O. Eibl

Structural and thermoelectric properties of epitaxially grown Bi2Te3 thin films and superlattices

Multi-quantum-well structures of Bi2Te3 are predicted to have a high thermoelectric figure of merit ZT. Bi2Te3 thin films and Bi2Te3∕Bi2(Te0.88Se0.12)3 superlattices (SLs) were grown epitaxially by molecular beam epitaxy on BaF2 substrates with periods of 12 and 6nm, respectively. Reflection high-energy electron diffraction confirmed a layer-by-layer growth, x-ray diffraction yielded the lattice parameters and SL periods and proved epitaxial growth. The in-plane transport coefficients were measured and the thin films and SL had power factors between 28 and 35μW∕cmK2. The lattice thermal conductivity varied between 1.60W∕mK for Bi2Te3 thin films and 1.01W∕mK for a 10nm SL. The best figures of merit ZT were achieved for the SL; however, the values are slightly smaller than those in bulk materials. Thin films and superlattices were investigated in plan view and cross section by transmission electron microscopy. In the Bi2Te3 thin film and SL the dislocation density was found to be 2×1010cm−2. Bending of the ...

Crystal structure of (Bi, Pb)2Sr2Can−1CunO4+2n+δ high-Tc superconductors

Abstract The crystal structure of (Bi, Pb) 2 Sr 2 Ca n −1 Cu n O 4+2 n +δ high-temperature superconducting phases was studied by transmission electron microscopy. The displacive modulation is largest for the n =1 material and yields displacement amplitudes of 70 pm with respect to the average structure. In all phases the largest distortions are found in the Bi-O double layers. The structural distortions were determined explicitly for the cations and amplitudes for the atomic modulation functions are given for the n =1 and n =2 phases. For Bi 2 Sr 2 CaCu 2 O 8+δ ( T c =85 K) the space group Pbnb was determined for the average structure and accounts for reflections which cannot be indexed by Bbmb. In material doped with Pb the periodicity of the modulation changes significantly, from 4.7 times the lattice parameter b for the undoped phases to 8 b for the doped n =2 phase and 8.8 b for the doped n =3 phase, respectively. For the doped phases systematic extinction conditions are compatible with the space group P B b 1 m 1 b 1 . Due to the modulation, shifts of atoms may lead to distortions of CuO polyhedra and should effect the electronic structure of the various phases.

Advanced Stacked Elemental Layer Process for Cu(InGa)Se 2 Thin Film Photovoltaic Devices

Targeting large area and low cost processing of highly efficient thin film solar modules an advanced stacked elemental layer process for Cu(InGa)Se 2 (CIGS) thin films is presented. Key process steps are i) barrier coating of the soda lime glass substrate combined with the addition of a sodium compound to the elemental Cu/In/Ga/Se-precursor stack and ii) rapid thermal processing (RTP) to form the CIGS compound. By this strategy exact impurity control is achieved and the advantageous influence of sodium on device performance and on CIGS film formation is demonstrated unambiguously by means of electrical characterisation, XRD, SEM, TEM and SIMS. Sodium enriched and sodium free precursor stacks were heated to intermediate states (300°C–500°C) of the RTPreaction process. The experiment clearly reveals that on the reaction pathway to the chalcopyrite semiconductor increased amounts of copper-selenide are formed, if sodium is added to the precursor films. TEM-electron diffraction unambiguously identifies the CuSe-phase which is localised at the surface of the forming CIGS-film. These experimental findings propose a sodium assisted quasi liquid growth model for the CIS formation taking into account that sodium promotes the existence of CuSe at higher temperatures and its effect as a flux agent. The model contributes to a better understanding of the observed superior crystal qualitiy for sodium enriched in contrast to sodium free CIGS films. Application of these experimental findings in the technique of the optimized and controlled sodium incorporation significantly improves process reproducibility, CIGS film homogenity over larger substrate areas and shifts the average efficiency of cells and modules to a significantly higher level. This is demonstrated by a 12-cell integrated series connected minimodule with an aperture area of 51 cm 2 and a confirmed efficiency of 11.75 %.

Epitaxial YBa2Cu3O7−δ films on silicon using combined YSZ/Y2O3 buffer layers: A comprehensive study

Abstract Epitaxial YBa 2 Cu 3 O 7−δ films with excellent structural and electronic properties were grown on silicon by reactive thermal coevaporation employing a combination of yttria-stabilized zirconia (YSZ) and Y 2 O 3 as a buffer layer. To provide optimum conditions, the YSZ growth was studied over a wide range of deposition parameter space. For the first time the effectiveness of the buffer has been derived quantitatively from SNMS profiles. Although the buffer layer can solve the problem of interdiffusion, the large differential thermal expansion between silicon and YBa 2 Cu 3 O 7−δ cannot be compensated in any way. In many cases this results in a fracture of films thicker than 70 nm and a fast degradation which imposes severe restrictions on the technical applications of silicon substrates. A thorough study of crack formation led to the conclusion that the theoretical limitation has not yet been reached and that there is still hope to overcome this problem.

Crystal defects in Bi2Sr2Can−1CunO4+2n+δ ceramics

Abstract Extended crystal defects in the high- T c superconducting phases Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n +δ ( n =2 and n =3) were investigated by high-resolution transmission electron microscopy. A high density of stacking faults corresponding to lamellae of the n =1 phase are observed as a consequence of an incomplete solid state reaction. In this reaction a heterogeneous multiphase sample consisting of the n =1 phase and other Sr-Ca-Cu-O compounds reacts to form the n =2 phase. Small stacking faults not bound to certain crystallographic planes are important for the diffusion of the extended chemical defects in this material. In material which contains the n =3 phase as the majority phase, intergrown lamellae consisting of n ≠3 phases are observed with high density. Nano-crystallites of the n =1 phase are coherently intergrown with the matrix on (001) planes. At the interfaces various extended crystal defects were observed. Among these were partial edge dislocations which transform two lamellae of phase n into one lamella of phase 2 n +2. The climb of these dislocations might be an important mechanism for the formation of phases with n > 3.

Displacive modulation and chemical composition of (Bi, Pb)2Sr2Can−1CunO2n+4 (n=2, 3) high-Tc superconductors

Abstract Electron diffraction and high-accuracy EDX microanalysis in the TEM clearlyrevealed the dependence of the displacive modulation on the Pb content in (Bi, Pb) 2 Sr 2 Ca n −1 Cu n O 2 n +4 high- T c superconductors. A general rule was derived which correlates the amount of Pb in the crystal structure to the wave length of the modulation. With increasing Pb concentration the wave length of the modulation increases linearly and the relation between the Pb content and the wave length of the modulation is the same for both the n =2 and the n =3 material. The maximum Pb fraction was as high as Bi:Pb=0.72:0.28 and resulted in an unmodulated crystal structure. The differences between the Pb modulation and the Bi modulation arise from a decrease of the displacement amplitudes parallel to the b direction and a zero component of the wave vector parallel to the c direction for the Pb modulation. Significant deviations from the ideal compositions 2212 and 2223 were detected by EDX microanalysis. A systematic (Sr+Ca) deficit was found for the n =2 and n =3 phases. EDX microanalysis also showed that as much as 20% of Ca can be substituted by Sr.

MgB2 bulk and tapes prepared by mechanical alloying: influence of the boron precursor powder

The influence of the quality of boron precursor powder on the microstructure and superconducting properties of MgB2 bulk samples and tapes was investigated. The nominal purity specified by the suppliers considers only metallic impurities and is not sufficient for the characterization of the boron precursor powder. Oxygen impurities and the grain size of the B precursor powder were found to affect Tc and the microstructure of the MgB2 tapes. The microstructure was investigated by SEM and TEM. Grains in the boron precursor powders were either nanocrystalline or crystalline, with grain sizes varying between 110 and 500 nm. MgB2 precursor powder was prepared by mechanical alloying, which resulted in a small, 20–60 nm, MgB2 grain size of bulk samples. Bulk samples showed the highest MgB2 phase fraction and a critical current density of 4.7 × 104 A cm−2 (at 20 K, 1 T) if boron precursor powder with small grain size and small fraction of metallic impurities was used. Such powder also yielded compact tapes and required lower annealing temperatures for the MgB2 phase formation. The typical critical current densities of the tapes were 5.0 × 104 A cm−2 (at 20 K, 3 T) and were significantly better than those of samples reported recently. These results underline the importance of mechanical alloying for enhancing the critical current density of MgB2 tapes. Summarizing, the phase content, the density and the superconducting properties of MgB2 bulk and tapes depend on the choice of boron precursor powder.

Chemically deposited La2Zr2O7 buffer layers for YBCO-coated conductors: film growth and microstructure

An adequate buffer layer architecture is of great importance for YBa2Cu3O7?? (YBCO)-coated conductor fabrication. We present a transmission electron microscopy (TEM) analysis of La2Zr2O7 (LZO) buffer layers on biaxially textured Ni?5?at.%W substrates for YBCO-coated conductors prepared by chemical solution deposition (CSD). The LZO thin films were heat-treated at 900 and 1050??C respectively. Electron diffraction patterns, and bright and dark-field images were used to determine the microstructure, texture and the nanoporosity of the films. By x-ray diffraction the films were found to be [100] oriented and strongly biaxially textured. Although x-ray diffraction suggests an epitaxial growth of LZO on Ni it was shown by TEM that this was not the case. The grain size of the films is between 100 and 300?nm and therefore much smaller than the Ni grain size of 40??m. Appropriate acquisition conditions for scanning electron microscopy (SEM) and TEM imaging are given to identify the nanogranularity of the films. For the film annealed at 1050??C high-resolution SEM images clearly show a polycrystalline LZO microstructure and the grain size can readily be determined. Electron diffraction rings are more pronounced than for the film annealed at 900??C, indicating a higher level of polycrystallinity in the film. SEM images of the film annealed at 900??C yield no evidence of a polycrystalline microstructure; only single misoriented LZO grains separated by 500?nm are observed. Nanovoids 10?40?nm in size were found in the LZO buffer layers with a high density. The voids had approximately cuboid shape, indicating an anisotropy of the surface energy in LZO. The surface planes of the voids were identified as {111} lattice planes. Despite the nanoporosity, which is a typical feature of CSD-grown buffer layers, the LZO buffer layers act as efficient Ni diffusion barriers. Energy dispersive x-ray microanalysis (EDX) in the transmission electron microscope yielded the composition of the films. In the LZO films no Ni-rich secondary phases were detected and significant C contamination occurred during spectrum acquisition.

Crystallography of (111) twins in BaTiO3

Abstract A transmission electron microscopy study has revealed (111) twins in cubic/tetragonal BaTiO3 ceramics. The relation between the lattices in the bicrystal is described by a coincident site lattice cell with Σ = 3. By considering the structure of BaTiO3, the formation of a twin can be understood as a shear operation with (111) planes successively translated by ⅓[1ζ2] vectors. Because of the small tetragonal distortion at room temperature, an α-δ fringe contrast is observed at the twin boundary for reflections which coincide, in the cubic phase, for matrix and twin. In the tetragonal phase, (111) planes parallel to the twin boundary remain parallel for matrix and twin. From structure images the width of the interface in the tetragonal phase is estimated to be less than three unit cells. The distortion of planes near the interface is caused by the presence of the tetragonal phase. For the structural model of an ideally coherent interface, a (Ba-O3) plane is proposed. This preserves the Ti-O octahedra...

Special grain boundaries in high-Tc Bi2Sr2CaCu2O8+δ

Abstract The structure of grain boundaries of highly dense Bi 2 Sr 2 CaCu 2 O 8+δ was investigated by high-resolution transmission electron microscopy. Special grain boundaries were observed for which the c -axes of adjacent grains were parallel to each other. Grains which are rotated with respect to each other by approximately 45° around the c -axis form coherent (001) grain boundaries. For (001) twin boundaries the rotation angle is 90° around the c -axis. For coherent (001) grain boundaries and twin boundaries, the interfaces lie between two adjacent (001) BiO layers. This indicates that the formation of (001) interfaces between these two layers is energetically most favourable due to the large bond lengths resulting in weak chemical bonds. Polytypoids, i.e. lamellae of Bi 2 Sr 2 Ca n −1 Cu n O 4+2 n +δ phases with ≠2, are frequently observed at twin boundaries and also occur at small angle tilt boundaries to accommodate for the mismatch. This yields compositional variations along the boundary. The dislocations in the boundary introduce either additional perovskite layers or Bi-O double layers. The latter dislocations occur less frequently and convert one lamella with n =3 or n =4 into two lamellae of n =1 phase. Incoherent grain boundaries show no evidence of intergranular phases. Thus, the weak-link behaviour of grain boundaries, which is observed in YBa 2 Cu 3 O 7− x might not occur in Bi 2 Sr 2 CaCu 2 O 8+δ ceramics.