Structure and optical properties of sol–gel derived Gd2O3 waveguide films

Pure and rare earth doped gadolinium oxide (Gd₂O₃) waveguide films were prepared by a simple sol–gel process and dip-coating method. Gd₂O₃ was successfully synthesized by hydrolysis of gadolinium acetate. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study the thermal chemistry properties of dried gel. Structure of Gd₂O₃ films annealed at different temperature ranging from 400 to 750 °C were investigated by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that Gd₂O₃ starts crystallizing at about 400 °C and the crystallite size increases with annealing temperature. Oriented growth of (4 0 0) face of Gd₂O₃ has been observed when the films were deposited on (1 0 0) Si substrate and annealed at 750 °C. The laser beam (λ=632.8 nm) was coupled into the film by a prism coupler and propagation loss of the film measured by scattering-detection method is about 2 dB/cm. Luminescence properties of europium ions doped films were measured and are discussed.     

插入二氧化铈薄膜提高MOD-YBa2Cu3O7-x厚膜超导性能的研究

YBa₂Cu₃O_7-x(YBCO)膜存在“厚度效应”: 随着厚度增加, YBCO薄膜的临界电流密度下降, 尤其是YBCO薄膜的厚度超过1 μm时, 它的临界电流密度急剧下降. 本文在YBCO薄膜之间引入极薄的二氧化铈(CeO₂)薄膜, 成功制备出结构为YBCO/YBCO/CeO₂/YBCO的超导厚膜. 所制备的厚度为2 μm的YBCO膜临界电流密度为1.36 MA/cm² (77 K, 自场), 其性能比相同厚度的纯YBCO膜有了较大幅度的提升. 研究表明CeO₂薄膜起到了传递织构、松弛应力的作用.     

硼膜制备工艺、微观结构及其在硼化镁超导约瑟夫森结中的应用

MgB_2材料具备临界转变温度较高、相干长度大、临界电流和临界磁场高等优点,被认为有替代Nb基超导材料的潜力.研究了不同温度下以化学气相沉积法制备的硼(B)薄膜的微观结构.实验结果表明:较低温度沉积的B先驱薄膜为无定形B膜,可以与Mg蒸气反应生成MgB_2超导薄膜;当沉积温度高于550?C时,所得硼薄膜为晶型薄膜;以晶型硼薄膜为先驱膜在镁蒸气中退火,不能生成硼化镁超导薄膜.利用晶型B膜的这一特点,成功制备了以晶型硼薄膜为介质层的硼化镁超导约瑟夫森结.     

Superconducting Nd1.85Ce0.15CuO4 films grown by the pulsed electron deposition technique

Nd_1.85Ce_0.15CuO_4−δ superconducting thin films were prepared on (1 0 0) SrTiO₃ substrates by pulsed electron deposition technique without reducing atmosphere. Oxygen content is finely controlled by high temperature vacuum annealing, and optimal superconductivity has been obtained. The deposition conditions of the film are discussed in details. Higher deposition temperature and lower gas pressure result in the loss of copper and the appearance of the foreign phase Ce_0.5Nd_0.5O_1.75. High quality Nd_1.85Ce_0.15CuO_4−δ epitaxial films are deposited at 840–870 °C in the mixed gas with a ratio of O₂:Ar = 1:3.     

On the origin of 1.5 μm luminescence in porous silicon coated with sol–gel derived erbium-doped Fe2O3 films

Sol–gel derived Fe₂O₃ films containing about 10 wt% of Er₂O₃ were deposited on porous silicon by dipping or by a spin-on technique followed by thermal processing at 1073 K for 15 min. The samples were characterized by means of PL, SEM and X-ray diffraction analyses. They exhibit strong room-temperature luminescence at 1.5 μm related to erbium in the sol–gel derived host. The luminescence intensity increases by a factor of 1000 when the samples are cooled from 300 to 4.2 K. After complete removal of the erbium-doped film by etching and partial etching the porous silicon, the erbium-related luminescence disappears. Following this, luminescence at 1.5 μm originating from optically active dislocations (“D-lines”) in porous silicon was detected. The influence of the conditions of synthesis on luminescence at 1.5 μm is discussed.     

MgB2超导膜的厚度与其Jc(5K,0T)的关系

通过混合物理化学气相沉积法 (hybrid physical-chemical vapor deposition, HPCVD), 在(000l) SiC 衬底上制得一系列从10 nm到8 μm的MgB₂超导膜样品, 并对它们的形貌、超导转变温度T_c 和临界电流密度J_c与膜厚度的关系进行了研究. 观察到T_c随膜厚度增加上升到最大值后, 尽管膜继续增厚, 但T_c值保持近乎平稳, 而J_c则先随膜厚度增加上升到最高值后, 继而则随膜的厚度的增加而下降. MgB₂膜的T_c(0)和T_c(onset)值与膜厚的关系基本一致, T_c(0)在膜厚为230 nm处达到最大值T_c(0)=41.4 K, 而J_c(5K,0T)在膜厚为100 nm时达到最大值, J_c (5 K, 0 T)=2.3×10⁸A·cm^-2, 这也说明了我们能用HPCVD方法制备出高质量干净MgB₂超导膜. 本文研究的超导膜厚度变化跨度非常大, 从10 nm级的超薄膜到100 nm级的薄膜, 再到几微米的厚膜, 如此T_c和J_c对膜厚度变化的依赖就有了较完整、成体系的研究. 并且本文的工作对MgB₂超导薄膜制备的厚度选取具有实际应用意义. 关键词： 2超导膜')" href="#">MgB₂超导膜 混合物理化学气相沉积法 厚度 临界电流密度     

Substrate dependent growth studies of MgB2 films by electrocrystallization

The comparative study of MgB₂ film growth on Al₂O₃ and glass substrate by electrocrystallization technique is discussed. The precursor magnesium films were deposited by vacuum evaporation method. These magnesium films were then used as electrode for the growth of MgB₂ films by electrocrystallization. The structural, morphological and superconducting properties of the electrocrystallized MgB₂ films on Al₂O₃ substrate were examined by using XRD, SEM and electrical resistivity measurement techniques and compared with that of MgB₂ films grown on glass substrate. The films deposited on Al₂O₃ substrates showed enhanced crystallinity and relatively higher T_c value compared to films deposited on glass substrates.     

Enhanced dispersion and stability of gold nanoparticles on stoichiometric and reduced TiO2(1 1 0) surface in the presence of molybdenum

Mo, Au and their coadsorbed layers were produced on nearly stoichiometric and oxygen-deficient titania surfaces by physical vapor deposition (PVD) and characterized by low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). The behavior of Au/Mo bimetallic layers was studied at different relative metal coverages and sample temperatures.

STM data indicated clearly that the deposition of Au on the Mo-covered stoichiometric TiO₂(1 1 0) surface results in an enhanced dispersion of gold at 300 K. The mean size of the Au nanoparticles formed at 300 K on the Mo-covered TiO₂(1 1 0) was significantly less than on the Mo-free titania surface (2 ± 0.5 nm and 4 ± 1 nm, respectively). Interestingly, the deposition of Mo at 300 K onto the stoichiometric TiO₂(1 1 0) surface covered by Au nanoparticles of 3–4 nm (0.5 ML) also resulted in an increased dispersity of gold. The driving force for the enhanced wetting at 300 K is that the Au–Mo bond energy is larger than the Au–Au bond energy in 3D gold particles formed on stoichiometric titania. In contrast, 2D gold nanoparticles produced on ion-sputtered titania were not disrupted in the presence of Mo at 300 K, indicating a considerable kinetic hindrance for breaking of the strong Au-TiO_x bond.

The annealing of the coadsorbed layer formed on a strongly reduced surface to 740 K did not cause a decrease in the wetting of titania surface by gold. The preserved dispersion of Au at higher temperatures is attributed to the presence of the oxygen-deficient sites of titania, which were retained through the reaction of molybdenum with the substrate. Our results suggest that using a Mo-load to titania, Au nanoparticles can be produced with high dispersion and high thermal stability, which offers the fabrication of an effective Au catalyst.     

MgB2超导薄膜的变温拉曼光谱研究

MgB₂作为迄今为止超导转变温度最高的合金超导体,由于其具有结构简单、相干长度长、晶界间不存在弱连接、上临界场很高、电-声散射时间短等特点,MgB₂超导薄膜在电子学领域有着广阔的的应用前景。拉曼光谱是研究电-声子相互作用和超导能带的一种有效方法,且已广泛用于分析MgB₂材料的电子、声子特征以及超导体能带结构,研究表明,样品质量、晶粒尺寸以及测试条件对MgB₂拉曼峰的峰位和峰形影响很大,其中拉曼光谱随温度的变化也是一个研究重点,但目前关于MgB₂变温拉曼光谱的研究,测试的温度范围相对较小,局限在83 K到室温区域或是转变温度附近。研究了大范围温度区间内MgB₂薄膜的拉曼光谱变化,采用混合物理化学沉积法在(0001)SiC衬底上制备了MgB₂多晶薄膜,薄膜的晶粒尺寸约为300 nm,超导转变温度为39.3 K,对其在10～293 K之间的拉曼光谱进行了测试,测量的波数范围为20～1 200 cm-1。变温拉曼光谱的测试结果显示,在高频620 cm-1附近以及低频80和110 cm-1附近存在MgB₂的拉曼峰。经分析,低频区域出现的两个拉曼峰的频率与超导能隙宽度相对应,表明MgB₂的双能隙特性。考虑到MgB₂中四种声子模式的拉曼活性,高频620 cm-1附近的拉曼峰应是由E_2g振动模所贡献的,且随着测试温度的降低,该拉曼峰的峰位未发生明显的偏移,但半高宽显著变小,从293 K时的380.7 cm-1减小到10 K时的155 .7 cm-1,分析表明E_2g声子与电子系统的非线性耦合所引起的非简谐效应可能是拉曼峰半高宽线性变小的主要原因。     

Bismuth oxide thin films prepared by chemical bath deposition (CBD) method: annealing effect

Bismuth oxide thin films have been deposited by room temperature chemical bath deposition (CBD) method and annealed at 623 K in air. They were characterized for structural, surface morphological, optical and electrical properties. From the X-ray diffraction patterns, it was found that after annealing a non-stoichiometric phase, Bi₂O_2.33, was removed and phase pure monoclinic Bi₂O₃ was obtained. Surface morphology of Bi₂O₃ film at lower magnification SEM showed rod-like structure, however, higher magnification showed a rectangular slice-like structure perpendicular to substrate, giving rise to microrods on the surface. The optical studies showed the decrease in band gap by 0.3 eV after annealing. The electrical resistivity variation showed semiconductor behavior and from thermoemf measurements, the electrical conductivity was found to be of n-type.     

Influence of fluctuations on thermal conductivity of high-Tc YBa2Cu3O7−δ superconductor

To study a behavior of the thermal conductivity near T_c specific heat and thermal diffusivity of the YBa₂Cu₃O_7−δ high-T_c ceramics were simultaneously measured. Close to T_c = 92.30 K the thermal diffusivity and the thermal conductivity discovered minima and the specific heat – maximum. Quantitative analysis of the influence of thermodynamical fluctuations showed the same power laws with Gaussian exponent equal to 0.5 and existing of crossover from the 3D Gaussian to 3D XY critical behavior in the specific heat and thermal conductivity at the approach to T_c. To explain the minimum in thermal conductivity at T_c we propose a mechanism of scattering of phonons on the superconducting fluctuations.     

Structural and electrical characterisation of La0.5Ca0.5MnO3 thin films grown by pulsed laser deposition

Thin films of perovskite manganite, with nominal composition La_0.5Ca_0.5MnO₃, have been prepared by pulsed laser deposition on (1 0 0) SrTiO₃, (1 0 0) LaAlO₃, (1 0 0) Si and YSZ/CeO₂-buffered (1 0 0) Si substrates. Structural and electrical characterisation was performed on the films. The magneto-transport properties of all the thin films depart from the bulk behaviour. The LCMO film grown on buffered Si shows an insulator–metallic transition around 130–150 K while the one deposited directly on Si displayed a similar behaviour under a melting field of 1 T. However, that transition is absent in the films grown on LAO and STO. We suggest that appropriate stress values induced by the substrate favour the formation of metallic percolative paths.     

In situ X-ray photoelectron spectroscopy study of the oxidation of CuGaSe2

The thermal and native oxidation of CuGaSe₂ thin films was studied by in situ X-ray photoelectron spectroscopy (XPS). The special design of the XPS chamber allowed to measure XP-spectra under oxidizing gas atmospheres at pressures of up to 5 mbar (in situ) or in ultra high vacuum (UHV). During thermal oxidation, the formation of predominantly Ga₂O₃ and some amount of SeO₂ were observed, but no copper oxides could be detected in the near surface region of the thin films. The same oxides were found after native oxidation in air under ambient conditions. Only after long term native oxidation for longer than 4 months Cu(OH)₂ was detected. An additional sodium oxide compound formed at the thin film surface, Na_xO and Na₂CO₃ after thermal and native oxidation, respectively. The amount of these sodium oxide compounds depends on the Na content on the as prepared surface. The formation of SeO₂ under humid conditions at 100 °C was found to depend on the surface composition of the thin film.     

Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication

High-density magnetic antidot arrays have been fabricated by deposition of Fe₂₀Ni₈₀ thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe₂₀Ni₈₀ thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95 nm (55 and 75 nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe₂₀Ni₈₀ antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores.     

Decomposition mechanism of triethylindium (TEI) on a GaP(0 0 1)-(2×1) surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of triethylindium (TEI: (C₂H₅)₃In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C₂H₅ to form P–C₂H₅ bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface.     

Bi2Te3拓扑绝缘体表面颗粒化铅膜诱导的超导邻近效应

拓扑绝缘体的出现为寻找拓扑超导体和Majorana费米子提供了一种可能的途径. 在拓扑绝缘体Bi₂Te₃表面沉积极薄的不连续铅膜, 试图通过邻近效应感应出大片的超导区, 为下一步研究拓扑超导电性创造条件.借助四引线电输运测量实验, 在0.25 K的低温下看到了超流现象, 表明沉积在Bi₂Te₃表面的厚度小于20 nm的颗粒化铅膜能够诱导邻近效应, 并且使大片Bi₂Te₃超导. 关键词： 超导邻近效应 S-N-S结 拓扑绝缘体     

Mechanism and energetics of dimerization of SiH2 radicals on H-terminated Si(0 0 1)-(2×1) surfaces

The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH₂ groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si₂H₄ species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH₂ radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500T700 K. The Si₂H₄ species resulting from the surface SiH₂ dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si₂H₄ configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH₂ radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1).     

Thermodynamic properties of the clathrate-type silver-oxide Ag6O8AgHF2

We report normal-state and superconducting properties of the clathrate-type silver-oxide Ag₆O₈AgHF₂. We present electrical resistivity, DC- and AC-susceptibility and specific-heat measurements of single crystalline Ag₆O₈AgHF₂. In the normal state, Ag₆O₈AgHF₂ exhibits metallic conductivity and a phase transition near 110 K, possibly a structural phase transition as observed in the related compound Ag₆O₈AgNO₃. The onset of superconductivity of our samples is observed around 1.2–1.5 K, and the H–T phase diagram is determined for the first time. The upper critical field H_c2(0) is estimated to be about 2000–2200 Oe and the coherence length ξ_GL(0) to be 40 nm.     

准一维Cr基超导体RbCr3As3的超导能隙

RbCr₃As₃是具有[（Cr₃As₃）^-]_∞线性链的准一维超导体,超导转变温度约为6.6 K.对RbCr₃As₃单晶进行了电输运和极低温热输运性质的研究.低温下,拟合了RbCr₃As₃正常态电阻率随温度的变化,发现其满足费米液体行为.通过拟合超导转变温度随磁场的关系,得到RbCr₃As₃单晶的上临界场约为25.6 T.对RbCr₃As₃进行了零场下的极低温热导率测量,得到其剩余线性项为7.5 μW·K^-2·cm^-1,占正常态热导率值的24%.测量不同磁场下RbCr₃As₃的热导率,发现与单带s波超导体相比较,RbCr₃As₃剩余线性项随磁场增加相对较快.这些结果表明RbCr₃As₃单晶很可能是有节点的非常规超导体.     

High-resolution electron microscopy study of Ni81Fe19 film with Co33Cr67 buffer layer

The anisotropic magnetoresistance (AMR) in permalloy Ni₈₁Fe₁₉ film deposited on a 1.2 nm Co₃₃Cr₆₇ buffer layer was significantly enhanced. The high-resolution electron microscopy was used to study the microstructure of Ni₈₁Fe₁₉ film with and without Co₃₃Cr₆₇ buffer layer. It was found that Co₃₃Cr₆₇ buffer layer can induce good (1 1 1) texture, while without Co₃₃Cr₆₇ buffer layer, Ni₈₁Fe₁₉ film show randomly oriented grain structure. The Δρ/ρ enhancement is attributed to the decrease in the resistivity ρ of the Ni₈₁Fe₁₉ film due to the formation of the large (1 1 1) textured grains in Ni₈₁Fe₁₉ film with Co₃₃Cr₆₇ buffer layer. However, the surface roughness of substrate may limit the (1 1 1) textured grain size and induce additional grain boundaries in Ni₈₁Fe₁₉ film with Co₃₃Cr₆₇ buffer layer, limit the enhancement of the AMR effect.