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Die amorphe Struktur der Phasenwechselmaterialien (PCM) GCT-9110 (Ge45Cu5Te50), GCT-7310 (Ge35Cu15Te50) und GCT-112 (Ge25Cu25Te50) auf der pseudo-binären Linie zwischen GeTe und CuTe wurden mittels anomaler Röntgenstreuung (AXS) und Röntgen-Absorptions\-feinstruktur (XAFS) untersucht. Mit Hilfe von Simulationen der Nahkanten\-absorption (XANES) und experimentellen Daten als Randbedingungen wurden mittels reverser Monte-Carlo Simulationen (RMC) dreidimensionale Modelle dieser Zusammensetzungen erzeugt. Bei niedrigen Cu-Konzentrationen, GCT-9110 und GCT-7310, bilden sich Cu-Cluster, die in die amorphe Matrix eingebettet sind und mit dieser über Cu-Te Bindungen verbunden sind. Dadurch ergibt sich eine mit der Cu-Konzentration steigende relative Te-Konzentration in der die Cluster umgebenden amorphen Matrix, wodurch die in diesem Konzentrationsbereich gefundene steigende Kristallisationstemperatur erklärt werden kann. Die amorphe Matrix besitzt eine bimodale Topologie, es bilden sich tetraedrische und defekt-oktaedrische Bindungseinheiten. Dadurch ergibt sich eine schwach ausgeprägte Ordnung auf höheren Längenskalen. Mit steigender Cu-Konzentration nimmt der Anteil der defekt-oktaedrischen Einheiten ab. Im Gegensatz dazu ist GCT-112 durch ein Ge-Cu-Te Netzwerk charakterisiert, dass sich ausschließlich über tetraedrische Strukturen beschreiben lässt und eine deutlich langreichweitigere Ordnung aufweist als GCT-9110 und GCT-7310.

Zugleich: Dissertation, Universität Kassel, 2022

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen, Report. IAM, Institute of General Mechanics 15, 1 Online-Ressource : Illustrationen (2022). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022, Due to accelerating climate changes and the presence of more extreme weather conditions, the topic of freezing-induced damage had become more important, especially in connection with infrastructure damages and slope stability. In cold regions, soils underneath pavement or concrete foundations may experience frost damage due tofreezing-thawing cycles. Besides, employing artificial ground freezing (AGF) technology in soft soils for providing temporary ground support during tunnel excavation or controlling the groundwater during geotechnical construction can lead to permanent heave-induced damages. On the other hand, recourse to various forms of renewable energy has become of great concern due to the limited non-renewable energy resources and increased environmental awareness. However, most renewable energy sources are intermittent, unpredictable, and only available at certain periods. Hence, efficient thermal energy storage is necessary to increase the overall efficiency and provide better reliability. Latent energy storage using phase change materials (PCMs) is considered one of the most efficient ways of storing energy. Since most PCMs suffer from low thermal conductivity, various techniques, such as highly conductive porous metals, could be incorporated to enhance their thermal conductivity and heat storage rates. This work aims to investigate coupled thermo-hydro-mechanical processes, which occur during the phase change in fluid-saturated porous media. In particular, a thermodynamically-consistent formulation is introduced leading to a novel phase-field thermo-hydro-mechanical (P-THM) approach. The model macroscopically describes a biphasic porous medium, consisting of a non-isothermal and incompressible porous matrix and a pore-fluid. The constituents are in a state of local thermal non-equilibrium, and the model proceeds from the small-strains assumption of the deformable elastic solid skeleton. The governing equations are based on the well-founded continuum mechanical theory of porous media (TPM) extended by the phase-field modeling (PFM) approach, where the model accounts for the temperature evolution, fluid phase transitions, fluid-solidinteractions, and the associated volumetric deformations. A key merit of the PFM approach is its feasibility within standard finite element frameworks, where it does not explicitly track the moving boundaries (interfaces) of the phase-change constituent. Of particular importance in the underlying work is the PFM-based unified kinematics treatment of the solid and liquid phases of the pore-fluid. Furthermore, a demonstration of the micro-cryo-suction mechanism during soil freezing and its realization in the continuum model through a phenomenological retention-curve-like formulation is presented. This enables the modeling of ice lens formation and stiffness degradation in the thermal (quasi-) steady state based on a suction-induced phase-field fracture approach. The model is further extended to describe the freezing process in partially saturated soil as a triphasic medium considering the water and gas (air) phases. The presented numerical examples and comparisons demonstrate the ability, reliability, and usefulness of theproposed modeling framework in describing the phase-change process in porous media under thermal loading., Published by Aachen

Ziel dieser Masterarbeit ist es, grundsätzliche Designs für das Nachrüsten von Dampfspeichern mit Phasenwechselmaterialbehältern mithilfe Simulationen hinsichtlich ihres strukturmechanischen Verhaltens einschließlich Ermüdungsfestigkeit zu bewerten. Um Wärme zu speichern beziehungsweise wieder verfügbar zu machen, werden diese Phasenwechselmaterialen wiederholt vom festen in den flüssigen Zustand übergeführt. Es wird eine Methode erarbeitet, bei der sowohl der Behälter als auch das Phasenwechselmaterial mit der Finite Elemente Methode für strukturmechanische Analysen modelliert werden, und deren Grenzen aufgezeigt.Die dafür notwendigen Werkstoffkennwerte werden, sofern vorhanden, aus Literaturquellen zusammengefügt oder aus vorhanden Werkstoffkennwerten hergeleitet und abgeschätzt.Für die strukturmechanischen Simulationen der Phasenwechselmaterialbehälter müssen Ausdehnung bzw. Schrumpfung des Phasenwechselmaterials, sowie die dadurch auftretende Spaltbildung berücksichtigt werden.Die entwickelte Methode zur Simulation der Phasenübergänge mit der Finiten Elemente Methode eignet sich nur solange sehr geringe Fließwege vom Phasenwechselmaterial zurückgelegt werden. Selbst bei kurzen Fließwegen könnenkeine komplexen Geometrien simuliert werden, da Konvergenzprobleme auftreten. Als Alternative könnte das Finite Elemente Modell mit einem CFD-Modell (Computational Fluid Dynamics) gekoppelt werden.Von den grundsätzlich vier möglichen Designvarianten können zwei aus wirtschaftlichen Gründen ausgeschlossen werden. Die Variante der geschlossenen vollständig gefüllten Module eignet sich aufgrund der einfachen Geometrie und den kurzen Fließwegen gut, um die entwickelte Methode zur Simulation von Phasenübergängen zu testen. Allerdings treten bei diesem Design große verformungsgesteuerte Ermüdungsbelastungen auf, wegen derer dieses Design ebenfalls ausgeschlossen werden muss. Die verbleibende Designvariante der offenen, teilweise gefüllten Module wird hinsichtlich des Kontaktverhaltens und ihren strukturmechanischen Implikationen auf den nachgerüsteten Dampfspeicher untersucht., This thesis aims to evaluate basic designs for retrofitting steam accumulators with phase change material containers using simulations regarding structural mechanics behavior including fatigue resistance. To store or to make heat available these phase change materials are repeatedly transition between liquid and solid state. A method is developed, with that as well the container as the phase change material are modelled with the finite element method for structural analyses and limits of the method are shown.The necessary material parameters are, if available, put together from literature or are derived and estimated from available parameters.For the structural simulations of the phase change material containers expansion or rather retraction of the phase change material and thereby occurring gaps need to be considered.The developed method for the simulation of phase transitions with the finite element method is only suited for applications with very short flow paths for the phase change material. Even with short flow paths only simple geometries can be simulated, because of problems with convergence. An alternative approach would be coupling finite elements models with a CFD model (Computational Fluid Dynamics).Of the four basic design variants two are out of question for economic reasons. The variant of closed fully filled modules is suited to test the developed method for the simulation of phase transitions because it has very short flow paths. Unfortunately, in this design large deformation-controlled fatigue loads occur, which lead to dismissing this variant too. The remaining design variant is then investigated for its contact behavior and the structural mechanical implications for the retrofitted steam accumulator.

Ziel dieser Masterarbeit ist es, das Verhalten von Phasenwechselmaterialien (PCM) während des Erstarrens und Schmelzens durch die Wahl von geeigneten Werkstoffmodellen zu simulieren. Dabei sollen nach Möglichkeit Effekte wie die Spaltbildung zwischen Behälter und Phasenwechselmaterial und die mechanischen Eigenschaften von festem Phasenwechselmaterial berücksichtigt werden und das Verhalten einer Flüssigkeit angenähert werden. Aufgrund der großen Schrumpfung beim Erstarren kommt es dabei im Phasenwechselmaterial zur Bildung von Vakuumeinschlüssen (Lunker). Dadurch kommt es beim erneuten Schmelzen des PCM und den damit verbundenen Wärmeausdehnungen zu Belastungen im Behälter.Hintergrund der Arbeit ist, dass Phasenwechselmaterialien zum Speichern von Wärme benötigt werden, da diese am Schmelzpunkt eine große Wärmemenge bei konstanter Temperatur abgeben. Die mit Phasenwechselmaterialien gefüllten Behälter sollen dabei auf Dampfspeicher montiert werden, um die Druck- und Temperaturschwankungen während der Dampfabgabe zu verringern und die speicherbare Wärmemenge zu erhöhen.Die dafür benötigten Werkstoffkennwerte wurden, wenn möglich, aus der Literatur entnommen. Für Werkstoffkennwerte, für die keine Zahlenwerte vorhanden waren, wurden die Werte abgeschätzt.Zur Simulation werden strukturmechanische Finite Elemente aus Standardsoftwareverwendet. Diese sind allerdings nicht in der Lage das Verhalten von Flüssigkeiten ausreichend genau zu beschreiben. Es können lediglich die Festigkeitskennwerte im flüssigen Zustand so weit abgesenkt werden, bis ein gelartiges Verhalten entsteht.Außerdem kann ein Schmelzpunkt nicht dargestellt werden. Daher findet das Erstarren und Schmelzen innerhalb eines Temperaturintervalls statt.In der Arbeit werden dazu vier Werkstoffmodelle vorgestellt, welche in der Lage sind,das Verhalten des Phasenwechselmaterials teilweise zu beschreiben. Aufgrund der Tatsache, dass für Phasenwechselmaterialien nur wenige Werkstoffkennwerte bekannt sind, war eine zusätzliche Anforderung an die Werkstoffmodelle, dass diese möglichst wenige Parameter benötigen. Deswegen wurde eines der vorgestelltenWerkstoffmodelle ausgeschlossen. Es wurde kein Werkstoffmodell gefunden, welchesin der Lage ist, das vollständige Verhalten von Phasenwechselmaterial zu beschreiben. Für die ausgewählten Werkstoffmodelle wurden anschließend die Werkstoffparameter festgelegt und Simulationen an einfachen Behältern durchgeführt und die Ergebnisse diskutiert., The aim of this master thesis is to simulate the behavior of phase-change-materials (PCM) during solidification and melting by choosing suitable material laws. Where possible, effects such as the gap formation between the container and the phasechange-material and the mechanical properties of solid phase-change-material are to be taken into account. Due to the large thermal shrinkage during solidification, vacuum inclusions are formed in the phase-change-material. This leads to stresses in the container when the PCM is remelted and the thermal expansions occur.The background of this thesis is that phase-change-materials are needed to store heat, as they release a large amount of heat at the melting point at a constant temperature. The containers filled with phase-change-materials are to be mounted on steam accumulators in order to reduce the pressure and temperature fluctuations during steam release and increase the amount of heat that can be stored.The material parameters required for the simulations were taken from the literature when possible. For material parameters for which no numerical values were available, the values were estimated.Structural finite elements from standard software were used for the simulation. However, these are not capable of describing the behavior of fluids with sufficient accuracy. Only the strength values in the liquid state can be reduced until a gel-like behavior is obtained. In this paper, four material laws are presented which are able to partially describe the behavior of the phase-change-material. Due to the fact that only a few material values are known for phase-change-materials, an additional requirement for the material laws was that they need as few parameters as possible. Therefore, one of the material laws was excluded. No material law was found that was capable of describing the complete behavior of phase-change-materials. The material parameters were then determined for the selected material laws and simulations were carried out on simple containers, followed by a discussion of the results.

Dissertation, RWTH Aachen University, 2020; Aachen 1 Online-Ressource (IX, 210 Seiten) : Illustrationen, Diagramme (2020). = Dissertation, RWTH Aachen University, 2020, With the introduction of the Optane memory in 2017, Intel and Micron made the first commercial use of the electrical properties of Phase Change Materials (PCM). PCMs can be switched between a high conductive crystalline state and a low conductive amorphous state by joule heating. These two states can be employed to store information. The unique properties of this material class are assumed to originate from their unconventional bonding mechanism, recently coined metavalent bonding. Metavalent bonding is characterized by electrons, which are on the verge of delocalization. In this thesis, the consequences of the tendency of the electrons towards delocalization are analyzed. Therefore three different effects of thin films are investigated in detail. The first effect is a spontaneous transition from an amorphous to a crystalline state at a critical film thickness in the range of a few atomic bilayers in PCM and metals at a constant temperature. After the transition, the crystalline film has an epitaxial relation to the substrate. During the transition the film crystallizes fully, there is no amorphous buffer layer. It is assumed that in thin films the electrons are so confined that it is not possible to develop the tendency needed for delocalization as employed in metavalent bonds. But this tendency is essential for metavalent bonding. Consequently, the thin film is bonded covalently. In literature, this transition is seen only for some combinations of substrates and PCMs. Moreover, a systematic analysis is lacking. In this work, it is shown that four out of five PCM undergo the transition on the Si(111)-7x7 surface. The only exception is Sb2Te3 due to its layered structure. In addition, a transition is demonstrated here for the first time in Ge2Sb2Te5 on Si(111)-7x7. One crucial but in literature not discussed factor for this transition is the growth mode. The transition is observed using Reflection High-Energy Electron Diffraction (RHEED). However, it can be shown that the needed lack of reflexes in RHEED occurs only during layer-by-layer growth. In addition it is shown that passivating the silicon surface with Sb forming the Si(111)-(sqrt(3) x sqrt(3)) R30° - Sb, prevents the transition in all cases studied and the film grows crystalline from the beginning. Moreover, the critical thickness of the transition depends on the substrate temperature. This dependency has not been reported so far. It is shown that the relation is based most likely on kinetic effects. The second effect which has been investigated concerns the in-plane lattice parameter at the start of the growth. The Si(111)-(sqrt(3) x sqrt(3)) R30° - Sb surface termination imprints a smaller lattice parameter on films like GeTe(111) or Ge2Sb2Te5 (111). Nevertheless, with RHEED a systematically too large lattice parameter has been found in all materials analyzed except Sb2Te3. The larger lattice parameter is assumed to be connected to the confined dimensions, influencing the bonding. Also, DFT calculations of free-standing films suggest a larger lattice parameter in thin films as compared to the bulk even without a substrate. As a third and last effect, the thin films are combined into multilayers. In literature, an unusual strain transfer is reported in multilayers of GeTe/Sb2Te3 or GeTe/Bi2Te3 stacks. The strain transfer is assumed to originate from the layered structure of, for example, Sb2Te3 and Bi2Te3. For the first time, GeTe and SnTe are combined into multilayers. Both materials have no layered structure in comparison to Sb2Te3 and Bi2Te3. Therefore, the strain transfer is of interest and is monitored via the in-plane lattice constant. Besides, the GeTe/SnTe superlattices are analyzed to determine the intermixing at different growth temperatures. Here, a rare combination of RHEED, X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Atom Probe Tomography (APT) give insights into the structure of the stacks. The origin of the breaking of the symmetry in the growth of SnTe (001) on Si(111) is analyzed using Angular RHEED (ARHEED). Taking the crystal structure of Si(111) into account, three equal rotational domains of the SnTe should occur. In the past, it was possible to use the miscut of the substrate surface and the silicon lattice to break this symmetry. In this case, two out of the three rotational domains are suppressed resulting in an almost single crystalline film. With ARHEED it is possible to determine the critical thickness of the suppression of the two domains in the range of 1.5 to 2 nm. In addition, the surface sensitivity of ARHEED is used to determine a previously unknown rotational domain at ±2° at small film thicknesses of SnTe. These domains vanish at roughly 4 nm. The calculations of the intersections of the two superimposed grids of Si and SnTe also confirm the occurrence of the rotation domainal at ±2°. This discovery underscores the ability of ARHEED to measure rotational domains at film thicknesses where other techniques, such as XRD, fail. The analysis of the three thin film effects and the discovery of the critical thickness for the suppression of two out of three rotational domains helps improving the understanding of metavalent bonding in phase change materials., Published by Aachen

Dissertation, RWTH Aachen University, 2019; Aachen 1 Online-Ressource (xviii, 137 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, RWTH Aachen University, 2019, Within the scope of this thesis, different chalcogenides with layered structures were considered and investigated for their electronic structure, stability and chemical bonding. Furthermore, the nature and strength of the interactions between the layers in such compounds has been characterized to better understand the relationship between structures and properties. First, a detailed introduction to the orbital-based bonding analysis of solids has been provided and, based on this, previous results of the bonding analysis of phase change materials are summarized and an antibonding region below the Fermi level was identified as a possible fingerprint of such materials. Furthermore, it was shown how the strength of various bonds in complex solid-state structures can be evaluated by means of ab initio methods and how the bonding analysis helps to understand the unusual structures of certain chalcogenides. In the results section, the layered structure of Ge₄Se₃Te was introduced. The strength of the Ch–Ch contacts between the layers and Ge–Ge contacts within the layers was investigated by means of various thought experiments. Subsequently, the transition from the α-GeTe structure to the structure of Ge₄Se₃Te was investigated by chemical bonding analysis in the context of the crystal orbital Hamiltonian population analysis. Furthermore, the structural diversity of In₂Se₃ was examined. Of the six stable phases at ambient pressure, there are three stable at room temperature and three at elevated temperatures. The experimental findings on the dynamic stability of In₂Se₃ could be confirmed by the calculation of the phonon dispersion and the different coordination polyhedra of the structures were then investigated in the context of chemical bonding and Mulliken charge analysis. In the last chapter of this work, the structure-property relationships of different chalcogenides with layered structures were investigated, and the strength of inter-layer interactions was related to the properties of hypothetical structures with two-dimensional defects. In summary, the structures of diverse layered chalcogenides were investigated in this work. Many functional chalcogenides, which are being investigated for applications as data storage materials, thermoelectrics, solar cells, and more, contain unusual structural elements that have been investigated by orbital-based bonding analysis., Published by Aachen

Phasenwechselmaterialien werden in optischen und elektrischen Datenspeichern eingesetzt. Der Schaltvorgang zum Schreiben der Daten erfolgt mit kurzen, intensiven Laser- oder Strompulsen zwischen einer amorphen und einer kristallinen Phase. Die optischen und elektrischen Eigenschaften dieser beiden Phasen unterscheiden sich signifikant voneinander, sodass die Daten mit schwachen Laser- oder Strompulsen ausgelesen werden können. Das Ziel der Arbeiten war Struktur-Eigenschaftsbeziehungen aufzuzeigen, die beim Design zukünftiger Datenspeichermaterialien helfen können. Es wurden dünne Filme von Phasenwechselmaterialien mit Cosputtern oder Coverdampfen auf unterschiedlichen Substraten hergestellt. Neben den literaturbekannten Verbindungen GeTe, Ge2Sb2Te5, Ge8Sb2Te11, Sn8Sb2Te11 und SnSb2Se4 wurden insgesamt neun weitere Materialien durch chemische Substitution einzelner Elemente synthetisiert. Verglichen mit den jeweiligen Ausgangsmaterialien wurden die Hybridisierung und die Ionizität der chemischen Bindungen variiert, welche für die Eigenschaften von Phasenwechselmaterialien essentiell und in der sogenannten Schatzkarte zur Identifizierung geeigneter Materialien aufgetragen sind. Die bisher unbekannten Verbindungen konnten umfassend charakterisiert und die Phasenwechseltemperaturen, die Kristallstrukturen, die optischen Konstanten, die elektronischen Bandlücken, der optische und elektrische Kontrast sowie das magnetoresistive Verhalten bestimmt werden. Laserschaltversuche dienten zur Überprüfung der Reversibilität des Phasenwechsels und der Schaltgeschwindigkeit. Phase change materials are used in optical and electrical data storage devices. These materials can rapidly and reversibly be switched between an amorphous and a crystalline state by applying short, intense laser pulses or high electrical current pulses. The optical and electrical properties of the two phases are remarkably different so that laser and current pulses can be used to read out information. The aim of this work was to characterize new compounds and to reveal structure-property relations which can help to design future data storage materials. All materials were prepared as thin films using sputter deposition or co-evaporation on different substrates. In addition to the well-known compounds GeTe, Ge2Sb2Te5, Ge8Sb2Te11, Sn8Sb2Te11 and SnSb2Se4, nine new compounds were synthesized via chemical substitution of single elements. Consequently, the hybridization and ionicity of bonding, which are essential for the materials properties, were altered and used to upgrade the so-called treasure map to identify suitable phase change materials. For each synthesized material, the phase change temperatures, the crystal structures, the optical constants, the electronic band gaps, the optical and electrical contrast as well as the magnetoresistive behavior were determined. Femtosecond laser pulses were used to investigate the reversibility of phase changes and switching speeds.

The dissertation presents the results of the investigation on atomic structure, crystallization and thin film growth behaviour of Ge-Sb-Te materials for phase-change applications. Thin layers of the target material were deposited by pulsed laser deposition under various treatment conditions, pre-characterised by X-ray diffraction methods and scanning beam electron microscopy, and subsequently prepared by focused ion beam technique for transmission electron microscopy investigation (TEM). The TEM investigation included common techniques such as dark-field imaging, selected area diffraction and secondary X-ray spectroscopy, as well as advanced techniques such as probe aberration-corrected imaging (Cs-corrected STEM) at various collection angles and scanning nano-beam diffraction. The results were evaluated in comparison with quantitative image simulations from model structures, and discussed in detail. Central conclusions include the modelling of characteristic disorder structures, refinement of the crystalline-crystalline structure transition, and close description of a quasi Van-der-Waals domain epitaxy growth window. In der Dissertation werden die Ergebnisse einer Untersuchung von atomarer Struktur, Kristallisationsverhalten und Dünnschichtwachstum an Ge-Sb-Te Materialien für Phasenwechselanwendungen präsentiert. Dünne Schichten des Target Materials wurden unter verschiedenen Konditionen mittels gepulster Laser Abscheidung deponiert, und anschließend mittels Röntgenbeugung und Rasterelektronenmikroskopie Vorcharakterisiert. Im Anschluss wurden die Proben mittels fokussierten Ionenstrahls für die Transmissionselektronenmikroskopie (TEM) präpariert. Die TEM Untersuchung erstreckte sich sowohl auf übliche Techniken wie Dunkelfeldabbildung, Feinbereichsbeugung und energiedispersive Röntgenspektroskopie, als auch fortgeschrittene Techniken wie Aberrationskorrigierte Raster-transmissionsabbildung (Cs-korrigiertes STEM) bei verschiedenen Sammelwinkeln und Raster-nanobeugung. Die detaillierte Diskussion der Ergebnisse beinhaltete den Vergleich mit quantitativen Bildsimulationen. Zentrale Schlussfolgerungen beinhalten die Modellierung von charakteristischen Defektstrukturen, eine nähere Beschreibung der Kristallinen Struktur-transformation, sowie die Beschreibung eines quasi Van-der-Waals Domänenepitaxie Wachstums in einem engen Abscheidungsfenster.

RWTH Aachen University, Diss., 2017; 125 Seiten(2017). = RWTH Aachen University, Diss., 2017, In this work, two methods to create material maps from first-principles density-functional-theory calculations are being presented. As coordinates for these maps serve novel mathematical indicators for chemical bonding analysis that have been developed. The predictive power of these maps is based on the fact that materials which fall into similar regions on these maps share characteristic attributes. Hence, it is possible to conclude the properties of previously unknown material candidates from their position on these maps alone. This enables a guided search for new materials of a specific property portfolio and aids in the design of novel compounds. The first-principles simulations of the compounds under investigation was done by utilizing the DFT program suite VASP. This yielded plane wave functions which were analytically projected onto a local basis set of atomic orbitals using the program LOBSTER and then scrutinized by the methods of the author’s creation. The first concept for creating material maps is solely based on three indicators that analyze the bonds according to their “sp”, “sp2” and “sp3” character. The second kind of material map is based on the “sp3” character of the bonds and the ionicity. The ionicity was computed via a novel method based on local electron densities as presented in this work. Before actualizing both mapping concepts, an important problem had to be solved: to ensure reliable results from the developed bonding indicators, the local basis set and the crystal structures of the simulated compounds have to be spatially aligned. For this, the basis functions have to be adjusted to the bonding polyhedron of each individual atom in a crystal. To this end, a mathematical apparatus has been devised which enables orbital rotation. This apparatus rests on a second analytical projection through which the local basis set in standard orientation is mapped onto a spatially rotated basis set. An algorithm was designed to harness this second projection scheme and automatically determine in which direction the basis functions have to be rotated to harmonize basis and bond vectors. Once structure and basis could be spatially aligned, the mapping concepts were put into action. The first mapping concept was utilized in the search for novel carbon-only functional materials and thus applied to theoretical and experimentally available carbon allotropes. Strong correlations between the allotropes’ positions on this map and their macroscopic properties, like the hardness of the materials, and the microscopic properties, like the crystal structure itself, underline the prediction power of this concept. To gain deeper knowledge, three hypothetical allotropes (T-carbon, TY-carbon and T-graphene) on this map were treated to an in-depth bonding analysis. The second mapping concept was used to generate a “treasure map” for phase change materials as new optical data storage materials. In contrast to previous concepts, this map allocates all phase change materials of industrial use to a characteristic “island-like” area. In addition to the compounds of industrial use, seven novel compounds fall into this characteristic region: SiSb4Te7, Sn2As2Te5, SiSb2Te4, Si2Sb2Te5, SiAs2Te4, PbAs2Te4 and PbAs4Te7. Hence, these materials are suggested to be candidates for new, previously unknown phase change materials that might find future industrial application., Published by Aachen

Diese Arbeit befasst sich mit dem Design und Aufbau eines Experiments zur Untersuchung der Unterk��hlung und Nukleation von Phasenwechselmaterialien f��r thermische Speicher. Weiters sollen auf Grundlage der T-History Methode kalorimetrische Messungen erm��glicht werden, die die Phasenwechselenthalpie und spezi sche W��rmekapazitat des Material bestimmen k��nnen. Der Bau dieses Messger��tes ist notwendig, da die zu vermessenden Probenmassen eine weitaus gr����ere Masse besitzen, als die in einem dynamischen Differenzkalorimeter (DSC) verwendeten. Dadurch kann der Grad der Unterk��hlung, der von der Masse abh��ngt, f��r thermische Speicher genau bestimmt werden. Anhand eines Vorexperiment konnten wichtige Informationen f��r die Konzeptionierung des Messger��tes gesammelt werden. So entstanden die ersten ��berlegungen zur Temperaturerfassung und zum Design des Probenraumes. Anhand einer Dymola-Simulation konnten wichtige Fuid- und thermodynamische Parameter berechnet werden. Diese geben Aufschluss ��ber das thermische Verhalten der Probe w��hrend einer Messung, dessen Schmelzdauer und wie viel Leistung notwendig ist, um die Luft auf eine vorgegebene Temperatur zu erw��rmen. Au��erdem wurden Berechnungen angestellt f��r eine Lochplatte, zur Erzeugung eines Druckabfalls und einer homogenen Str��mung im Probenraum. Das Ergebnis war, dass diese Platte f��r einen gen��gend gro��en Druckabfall zu wenige und zu kleine L��cher hatte - die Stromung wurde nicht gleichma��ig in den Probenraum flie��en. Deswegen wurde ein Fluidisierungskissen gew��hlt, das einen vorgegebenen Druckverlustbeiwert besitzt. Aufbauend auf den Erkenntnissen durch das Vorexperiment und den errechneten Parametern in der Dymola-Simulation wurden die einzelnen Komponenten des Messger��tes und der allgemeine Aufbau pr��sentiert. Da die Messwertaufzeichnung einen wichtigen Bestandteil des Messger��tes darstellt, wurden die Eigenschaften einer Temperaturaufzeichnung und dessen Programmierung besprochen. Testmessungen und Untersuchungen gaben die M��glichkeit, Fehler in der Konstruktion bzw. bei der Temperaturaufzeichnung festzustellen. Dabei wurde besonderes Augenmerk auf die Temperaturaufzeichnung durch die Infrarot-Sensoren, die Verteilung der Probenraumtemperatur und dem Rauschen in der Aufzeichnung gelegt. Messungen an den Zuckeralkoholen Erythrit und Mannit, die sowohl aktiv als auch passiv gek��hlt wurden, gaben erste Einsicht in die Unterk��hlung der Phasenwechselmaterialien. Weiters wurde der Einfluss der thermischen Strahlung auf eine kalorimetrische Messung besprochen. Anhand den Testmessungen werden Verbesserungsvorschlage f��r die auftretenden Fehler und einem optimalen Messprozess aufgez��hlt. Das Messger��t eignet sich sehr gut zur Untersuchung der Unterk��hlung, der Schmelzdauer und Temperaturerh��hung durch die Kristallisationsw��rme. F��r eine kalorimetrische Messung m��ssen noch die im Kapitel Verbesserungsvorschl��ge erw��hnten Probleme ausgebessert werden. F��hrt man dies durch, so erh��lt man wichtige Informationen ��ber die Phasenwechselenthalpie und spezi sche Warmekapazit��t f��r inhomogene gro��e Massen., This thesis deals with the design and construction of an experiment for the investigation of the cooling and nucleation of phase-change materials for thermal storage. Furthermore, calorimetric measurements, based on the T-history method to determine the phase-change enthalpy and specific thermal capacitances of the material, should be able. The construction of this measuring instrument is necessary, since the sample masses to be measured have a much larger mass than those in a dynamic Differential scanning calorimetry (DSC). As a result, the degree of subcooling, which depends on the mass, can be exactly determined for thermal storage. On the basis of a preliminary experiment, important information for the conception of the Measuring instruments were optained. Thus, the first considerations about the temperature detection and the design of the sample chamber were developed. By means of a Dymola simulation, important fluid and thermodynamic parameters could be calculated. These provide information on the thermal behavior of the sample during a measurement the melting time and how much power is necessary to heat the air to a predetermined temperature. In addition, calculations were made adjusted for a perforated plate, for generating a pressure drop and a homogeneous flow in the sample chamber. The result was that this plate had too few and too small holes for a sufficiently large pressure drop - the flow would not flow smoothly into the sample chamber. Therefore, a fluidization cushion was selected, which had a predetermined pressure loss coefficient. Based on the findings of the preliminary experiment and the calculated parameters in the Dymola simulation, the individual components of the measuring instrument and the general design were presented. Since the recording of the measured value is an important component of the measuring instrument, the characteristics of a temperature recording and its programming were discussed. Test measurements and examinations gave the possibility of establishing errors in design and temperature recording. Particular attention was paid to the temperature recording by the infrared sensors, the distribution of the sample chamber temperature and the noise in the recording. Measurements on the sugar alcohols Erythritol and Mannitol, both active and passive cooled, showed significant subcooling for the phase-change material. Furthermore, the influence of the thermal radiation on a calorimetric measurement was discussed. On the basis of the test measurements, improvement suggestions for the occurring errors and an optimal measuring process are enumerated. The measuring device is very well suited for the investigation of subcooling, the melting time and the temperature increase by the crystallization heat.

Phase change materials combine a pronounced contrast in resistivity and reflectivity between their disordered amorphous and ordered crystalline state with very fast crystallization kinetics. Due to this exceptional combination of properties phase-change materials find broad application in non-volatile optical memories such as CD, DVD or Bluray Disc. Furthermore, this class of materials demonstrates remarkable electrical transport phenomena in their disordered state, which have shown to be crucial for their application in electronic storage devices. The threshold switching phenomenon denotes the sudden decrease in resistivity beyond a critical electrical threshold field. The threshold switching phenomenon facilitates the phase transitions at practical small voltages. Below this threshold the amorphous state resistivity is thermally activated and is observed to increase with time. This effect known as resistance drift seriously hampers the development of multi-level storage devices. Hence, understanding the physical origins of threshold switching and resistance drift phenomena is crucial to improve non-volatile phase-change memories. Even though both phenomena are often attributed to localized defect states in the band gap, the defect state density in amorphous phase-change materials has remained poorly studied. This thesis presents defects state densities measured on different amorphous phase-change materials and chalcogenides showing electrical threshold switching. On the basis of Modulated Photo Current (MPC) Experiments and Photothermal Spectroscopy a sophisticated band model for a-GeTe has been developed, which is shown to consist of defect bands and band tail states. This study on a-GeTe has shown that the data analysis within MPC experiments can be drastically improved by taking the temperature dependence of the optical band gap into account. To get a better understanding of resistance drift phenomena this study focuses on the evolution of resistivity on heating and ageing, activation energy of electronic conduction, optical band gap, defect state density, mechanical stress and nearest neighbour ordering in a-GeTe thin films. After heating the samples one hour at 140°C the activation energy for electric conduction increases by 30 meV, while the optical band gap increases by 60 meV. Additionally, MPC experiments revealed a decreasing concentration of mid gap states in aged a-GeTe thin films. These findings demonstrate the impact of band gap opening and defect annihilation on resistance drift. Furthermore, the stoichiometric dependence of resistance drift phenomena in a-GeSnTe phase-change alloys is studied in this thesis. A systematic decrease in the amorphous state resistivity, activation energy for electric conduction, optical band gap and defect density is observed with increasing tin content resulting in a low resistance drift for tin rich compositions such as a-Ge2Sn2Te4. This study on GeSnTe systems demonstrates, that phase change alloys showing a more stable amorphous state resistivity are characterized by a low activation energy of electronic conduction. This finding found in GeSnTe alloys holds also true for GeSbTe and AgInSbTe systems. On the example of a-Ge2Sn2Te4 and a-GeTe exhibiting a strong resistance drift, the evolution of the amorphous state resistivity is shown to be closely linked to the relaxation of internal mechanical stresses resulting in an improving structural ordering of the amorphous phase. For the investigated alloys showing electrical switching, the measured density of midgap states is observed to decreases with decreasing threshold field known from literature. This result favours a generation-recombination model behind electrical switching in amorphous chalcogenides as originally proposed by D. Adler.

The integration of phase change materials into semiconductor heterostructures may lead to the development of a new generation of high density non-volatile phase change memories. Epitaxial phase change materials allow to study the detailed structural changes during the phase transition and to determine the scaling limits of the memory. This work is dedicated to the epitaxial growth of Ge-Sb-Te phase change alloys on GaSb(001). We deposit Ge-Sb-Te (GST) films on GaSb(001) substrates by means of molecular beam epitaxy (MBE). The film orientation and lattice constant evolution is determined in real time during growth using grazing incidence X-ray diffraction (GID). The nucleation stage of the growth is studied \emph{in situ} using reflection high energy electron diffraction (RHEED)., Die monolithische Integration von Phasenwechselmaterialien mit Halbleiter-Hetero\-strukturen er\"offnet neue Perspektiven f\"ur zuk\"unftige Generationen von nichtfl\"uchtigen Speicherbauelementen. %Epitaktische Phasenwechselmaterialien erm������glichen detaillierte %Studien der strukturellen ������nderungen w������hrend des Phasen������bergangs und %erlauben eine Bestimmung der Skalierungslimits zuk������nftiger %Datenspeicher. Diese Arbeit befasst sich mit dem epitaktischen Wachstum von Ge-Sb-Te Phasenwechselmaterialien. Dazu wurden Ge-Sb-Te(GST) Schichten mittels Molekularstrahlepitaxie (MBE) auf GaSb(001)-Substraten abgeschieden. Die kristallografische Orientierung und die Ver\"anderungen der Gitterkonstante w\"ahrend des Wachstums wurden mittels R\"ontgenbeugung unter streifendem Einfall (GIXRD) bestimmt. Das Nukleationsverhalten zu Beginn des Wachstums wurde mittels Hochenergie-Elektronenbeugung unter streifendem Einfall (RHEED) untersucht.

Phase-change materials belong to a group of materials, that are suitable for optical and electronic data storage applications due to their unusual combination of properties. Within very short time, phase-change materials can reversibly be switched between a disordered, amorphous and an ordered, crystalline phase, and both the crystallization and the amorphization can be induced with laser pulses or electrical pulses. Because of the high optical and/or electrical contrast between both phases, using laser radiation or electrical pulses also the state of the material can reliably be detected. The knowledge of the structure of the amorphous and crystalline phases, which are relevant for data storage devices, is important to understand the origin of the high contrast and to be able to link the stoichiometry of the material with its physical properties. Therefore, in this study the structure of different phase-change materials is investigated by means of extended x-ray absorption fine structure(EXAFS) measurements. After a short introduction to phase-change materials, the theory of x-ray absorption measurements is discussed and the methods, that are used to analyse the EXAFS spectra, are explained. Subsequent, the experimental equipment to prepare and characterize the samples is presented. Within the scope of this thesis, the structure of the crystalline and the amorphous phase of three alloys, GeTe, Ge8Sb2Te11 and Ge1Sb2Te4, is studied. These materials lie along the pseudo-binary line, which ranges from GeTe to Sb2Te3. After the detailed description of the analysis of the EXAFS spectra and the results, the structures of the investigated materials are compared and discussed. The different local order in the amorphous and the crystalline phase, which is already apparent in the EXAFS spectra, is characteristic for phase-change materials. In the Fourier transformed display of the EXAFS spectra in real space, not only the amplitudes but also the peak positions differ in both phases. The analysis of the EXAFS spectra yields, that in the studied materials the bond lengths are shorter in the amorphous phase as compared to the crystalline phase. Investigating the crystalline phase of the phase-change materials it can be seen, that the temperature has an exceptionally large influence on the EXAFS spectra. The information content increases significantly, if the recording temperature is lowered to 10 K, and the spectra contain information on interatomic distances up to approximately 6Å (compared to ca. 3Å at room temperature). The large impact of the temperature can be associated with the large static disorder in the crystalline phase. Although at 10K contributions from thermal oscillations on the EXAFS Debye-Waller factor can be neglected, these factors are large for the crystalline phase, even larger than for the amorphous phase. The results of the Reverse Monte Carlo simulations provide the same information: the pair correlation functions for the crystalline configurations have much broader peaks than for the respective amorphous configurations. This means, that in contrast to common semiconductors, like e.g. Ge, the local order in the investigated phase-change materials is smaller in the crystalline phase than in the amorphous phase, even though no long range, periodic order exists in the amorphous phase.

In this thesis, the electrical transport properties of crystalline phase change materials are discussed. Phase change materials (PCM) are a special class of semiconducting and metallic thin film alloys, typically with a high amount of the group five element antimony or the group six element tellurium, such as Ge2Sb2Te5. The unique property portfolio of this material class makes it suitable for memory applications. PCMs reveal fast switching between two stable room-temperature phases (amorphous and crystalline) realized by optical laser or electrical current pulses in memory devices. Additionally, a pronounced property contrast in form of optical reflectivity and electrical conductivity between the amorphous and cyrstalline phase is the characteristic fingerprint of PCMs. The emerging electrical solid state memory PCRAM is a very promising candidate to replace Flash memory in the near future or to even become a universal memory, which is non-volatile and shows the speed and cyclability of DRAM. One of the main technological challenges is the switching process into the amorphous state, which is the most power demanding step. In order to reduce the switching power, the crystalline resistivity needs to be increased at a given voltage. Thus understanding and tayloring of this property is mandatory. In this work, first the technological relevance, i.e. optical and electrical memory concepts based on PCMs are introduced (chapter 2.1). Subsequently a description of the physical properties of PCMs in four categories is given. Namely, structure (section 2.2.1), kinetics (section 2.2.2), optical properties (section 2.2.3) and electrical properties (section 2.2.4) are discussed. In chapter 2.3 important recent developments such as the identfication of resonant bonding in crystalline PCMs and a property predicting coordination scheme are briefly reviewed. Chapter 3 deals with the theoretical background of electrical transport, while chapter 4 introduces the experimental techniques: Sputtering, XRR, XRD, DSC, thermal annealing, profilometry, ellipsometry, FTIR spectroscopy, van der Pauw, Hall, and phase change optical and electrical switching techniques which are used to obtain the results in chapter 5. The latter chapter is split into two parts. In the first one the material class of doped Sb2Te is investigated with respect to structural, kinetic, optical and electrical properties. The second part covers the study of the systematic variation of the resistivity of pseudo-binary GeTe-Sb2Te3 alloys upon thermal annealing. This variation is discussed in the framework of an insulator to metal transition with the help of results from a combined van der Pauw, XRD, Hall, STM and FTIR study.

Starting from a brief introduction into the physics of phase change materials their applications in the field of data storage are reviewed. Without the latter there would certainly be less attention paid to the topic today, even in basic research. For years now, optical data storage media based on phase change materials have been successfully produced for the mass market. Despite its maturity there are still new ideas on how to further improve this technology in order to stay competible with other storage media in the future. In contrast, for the application of phase change materials in electronic memories the situation is completely different. Although proposed decades ago, today it is still a rather young and promising technology. In this work both applications are analyzed with respect to their requirements to the incorporated phase change material, with an emphasis on the second one. For optical storage and especially in the case of electronic memories the author arrives at the conclusion, that the crystallization kinetics of phase change materials is the most urgent and fundamental problem that needs to be solved. The emphasis on a deep and quantitative understanding of this phenomenon implicitely criticizes the popular approach of studying the switching of electronic cells for phase change based memories without an in-depth research of the crystallization kinetics. After this identification of the research objective, the result of a thorough review of the literature on the theory of crystallization is presented. The understanding of glass formation turns out to be extremely important, since it deals with the stability of an amorphous solid or undercooled liquid against structural reconfiguration. Consequently, a separate chapter is dedicated to the theory of glass formation. Based on the knowledge of the theoretical connections between glass transition and crystallization it is investigated how meaningful a calculation of the enthalpy of atomization is for a prediction of stoichiometric trends not only of glass transition, but also of crystallization. The sparce experimental evidence on the glass transition temperature of phase change materials is compared to the calculated enthalpies. The same is done for a series of measurements of crystallization. These comparisons show that the proposed strategy works well for predicting the influence of a stoichiometric variation of a phase change material on its stability against crystallization. The author's critical review of the literature on crystallization kinetics reveals that the widely used classical crystallization theory still lacks a rigorous experimental prove of its validity. The latter is difficult, because the multitude of quasi-free parameters generally ensures a good mathematical agreement between theory and the often very limited experimental data. Such a check of validity of the theory is especially challenging for phase change materials: In a wide range of high temperatures crystallization proceeds so fast, that until now an experimental quantification of nucleation rate and crystal growth velocity was only possible in a rather limited regime of low temperatures. The extrapolation of such data over the whole temperature range up to the melting temperature by application of the equations provided by the classical theory is assessed to be too uncertain to be trusted. To close that gap with experimental evidence and to advance therewith towards a disentanglement of electronic and thermal effects involved in the switching of an electrical cell, a new experimental setup has been designed and realized. It combines laser induced annealing experiments with the capability to apply and measure fast electrical pulses. The implementation of a control of the sample's base temperature is an additional, valuable component. Each of the sections of the new setup on its own is already a sophisticated tool, that enables its user to investigate phase change materials on very short time scales. Examples for this are unprecedented laser experiments that have been performed by the author. Some of those innovatively separate crystal nucleation and growth. Others demonstrate a path towards a quantitative measurement of crystallization kinetics in the melt-quenched amorphous state. The latter is highly relevant for technology, since it is that amorphous phase, that is realized in applications. But the new setup is more than just the sum of its parts. The combination of optical, electrical and thermal experiments opens up a wide field of new possibilities. This is indicated by the demonstration of an electrical experiment for the investigation of the threshold-switching effect, the phenomenon describing an abrupt breakdown of the resistivity of the amorphous phase upon application of a critical electric field. The initialization of the sample by laser annealing allows for a clean'' experiment in which the starting conditions are not dependent on the property of interest itself, in this case the electrical behaviour. Such a successive approach from purely thermal experiments towards the testing of the realistic but complex cells of a phase change based electronic memory is necessary to stepwise synchronize the numerical simulations. This strategy is essential for achieving a deep understanding of the physical processes involved in the switching of such cells. Beyond these technologically important measurements the new setup is pioneering for a multitude of further optical and electrical experiments that are likely to make valuable contributions to the research of the physical properties of phase change materials. Examples for such experiments are proposed at the end of this work.