关于YBCO薄膜过热现象的研究

在用YBa₂Cu₃O_z(YBCO)种膜液相外延生长Nd_1+xBa_2-xCuO_z(NdBCO)厚膜的过程中，YBCO晶体在高于熔点的温度下保持不熔化并且起到了外延种子的作用.采用高温金相显微镜,我们实时观察YBCO薄膜的熔化过程，发现了超导氧化物薄膜的过热现象，并且结合XRD极图的分析和Ba-Cu-O熔体的不润湿性现象合理解释了YBCO形成过热的机制.另外，通过对具有不同微观结构的YBCO薄膜熔化行为的横向比较，研究YBCO薄膜品质对于其过热度的影响，并用半共格界面能理论很好地解释了AFM和XRD分析及实时观察熔化过程的实验结果. 关键词： 过热 YBCO薄膜 熔化形核     

面内和面外取向对RBa_2Cu_3O_z薄膜热稳定性影响的研究

采用高温金相显微镜,实时观察RBa2Cu3Oz(RBCO,R代表稀土元素)薄膜的熔化过程.结合X射线衍射(XRD)极图的分析,对不同面内取向和面外取向的RBCO薄膜熔化行为进行横向比较,研究了RBCO微结构对于其热稳定性的影响,并用半共格界面能理论很好地解释了XRD分析和实时观察熔化过程的实验结果.对不同面内取向YBa2Cu3Oz(YBCO)薄膜的研究结果表明,空气中0°晶粒的热稳定性优于45°晶粒;此外还研究了c轴取向的SmBa2Cu3Oz(SmBCO)薄膜的热稳定性,结果表明微量a轴晶粒对SmBCO薄膜的熔化行为以及其热稳定性有明显影响.     

LPE growth of PZNT film using superheating YBCO thin film as seed layer

A superheating YBa₂Cu₃O_7-δ (YBCO or Y123) thin film was applied as a seed layer to grow Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT) films by a liquid phase epitaxy (LPE) process. In the present work, the YBCO thin film underwent a growth temperature of 1050 °C, which was about 40 °C higher than its peritectic temperature. It is very interesting that the superheated YBCO seed film could be used to grow not only Nd_1+xBa_2-xCu₃O_7-δ (NBCO) films with similar compositions but also PZNT films with completely different compositions. The XRD analysis confirmed that the PZNT film grown on the YBCO seeded MgO substrate had a good epitaxial relationship of [100](001)_PZNT//[100](001)_YBCO//[100] (001)_MgO. Compared with the PZNT films directly grown on MgO substrates, the LPE PZNT film on YBCO/MgO presented a better surface morphology. It was found that the superheating YBCO seed film plays a crucial role for the LPE growth of PZNT in the process. Furthermore, the superheating mechanism was discussed in terms of thermodynamic theories as well. PACS 77.84.-s; 61.10.Nz; 68.37.Hk; 81.15.Lm; 82.45.Mp     

Epitaxial growth of YSZ buffer layer for YBa2Cu3O7−δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO₂ buffered biaxially textured Ni-W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa₂Cu₃O_7−δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO₂/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3-4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.     

The analysis of superconducting thin films modified by AFM lithography with a spectroscopic imaging technique

A practical application of nanolithography using atomic force microscopy (AFM) was accomplished in fabricating superconducting flux flow transistors (SFFTs). It was found that it is essential to oxidize a superconducting thin film, grown on LaAlO₃ substrates by a thermal CVD process, by an applied bias voltage between a conducting AFM tip and the films, since I/V characteristics of the device were mainly controlled by the modified gate area in the SFFT. After AFM lithography, the critical current of an YBCO thin film was found to be degraded. Raman lines in the modified YBCO film were observed at 340, 502, and 632 cm⁻¹ in Ar the laser system and 142, 225, and 585 cm⁻¹ in the He-Ne laser system. Raman fluorescence images were also produced by mapping the Raman peaks. A strain image of the peak at 142 cm⁻¹ was most clear, which means that a surface of the YBCO thin film was changed into the YBa₂Cu₃O₆ insulator. AFM nanolithography enables us to fabricate a channel between a source and a drain in SFFT in order to get I/V characteristics.     

Effects of Mn doping on BaTiO3 thin films grown on highly oriented pyrolytic graphite substrates

We report multiferroelectric properties of Mn-doped BaTiO₃ (MBTO) thin films on highly oriented pyrolytic graphite (HOPG) substrates. The MBTO thin films were grown on the HOPG substrate by pulse laser deposition. For comparison purpose, undoped BaTiO₃ (BTO) thin films were also prepared under same experimental conditions. The BTO and MBTO thin films were polycrystalline, indicating that the MBTO thin film has better crystallinity than the BTO thin film. The leakage current of the MBTO thin film was reduced due to the Mn doping substitution. In addition, the MBTO thin film exhibited better than the BTO thin film in ferroelectric and magnetic behaviors. We suggest that the Mn doping bring about the improvements of ferroelectric and ferromagnetic properties of the BTO thin films. Based on atomic force microscopy (AFM) and conducting AFM (CAFM) studies, the grain size of MBTO thin film was much larger than that of BTO thin film.     

Chiral surfaces and metal/ceramic heteroepitaxy in the Pt/SrTiO3(621) system

Low-energy electron diffraction (LEED), atomic force microscopy (AFM), and X-ray diffraction (XRD) have been used to investigate the structural and morphological character of a naturally chiral ceramic SrTiO₃(621) substrate and of Pt and Cu thin films deposited on its surface. AFM experiments showed that as-received chirally-oriented SrTiO₃(621) substrates display atomically smooth surface morphologies, while LEED patterns revealed that the surface structure has a net chirality. Pt(621) and Cu(621) thin films were grown heteroepitaxially on SrTiO₃(621) substrates, as confirmed by XRD. AFM showed that the film surfaces were atomically smooth and LEED illustrated that the Pt films exhibit surface chirality, and by implication that the atomically-flat chirally-oriented Cu films also have chiral surfaces. The characteristics of the observed LEED patterns, where splitting of diffraction spots is considered to arise from the kinked step features of naturally chiral fcc metal surfaces, are discussed with respect to existing models. These results indicate that the chiral SrTiO₃(621) ceramic surface drives the growth of single-enantiomer, chiral, metal (621) thin films.     

Periodic step structure of clean and YBa2Cu3O7 − δ-covered vicinal SrTiO3(001) surfaces

We have studied the structure of a stable vicinal SrTiO₃(001) surface by UHV scanning tunneling microscopy. We find that the regular nanoscale step structure on the vicinal surface strongly influences the growth of c-axis-orientedYBa₂Cu₃O_{7 − δ} (YBCO) thin films. It generates an almost periodic surface structure of the YBCO films. This substrate-mediated control of the film growth is particularly important, because the resultant YBCO microstructure leads to an in-plane anisotropy of the transport properties in the high-temperature superconducting material.     

Surface characterization of TiO2 thin films obtained by high-energy reactive magnetron sputtering

This paper presents the results of surface characterization of TiO₂ thin films deposited on different substrates by the use of high-energy reactive magnetron sputtering. Structural investigations carried out by X-ray diffraction (XRD) and atomic force microscopy (AFM) have shown a strong influence of both the substrate type, and its placement in the deposition chamber (relative to the sputtering target), on the structural properties of the films. In all cases, there is evidence for pseudoepitaxial growth. XRD examination showed existence of TiO₂-rutile phase with preferred (1 1 0) orientation and AFM measurements revealed nanocrystalline structure directly after deposition. X-ray photoelectron spectroscopy analysis showed that the TiO₂ films have stoichiometric composition.     

SiO2和HfO2薄膜光学性能的椭偏光谱测量

结合XRD和原子力显微镜等方法,利用椭圆偏振光谱仪测试了单层SiO2薄膜（K9基片）和单层HfO2薄膜（K9基片）的椭偏参数,并用Sellmeier模型和Cauchy模型对两种薄膜进行拟合,获得了SiO2薄膜和HfO2薄膜在300-800nm波段内的色散关系。用X射线衍射仪确定薄膜结构,并用原子力显微镜观察薄膜的微观形貌,分析表明：SiO2薄膜晶相结构呈现无定型结构,HfO2薄膜的晶相结构呈现单斜相结构;薄膜光学常数的大小和薄膜的表面形貌有关;Sellmeier和Cauchy模型较好地描述了该波段内薄膜的光学性能,并得到薄膜的折射率和消光系数等光学常数随波长的变化规律。     

Growth and structure of Bi0.5(Na0.7K0.2Li0.1)0.5TiO3 thin films prepared by pulsed laser deposition technique

Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5TiO₃ (BNKLT) thin films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition (PLD) technique. The films prepared were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of the processing parameters, such as oxygen pressure, substrate temperature and laser power, on the crystal structure, surface morphology, roughness and deposition rates of the thin films were investigated. It was found that the substrate temperature of 600 °C and oxygen pressure of 30 Pa are the optimized technical parameters for the growth of textured film, and all the thin films prepared have granular structure, homogeneous grain size and smooth surfaces.     

Structural characterization of polycrystalline Ag-In-Se thin films deposited by e-beam technique

The Ag-In-Se thin films were deposited by e-beam evaporation of the Ag₃In₅Se₉ single crystal powder under high vacuum without intentional doping. Energy dispersive X-ray analysis (EDXA) showed the decreasing behavior of Se and Ag in the structure depending on the annealing. X-ray diffraction (XRD) analysis showed that as-grown films have amorphous structure while annealing the films under nitrogen environment at 200 °C transformed from the amorphous to polycrystalline structure. The crystallinity of the films improved as annealing temperature increases up to 400 °C by 100 °C-step. The polycrystalline films show mixed binary and ternary crystalline phases. Each phase was determined by comparing XRD patterns with complete data cards as Ag₃In₅Se₉, AgInSe₂, In₄Se₃, In₂Se₃, InSe, Se₆ and Se. The existence of Se segregation was supported by the formation of Se aggregates in crystalline phases of Se₆ and Se. The X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis have been carried out in order to obtain detailed information about the atomic composition, chemical states and morphology of the thin film surface. The decomposition of In 4d, Se 3d and Ag 3d photoelectron peaks revealed the existence of In-In, In-Se, In-Ag, Se-Se and Ag-Ag bondings in as-grown thin films. After annealing the thin films at different temperatures, the concentration of In-Se and In-Ag bonds decreases significantly, which results in an In-rich, but Ag- and Se-deficient thin film structure. The roughness of the film surface as a result of application of post-annealing in between 200 and 400 °C monitored by AFM technique was observed to change from 1.81 to 22.89 nm.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also 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.     

Effects of deposition temperature on the structural and morphological properties of SnO2 films fabricated by pulsed laser deposition

Tin oxide (SnO₂) thin films were grown on Si (1 0 0) substrates using pulsed laser deposition (PLD) in O₂ gas ambient (10 Pa) and at different substrate temperatures (RT, 150, 300 and 400 °C). The influence of the substrate temperature on the structural and morphological properties of the films was investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). XRD measurements showed that the almost amorphous microstructure transformed into a polycrystalline SnO₂ phase. The film deposited at 400 °C has the best crystalline properties, i.e. optimum growth conditions. However, the film grown at 300 °C has minimum average root mean square (RMS) roughness of 3.1 nm with average grain size of 6.958 nm. The thickness of the thin films determined by the ellipsometer data is also presented and discussed.     

Growth and characterization of nanostructured anatase phase TiO<Subscript>2</Subscript> thin films prepared by DC reactive unbalanced magnetron sputtering

Titanium dioxide thin films were deposited on three different unheated substrates by unbalanced magnetron sputtering. The effects of the sputtering current and deposition time on the crystallization of TiO₂ thin films were studied. The TiO₂ thin films were deposited at three sputtering current values of 0.50, 0.75, and 1.00 A with different deposition times of 25, 35, and 45 min, respectively. The surface morphology of the films was investigated by atomic force microscopy (AFM). The structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The film thickness was determined by field emission scanning electron microscopy (FE-SEM), and the optical property was evaluated with spectroscopic ellipsometry. The results show that polycrystalline anatase films were obtained at a low sputtering current value. The crystallinity of the anatase phase increases as the sputtering current increases. Furthermore, nanostructured anatase phase TiO₂ thin films were obtained for all deposition conditions. The grain size of TiO₂ thin films was in the range 10–30 nm. In addition, the grain size increases as the sputtering current and deposition time increase.     

Pulsed laser deposition of BaGa<Subscript>2</Subscript>O<Subscript>4</Subscript>

This paper reports the first results obtained on monobarium gallate thin films grown on silicon and platinum coated substrates by pulsed laser deposition. The influence of oxygen background pressure and substrate (or post-annealing) temperature on the film properties was studied. The films were characterized by XRD, RHEED, AFM, photoelectron and electrical impedance spectroscopy. The structure analysis showed that the films crystallized into a hexagonal phase, most probably into (metastable) α-BaGa₂O₄. Depending on deposition conditions, films with different (from nearly epitaxial to polycrystalline) textures were obtained.     

The role of target-to-substrate distance on the DC magnetron sputtered zirconia thin films’ bioactivity

Zirconium dioxide thin films were deposited on 316L-stainless steel type substrates using DC unbalanced magnetron sputtering. The process parameter of this work was the target-to-substrate distance (d_t-s), which was varied from 60 to 120 mm. The crystal structure and surface topography of zirconium dioxide thin films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results demonstrate that all of the ZrO₂ thin films are composed monoclinic phase. The film sputtered at short d_t-s (60 mm) shows a rather heterogeneous, uneven surface. The grain size, roughness, and thickness of thin films were decreased by increasing d_t-s. The bioactivity was assessed by investigating the formation of hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) on the thin film surface soaked in simulated body fluids (SBF) for 7 days. XRD and scanning electron microscopy (SEM) were used to verify the formation of apatite layers on the samples. Bone-like apatites were formed on the surface of the ZrO₂ thin film in SBF immersion experiments. A nanocrystalline hydroxyapatite (HA) with a particle size of 2-4 μm was deposited. Higher crystallinity of HA on the surface was observed when the distance d_t-s increased to more than 80 mm. Therefore, it seems that a d_t-s greater than 80 mm is an important sputtering condition for inducing HA on the zirconia film.     

Pure UV photoluminescence from ZnO thin film by thermal retardation and using an amorphous SiO2 buffer layer

ZnO/SiO₂ thin films were fabricated on Si substrates by E-beam evaporation with thermal retardation. The as-prepared films were annealed for 2 h every 100 °C in the temperature range 400-800 °C under ambient air. The structural and optical properties were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL). The XRD analysis indicated that all ZnO thin films had a highly preferred orientation with the c-axis perpendicular to the substrate. From AFM images (AFM scan size is 1 μm×1 μm), the RMS roughnesses of the films were 3.82, 5.18, 3.65, 3.40 and 13.2 nm, respectively. PL measurements indicated that UV luminescence at only 374 nm was observed for all samples. The optical quality of the ZnO film was increased by thermal retardation and by using an amorphous SiO₂ buffer layer.     

Structural investigations of TiO2:Tb thin films by X-ray diffraction and atomic force microscopy

In this work, structural investigations of TiO₂ thin films doped with Tb at the amount of 0.4, 2 and 2.6 at.% have been outlined. Thin films were deposited on Si and SiO₂ substrates by high energy reactive magnetron sputtering from mosaic Ti-Tb target. The influence of Tb dopant amount, post-annealing treatment and kind of applied substrate on microstructure has been discussed. Thin films were investigated by means of X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD analysis revealed the existence of crystalline TiO₂ in anatase and rutile forms, depending on Tb amount in examined samples. AFM images show that as-deposited samples with 0.4 at.% concentration of terbium (anatase structure) have bigger crystallites as compared to 2% and 2.6 at.% of Tb (rutile structure). The additional annealing at 1070 K results in a mixed anatase (77%) and rutile (23%) structure.