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- ItemAnalysis of catalyst surface wetting: The early stage of epitaxial germanium nanowire growth(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2020) Ernst, Owen C.; Lange, Felix; Uebel, David; Teubner, Thomas; Boeck, TorstenThe dewetting process is crucial for several applications in nanotechnology. Even though not all dewetting phenomena are fully understood yet, especially regarding metallic fluids, it is clear that the formation of nanometre-sized particles, droplets, and clusters as well as their movement are strongly linked to their wetting behaviour. For this reason, the thermodynamic stability of thin metal layers (0.1-100 nm) with respect to their free energy is examined here. The decisive factor for the theoretical considerations is the interfacial energy. In order to achieve a better understanding of the interfacial interactions, three different models for estimating the interfacial energy are presented here: (i) fully theoretical, (ii) empirical, and (iii) semi-empirical models. The formation of nanometre-sized gold particles on silicon and silicon oxide substrates is investigated in detail. In addition, the strengths and weaknesses of the three models are elucidated, the different substrates used are compared, and the possibility to further process the obtained particles as nanocatalysts is verified. The importance of a persistent thin communication wetting layer between the particles and its effects on particle size and number is also clarified here. In particular, the intrinsic reduction of the Laplace pressure of the system due to material re-evaporation and Ostwald ripening describes the theoretically predicted and experimentally obtained results. Thus, dewetting phenomena of thin metal layers can be used to manufacture nanostructured devices. From this point of view, the application of gold droplets as catalysts to grow germanium nanowires on different substrates is described. © 2020 Ernst et al.
- ItemOn the relationship between SiF4plasma species and sample properties in ultra low-k etching processes(New York, NY : American Inst. of Physics, 2020) Haase, Micha; Melzer, Marcel; Lang, Norbert; Ecke, Ramona; Zimmermann, Sven; van Helden, Jean-Pierre H.; Schulz, Stefan E.The temporal behavior of the molecular etching product SiF4 in fluorocarbon-based plasmas used for the dry etching of ultra low-k (ULK) materials has been brought into connection with the polymer deposition on the surface during plasma treatment within the scope of this work. For this purpose, time-resolved measurements of the density of SiF4 have been performed by quantum cascade laser absorption spectroscopy. A quantification of the non-linear time dependence was achieved by its characterization via a time constant of the decreasing SiF4 density over the process time. The time constant predicts how fast the stationary SiF4 density is reached. The higher the time constant is, the thicker the polymer film on top of the treated ultra low-k surface. A correlation between the time constant and the ULK damage was also found. ULK damage and polymer deposition were proven by Variable Angle Spectroscopic Ellipsometry and X-ray Photoelectron Spectroscopy. In summary, the observed decay of the etching product concentration over process time is caused by the suppressed desorption of the SiF4 molecules due to a more dominant adsorption of polymers. © 2020 Author(s).
- ItemHigh-temperature annealing of AlN films grown on 4H-SiC(New York, NY : American Inst. of Physics, 2020) Brunner, F.; Cancellara, L.; Hagedorn, S.; Albrecht, M.; Weyers, M.The effect of high-temperature annealing (HTA) at 1700 °C on AlN films grown on 4H-SiC substrates by metalorganic vapor phase epitaxy has been studied. It is shown that the structural quality of the AlN layers improves significantly after HTA similar to what has been demonstrated for AlN grown on sapphire. Dislocation densities reduce by one order of magnitude resulting in 8 × 108 cm-2 for a-type and 1 × 108 cm-2 for c-type dislocations. The high-temperature treatment removes pits from the surface by dissolving nanotubes and dislocations in the material. XRD measurements prove that the residual strain in AlN/4H-SiC is further relaxed after annealing. AlN films grown at higher temperature resulting in a lower as-grown defect density show only a marginal reduction in dislocation density after annealing. Secondary ion mass spectrometry investigation of impurity concentrations reveals an increase of Si after HTA probably due to in-diffusion from the SiC substrate. However, C concentration reduces considerably with HTA that points to an efficient carbon removal process (i.e., CO formation). © 2020 Author(s).
- ItemOn the Impact of Strained PECVD Nitride Layers on Oxide Precipitate Nucleation in Silicon(Pennington, NJ : ECS, 2019) Kissinger, G.; Kot, D.; Costina, I.; Lisker, M.PECVD nitride layers with different layer stress ranging from about 315 MPa to −1735 MPa were deposited on silicon wafers with similar concentration of interstitial oxygen. After a thermal treatment consisting of nucleation at 650°C for 4 h or 8 h followed annealing 780°C 3 h + 1000°C 16 h in nitrogen, the profiles of the oxide precipitate density were investigated. The binding states of hydrogen in the layers was investigated by FTIR. There is a clear effect of the layer stress on oxide precipitate nucleation. The higher the compressive layer stress is the higher is a BMD peak below the front surface. If the nitride layer is removed after the nucleation anneal the BMD peak below the front surface becomes lower. It is possible to model the BMD peak below the surface by vacancy in-diffusion from the silicon/nitride interface. With increasing duration of the nucleation anneal the vacancy injection from the silicon/nitride interface decreases and with increasing compressive layer stress it increases. © The Author(s) 2019.
- ItemHigh-temperature electromechanical loss in piezoelectric langasite and catangasite crystals(Melville, NY : American Inst. of Physics, 2021) Suhak, Yuriy; Fritze, Holger; Sotnikov, Andrei; Schmidt, Hagen; Johnson, Ward L.Temperature-dependent acoustic loss Q−1 is studied in partially disordered langasite (LGS, La3Ga5SiO14) and ordered catangasite (CTGS, Ca3TaGa3Si2O14) crystals and compared with previously reported CTGS and langatate (LGT, La3Ga5.5Ta0.5O14) data. Two independent techniques, a contactless tone-burst excitation technique and contacting resonant piezoelectric spectroscopy, are used in this study. Contributions to the measured Q−1(T) are determined through fitting to physics-based functions, and the extracted fit parameters, including the activation energies of the processes, are discussed. It is shown that losses in LGS and CTGS are caused by a superposition of several mechanisms, including intrinsic phonon–phonon loss, point-defect relaxations, and conductivity-related relaxations.
- ItemOptical properties of silicon nanowire arrays formed by metal-assisted chemical etching: Evidences for light localization effect(New York, NY [u.a.] : Springer, 2012) Osminkina, L.A.; Gonchar, K.A.; Marshov, V.S.; Bunkov, K.V.; Petrov, D.V.; Golovan, L.A.; Talkenberg, F.; Sivakov, V.A.; Timoshenko, V.Y.We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with a mean diameter of approximately 100 nm and length of about 1-25 μm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infrared spectral region, the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity, approximately 75%), while a strong light scattering for wavelength of 0.3 ÷ 1 μm results in a decrease of the total reflectance of 1%-5%, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at approximately 1 μm increases strongly in the sample with SiNWs in comparison with that in c-Si substrate. This effect is related to an increase of the light-matter interaction time due to the strong scattering of the excitation light in SiNW array. The prepared SiNWs are discussed as a kind of 'black silicon', which can be formed in a large scale and can be used for photonic applications as well as in molecular sensing.
- ItemAnalyzer-free, intensity-based, wide-field magneto-optical microscopy(Melville, NY : American Inst. of Physics, 2021) Schäfer, Rudolf; Oppeneer, Peter M.; Ognev, Alexey; Samardak, Alexander; Soldatov, Ivan V.In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain-boundary contrast is due to the magneto-optical gradient effect. A domain-boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell-Fresnel theory. © 2021 Author(s).
- ItemOn the Impact of Strained PECVD Oxide Layers on Oxide Precipitation in Silicon(Pennington, NJ : ECS, 2019) Kissinger, G.; Kot, D.; Lisker, M.; Sattler, A.PECVD oxide layers with different layer stress ranging from about −305.2 MPa to 39.9 MPa were deposited on silicon wafers with similar concentration of interstitial oxygen. After a thermal treatment consisting of rapid thermal annealing (RTA) and furnace annealing 780°C 3 h + 1000°C 16 h in nitrogen the profiles of the oxide precipitate density were investigated. Supersaturations of self-interstitials as function of layer stress were determined by adjusting modelling results to measured depth profiles of bulk microdefects. The self-interstitial supersaturation generated by RTA at 1250°C and 1175°C at the silicon/oxide interface is increasing linearly with increasing layer stress. Values for self-interstitial supersaturation determined on deposited oxide layers after RTA at 1250°C and 1175°C are very similar to values published for RTO by Sudo et al. An RTA at 1175°C with a PECVD oxide on top of the wafer is a method to effectively suppress oxygen precipitation in silicon wafers. Nucleation anneals carried out at 650°C for 4 h and 8 h did not show any effect of PECVD oxide layers on oxide precipitate nucleation. © The Author(s) 2019.
- ItemCrossover of skyrmion and helical modulations in noncentrosymmetric ferromagnets(Bristol : Institute of Physics Publishing, 2018) Leonov, A.O.; Bogdanov, A.N.The coupling between angular (twisting) and longitudinal modulations arising near the ordering temperature of noncentrosymmetric ferromagnets strongly influences the structure of skyrmion states and their evolution in an applied magnetic field. In the precursor states of cubic helimagnets, a continuous transformation of skyrmion lattices into the saturated state is replaced by the first-order processes accompanied by the formation of multidomain states. Recently the effects imposed by dominant longitudinal modulations have been reported in bulk MnSi and FeGe. Similar phenomena can be observed in the precursor regions of cubic helimagnet epilayers and in easy-plane chiral ferromagnets (e.g. in the hexagonal helimagnet CrNb3S6).
- ItemValence effect on the thermopower of Eu systems(College Park, MD : American Physical Society, 2020) Stockert, U.; Seiro, S.; Seiro, S.; Caroca-Canales, N.; Hassinger, E.; Hassinger, E.; Geibel, C.We investigated the thermoelectric transport properties of EuNi2P2 and EuIr2Si2 to evaluate the relevance of Kondo interaction and valence fluctuations in these materials. While the thermal conductivities behave conventionally, the thermopower curves exhibit large values with pronounced maxima as typically observed in Ce- and Yb-based heavy-fermion materials. However, neither the positions of these maxima nor the absolute thermopower values at low temperature are in line with the heavy-fermion scenario and the moderately enhanced effective charge carrier masses. Instead, we may relate the thermopower in our materials to the temperature-dependent Eu valence by taking into account changes in the chemical potential. Our analysis confirms that valence fluctuations play an important role in EuNi2P2 and EuIr2Si2.