Olaf Bremert

Pulsed laser deposition of thin metallic alloys

The pulsed KrF excimer laser ablation was applied for the preparation of thin metallic alloys. Above an ablation threshold of about 5 J/cm2, an explosive evaporation of the target material occurs leading to high deposition rates of up to 3 nm/s and a stoichiometry transfer between the target and the deposited film. The surfaces of the grown amorphous and polycrystalline films are smooth except for a small number of droplets. The pulsed laser ablation was found to be an attractive alternative to other film deposition techniques, not only for high‐temperature superconductors, semiconductors, and insulators, but also for metallic alloys.

Laser deposition of metallic alloys and multilayers

Abstract The characteristic behaviours of pulsed excimer laser deposition (at 248 nm) of metallic alloys and multilayers are: high deposition rates of up to 3 nm/s above an ablation threshold of about 5 J/cm 2 , a nearly congruent transfer between target and film, droplets on the film surface, and a strong thickness dependence of the obtained films. Compared to sputtered or evaporated films the alloys obtained are characterized by similar structures in most cases, but often larger extensions of single-phase regions and enlarged lattice parameters in the growth direction, while multilayers sometimes have different textures, often a better alignment of the grains in the growth direction and in systems with a negative heat of mixing sometimes a different critical bilayer thickness for the formation of fully amorphous films. Therefore, the pulsed laser deposition is an attractive alternative thin-film technique for the deposition of metallic alloys and multilayers.

Structural properties of laser deposited metallic alloys and multilayers

Abstract Pulsed laser deposited (PLD) metallic alloys and multilayers are characterized by the formation of amorphous or metastable nanocrystalline phases with high solid solubilities, unusually enlarged lattice spacings in growth direction and intermixed interfaces. The differences to sputtered and evaporated samples are discussed with respect to the high instantaneous deposition rate, which is about 105 times larger than during sputtering or thermal evaporation, and the high kinetic energy of the deposited particles of up to more than 100 eV at high laser fluences inducing atomic mixing, a large number of defects and a high stress in the deposited films.

Structure of the Fe‐doped high‐temperature superconductors Y1Ba2Cu3Oy and Bi2Sr2Ca1Cu2Oy

The high‐temperature superconductors YBa2(Cu1−xFex)3Oy and Bi2Sr2Ca1(Cu1−xFex)2Oy have been examined by resistivity measurements, x‐ray diffraction, and Mossbauer experiments in order to understand the influence of Fe on the structure of the perovskites. The concentration dependence of the Fe substitution and the results of the annealing experiments are consistent with the following model which explains the observed structural changes with Fe composition in YBa2Cu3Oy. For low Fe content, the Fe atoms mainly substitute on the Cu(I) sites leading only to a slight decrease of Tc, but to an attraction of more oxygen ions, which are introduced into the (001) plane. The Fe ions with five‐ or sixfold coordination act like local symmetry points inducing the formation of short orthogonal chains crossing at the Fe ions. This leads to the orthorhombic‐to‐tetragonal transition at an Fe concentration of 2.3%. For higher Fe content, Tc more rapidly decreases, probably due to a partial substitution of Fe also on the Cu(...

Comparison of the two orthorhombic-tetragonal transitions in iron-doped Y1Ba2Cu3Oy☆

Abstract Undoped and iron-doped Y1Ba2Cu3Oy samples were investigated by in situ X-ray experiments, differential scanning calorimetry, thermogravimetrical analysis, Mossbauer spectroscopy and resistivity measurements. In these systems two different orthorhombic-tetragonal (O-T) transitions occur. The first — for undoped samples or those with low iron content — is induced by annealing experiments with an oxygen content of about 6.5. The decrease of oxygen content leads to a drastic reduction of Tc and large changes of the Mossbauer spectra. The second O-T transition occurs at an iron content of about 2.3%. The substitution by iron in the (001) plane does not greatly affect the superconducting properties, but leads to the attraction of additional oxygen, which is strongly bonded to the lattice, and to the formation of a microdomain structure. This increases the heat of absorption and the activation energy for diffusion.

Comparison of the structure of laser deposited and sputtered metallic alloys

Abstract The structure of metallic elements and alloy thin films grown by pulsed KrF laser deposition was examined by X-ray diffraction and Mossbauer spectroscopy, and compared with results on triode sputtered films. While, in general, the obtained phases and structures are similar, laser ablated films often exhibit smaller grain sizes, enlarged lattice parameters in the growth direction, larger extensions of the single-phase regions and sometimes a better alignment of the grains. It can be concluded, that often laser deposited metallic material is in a state further away from equilibrium than after preparation by classical thin film techniques . The differences can be explained by the 10 5 –10 6 times higher momentary deposition rate of the pulsed laser ablation process.

Structure of Fe-doped Y1Ba2Cu3Oy and Bi2Sr2Ca1Cu2Oy

Abstract The high-temperature superconductors Y 1 Ba 2 (Cu 1−x Fe x ) 3 O y and Bi 2 Sr 2 Ca 1 (Cu 1−x Fe x ) 2 O y were investigated by X-ray diffraction and Mossbauer experiments, resistivity measurements and thermo-gravimetrical analysis. In the doped “123”-compound the prefered substitution of Fe on the Cu(I) site does not affect the superconducting transition T c at low Fe content much, but leads to an attraction of additional oxygen and to the formation of a microdomain structure. In the Bi-compound the substitution of Fe in the Cu O 2 planes immediately leads to a reduction of T c and to systematically structural changes. In this structure several different Fe-environments exists as well.

Structure of laser-deposited metallic alloys and multilayers

Abstract The structure of metallic alloys and multilayers prepared by pulsed laser deposition using KrF radiation (248 nm) and a pulse duration of 30 ns was investigated by X-ray diffraction. In comparison with conventional deposition techniques, the structural properties of laser-deposited FeNb and FeAg films can be characterized by the formation of amorphous or metastable nanocrystalline phases with higher solubilities, unusually enlarged lattice spacings in the growth direction and different film textures. Often these films are in a state further away from the equilibrium state than sputtered or evaporated samples. Fe Nb multilayers exhibit much broader amorphous interfaces, while Fe Ag superlattices show a different texture and sharp interfaces visible by satellite reflections in the X-ray spectra. These differences can be understood by the about 105 times higher instantaneous deposition rate and the high kinetic energy of the deposited particles in the range of up to 100 eV.

Pulsed Laser Deposition of Thin Metallic Multilayers and Amorphous Films

The method of pulsed excimer laser ablation using KrF radiation was applied for the deposition of thin metallic elementary multilayers. Above an ablation threshold of about 5 J/cm 2 an ‘explosive’ evaporation of the metallic targets occurs leading to high deposition rates of up to 5 nm/s. For different metals, the ablation threshold slightly varies leading at the same laser fluence to different growth rates as shown for Ag, Fe, Zr and Nb. By using two elementary targets and adjusting the dwelling times on both targets, Fe/Ag, Fe/Zr and Fe/Nb multilayers of different bilayer thicknesses were deposited. While Fe/Ag superstructures show crystalline phases down to a periodicity of 1 nm, Fe/Zr and Fe/Nb films are amorphous at such wavelengths. On the other side, Fe/Nb multilayers can also be amorphized by a solid state interdiffusion reaction of the elementary multilayers. The surfaces of the grown films are smooth except for a small number of droplets on the film surface.

Two orthorhombic-tetragonal transitions in Fe-doped YBa2Cu3Oy superconductors

Fe doped Y 1 Ba 2 Cu 3 O y samples were investigated by in-situ X-ray experiments, Mossbauer spectroscopy and resistivity measurements. Two different orthorhombic-to-tetragonal transitions - induced by annealing experiments at the oxygen content of about 6.5 and by a Fe-substitution of about 2.3 %, respectively - occur. In the first case the decrease of the oxygen content leads to a drastic reduction of T c and in the Mossbauer spectra only one Fe site remains. Contrary to that with increasing Fe-concentration T c only slightly decreases at the orthorhombic-to-tetragonal transition and the occupation of the different Fe sites changes.