a remanent magnetization of 3.30 T, a squareness ratio (Mr/Ms) of 0.81, and a coercivity of 0.40 T. The results show that the BaM thick film has potential for use in self-biasing microwave devices, and also proves that the tape casting technique is capable of fabricating high-quality barium ferrite films, thus providing a unique opportunity to realize the large area production of thick film.