Influence of the transparency of tunnel barriers in Nb/Al2O3/Al/Al2O3/Nb junctions on transport properties

The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, I_C, and normal resistance, R_N, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al₂O₃/Nb and Nb/Al₂O₃/Al/Al₂O₃/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al₂O₃/Al/Al₂O₃/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm². At the same conditions the current density of SINIS devices revealed 1–100 A/cm² with non-hysteretic IVC and characteristic voltages, I_CR_N, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the I_CR_N product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γ_eff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I–V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.     

Fe/Al2O3/Fe隧道结特性分析

用离子束溅射方法制备磁性隧道结(MTJ). 研究MTJ样品的隧道结磁电阻(TMR)效应.用X射线 光电子能谱分析了MTJ的软、硬磁层和非磁层及其界面的化学组成与微结构.研究了MTJ的微 结构对氧化铝势垒高度与有效宽度和TMR效应的影响. 关键词： 磁性隧道结 X射线光电子能谱 隧道结磁电阻     

Investigation on densification of Al2O3/B4C ceramic by ab initio calculation and experiment

Compared to experiment, the adsorption energies, bonding properties, and electronic structure of two different Al₂O₃/B₄C bridge sites with seven different Al₂O₃ surfaces are investigated by ab initio periodic density functional theory. The Al₂O₃/B₄C ceramic sintered in Ar is synthesized and measured by XRD and TEM. The calculated results reveal that the densification of O_bridge site of Al₂O₃/B₄C surface is better than that of Al_bridge. The Al₂O₃ (1 1 3)/B₄C with O_bridge is the most favorable and stable. The electronic structure shows that the electron hybridization exists between Al, O atoms and C, B atoms. The results indicate that the calculated results are in good agreement with the experiment.     

Farinfrared absorption in Al2O3 and MgO

Far-infrared absorption measurements performed in Al₂O₃ and MgO between 300 and 1 500 K with a Fourier scanning interferometer are reported. A temperature analysis of results based on a phonon self-energy model allows to assign the absorption in this region to 2- and 3-phonon difference processes. The contributions of these processes are separated and discussed. In Al₂O₃, the frequency dependence of the absorption is also analyzed. A good overall agreement between theory and experiment is evidenced.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

MoO3/Al2O3催化剂中Mo分散的正电子研究

用浸渍法制备了一系列不同Mo含量的MoO₃/Al₂O₃催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al₂O₃载体中存在两种不同尺寸的孔洞.掺入MoO₃之后,Mo原子主要进入Al₂O₃的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO 关键词： 3/Al₂O₃催化剂')" href="#">MoO₃/Al₂O₃催化剂 正电子湮没寿命谱 符合多普勒展宽 Mo 分散     

Interfacial reactions and oxidation behavior of Al2O3 and Al2O3/Al coatings on an orthorhombic Ti2AlNb alloy

The uniform and dense Al₂O₃ and Al₂O₃/Al coatings were deposited on an orthorhombic Ti₂AlNb alloy by filtered arc ion plating. The interfacial reactions of the Al₂O₃/Ti₂AlNb and Al₂O₃/Al/Ti₂AlNb specimens after vacuum annealing at 750 °C were studied. In the Al₂O₃/Ti₂AlNb specimens, the Al₂O₃ coating decomposed significantly due to reaction between the Al₂O₃ coating and the O-Ti₂AlNb substrate. In the Al₂O₃/Al/Ti₂AlNb specimens, a γ-TiAl layer and an Nb-rich zone came into being by interdiffusion between the Al layer and the O-Ti₂AlNb substrate. The γ-TiAl layer is chemically compatible with Al₂O₃, with no decomposition of Al₂O₃ being detected. No internal oxidation or oxygen and nitrogen dissolution zone was observed in the O-Ti₂AlNb alloy. The Al₂O₃/Al/Ti₂AlNb specimens exhibited excellent oxidation resistance at 750 °C.     

Stabilization of a very high-k crystalline ZrO2 phase by post deposition annealing of atomic layer deposited ZrO2/La2O3 dielectrics on germanium

The impact of the ZrO₂/La₂O₃ film thickness ratio and the post deposition annealing in the temperature range between 400 °C and 600 °C on the electrical properties of ultrathin ZrO₂/La₂O₃ high-k dielectrics grown by atomic layer deposition on (1 0 0) germanium is investigated. As-deposited stacks have a relative dielectric constant of 24 which is increased to a value of 35 after annealing at 500 °C due to the stabilization of tetragonal/cubic ZrO₂ phases. This effect depends on the absolute thickness of ZrO₂ within the dielectric stack and is limited due to possible interfacial reactions at the oxide/Ge interface. We show that adequate processing leads to very high-k dielectrics with EOT values below 1 nm, leakage current densities in the range of 0.01 A/cm², and interface trap densities in the range of 2-5 × 10¹² eV⁻¹ cm⁻².     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

Characterization of CuAlO2 thin film prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure

CuAlO₂ thin film was successfully prepared by rapid thermal annealing of an Al₂O₃/Cu₂O/sapphire structure in air above 1000 °C. The film was mostly with single crystalline structure as verified by X-ray diffraction methods. We found that crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. The highest conductivity obtained in this work was 0.57 S/cm. Optical gap of this film was determined to be 3.75 eV.     

Effect of γ-Al2O3 substrate on NO2 interaction with supported BaO clusters

BaO oxide is the main storage component of the NO_x storage and reduction catalysts. Herein, the interactions between the NO₂ molecule and the unsupported as well as γ-Al₂O₃ supported BaO clusters have been studied using the first principle density functional theory calculation. Our results indicated that there is a strong synergetic effect involving both the BaO clusters and the surface of the γ-Al₂O₃ substrate toward NO₂ adsorption. The interfacial region between the monodispersed BaO cluster and the substrate surface that allows NO₂ to bond with the cluster and the surface simultaneously was shown to be optimal for NO₂ adsorption.     

Electronic properties of thin films of laser-ablated Al2O3

Laser ablation coupled to mass quadrupole spectrometry (LAMQS) has been used to prepare thin films of aluminum oxide deposited on Si substrates starting from commercial Al₂O₃ polycrystalline targets. X-ray photoemission (XPS) and reflection electron energy loss spectroscopy (REELS) have allowed the investigation of the electronic properties of the produced films. In particular, it was found that the Al/O atomic ratio assumes a value very near to 0.7 (stoichiometric ratio) only for films deposited normally with respect to the target surface, while films grown at larger deposition angles are more rich in oxygen content.The composition, the mass density, the optical energy gap, the complex dielectric function and refraction index of the films have been calculated and compared with the results obtained from our starting target material and with the literature. The morphology of the deposited samples has been analyzed by the AFM technique.     

The microwave response of MgB2 /Al2O3 superconducting thin films

Microwave characteristics of MgB2/Al2O3 superconducting thin films were investigated by coplanar resonator technique. The thin films studied have different grain sizes resulting from different growth techniques. The experimental results can be described very well by a grain-size model which combines coplanar resonator theory and Josephson junction network model. It was found that the penetration depth and surface resistance of thin films with smaller grain sizes are larger than those of thin films with larger grain sizes.     

Effect of TiO2/Al2O3 film coated diamond abrasive particles by sol-gel technique

The diamond abrasive particles were coated with the TiO₂/Al₂O₃ film by the sol-gel technique. Compared with the uncoated diamonds, the TiO₂/Al₂O₃ film was excellent material for the protection of the diamonds. The results showed that the incipient oxidation temperature of the TiO₂/Al₂O₃ film coated diamonds in air atmosphere was 775 °C, which was higher 175 °C than that of the uncoated diamonds. And the coated diamonds also had better the diamond's single particle compressive strength and the impact toughness than that of uncoated diamonds after sintering at 750 °C. For the vitrified bond grinding wheels, replacing the uncoated diamonds with the TiO₂/Al₂O₃ film coated diamonds, the volume expansion of the grinding wheels decreased from 6.2% to 3.4%, the porosity decreased from 35.7% to 25.7%, the hardness increased from 61.2H_RC to 66.5H_RC and the grinding ratio of the vitrified bond grinding wheels to carbide alloy (YG8) increased from 11.5 to 19.1.     

Promotion of surface SOx on the selective catalytic reduction of NO by hydrocarbons over Ag/Al2O3

The promotion of sulfur oxides on the selective catalytic reduction (SCR) of NO by hydrocarbons in the presence of a low concentration of sulfur oxides over Ag/Al₂O₃ has been investigated by a flow reaction test and in situ infrared spectroscopy. When the C₃H₆ (or C₁₀H₂₂) + NO + O₂ feed-flow reaction was tested, maximum NO reduction was below 30% over fresh Ag/Al₂O₃. After the addition of SO₂ to the feed flow, conversion increased slightly. Conversion increased further after SO₂ was cut-off from the feed flow. This demonstrated that the increase in NO reduction activity of the catalyst was related to SO_x adsorbed on the catalyst. SO_x adsorbed on the catalytic surface (1375 cm⁻¹) was detected by IR spectroscopy and was stable within the temperature range. NCO species, as an intermediate in NO reduction, on SO_x-adsorbed Ag/Al₂O₃ in a C₃H₆ + NO + O₂ feed flow was observed in in situ IR spectra during the elevation of the reaction temperature from 473 to 673 K, while it was only observed at 673 K on fresh Ag/Al₂O₃ under the same experimental conditions. We suggest that SO_x in low concentrations depressed the combustion of reductants by contaminating hydrocarbon combustion active sites on the catalyst, resulting in an increase in NO reduction efficiency of the reductants.     

Correlation between Al2O3 particles and interface of Al-Al2O3 coatings by cold spray

Al-Al₂O₃ composite coatings with different Al₂O₃ particle shapes were prepared on Si and Al substrate by cold spray. The powder compositions of metal (Al) and ceramic (Al₂O₃) having different sizes and agglomerations were varied into ratios of 10:1 wt% and 1:1 wt%. Al₂O₃ particles were successfully incorporated into the soft metal matrix of Al. It was found that crater formation between the coatings and substrate, which is typical characteristic signature of cold spray could be affected by initial starting Al₂O₃ particles. In addition, when the large hard particles of fused Al₂O₃ were employed, the deep and big craters were generated at the interface between coatings and hard substrates. In the case of pure soft metal coating such as Al on hard substrate, it is very hard to get proper adhesion due to lack of crater formation. Therefore, the composite coating would have certain advantages.     

P-n codoping induced enhancement of ferromagnetism in Mn-doped In2O3: A first-principles study

We have performed first-principles density functional theory calculation in order to investigate the feasibility of “p-n codoping method” in improving magnetic property of In₂O₃ based diluted magnetic semiconductors. We find that the ferromagnetic state is favored in Mn-doped In₂O₃, and Sn doping can increase magnetic moment in Mn-doped In₂O₃. These findings are in line with our earlier experimental observation. Along with previous works, we now have enough evidences to support that p-n codoping is a valid method to improve magnetism of oxides based diluted magnetic semiconductors.     

Dielectric and impedance measurements on (1−x)Ba(Fe1/2Ta1/2)O3-xBa(Zn1/3Ta2/3)O3 ceramics

In this work, ((1−x)Ba(Fe_1/2Ta_1/2)O₃-xBa(Zn_1/3Ta_2/3)O₃), ((1−x)BFT-xBZT) ceramics with x = 0.00–0.12 were synthesized by the solid–state reaction method. X-ray diffraction data revealed that both the powders and ceramics were of a pure-phase cubic perovskite structure. All ceramics showed large dielectric constants. For the x = 0.12 sample, a very high dielectric constant (>20,600) was observed. A lowering in the dielectric loss compared to pure BFT ceramics was observed with the BZT addition. The impedance measurements indicated that BZT has a strong effect on the bulk grain and grain boundary resistance of BFT ceramics. These results are in agreement with the measured dielectric properties. Based on dielectric and impedance results, (1−x)BFT-xBZT ceramics could be of great interest for high performance dielectric materials applications due their giant dielectric constant behavior.     

Comparative study of heterogeneous magnetic state above TC in La0.82Sr0.18CoO3 cobaltite and La0.83Sr0.17MnO3 manganite

The magnetic, transport and structural properties are studied for La_0.83Sr_0.17MnO₃ and La_0.82Sr_0.18CoO₃ single crystals with nearly the same doping and the metallic ground state. Their comparisons have shown that ferromagnetic clusters originate in the paramagnetic matrix below Т^?>T_C in both samples and exhibit similar properties. This suggests the possible universality of such phenomena in doped mixed-valence oxides of transition metals with the perovskite-type structure. The cluster density increases on cooling and plays an important role on the physical properties of these systems. The differences in cluster evolutions and scenarios of their insulator–metal transitions are related to different magnetic behaviors of the matrixes in these crystals that is mainly due to distinct spin states of the Mn³⁺ and Co³⁺ ions.