@misc{AckerLangnerMeineletal., author = {Acker, J{\"o}rg and Langner, Thomas and Meinel, Birgit and Sieber, Tim}, title = {Saw Damage as an Etch Mask for the Acidic Texturization of Multicrystalline Silicon Wafers}, series = {Materials Science in Semiconductor Processing}, volume = {74}, journal = {Materials Science in Semiconductor Processing}, issn = {1369-8001}, doi = {10.1016/j.mssp.2017.09.039}, pages = {238 -- 248}, abstract = {The surface of multicrystalline silicon solar cells are etched by mixtures of HF, HNO3 and H2SiF6 in order to remove saw damage caused by wafer slicing, as well as to create a water surface topography that provides a low reflectance for incident light, otherwise known as the texture. Topographically analyzing wafer surfaces before and after etching has revealed that the saw damage controls the texturized wafer surface's final topography.The first key factor is the dimension and magnitude of the plastic stress field introduced by indenting SiC grains into the wafer surface during the wafering process. The second key factor is that lattice-stressed silicon is etched at a higher rate than unstressed bulk silicon. At the wire entrance, side sharp and large SiC grains create the deepest indention pits, and therefore the deepest of the water surface stress fields. The lattice-disturbed silicon inside these pits is etched at a higher rate compared to the pit's side walls, which are uniformly attacked across the wafer area. Consequentially, existing pits deepen, and these areas generate the wafer's lowest reflectivity. At the wire exit side, a higher number of smaller and rounder SiC particles indent the surface and create more numerous and shallower indention pits compared to the wire entrance side. The resulting stress field is less deep, so less silicon is removed from inside of these pits during etching compared to the wire entrance side. This yields to a wafer surface region consisting of shallowly etched pits and higher reflectance. It is concluded that the saw damage acts like an etch mask in the texturization of multicrystalline silicon wafers.}, language = {en} } @misc{AckerSieberLangneretal., author = {Acker, J{\"o}rg and Sieber, Tim and Langner, Thomas and Herold, Steven}, title = {The impact of lattice strain on the reactivity of silicon}, series = {Silicon for the Chemical and Solar Industry XIV}, journal = {Silicon for the Chemical and Solar Industry XIV}, editor = {Andresen, Birger and Nygaard, Lars and Rong, Harry and Tangstad, Merete and Tveit, Halvard and Page, Ingrid Gamst}, publisher = {The Norwegian University of Science and Technology}, address = {Trondheim}, pages = {11 -- 20}, abstract = {The present study is focused on the question of how lattice strain mechanically introduced into silicon alters the chemical reactivity of the silicon atoms that are affected by the strain field on a microscopic length scale. The magnitude and local distribution of lattice strain are extracted from confocal Raman microscopy measurements. The reactivity of Si is expressed by the etch rate of Si after treatment with HF-HNO3-H2SiF6 mixtures. Then, the local etch rate is calculated from the local etch depth as determined by confocal microscopy. It has been found that tensile strain leads to the highest enhancement of the etch rate, followed by a compressive strain increase in the etch rate.}, language = {en} } @misc{LangnerSieberAcker, author = {Langner, Thomas and Sieber, Tim and Acker, J{\"o}rg}, title = {Etching Shapes the Topography of Silicon Wafers: Lattice-Strain Enhanced Chemical Reactivity of Silicon for Efficient Solar Cells}, series = {ACS Applied Nano Materials}, volume = {1}, journal = {ACS Applied Nano Materials}, number = {8}, doi = {10.1021/acsanm.8b00906}, pages = {4135 -- 4144}, abstract = {Multiwire sawing of silicon (Si) bricks is the state-of-the-art technology to produce multicrystalline Si solar wafers. The massive indentation of the abrasive Si carbide or diamond particles used leads to a heavily mechanically damaged layer on the wafer surface. Etching the surface layer using typical HF-HNO3-H2SiF6 acid mixtures reveals an unevenly distributed etch attack with etch rates several times higher than known for bulk Si etching. The present study follows the hypothesis that lattice strain, introduced by the sawing process, leads to an increase of the etch rate and determines the topography of the etched wafer, the so-called texture. Scratches were introduced into single crystalline Si surfaces in model experiments, and the magnitude and local distribution of lattice strain were extracted from confocal Raman microscopy measurements. The essential parameter used to describe the local reactivity of Si is the local etch rate, which was derived by confocal microscopy from the local height before and after etching. It was found that the reactivity of Si increases linearly with the magnitude of lattice strain. An increase in tensile strain raises the reactivity of Si significantly higher than an increase of compressive strain. The second decisive parameter is the reactivity of the etch mixture that correlates with the total concentration of the acid mixtures. Diluted acid mixtures with a low reactivity attack only the highest strained Si, whereas more concentrated and, therefore, more reactive acid mixtures can 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. The presence of unevenly distributed lattice strain of different magnitude and the resulting unevenly distributed reactivity of Si explain the features of a heterogeneous etch attack observed and the resulting topography of the etched wafer surface.}, language = {en} } @inproceedings{MeissnerAcker, author = {Meißner, Andr{\´e} and Acker, J{\"o}rg}, title = {Raman spectroscopic study on the formation of Cu3Si}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, abstract = {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.}, language = {en} } @inproceedings{LangnerSieberAcker, author = {Langner, Thomas and Sieber, Tim and Acker, J{\"o}rg}, title = {Lattice strain controls the etching of solar wafer surfaces}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 201}, abstract = {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.}, language = {en} } @inproceedings{SchoenekerlAcker, author = {Sch{\"o}nekerl, Stefan and Acker, J{\"o}rg}, title = {Behavior of electroless copper deposition onto multi-crystalline silicon in diluted hydrofluoric acid solutions}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 82}, abstract = {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.}, language = {en} } @inproceedings{HeroldAcker, author = {Herold, Steven and Acker, J{\"o}rg}, title = {The influence of lattice deformations on the etch rates of potassium hydroxide on silicon}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 198}, abstract = {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.}, language = {en} } @inproceedings{HeroldAcker, author = {Herold, Steven and Acker, J{\"o}rg}, title = {Measurement of the temperature dependence of lattice deformations in silicon using Raman microscopy}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 197}, abstract = {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.}, language = {en} } @inproceedings{SieberDuckeAcker, author = {Sieber, Tim and Ducke, Jana and Acker, J{\"o}rg}, title = {Degradation of Li(Ni₀.₃₃MnCo₀.₃₃)O₂ in the recycling of lithium battery cathodes}, series = {6th Dresden Nanoanalysis Symposium - Abstract Booklet}, booktitle = {6th Dresden Nanoanalysis Symposium - Abstract Booklet}, editor = {Zschech, Ehrenfried}, publisher = {Fraunhofer IKTS Dresden}, address = {Dresden}, pages = {S. 52}, abstract = {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.}, language = {en} } @inproceedings{RietigLangnerAcker, author = {Rietig, Anja and Langner, Thomas and Acker, J{\"o}rg}, title = {Dissolution of silicon in HF/HNO3 mixtures: A revised model}, series = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, booktitle = {9th European Silicon Days, 9-12 September 2018, Saarbr{\"u}cken, Germany, Book of abstracts}, editor = {Scheschkewitz, David and Kickelbick, Guido}, publisher = {Universit{\"a}t des Saarlandes}, address = {Saarbr{\"u}cken}, pages = {S. 199}, abstract = {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.}, language = {en} }