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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.
- ItemTopography evolution of germanium thin films synthesized by pulsed laser deposition(New York, NY : American Inst. of Physics, 2017) Schumacher, P.; Mayr, S.G.; Rauschenbach, B.Germanium thin films were deposited by Pulsed Laser Deposition (PLD) onto single crystal Ge (100) and Si (100) substrates with a native oxide film on the surface. The topography of the surface was investigated by Atomic Force Microscopy (AFM) to evaluate the scaling behavior of the surface roughness of amorphous and polycrystalline Ge films grown on substrates with different roughnesses. Roughness evolution was interpreted within the framework of stochastic rate equations for thin film growth. Here the Kardar-Parisi-Zhang equation was used to describe the smoothening process. Additionally, a roughening regime was observed in which 3-dimensional growth occurred. Diffusion of the deposited Ge adatoms controlled the growth of the amorphous Ge thin films. The growth of polycrystalline thin Ge films was dominated by diffusion processes only in the initial stage of the growth.
- ItemAtomically controlled CVD processing of group IV semiconductors for ultra-large-scale integrations(Bristol : IOP Publishing, 2012) Murota, Junichi; Sakuraba, Masao; Tillack, BerndOne of the main requirements for ultra-large-scale integrations (ULSIs) is atomic-order control of process technology. Our concept of atomically controlled processing is based on atomic-order surface reaction control by CVD. By ultraclean low-pressure CVD using SiH4 and GeH4 gases, high-quality low-temperature epitaxial growth of Si1−xGex (100) (x=0–1) with atomically flat surfaces and interfaces on Si(100) is achieved. Self-limiting formation of 1–3 atomic layers of group IV or related atoms in the thermal adsorption and reaction of hydride gases on Si1-xGex (100) are generalized based on the Langmuir-type model. By the Si epitaxial growth on top of the material already-formed on Si(100), N, B and C atoms are confined within about a 1 nm thick layer. In Si cap layer growth on the P atomic layer formed on Si1−xGex (100), segregation of P atoms is suppressed by using Si2H6 instead of SiH4 at a low temperature of 450 °C. Heavy C atomic-layer doping suppresses strain relaxation as well as intermixing between Si and Ge at the Si1−xGex/Si heterointerface. It is confirmed that higher carrier concentration and higher carrier mobility are achieved by atomic-layer doping. These results open the way to atomically controlled technology for ULSIs.
- ItemQuasi-Transient Calculation of Czochralski Growth of Ge Crystals Using the Software Elmer(Basel : MDPI, 2019) Miller, Wolfram; Abrosimov, Nikolay; Fischer, Jörg; Gybin, Alexander; Juda, Uta; Kayser, Stefan; Janicskó-Csáthy, JószefA numerical scheme was developed to compute the thermal and stress fields of the Czochralski process in a quasi-time dependent mode. The growth velocity was computed from the geometrical changes in melt and crystal due to pulling for every stage, for which the thermal and stress fields were computed by using the open source software Elmer. The method was applied to the Czochralski growth of Ge crystals by inductive heating. From a series of growth experiments, we chose one as a reference to check the validity of the scheme with respect to this Czochralski process. A good agreement both for the shapes of the melt/crystal interface at various time steps and the change in power consumption with process time was observed. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemDiffusion and interface effects during preparation of all-solid microstructured fibers(Basel : MDPI AG, 2014) Kobelke, J.; Bierlich, J.; Wondraczek, K.; Aichele, C.; Pan, Z.; Unger, S.; Schuster, K.; Bartelt, H.All-solid microstructured optical fibers (MOF) allow the realization of very flexible optical waveguide designs. They are prepared by stacking of doped silica rods or canes in complex arrangements. Typical dopants in silica matrices are germanium and phosphorus to increase the refractive index (RI), or boron and fluorine to decrease the RI. However, the direct interface contact of stacking elements often causes interrelated chemical reactions or evaporation during thermal processing. The obtained fiber structures after the final drawing step thus tend to deviate from the targeted structure risking degrading their favored optical functionality. Dopant profiles and design parameters(e.g., the RI homogeneity of the cladding) are controlled by the combination of diffusion and equilibrium conditions of evaporation reactions. We show simulation results of diffusion and thermal dissociation in germanium and fluorine doped silica rod arrangements according to the monitored geometrical disturbances in stretched canes or drawn fibers. The paper indicates geometrical limits of dopant structures in sub-μm-level depending on the dopant concentration and the thermal conditions during the drawing process. The presented results thus enable an optimized planning of the preform parameters avoiding unwanted alterations in dopant concentration profiles or in design parameters encountered during the drawing process.
- ItemTemperature dependence of strain–phonon coefficient in epitaxial Ge/Si(001): A comprehensive analysis(Chichester [u.a.] : Wiley, 2020) Manganelli, C.L.; Virgilio, M.; Skibitzki, O.; Salvalaglio, M.; Spirito, D.; Zaumseil, P.; Yamamoto, Y.; Montanari, M.; Klesse, W.M.; Capellini, G.We investigate the temperature dependence of the Ge Raman mode strain–phonon coefficient in Ge/Si heteroepitaxial layers. By analyzing the temperature-dependent evolution of both the Raman Ge-Ge line and of the Ge lattice strain, we obtain a linear dependence of the strain–phonon coefficient as a function of temperature. Our findings provide an efficient method for capturing the temperature-dependent strain relaxation mechanism in heteroepitaxial systems. Furthermore, we show that the rather large variability reported in the literature for the strain–phonon coefficient values might be due to the local heating of the sample due to the excitation laser used in µ-Raman experiments. © 2020 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd
- ItemNanoscopic tip sensors fabricated by gas phase etching of optical glass fibers(Heidelberg : Springer, 2012) Bierlich, J.; Kobelke, J.; Brand, D.; Kirsch, K.; Dellith, J.; Bartelt, H.Silica-based fiber tips are used in a variety of spectroscopic, micro- or nano-scopic optical sensor applications and photonic micro-devices. The miniaturization of optical sensor systems and the technical implementation using optical fibers can provide new sensor designs with improved properties and functionality for new applications. The selective-etching of specifically doped silica fibers is a promising method in order to form complex photonic micro structures at the end or within fibers such as tips and cavities in various shapes useful for the all-fiber sensor and imaging applications. In the present study, we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations into surface plasmon resonance effects, the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.
- ItemControl of etch pit formation for epitaxial growth of graphene on germanium(Melville, NY : American Inst. of Physics, 2019) Becker, Andreas; Wenger, Christian; Dabrowski, JarekGraphene epitaxy on germanium by chemical vapor deposition is a promising approach to integrate graphene into microelectronics, but the synthesis is still accompanied by several challenges such as the high process temperature, the reproducibility of growth, and the formation of etch pits during the process. We show that the substrate cleaning by preannealing in molecular hydrogen, which is crucial to successful and reproducible graphene growth, requires a high temperature and dose. During both substrate cleaning and graphene growth, etch pits can develop under certain conditions and disrupt the synthesis process. We explain the mechanisms how these etch pits may form by preferential evaporation of substrate, how substrate topography is related to the state of the cleaning process, and how etch pit formation during graphene growth can be controlled by choice of a sufficiently high precursor flow. Our study explains how graphene can be grown reliably on germanium at high temperature and thereby lays the foundation for further optimization of the growth process. © 2019 Author(s).
- ItemRipple coarsening on ion beam-eroded surfaces(New York, NY [u.a.] : Springer, 2014) Teichmann, M.; Lorbeer, J.; Frost, F.; Rauschenbach, B.Abstract: The temporal evolution of ripple pattern on Ge, Si, Al2O3, and SiO2 by low-energy ion beam erosion with Xe + ions is studied. The experiments focus on the ripple dynamics in a fluence range from 1.1 × 1017 cm-2 to 1.3 × 1019 cm-2 at ion incidence angles of 65° and 75° and ion energies of 600 and 1,200 eV. At low fluences a short-wavelength ripple structure emerges on the surface that is superimposed and later on dominated by long wavelength structures for increasing fluences. The coarsening of short wavelength ripples depends on the material system and angle of incidence. These observations are associated with the influence of reflected primary ions and gradient-dependent sputtering. The investigations reveal that coarsening of the pattern is a universal behavior for all investigated materials, just at the earliest accessible stage of surface evolution.
- ItemPattern formation on Ge by low energy ion beam erosion(Bristol : IOP, 2013) Teichmann, M.; Lorbeer, J.; Ziberi, B.; Frost, F.; Rauschenbach, B.Modification of nanoscale surface topography is inherent to low-energy ion beam erosion processes and is one of the most important fields of nanotechnology. In this report a comprehensive study of surface smoothing and self-organized pattern formation on Ge(100) by using different noble gases ion beam erosion is presented. The investigations focus on low ion energies ( 2000 eV) and include the entire range of ion incidence angles. It is found that for ions (Ne, Ar) with masses lower than the mass of the Ge target atoms, no pattern formation occurs and surface smoothing is observed for all angles of ion incidence. In contrast, for erosion with higher mass ions (Kr, Xe), ripple formation starts at incidence angles of about 65° depending on ion energy. At smaller incident angles surface smoothing occurs again. Investigations of the surface dynamics for specific ion incidence angles by changing the ion fluence over two orders of magnitude gives a clear evidence for coarsening and faceting of the surface pattern. Both observations indicate that gradient-dependent sputtering and reflection of primary ions play crucial role in the pattern evolution, just at the lowest accessible fluences. The results are discussed in relation to recently proposed redistributive or stress-induced models for pattern formation. In addition, it is argued that a large angular variation of the sputter yield and reflected primary ions can significantly contribute to pattern formation and evolution as nonlinear and non-local processes as supported by simulation of sputtering and ion reflection.