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.     

Adsorption of hydrogen on evaporated cobalt films

Hydrogen adsorption on evaporated Co films has been studied by means of measurements of the surface potential changes that occur during this process, and analysis of the desorption spectrum of hydrogen. It has been observed that hydrogen adsorbed at 78 K on Co films exists in three forms with essentially different electrical properties: atomic, electronegatively polarized β^? form; atomic, electropositively polarized β⁺ form and reversibly adsorbed, molecular, positively polarized α form. The β^? form is not homogeneous from the point of view of the bond energy with the metal surface and consists of the states β_s^? and β^? characterized by activation energy of desorption 10.0 and 18.8 kcal/mol H₂ correspondingly. The Activation energy of desorption of the β⁺ form is low, i.e. 2.1 $\text{kcal}\text{mol}$ H₂.     

Post-deposition atomic terraces growth of ZnO thin films deposited on epi-GaN templates

We demonstrate how growth processes affect on ZnO film properties, which are to be essential guides to prevent defect formation in order to synthesize reproducible high quality ZnO films. First, we reveal that deposition at a low temperature is indispensable to transfer underlying GaN atomic terraces to ZnO surface. As the film thickness is increased, however, the terraces disappear to develop island morphology. It is found that the thick film surface is smoothed to the extent that atomic terraces can be seen after an appropriate thermal treatment. Adverse effects associated with high annealing temperatures are then demonstrated as evidenced by cracks formation, increased yellow cathode-luminescence and intermixing at the interface.     

Dielectric properties of evaporated films

The evidence for vacancy-pair formation in alkali halide crystals is reviewed. Existing information on the dielectric properties and structure of thin films is discussed with a view to using the high defect concentrations in vacuum-deposited films to confirm vacancy-pair orientation effects. Experimental measurements of the dielectric constant and dissipation factor of evaporated films have been made at room temperature over a range of frequencies from 100 c.p.s. to 100 kc/s. Even under vacuum these films show pronounced losses at low frequencies which are attributed to excess vacancy concentrations. Such losses are contrary to the theory of ideal lattices and are not shown by single crystals.

In spite of the excess defect concentrations no evidence of vacancy-pair orientation has been found. Ageing effects have been observed in all cases, the losses decreasing with time as the defect concentration decreases. In the alkali halides the magnitude of the losses depends on the cation mobility but they decrease at a rate determined by the anion mobility. This leads to a proposed dielectric relaxation mechanism in which individual crystallites form the polarizable units, becoming polarized by migration of the cation vacancies towards one end. The losses decrease as the defects are gradually eliminated by simultaneous condensation of positive and negative vacancies at grain boundaries. This is essential to maintain electrical neutrality and the rate is determined by the diffusion of the slower anion vacancies. The measured rates are in agreement with anion activation energies obtained by tracer methods. These results cannot be explained by vacancy-pair formation even if it is assumed that vacancy pairs can form but are incapable of orientation and hence we must conclude that there is little or no vacancy-pair formation.

Measurements at atmospheric pressure show that moisture has a pronounced effect in all cases, producing dielectric losses which completely obscure the vacancy effects. The changes in dielectric properties during and after adsorption cannot be explained as conductivity effects and are in complete opposition to any modification of the Maxwell-Wagner theory. The ageing effects show that after the initial adsorption, the water molecules migrate over crystallite surfaces to positions where they are more strongly bound and contribute to the dielectric polarization by a form of hindered rotation closely analogous to mechanisms proposed for ice. At these equilibrium sites, H-bonds are frequently formed between the adsorbed molecules and the halide ions of the crystal lattice.     

微量Ni~(2+)掺杂MOD-YBCO薄膜性能的研究

采用无氟高分子辅助金属有机物沉积法(PA-MOD)在La Al O3(LAO)单晶衬底上制备了一系列YBa2Cu3-xNixO7-z(x=0、0.0005、0.001、0.0015、0.002)超导薄膜。利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)及综合物性测量系统(PPMS)分别研究了微量Ni2+掺杂YBCO薄膜的晶体结构、表面形貌及超导性能。结果表明,微量Ni2+掺杂明显提高YBCO薄膜在外加磁场下的临界电流密度,说明微量Ni2+掺杂增强了YBCO薄膜的磁通钉扎性能。     

Texture analysis of RF-sputtered CeO2 buffer layers and superconducting MOD-YBCO films

CeO₂ buffer layers were deposited on YSZ single-crystal substrates using an RF-sputtering method. The development of crystalline textures of sputtered CeO₂ films at different sputtering pressure and their effects on YBCO films, deposited by Metal Organic Deposition (MOD), were investigated. Both CeO₂ and subsequent YBCO films grew well epitaxially. The relative XRD peak intensities of CeO₂ (2 0 0) to substrate YSZ (2 0 0) increased with deposition pressure in the range of 3–5 mTorr and were inversely proportional to the θ–2θ scan FWHM values of CeO₂ (2 0 0). Also, the reaction layers of BaCeO₃ were thicker in the samples with lower CeO₂ (2 0 0) intensities and poor out-of-plane alignment when CeO₂ were deposited at the lower pressure of 3.3 mTorr. It is noted, however, that the superconducting layer grew well epitaxially on these BaCeO₃ layers, possibly due to the epitaxial relation between CeO₂ and YBCO. The superconducting critical currents of MOD-YBCO films showed an increasing tendency as both the Δ2θ (CeO₂) and BaCeO₃ peak intensities decreased.     

Electrical properties of evaporated a-CdTe films

The electrical dc conductivity of amorphous CdTe thin films evaporated in a high vacuum system has been measured in situ as deposited on a cold substrate and after various annealings from 100 K up to 365 K. The investigated features are analysed according to the Mott model.     

Doping of evaporated amorphous silicon films

It is shown that evaporated amorphous silicon films can be doped interstitially by ion implantation and in-diffusion of lithium. The doping effect is smaller than for glow discharge silicon films, and the doped state is less stable at elevated temperature. This is explained by precipitation as well as out-diffusion effects in connection with the high defect concentration in evaporated films.     

Effects of thickness on superconducting properties and structures of Y_2O_3/BZO-doped MOD-YBCO films

We report the thickness dependence of critical current density(J c) in YBa2Cu3O7-x(YBCO) films with BaZrO3(BZO) and Y2O3 additions grown on single crystal LaAlO3 substrates by metalorganic deposition using trifluoroacetates(TFA-MOD). Comparing with pure YBCO films, the J c of BZO/Y2O3-doped YBCO films was significantly enhanced. It was also found that with the increase of the thickness of YBCO film from 0.25 μm to 1.5 μm, the I c of BZO/Y2O3-doped YBCO film increased from 130 A/cm to 250 A/cm and yet J c of YBCO film decreased from 6.5 MA/cm2 to 2.5 M A/cm2.The thick BZO/Y2O3-doped MOD-YBCO film showed lower J c, which is mainly attributed to the formation of a-axis grains and pores.     

Photoconductive studies on electron beam evaporated CdSe films

Thin CdSe films were electron beam evaporated. The CdSe powder synthesized in the laboratory by a chemical method was used as source for the deposition of films. Clean glass and titanium substrates were used as substrates. The substrate temperature was varied in the range of 30-250 °C. X-ray diffraction studies indicated polycrystalline hexagonal structure. The band gap was 1.65 eV. The grain size was 15-30 nm with increase of substrate temperature. Photoconductive cells fabricated with the doped and undoped films have exhibited high photosensitivity and high signal to noise ratio. The current voltage characteristics were linear.     

Impact of nitrogen doping on growth and hydrogen impurity incorporation of thick nanocrystalline diamond films

A much larger amount of bonded hydrogen was found in thick nanocrystalline diamond(NCD) films produced by only adding 0.24% N2 into 4% CH4 /H2 plasma,as compared to the high quality transparent microcrystalline diamond(MCD) films,grown using the same growth parameters except for nitrogen.These experimental results clearly evidence that defect formation and impurity incorporation(for example,N and H) impeding diamond grain growth is the main formation mechanism of NCD upon nitrogen doping and strongly support the model proposed in the literature that nitrogen competes with CH x(x=1,2,3) growth species for adsorption sites.     

Thermoelectric power of vacuum evaporated PbTe films

Variation of thermoelectric power with thickness of the vacuum evaporated PbTe films has been investigated. The films were deposited on mica, glass, quartz and LiF substrates, under identical conditions. Attempts have been made to derive a relation which represents the variation of thermoelectric power with thickness of the film. The temperature variation of thermoelectric power has also bsen studied.     

Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.     

Structural and electrical properties of evaporated Fe thin films

Series of Fe thin films have been prepared by thermal evaporation onto glass and Si(1 0 0) substrates. The Rutherford backscattering (RBS), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and the four point probe techniques have been used to investigate the structural and electrical properties of these Fe thin films as a function of the substrate, the Fe thickness t in the 76-431 nm range and the deposition rate. The Fe/Si samples have a 〈1 1 0〉 for all thicknesses, whereas the Fe/glass grows with a strong 〈1 0 0〉 texture; as t increases (>100 nm), the preferred orientation changes to 〈1 1 0〉. The compressive stress in Fe/Si remains constant over the whole thickness range and is greater than the one in Fe/glass which is relieved when t > 100 nm. The grain size D values are between 9.2 and 30 nm. The Fe/glass films are more electrically resistive than the Fe/Si(1 0 0) ones. Diffusion at the grain boundary seems to be the predominant factor in the electrical resistivity ρ values with the reflection coefficient R greater in Fe/glass than in Fe/Si. For the same thickness (100 nm), the decrease of the deposition rate from 4.3 to 0.3 Å/s did not affect the texture and the reflection coefficient R but led to an increase in D and a decrease in the strain and in ρ for both Fe/glass and Fe/Si systems. On the other hand, keeping the same deposition rate (0.3 Å/s) and increasing the thickness t from 76 to 100 nm induced different changes in the two systems.     

沉积温度对热舟蒸发MgF2薄膜性能的影响

 采用热舟蒸发方法沉积了氟化镁(MgF₂)材料的单层膜,沉积温度从200 ℃上升到350 ℃,间隔为50 ℃。测量了样品的透射率和反射率光谱曲线,进行了表面粗糙度的标定,并在此基础上进行了光学损耗及散射损耗的计算。同时对355 nm波长处的激光诱导损伤阈值进行了测量。结果表明:随着沉积温度的升高,光学损耗增加;在短波长范围散射损耗在光学损耗中所占比例很小,光学损耗的增加主要由吸收损耗引起;在355 nm波长处的损伤阈值变化与吸收损耗的变化趋势相关,损伤机制主要是吸收起主导作用。样品的微缺陷密度也是影响损伤阈值的一个重要因素,损伤阈值随缺陷密度的增加而降低。     

Surface-enhanced Raman scattering of silicon evaporated on Ag island films

Enhanced Raman scattering of Si films from 1 to 25 Å thick deposited on Ag island films is presented. It is found that Raman spectra of Si deposits have a spectral shape similar to those of amorphous Si.