Zeitschriftenaufsätze und Buchbeiträge

Reactive multilayer systems consisting of alternating nanoscale Al and Ni layers are applicable in joining, various pyrotechnic applications and thermal batteries. Since diffusion based high-temperature synthesis occurs without the presence of air, efforts have focused on investigating the understanding of the fundamental reaction processes and characteristics. The aim of this study is to expose the reactive multilayers to extrinsic induced tensile stress so that the self-propagating synthesis can proceed under these conditions. Further, the properties during and after the reaction will be investigated. Multilayers deposited by sputtering on Kapton® substrates with different bilayer- and total thicknesses as well as commercial Nanofoils® with thicknesses of 40 µm and 60 µm were used as samples. The investigations focused on the propagation velocity measured with a high-speed camera, the temperature regime determined with a high-speed pyrometer, and the formed phases after the synthesis examined via X-ray diffraction. The gained results of this study reveal important insights for the application of the reactive Al/Ni multilayer system in terms of stability or reliability related to propagation front velocity, maximum temperature and formed phases under induced external tensile stresses.

Emission 57Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive 57Mn+ ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO3 (BFO) thin films. 57Mn β decays (t1/2 = 90 s) to the 14.4 keV Mössbauer state of 57Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K-700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe3+ ions located in octahedral sites (Bhf); and (ii) to Fe3+ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field BDistr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of Bhf = 44.5(9) T. At higher sample temperatures, the Bhf becomes much weaker, with the Fe3+ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba-BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.

Photosensitive glasses for radiation-induced 3D microstructuring, due to their optical transparency and thermal, mechanical, and chemical resistance, enable the use of new strategies for numerous microscale applications, ranging from optics to biomedical systems. In this context, we investigated the plasma etching of photosensitive glasses after their exposure and compared it to the established wet chemical etching method, which offers new degrees of freedom in microstructuring control and microsystem fabrication. A CF4/H2 etching gas mixture with a constant volumetric flow of 30 sccm and a variable H2 concentration from 0% to 40% was utilized for plasma-based etching, while for wet chemical etching, diluted hydrofluoric acid (1% ≤ cHF ≤ 20%) was used. Therefore, both etching processes are based on a chemical etching attack involving fluorine ions. A key result is the observed reversion of the etch selectivity between the initial glassy and partially crystallized parts that evolve after UV exposure and thermal treatment. The crystallized parts were found to be 27 times more soluble than the unexposed glass parts during wet chemical etching. During the plasma etching process, the glassy components dissolve approximately 2.5 times faster than the partially crystalline components. Unlike wet chemical etching, the surfaces of plasma etched photostructured samples showed cone- and truncated-cone-shaped topographies, which supposedly resulted from self-masking effects during plasma etching, as well as a distinct physical contribution from the plasma etching process. The influences of various water species on the etching behaviors of the homogeneous glass and partially crystallized material are discussed based on FTIR-ATR and in relation to the respective etch rates and SNMS measurements.

The proposed study demonstrates a single-step CVD method for synthesizing three-dimensional vertical MoS2 nanosheets. The postulated synthesizing approach employs a temperature ramp with a continuous N2 gas flow during the deposition process. The distinctive signals of MoS2 were revealed via Raman spectroscopy study, and the substantial frequency difference in the characteristic signals supported the bulk nature of the synthesized material. Additionally, XRD measurements sustained the material’s crystallinity and its 2H-MoS2 nature. The FIB cross-sectional analysis provided information on the origin and evolution of the vertical MoS2 structures and their growth mechanisms. The strain energy produced by the compression between MoS2 islands is assumed to primarily drive the formation of vertical MoS2 nanosheets. In addition, vertical MoS2 structures that emerge from micro fissures (cracks) on individual MoS2 islands were observed and examined. For the evaluation of electrical properties, field-effect transistor structures were fabricated on the synthesized material employing standard semiconductor technology. The lateral back-gated field-effect transistors fabricated on the synthesized material showed an n-type behavior with field-effect mobility of 1.46 cm2 V^-1 s^-1 and an estimated carrier concentration of 4.5 × 10^12 cm^-2. Furthermore, the effects of a back-gate voltage bias and channel dimensions on the hysteresis effect of FET devices were investigated and quantified.

Silicon carbide layers were fabricated using self-propagating high-temperature synthesis of binary silicon-carbon based reactive multilayers. The silicon and carbon bilayers were fabricated with two different bilayer thicknesses. They are deposited by magnetron sputtering in an alternating layer system with a total thickness of 1 μm. The entire system is annealed by rapid thermal annealing at different temperatures ranging from 500 to 1100 ˚C. From XRD analysis we could find that the formation of the silicon carbide phase was initiated from 700 ˚C. With increasing bilayer thickness the silicon carbide phase formation was partially suppressed by the silicon recrystallization due to resulting lower carbon diffusion into silicon. The transformation process proceeds in a four-step process: densification/recrystallization, interdiffusion, nucleation and transformation. From this, it was noted that when compared to low bilayer thickness samples, the formation of the silicon carbide phase is delayed with increasing bilayer thickness and needs higher reaction initiation temperatures.