Excellent C-axis Orientation Research Articles

Highly epitaxial YBa2Cu3O7−δ films grown on gradient La2−x Gd x Zr2O7-buffered NiW-RABiTS using all sol–gel process

The overall purpose of this work is to develop a reliable and low-cost technique for fabrication of coated conductors (CCs) on the flexible substrates, and to understand the effects of oriented growth and microstructure on the superconducting performance of CCs in-depth. The investigations of gradient La2−x Gd x Zr2O7 buffer architecture and YBa2Cu3O7−δ (YBCO) film on Ni-5at%W rolling-assisted biaxially textured substrate using all sol–gel process are reported. Combining x-ray diffraction and transmission electron microscopy (TEM) analysis, it is revealed that the gradient buffer architecture with increased lattice constant along the direction of film growth has an excellent c-axis orientation, while the inversed gradient architecture with decreased lattice constant shows the deteriorated orientation, indicating that the highly epitaxial growth mechanism most likely related to the interfacial lattice matching degree. The detailed analysis of YBCO films grown on gradient La2−x Gd x Zr2O7 under different deposition parameters, and the influence of microstructure on electrical properties are systematically discussed using x-ray diffraction, rocking curve test, scanning electron microscopy and TEM observations. Finally, the excellent biaxial texture YBCO film on gradient La2−x Gd x Zr2O7 buffer architecture is prepared at a proper heat-treatment temperature of 780 °C, and the full width at half maximum of ϕ-scan and ω-scan are 5.9° and 6.4°, respectively. The critical transition temperature T c,onset of the as-prepared YBCO film is 90 K. With elevating the temperatures, the appearance of a large number of weak-link and amorphous regions, being indicative of the microstructural collapse, is mainly responsible for the reduced superconductivity.

Fabrication of highly textured Ca3Co4O9 ceramics with controlled density and high thermoelectric power factors

Ca3Co4O9 ceramics have been studied as an alternative p-type thermoelectric material. Thermoelectric properties of the ceramics would be improved by either orientation of grains or introduction of pores. In this study, we fabricated textured Ca3Co4O9 ceramics with controlled density by a reactive-templated grain growth method combined with a hot-forging technique. A powder precursor obtained by mixing β-Co(OH)2 as a template and CaCO3 as a matrix was uniaxially pressed into pellets and sintered under hot-forging pressures up to 5.0 MPa. The relative density of the resulting ceramics was varied between 41.0 and 83.8 % while all the ceramics showed excellent c-axis orientation. The in-plane electrical conductivity of our ceramics could be kept relatively higher than that ever reported previously due to the orientation. Because Seebeck coefficient did not depend on the relative density, the higher electrical conductivity of our ceramics led directly to improved thermoelectric power factors between 67.0 and 409 μW·m−1 K−2.

Stoichiometry, phase, and texture evolution in PLD-Grown hexagonal barium ferrite films as a function of laser process parameters

Barium hexaferrite (BaFe12O19 or BaM) films were grown on c-plane sapphire (0001) substrates by pulsed laser deposition (PLD) to evaluate the effects of laser fluence on their composition, structure, and magnetic properties. A Continuum Surelite pulsed 266 nm Nd:YAG laser was employed, and the laser fluence was varied systemically between 1 and 5.7 [J/cm2]. As the laser fluence changed the stoichiometric transfer between the BaM targets and deposited thin films were affected. The Fe to Ba ratio in the films increased with laser fluence. The films deposited at laser fluences below 4 J/cm2 showed undesirable 3-dimensional island growth. Moreover, an insufficient laser energy resulted in deposition of secondary phases corresponding to Barium monoferrite (BaFe2O4) and Magnetite (Fe3O4). Alternatively, laser fluences above 5 J/cm2 promoted resputtering of the growing film at the substrate and degraded film quality, structure, and magnetic properties. BaM films deposited at 4.8 J/cm2 - the optimal laser fluence - showed excellent c-axis orientation perpendicular to the film plane with an anisotropy field of 16 kOe and saturation magnetization of 4.39 kG. These results clearly demonstrate a strong influence of laser parameters on PLD-grown hexaferrite films and provide a path for high-yield production using pulsed laser depositions systems.

Further improvements in conducting and transparent properties of ZnO:Ga films with perpetual c-axis orientation: Materials optimization and application in silicon solar cells

Technologically appropriate device friendly ZnO:Ga films have been prepared at a low growth temperature (100°C) by changing the RF power (P) applied to the magnetron plasma. Structurally preferred c-axis orientation of the ZnO:Ga network has been attained with I〈002〉/I〈103〉>5. The c-axis oriented grains of wurtzite ZnO:Ga grows geometrically and settles in tangentially, providing favorable conduction path for stacked layer devices. Nano-sheet like structures produced at the surface are interconnected and provide conducting path across the surface; however, those accommodate a lot of pores in between that help better light trapping and reduce the reflection loss. The optimized ZnO:Ga thin film prepared at RF power of 200W has 〈002〉 oriented grains of average size ∼10nm and exhibits a very high conductivity ∼200Scm−1 and elevated transmission (∼93% at 500nm) in the visible range. The optimized ZnO:Ga film has been used as the transparent conducting oxide (TCO) window layer of RF-PECVD grown silicon thin film solar cells in glass/TCO/p-i-n-Si/Al configuration. The characteristics of identically prepared p-i-n-Si solar cells are compared by replacing presently developed ZnO:Ga TCO with the best quality U-type SnO2 coated Asahi glass substrates. The ZnO:Ga coated glass substrate offers a higher open circuit voltage (VOC) and the higher fill factor (FF). The ZnO:Ga film being more stable in hydrogen plasma than its SnO2 counterpart, maintains a high transparency to the solar radiation and improves the VOC, while reduced diffusion of Zn across the p-layer creates less defects at the p-i interface in Si:H cells and thereby, increases the FF. Nearly identical conversion efficiency is preserved for both TCO substrates. Excellent c-axis orientation even at low growth temperature promises improved device performance by extended parametric optimization.

Properties of Ba-Hexaferrite Thin Films with Different Layer Structures

M-type Ba-hexaferrite (BaM) thin films with two different structures (single layered and double layered) were deposited on (001) Al2O3 substrates by RF magnetron sputtering. The changes in microstructural and magnetic properties of the films corresponding to different layer structures and substrate temperatures (Ts) were investigated. Experimental results indicated that for the single layered films deposited directly on substrates at Ts = 300 °C and Ts = 500 °C, most of the grains are acicular type grains with their c-axis in-plane and/or randomly oriented. However, in the double layered film with first interfacial layer deposited at Ts = 300 °C and second layer deposited at Ts = 500 °C, good crystallographic characteristics and excellent perpendicular c-axis orientation were obtained. The c-axis dispersion angle (Δθc) decreased to 0.49°, while the squareness ratio and coercivity of the out-of-plane respectively increased to 0.85 and 4.67 kOe in the double layered film. The mechanism for improving perpendicular c-axis orientation with the interfacial BaM layer was attributed to an increase in the perpendicularly oriented nucleation sites and the release of the stress that comes from the film-substrate interface.

Effects of BaM Interfacial Layer on the $c$-Axis Orientation of BaM Thin Films Deposited on SiO$_{2}$/Si Substrates

M-type barium ferrite (BaM) thin films with two different structures (single layered and double layered) were deposited on thermally oxidized silicon (SiO2/Si) substrates by radio frequency (RF) magnetron sputtering. The changes in surface morphologies, crystallographic and magnetic properties of the films corresponding to different structures and substrate temperatures (Ts) were investigated in detail. In the single layered films deposited directly on SiO2/Si substrates at Ts = 300°C and Ts = 500°C respectively, most of the grains are acicular type. XRD data and magnetic properties measurement confirmed that they are both weakly c-axis perpendicularly oriented. However, in the double layered film with first interfacial layer deposited at Ts = 300°C and second layer deposited at Ts = 500°C, good crystallographic characteristics and excellent perpendicular c-axis orientation were obtained. The c-axis dispersion angle (Δθc) decreased to 0.51°C; the lattice parameters were comparable to those of the bulk BaM; the squareness ratio and coercivity of the out-of-plane increased to 0.85 and 338 kA/m, respectively, for the double layered film. The mechanism for improving perpendicular -axis orientation with an interfacial BaM layer was attributed to two reasons. One is a relative increase in the nucleation sites for perpendicularly oriented grains over the nucleation sites for in-plane and/or randomly oriented grains, and the other is the release of stress that comes from the interface between BaM thin film and SiO2/Si substrate.

Deposition of perpendicular c-axis oriented BaM thin films on (001) Al2O3 substrates by introducing an interfacial BaM buffer layer

M-type barium ferrite (BaM) thin films with two different structures (single layered and double layered) were deposited on (001) Al2O3 single crystal substrates using an RF magnetron sputtering system. The changes in the surface morphologies, crystallographic and magnetic properties of BaM thin films corresponding to the different structures and substrate temperatures (Ts) were investigated in detail. The results indicated that the interfacial BaM buffer layer plays an important role in improving the crystallographic and magnetic properties of BaM thin films. The single layered BaM thin films deposited at Ts=300°C and Ts=500°C are both randomly c-axis oriented. Whereas, by introducing a thin BaM layer which acts as an interfacial buffer layer, an excellent perpendicular c-axis orientation with an epitaxial structure for the double layered BaM thin film was obtained. The c-axis dispersion angle (Δθc) decreased to 0.49°, and the perpendicular squareness ratio increased to 0.85 for the double layered film. The mechanism for improving the perpendicular c-axis orientation with an interfacial BaM buffer layer was attributed to two reasons. One is a relative increase in the nucleation sites for perpendicularly oriented grains over the nucleation sites for in-plane and/or randomly oriented grains, and the other is the release of the stress that comes from the interface between BaM thin film and Al2O3 substrate.

Room-temperature remote-plasma sputtering of c-axis oriented zinc oxide thin films

Highly c-axis oriented ZnO films have been deposited at room temperature with high rates (∼50 nm·min−1) using an innovative remote plasma sputtering configuration, which allows independent control of the plasma density and the sputtering ion energy. The ZnO films deposited possess excellent crystallographic orientation, high resistivity (>109 Ω·m), and exhibit very low surface roughness. The ability to increase the sputtering ion energy without causing unwanted Ar+ bombardment onto the substrate has been shown to be crucial for the growth of films with excellent c-axis orientation without the need of substrate heating. In addition, the elimination of the Ar+ bombardment has facilitated the growth of films with very low defect density and hence very low intrinsic stress (<100 MPa for 3 μm-thick films). This is over an order of magnitude lower than films grown with a standard magnetron sputtering system.

Microfluidics based on ZnO/nanocrystalline diamond surface acoustic wave devices.

Surface acoustic wave (SAW) devices with 64 μm wavelength were fabricated on a zinc oxide (ZnO) film deposited on top of an ultra-smooth nanocrystalline diamond (UNCD) layer. The smooth surface of the UNCD film allowed the growth of the ZnO film with excellent c-axis orientation and low surface roughness, suitable for SAW fabrication, and could restrain the wave from significantly dissipating into the substrate. The frequency response of the fabricated devices was characterized and a Rayleigh mode was observed at ∼65.4 MHz. This mode was utilised to demonstrate that the ZnO/UNCD SAW device can be successfully used for microfluidic applications. Streaming, pumping, and jetting using microdroplets of 0.5 and 20 μl were achieved and characterized under different powers applied to the SAW device, focusing more on the jetting behaviors induced by the ZnO SAW.

The Preparation and Characteristics of InxGa1−xN (0.06 ≤ x ≤ 0.58) Films Deposited by ECR-PEMOCVD

We investigate the structural property and surface morphology of InxGa1−xN films for In compositions ranging from 0.06 to 0.58, which are deposited by electron cyclotron resonance plasma enhanced metal organic chemical vapor deposition (ECR-PEMOCVD). The results of x-ray diffraction (XRD) in InGaN films confirm that they have excellent c-axis orientation. The In content in the InGaN epilayers is checked by electron probe microanalysis (EPMA), which reveals that In fractions determined by XRD are in good agreement with the EPMA results. Atomic force microscopy measurements reveal that the grown films have a surface roughness that varies between 4.16 and 8.14nm. The results suggest that it is possible to deposit high-c-axis-orientation InGaN films with different In contents.

Effect of CeO<sub>2</sub> Deposition Temperature on the Properties of Tl-2212 Superconducting Films Grown on MgO Substrates

The preparation and superconductivity of Tl-2212 films grown on CeO2-buffered MgO substrates were investigated. AFM showed that the annealing of MgO substrates remarkably improved their crystal quality of surface layers, and led to the surface morphology of substrates evolving into unattached outgrowth structure with ultra-smooth surface. XRD measurements showed that CeO2 buffer layers grown on MgO annealed at the suitable conditions were pure (00l) phase. The 500 nm thick Tl-2212 films grown on these CeO2 films subsequently possessed excellent c-axis orientation and superconductivity. The best Tl-2212 film reached a low surface resistance of about 324 μΩ (10 GHz, 77 K), a critical current density as high as 3.5 MA/cm2 (77 K, 0 T) and a critical transition temperature of about 108.4 K.

Growth of Tl-2212 Films on CeO<sub>2</sub>-Buffered Sapphire Substrates

Tl2Ba2CaCu2O8 (Tl-2212) films were prepared on r-cut sapphire substrates buffered with CeO2 by dc magnetron sputtering and post-annealing method. The in situ two-temperature process was used to grow CeO2 buffer layers. XRD and AFM measurements showed that CeO2 films deposited at temperature of 370-470°C were excellent c-axis orientation and had smooth surface. SEM observations demonstrated that the Tl-2212 films’ surface morphology was changed from condensing crystal structure to plate-like structure when the CeO2 deposition temperature was increased. The best Tl-2212 film’s critical temperature Tc can reach to 108.3 K, and critical current density Jc obtained at 5.33 MA/cm2 (77 K, 0 T).

Magnetic Anisotropy Energy of L1<SUB>0</SUB> CoPt-B Thin Films Elongated c-axis

Very small amount of B promotes the disorder/order transformation of L10 CoPt at low annealing temperature along with excellent c-axis orientation. The c-axis is slightly expanded by B atoms interstitially incorporated into the octahedral sites in the L10 CoPt lattice. Magnetic measurements using Hall magnetometry have revealed that the magnetic anisotropy energy (MAE) is enhanced by elongation of the c-axis. This result is in harmony with the first principles calculation which predicts that the c-axis elongation enhances the MAE of L10 CoPt due to the d band splitting of Co near the Fermi level in the minority spin state.

Characterization of the ZnO thin film prepared by single source chemical vapor deposition under low vacuum condition

A novel technique is developed for growing high quality ZnO thin films by means of single source chemical vapor deposition (SS CVD) under low vacuum conditions with the precursor of zinc carbamate Zn4O(CO2Net2)6. SEM, AFM and XRD studies show that the resultant thin films have high density, smooth surface, uniform polycrystalline structure and excellent c-axis orientation. XPS investigation indicates that the ZnO films are free of decomposed precursor residues in the bulk. Careful quantitative XPS analysis reveals that the ZnO films are stoichiometric with O/Zn atomic ratio very close to that of ZnO single crystal.

TlSr2CaCu2Oy template for the growth of superconducting Tl(Ba, Sr)2Ca2Cu3Oy thin films on CeO2 buffered sapphire

Thin films of Tl(Ba, Sr)2Ca2Cu3Oy superconductor have been grown on CeO2 buffered r-cut sapphire substrate with the use of a template TlSr2CaCu2Oy by the amorphous phase epitaxy method. By using this template containing Sr, we have overcome the problem of chemical reactions between Ba and Ce present in the superconducting and buffer layers, respectively, at the normal processing temperatures of 840–890 °C. Films grown by this method have an excellent c-axis orientation as well as in-plane alignment with the TlSr2CaCu2Oy template, which in turn is aligned with the CeO2 layers. These films have superconducting critical temperature values up to 104 K and critical current density values as high as 1 MA cm−2 at 77.3 K and 0 T.

Formation processes of Bi-2212 films prepared on Ag( [formula omitted]) substrate by an atomization technique

We have studied in detail the growth of Bi-2212 ribbon-like thin film by melting Bi-2212 powders dispersed on flat, sputter-deposited Ag(0 0 1) films with order of tens of nm root-mean-square surface roughness. The formation processes of Bi-2212 ribbon-like thin films were studied by in situ high-temperature microscope observations. Because the powders melt incongruently, the liquid phase disperses with residual solid phases on the substrate. The residual solid phases act as the barrier for the melting phase to diffuse. Nearly monophasic Bi-2212 with excellent c-axis orientation in these thin films, proved by X-ray diffraction results, is the result of shortened diffusion length of the liquid phase. These techniques can be used especially to synthesize high quality Bi-based superconducting thin films for intrinsic Josephson junction devices.

Perpendicular magnetic recording media using hexagonal ferrite thin films deposited on Pt underlayers and interlayers

Ba ferrite (BaM) thin films with excellent c-axis orientation were deposited on Pt(1 1 1) underlayers using Facing Targets Sputtering system. BaM thin films with extremely low thickness, as thin as 8 nm, exhibited apparent c-axis orientation. Such thin BaM layers exhibited small grain size. BaM/Pt multilayered structure was also effective to introduce exchange interaction among BaM crystallites.

BaM/Pt multilayered films with fine grains and large perpendicular magnetic anisotropy for high density recording media

A magnetoplumbite type of Ba ferrite (BaM) thin layer was deposited on a 9 nm thick Pt underlayer, and excellent c-axis orientation was observed even for an 8 nm thick BaM layer, which corresponds to only three or four BaM unit cells. The grain size was almost in the same range of 60–85 nm even when the BaM layer thickness tBaM decreased from 60 to 17 nm, and tBaM should be reduced below 10 nm to make a grain size smaller than 50 nm. However, the perpendicular coercivity Hc⊥ and squareness S⊥ decreased drastically from 2.6 to 0.5 kOe and from 0.6 to 0.2, respectively, with the decrease of tBaM from 60 to 8 nm because of higher demagnetizing field and susceptibility to thermal fluctuation. On the other hand, the [BaM(5–24 nm)/Pt(9 nm)]3 multilayer exhibited higher Hc⊥ and larger S⊥ than the BaM/Pt bilayer for the same BaM layer thickness and Hc⊥ and S⊥ of the [BaM(8 nm)/Pt(9 nm)]3 multilayer was 2.0 kOe and 0.6, respectively. It was clarified that the deposition of the BaM/Pt multilayer was very effective for achieving a high perpendicular magnetic anisotropy constant even for the ultrathin BaM layer.

Twinned LaAlO 3 substrate effect on epitaxially grown La–Ca–Mn–O thin film crystalline structure

La–Ca–Mn–O (LCMO) thin film was epitaxially grown on LaAlO 3 (1 0 0) substrate using RF magnetron sputtering. The epitaxial relationship between LCMO and LaAlO 3 (LAO), was characterized using MeV 4 He 2+ backscattering (BS)/channeling and a 4-circle X-ray diffractometer (XRD). The LCMO film deposited at 600°C with an RF power of 100 W showed the channeling minimum yield of 4.98% indicating its excellent crystallinity and high c-axis orientation. Through channeling angular scan measurement, it is confirmed that the 145 nm thick LCMO thin film conserved the compressive stress. The FWHM value of LCMO (2 0 0) peak in XRD θ-rocking is mainly influenced by the twin structure of LAO substrate. It is also interesting that the twin angle of LAO substrate, 0.18°, is very close to the separation of two peaks, 0.20°, in XRD θ-rocking scan on LCMO (2 0 0) peak. From these results, it is suggested that these two subpeaks in XRD θ–2 θ curve were originated from each twinned plane rather than coexistence of strained and relaxed layers. In addition, the FWHM of LCMO (2 0 0) peak obtained from high-resolution XRD θ-rocking, 0.147°, is smaller than any value ever reported.

Crystallization of Amorphous (Pb, La)(Zr, Ti)O3 Thin Film and Its Electrical Properties.

Dense amorphous PLZT films with composition (Pb0.925La0.075) (Zr0.4Ti0.6)O3 were deposited on Pt/MgO (100) substrates by rf-magnetron sputtering without heating the substrates and were subsequently crystallized by postdeposition annealing. The optimum conditions in the two-step annealing process to crystallize the amorphous PLZT films were investigated. The post-deposition-annealed films showed excellent crystallinity and c-axis orientation when the amorphous PLZT films were annealed in an oxygen flow at a soaking temperature for the initial nucleation process of T1=300°C, a temperature for crystal growth process of T2=750°C, and the T2-soaking time of t2=60min. Conventional Pt/PLZT/Pt capacitors were fabricated by forming Pt top electrodes on crystallized 300-nm-thick PLZT films. The remanent polarization and the coercive field of the present capacitors, which indicated almost symmetric P-E hysteresis loops, were 2Pr≅30μC/cm2 and 2Ec≅90kV/cm, respectively. The high-endurance PLZT capacitors with excellent ferroelectric properties produced by the present optimized crystallization have great potential for application to nonvolatile memory devices.