TY - CHAP A1 - Acker, Jörg A1 - Ducke, Jana A1 - Rietig, Anja A1 - Müller, Tim A1 - Eisert, Stefan A1 - Reichenbach, Birk A1 - Löser, Wolfgang ED - Oye, Harald A. ED - Brekken, Harald ED - Rong, Harry ED - Tangstad, Merete ED - Tveit, Halvard T1 - Segregation, grain boundary milling, and chemical leaching for the refinement of metallurgical-grade silicon for photovoltaic application T2 - Silicon for the Chemical and Solar Industry XII, Trondheim, 2014 N2 - The present work describes a completely new approach to the solidification refinement of metallurgical-grade silicon. The new process comprises the following steps: (i) The first step involves adding auxiliary metals to the molten silicon in order to segregate the metallic and non-metallic impurities in the secondary phase after cooling. (ii) The melt is rapidly cooled in the cellular solidification regime. This generates a Si microstructure with a defined cell size in which all cell boundaries are surrounded by the secondary phase. Furthermore, the secondary phase should form an interconnected three-dimensional network. (iii) The solids are crushed by shockwaves using electrohydraulic fragmentation techniques. The shockwaves lead to preferential crushing at the interface between the silicon and the secondary phase. (iv) The secondary phases are fast and effectively removed by microwave-assisted high-pressure leaching that was newly developed for this process. The potential of the new refinement procedure is demonstrated with auxiliary metals Ca, Al, and Ti. This new procedure yields a significant decrease in phosphorous and metal impurities. KW - silicon KW - leaching KW - hydrometallurgy KW - solar cell KW - segregation KW - etching Y1 - 2014 SN - 978-82-997357-8-0 SP - 177 EP - 188 PB - Department of Materials Science and Engineering, Norwegian University of Science and Technology CY - Trondheim ER - TY - CHAP A1 - Rietig, Anja A1 - Acker, Jörg ED - Nygaard, Lars ED - Pachaly, Bernd ED - Page, Ingrid Gamst ED - Rong, Harry ED - Tangstad, Merete ED - Tveit, Halvard T1 - A new and fast method for determination of boron, phosphorus and other trace elements in metallurgical grade silicon T2 - Silicon for the Chemical and Solar Industry XIII, Kristiansand, 2016 N2 - A new method for accurate and precise determination of non-metallic and metallic impurities in silicon was developed and statistically validated. The first step is the fast dissolution of silicon in a microwave-assisted high pressure system to minimize a loss of phosphorus. The essential innovation is the use of the concentrated digestion solution for ICP-OES measurements. This approach avoids the common removal of the silicon and acid matrix by volatilization, which can cause considerable losses of boron. Finally, for the ICP-OES measurements in such high-silicon matrices the optimum measuring conditions were determined and a careful selection of emission lines with respect to selectivity, spectral and non-spectral inferences and matrix effects was performed. The method of matrix matched calibration (MMC) is used for quantification of the impurities’ contents. For Al, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zr and P the validation was performed against certified reference materials (IPT134, IPT135, NIST57b). To validate the determination of boron 9 silicon samples of different boron contents from three interlaboratory comparisons were used. The new procedure allows the determination of impurities of 4N-silicon (12 elements) with high precision and accuracy. KW - silicon KW - ICP-OES KW - impurity KW - chemical analysis KW - boron KW - phosphorus Y1 - 2016 UR - https://www.ntnu.no/trykk/publikasjoner/Silicon%20for%20the%20chemical%20and%20solar%20industry%20XIII/ SP - 95 EP - 106 PB - Department of Materials Science and Engineering, Norwegian University of Science and Technology CY - Trondheim ER - TY - CHAP A1 - Meißner, André A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Raman spectroscopic study on the formation of Cu3Si T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - The term “direct synthesis” is defined in the literature as the reactions between silicon and methyl chloride, hydrogen chloride and other reagents like chlorobenzene and ethyl chloride to yield various alkyl- or aryl substituted chlorosilanes.[1] These reactions have two features in common: (1) The reactivity of silicon - in terms of reaction start temperature, reaction rate and silane product distribution - is originated and controlled by the interaction with metals. (2) The reaction is fully under kinetic control since the formed silanes are the thermodynamically least stable products in the system Si-H-Cl-C.[2] Several authors consider Cu3Si as the catalytically active phase in the direct synthesis. It is assumed, that CuCl formed under the conditions of the direct synthesis reacts with Si according to Eq. 1 and 2 to yield Cu3Si.[1] (1) and (2) The present work describes a Raman microscopic study of the reaction of Si with Cu and CuCl with special emphasize given to the identification of the Cu3Si phase and the processes occurring in the surrounding bulk Si. There is one pathway in which a solid state reaction[3] between Si and CuCl leads to a massive nucleation of Cu3Si exactly at the position of the Si/CuCl solid-solid interface. The nucleation of Cu3Si creates such an enormous lattice strain so that several high-pressure modifications of Si can be identified at the reaction site and around. The second reaction pathway is controlled by a gas phase transport of CuCl at low temperatures. This transport pathway leads to a spread of Cu in nearest neighborhood close to the CuCl particles as well to a long range transport leading to a nucleation of microscopic Cu3Si precipitates away from the CuCl particles. Further studies on the reactivity of the Cu3Si containing reaction sites were performed and will be discussed in the presentation. KW - Raman spectroscopy KW - silicon KW - lattice strain KW - copper silicide KW - direct synthesis Y1 - 2018 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Langner, Thomas A1 - Sieber, Tim A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Lattice strain controls the etching of solar wafer surfaces T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - Multi-wire sawing using an abrasive SiC slurry or diamond wires constitutes the main slicing techniques for multi- and monocrystalline silicon crystals in photovoltaics. The massive mechanical load during the sawing process creates a wafer surface layer characterized by lattice defects, pits, fractures, rifts, cracks, amorphous Si and even some high-pressure Si modifications, otherwise known as saw damage.[1] This highly defect-rich surface causes the rapid recombination of electron-hole pairs, requiring that it be removed by etching in order to manufacture solar cells and to generate a surface morphology having a low reflectivity which directly affects the solar cell’s efficiency. However, etching of the saw damage features of a heterogeneous and laterally unevenly distributed etch attack and a significantly higher etch rate compared to the underlying bulk silicon.[2,3] The present study is focused on the question of how mechanically introduced lattice strain in single-crystalline silicon alters the chemical reactivity of the silicon atoms affected by the strain field on a microscopic length scale. The magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements according to Ref. 4. One of the parameters used to describe the reactivity of silicon is the local etch rate, which was derived from the local removal before and after etching by confocal microscopy. Wet-chemical etching was performed with HF-HNO3-H2SiF6 acid mixtures of different concentrations. It was found, that the reactivity of silicon increased linearly with the magnitude of lattice strain. In particular, an increase in tensile strain led to a higher increase in reactivity compared to the increase observed with growing compressive strain. The second decisive parameter is the reactivity of the etch mixture. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and therefore more reactive acid mixtures are able to attack even slightly strained Si. Side effects, such as the behavior of amorphous or nanocrystalline Si and the generation of highly reactive intermediary species while etching, are discussed. KW - Raman spectroscopy KW - silicon KW - lattice strain KW - etching KW - confocal microscopy Y1 - 2018 SP - S. 201 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Schönekerl, Stefan A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Behavior of electroless copper deposition onto multi-crystalline silicon in diluted hydrofluoric acid solutions T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - The metal-assisted etching of Si is a method which has been studied for many years with regard to the creation of nanoscale surface structures, but much less research was done to elucidate the reaction processes. Most publications argue that either reduction of the metal cation could result in double charge transfer with concomitant hydrogen release or four-fold charge transfer without H2 formation [1]. Following, SiO2 is to be formed, which is subsequently converted into H2SiF6 by F-, HF2- or HF [2]. Due to the few reliable findings, own experiments were carried out. In this context, the deposition of Cu on multi-crystalline Si at various Cu2+ activities and different HF levels was investigated, and the H2 emission for these processes analysed. The results of the series of experiments indicate a different reaction behavior from the theory described above. Apparently, a slightly higher redox potential of the Cu2+/Cu+ half-cell compared to 2H+/H2 redox couple is sufficient to initiate the silicon dissolution process. The stoichiometric ratio between Cu deposition and Si dissolution process is strongly affected by the Cu2+ activity, but it is obviously not influenced by HF activity. At Cu2+ activities of < 2∙10-5 mol/kg less than one electron is nominally exchanged between metal cation and Si, and at activities of approx. 1∙10-2 mol/kg there is an almost four electron charge transfer. At activities > 1∙10-2 mol/kg the stoichiometric ratio and the Cu deposition and Si dissolution kinetics decrease, presumably due to the fact the compact Cu layer inhibits the transition of the dissolved Si into the etching solution. The shift in the stoichiometric ratio suggests the first Cu2+ based oxidative attack on Si enables the reaction of a further oxidizing agent. It is likely water will attack the silicon as second oxidant, since there is no dependence between the content of HF species in the etching solutions and the Si dissolution kinetics. This second reaction step seems to be associated with hydrogen evolution. The amount of hydrogen formation indicates that at a Cu2+ activity of < 2∙10-5 mol/kg calculative only one electron is transferred from Si to Cu2+ and H+, and at a Cu2+ activity of 1∙10-2 mol/kg four electrons in total. However, HF is necessary to convert the oxidized Si to the supposed formation of H3SiF, H2SiF2, HSiF3, and SiF4 and H2SiF6 respectively. KW - silicon KW - metal assisted etching KW - deposition KW - copper KW - dissolution Y1 - 2018 SP - S. 82 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Herold, Steven A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - The influence of lattice deformations on the etch rates of potassium hydroxide on silicon T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - One of the most common methods to analyse silicon materials is Raman-spectroscopy.[1] More recently there is a focus in the analysis of lattice deformations during sample preparations.[2] These deformations not only increase the chance of crack formation but also increases the chemical reactivity of the silicon towards etch processes or oxidation processes.[3] In this contribution, we will present our latest results on the influence of lattice deformations induced by scratches and saw damage on the etch rate of potassium hydroxide. The lattice deformation will be measured using Raman-microscopy and correlated with the height difference measured with confocal microscopy. It will be shown that the etch rate strongly depends on the lattice deformation and that the etch rate will increase by up to the factor 10 in areas with tensile deformation. KW - silicon KW - etching KW - Raman spectroscopy KW - lattice strain KW - saw damage KW - confocal microscopy Y1 - 2018 SP - S. 198 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Herold, Steven A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Measurement of the temperature dependence of lattice deformations in silicon using Raman microscopy T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - One of the most common methods to analyse silicon materials is Raman-spectroscopy.[1] More recently there is a focus in the analysis of lattice deformations during sample preparations.[2] These deformations not only increase the chance of crack formation but also increases the chemical reactivity of the silicon towards etch processes or oxidation processes.[3] In this contribution, we will present our latest Raman evaluation technique for silicon lattice deformation and a way to control them using rapid thermal annealing. The transitions and relaxation of different lattice deformation states after the rapid heating to up to 900°C will be analysed in detail, while also the recrystallisation of amorphous silicon will be discussed from the aspects of lattice deformations. It will be shown that during the tempering the lattice deformation on scratched, indented or diamond wire sawn samples homogenises to a mainly weak tensile deformed state and that these homogenisation processes happen only up to certain critical temperatures. KW - silicon KW - lattice strain KW - Raman microscopy KW - phase transition Y1 - 2018 SP - S. 197 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Sieber, Tim A1 - Ducke, Jana A1 - Acker, Jörg ED - Zschech, Ehrenfried T1 - Degradation of Li(Ni₀.₃₃MnCo₀.₃₃)O₂ in the recycling of lithium battery cathodes T2 - 6th Dresden Nanoanalysis Symposium - Abstract Booklet N2 - The compound Li(Ni₀.₃₃Mn₀.₃₃Co₀.₃₃)O₂(NMC) is the state-of-the-art lithium-ion battery cathode material. Due to the increasing demand NMC is of crucial economically importance for the worldwide emerging market of electromobility. Recycling of end-of-life lithium-ion batteries to recover NMC, in particular of batteries from automotive vehicles, is one future strategy to save costs and to become more independent from the supply of the essential elements Co and Mn. Several concepts for NMC recycling from lithium-ion batteries are based on wet-chemical process steps, in particular, to separate the NMC containing cathode layer from the underlying metal foil. However, NMC is very sensitive against the attack by water and reagents that are added to promote the separation process. The present study deals with the wet-chemical recycling of NMC using aqueous reagent solutions in a under varying process conditions. The recovered NMC samples are characterized in order to study the ongoing degradation at the surface of the NMC particles. In particular, two major degradation pathways are identified: (i) a preferential loss of lithium and nickel and (ii) the formation of passivation layers due to unwanted side reactions. DRIFT measurements are performed to study the NMC surface species after the recovery processes. SEM/EDX mappings are used to detect changes in the chemical composition in the surface region of the chemically treated NMC particles. Finally, a detailed study of the changes in the chemical state at the NMC particle surface is done by Raman microscopy by means of the deconvolution of the recorded spectra into their A1G component (representing the metal-oxide phonons) and into the Eg component (representing the oxide-metal-oxide phonons). As result of this study, the consequences of different wet-chemical process conditions on the quality of the recovered NMC material are discussed. KW - lithium battery KW - recycling KW - nickel manganese cobalt oxide KW - degradation KW - Raman microscopy KW - chemical analysis Y1 - 2018 UR - https://www.nanoanalytik.fraunhofer.de/content/dam/nanoanalytik/de/documents/6th_Dresden_Nanoanalysis_Symposium_abstracts.pdf SP - S. 52 PB - Fraunhofer IKTS Dresden CY - Dresden ER - TY - CHAP A1 - Rietig, Anja A1 - Langner, Thomas A1 - Acker, Jörg ED - Scheschkewitz, David ED - Kickelbick, Guido T1 - Dissolution of silicon in HF/HNO3 mixtures: A revised model T2 - 9th European Silicon Days, 9-12 September 2018, Saarbrücken, Germany, Book of abstracts N2 - The dissolution of Si in HF/HNO3 consists of a set of complex reactions and thus a large number of reaction products. The most comprehensive picture of this reaction, the role of the involved reaction products and the reactivity of the HF/HNO3 mixtures depending on their composition is drawn by Steinert et al..[1]-[3] Based on the first systematic investigations on hydrogen formation by Hoffmann et al.[4], Acker et al. succeeded a first mass and electron balance for the reaction of silicon in HF/HNO3.[5] However, there is still a lack in interpretation of the mass end electron balances arising from several nitrous oxides. So far, the identified nitrogen oxides NO, NO2 and N2O were considered in sum[5] and neither separated nor individually studied or quantified. The aim of this work is to complete the mass and electron balance by the contribution of the individual nitrous oxides and to identify their individual formation pathways. Kinetic measurements of the NO and NO2 formation during the dissolution of Si, NO2 turns out as a result from the oxidation of the primary product NO by the HNO3 in the etching mixtures. Subsequently, NO and NO2 react to N2O3 dissolved in the acid mixture. The kinetics of both reactions were individually studied by bubbling NO in HF/HNO3 mixtures of different composition. The already identified intermediary species N4O62+ turns out to be formed by disproportionation of dissolved NO2 via N2O4 without dissolution of silicon. A detailed kinetic studied showed, that only dissolved N2O3 and not the intermediate N4O62+ contribute to the dissolution rate of silicon in HF/HNO3 acid mixtures. Finally, kinetic measurements revealed that the formed H2 reduces gaseous NO yielding to the final gaseous reaction products N2, N2O as well as to ammonium ions which all are formed with identical reaction rate. This reaction is assumed to proceed via NH2OH as intermediate. As result of the identification and quantification of intermediary and final reaction products a new reaction scheme needs to be established leading to a new approach to the mass and electron balance for the oxidation of silicon during the dissolution in HF/HNO3 mixtures. KW - silicon KW - etching KW - Raman spectroscopy KW - mass spectrometry KW - gas analysis KW - reaction mechanism Y1 - 2018 SP - S. 199 PB - Universität des Saarlandes CY - Saarbrücken ER - TY - CHAP A1 - Acker, Jörg A1 - Langner, Thomas A1 - Koschwitz, Tim ED - Marciniec, Bogdan T1 - Lattice-strain induced chemical reactivity of silicon T2 - 8th European Silicon Days 2018, Conference Proceedings N2 - Silicon wafer for solar application are produced by multi-wire sawing from 12x12 cm2 silicon bricks. After slicing the wafer surface consists of a several micrometer surface layer of very heterogeneous constitution, the so called saw damage. The topmost layer of the saw damage consists of debris, amorphous silicon and high-pressure silicon phases followed by a very defect-rich and lattice-strained region of fractures, cracks, and rifts caused by the rupture of the silicon lattice during the slicing process [1,2]. Such a damaged surface exhibits very poor semiconductor properties; therefore the saw damage is removed by chemical etching using mixtures of HF, HNO3 and H2SiF6 in order to produce solar cells. Recent investigations showed that the etching of the saw damage is a very heterogeneous process [3-5]. The surface consists of spots at which the etching forms very rapidly deep grooves while other spots remain unetched over a considerably long time. Some of the rapidly formed grooves seem to remain their shape while others grow because of an attack of the side walls, however, without making these grooves significantly deeper. So far there is no explanation for this behavior. The present work is the first study about the locally resolved etching behavior of lattice-strained silicon. The surface of polished single-crystal wafers were scratched with a diamond tip under defined conditions. By means of confocal Raman microscopy the local state of the silicon lattice in and nearby the scratches were characterized in terms of compression and tensile stress with a lateral resolution of 2 μm. Then, the scratches were etched stepwise using HF-HNO3-H2SiF6 mixtures and measured by confocal microscopy to quantify the local removal of silicon and measured by confocal Raman microscopy to monitor the lattice state. For the first time this study reveals and quantifies the impact of tensile and compressive lattice stress on the etch rate of silicon with the major outcome, that stress leads to a significantly anisotropic etching behavior. From the time dependent development of the scratch profiles and the topography of the surrounding wafer areas a detailed picture about the formation of highly reactive species and etching behavior against unstrained silicon is deduced. KW - silicon KW - lattice strain KW - reactivity KW - etching KW - Raman microscopy KW - confocal microscopy Y1 - 2016 SP - S. 124 PB - Wielkopolska Centre for Advanced Technologies, Adam Mickiewicz University CY - Poznań, Poland ER -