Stepan Kyrsta

Ion-induced formation of regular nanostructures on amorphous GaSb surfaces

Crystalline and amorphous GaSb surfaces are compared concerning their response to sputter erosion with low energy Ar+ ions under normal incidence. We show that the formation of regular nanostructures on GaSb is basically independent of whether the initial material is crystalline or amorphous. The similarity in the temporal and spatial evolution demonstrates that the dynamics of the morphology evolution is entirely controlled by a thin amorphous surface layer.

Characterization of Ge-Sb-Te thin films deposited using a composition-spread approach

Abstract Ge–Sb–Te Thin films for rewritable phase change optical data storage applications were deposited by magnetron sputtering using a composition-spread approach. The deposition took place in an UHV chamber by cosputtering of three magnetron cathodes equipped with pure Ge-, Sb- and Te-targets in a DC–argon plasma. To investigate the influence of the chemical composition of the phase change material on its optical properties, films with lateral compositional gradients of up to 30 at.% were deposited. The composition, binding states and structure of the films were investigated by EPMA mappings, XPS, AES, RHEED and GIXRD on plain Si wafers, whereas the phase change velocity of Ge–Sb–Te was determined on Si–Al–SiO 2 stacks. The change between amorphous and crystalline phases of the films was induced and characterized with a static tester consisting of an optical microscope with an integrated high power laser diode. The change in reflectivity induced by the laser pulses was measured by a high sensitivity photo detector. Depending on the composition, the duration for the initial crystallization was determined between 220 and 500 ns, while the re-amorphization required between 20 and 120 ns. Structural analyses proved the existence of two crystalline phases with cubic and hexagonal structure in the initialized films.

Sputter deposition and film characterization of NiAl on sapphire fibres

Abstract Once the fibre–matrix bonding has been optimized to meet the different requirements during fabrication and operation of the subsequent composite component, sapphire fibre-reinforced NiAl will be a potential candidate to substitute conventional superalloys as structural material for gas turbine blades. To improve the composite fabrication process, a direct deposition of the intermetallic matrix material onto hBN-coated sapphire fibres prior to the consolidation of the fibre–matrix composite is proposed. It is believed that this will simplify the fabrication process and prevent pore formation during diffusion bonding. In addition, the fibre volume fraction can be quite easily adjusted by varying the NiAl coating thickness. For this, a high-rate deposition of NiAl is in any case necessary. It has been achieved by a pulsed DC magnetron sputtering of combined Al–Ni targets with the fibres rotating between the two facing cathodes. The obtained nickel aluminide coatings were analyzed as to structure and composition by means of X-ray (GIXRD) as well as electron diffraction (RHEED) and X-ray photoelectron spectroscopy as well as electron probe microanalysis, respectively. The morphology of the NiAl coatings was examined by scanning electron microscopy.

Ge-Sb-Te system for rewritable optical data storage by a composition-spread approach

Because of its fast reversible phase change between a crystalline and an amorphous phase and a corresponding change in optical properties, Ge-Sb-Te alloys are well known as materials for phase change optical data storage [1]. Especially the stoichiometric Ge 2 Sb 2 Te 5 of the GeTe-Sb 2 Te 3 pseudobinary line is suited for this purpose and already commercially used [2]. Nevertheless, the physical principles of this technique are not yet completely understood. In the presented paper a composition-spread approach was used to deposit Ge-Sb-Te films with compositions around the ternary phase Ge 2 Sb 2 Te 5 . The deposition took place in a UHV sputtering chamber using three magnetron cathodes equipped with pure Ge-, Sb- and Te-targets, respectively, for film deposition. Films were deposited on Si-wafers as well as on Si-Al-SiO 2 stacks. The resulting composition-spread was analyzed by EPMA-mappings and GI-XRD with respect to composition and structure. The velocity of the phase change was determined using a static tester. The correlation between film constitution and kinetics of the phase change revealed that the change from the initialized crystalline phase to the amorphous phase could be achieved in about 20 ns for optimized compositions. Even slight deviations from this composition resulted in a strong decrease of the phase change velocity. Structural analysis proved the existence of two crystalline phases with cubic and hexagonal structure in the initialized films.