Düsseldorf, Universiẗat, Diss., 2000.

This thesis is a sandwich thesis composed of three papers that are published in refereed journals or conferences. The first paper is a systematic experimental study conducted to identify the first phase to form during cobalt germanidation. Hexagonal β-Co5Ge3 was the first phase to form at temperatures as low as 227°C followed by monoclinic CoGe as the second phase at the same temperature. We also report for the first time that both phases that formed were highly ordered partial epitaxial crystal orientations suggesting that both of those low-temperature phases could potentially serve as high quality contacts for germanium based devices with a very low thermal budget which is advantageous for the process design. Those results contributed to a better understanding of cobalt germanidation leading to the first multiphase technology computer aided design model presented in the second paper. This kinetic model for cobalt germanide growth can predict the resulting phase based on anneal time, temperature, and ambient. The model has been calibrated to experimental results. This predictive model can help in the design of cobalt germanide contacts with low resistance and can serve as a general modeling framework for multiphase solid state reaction binary systems. A comprehensive survey of the experimental results for formation of cobalt germanides is discussed and the data are reconciled in the third paper. Factors affecting the resulting phases and their quality are identified and some optimum choices for the experimental parameters are pointed based on the survey. The role of germanium crystal orientation in ohmic and Schottky properties of the contact is analyzed. Fermi level pinning plays a role mainly on metal/(100) n-type Ge interfaces and its role is minimal on p-type Ge and other crystalline orientations. Schottky Barrier Heights for cobalt germanide contacts reported in the literature are surveyed. Crystalline cobalt germanides, forming when Co is deposited at high temperatures, are expected to have lower interface resistivities compared to those reported. The work is important because contact resistance has become one of the most important factors in advanced complementary metal oxide semiconductor (CMOS) technology and advanced devices already include germanium (Ge) in the source/drain regions of devices. It is also important because heating at the interface due to contact resistance is one of the key challenges in power devices and cobalt germanide can be used both for Si and Ge based devices as well as for gallium nitride (GaN) devices. The latter application is possible because cobalt germanide is lattice-matched to GaN. / Thesis / Doctor of Philosophy (PhD) / The main goal of this thesis is to create predictive empirical, mathematical, and physical models to help the designer of the semiconductor process technology to design high quality electric contacts, namely cobalt germanides, to their semiconductor devices, germanium based. The choice of cobalt germanides is motivated by their expected superior quality given the possibility of growing them in crystalline form. We settled a theoretical and experimental controversy regarding the first phase to form by conducting experiments demonstrating that low-temperature forming cobalt germanide phases are highly ordered and could serve as high quality contacts. A predictive physical based mathematical model was developed to assist the designer in obtaining the desired cobalt germanide phase for its needed electrical properties by design. Factors affecting the quality of the germanide were identified based on an extensive survey and the optimum choices for the parameters to obtain high quality contact were pointed.

Kernpunkt der Dissertationen ist aufbauend auf langjährigen Arbeiten zur Silicumchemie die Untersuchung analoger Hochtemperaturreaktionen in dem quaternären System Ge-Cl-H-Metall. Als Metalle wurden die germanidbildenden Metalle Ni, Cu und Fe und die primär chloridbildenden Metalle La, Mg und Ca in die Untersuchungen einbezogen. Zunächst werden die Eigenschaften der binären, folgend der ternären und schließlich der quaternären Systeme untersucht. Die Experimente beinhalten Umsetzungen im Strömungsreaktor, isotherme und anisotherme DSC-Messungen mit kinetischer und thermodynamischer Zielstellung teilweise in Kopplung mit FT-IR und MS sowie die entsprechenden Charakterisierungsmethoden (XRD, REM-EDX, chemische Analyse, NMR, FT-IR). Versuchsplanung und Interpretation der Messergebnisse werden begleitet durch umfangreiche thermodynamische Berechnungen. Es resultieren neuartige Ergebnisse zur Reaktivität in den stofflichen Systemen.

The semiconducting silicides offer significant potential for use in optoelectronic devices. Full implementation of the materials, however, requires the ability to tailor the energy gap and band structure to permit the synthesis of heterojunctions. One promising approach is to alloy the silicides with Ge. As part of an investigation into the synthesis of semiconducting silicide heterostructures, a series of β-Fe(Si1−xGex)2 epilayer samples, with nominal alloy content in the range 0 < x < 0.15, have been prepared by molecular beam epitaxy on Si(100). I present results of the epitaxial and crystalline quality of the films, as determined by reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, and double crystal x-ray diffraction, and of the band gap dependence on the alloy composition, as determined by Fourier transform infrared spectroscopy. A reduction in band gap was observed with increasing Ge content, in agreement with previous theoretical predictions. However Ge segregation was also observed in β-Fe(Si1−xGex)2 epilayers when x > 0.04. Osmium silicide films have been grown by molecular beam epitaxy on Si(100). The silicides have been grown using e-beam evaporation sources for both Os and Si onto Si(100) substrates at varying growth rates and temperatures ranging from 600-700ºC. The resulting films have been analyzed using reflection high-energy electron diffraction, Raman spectroscopy, reflectivity measurements, in-plane and out of plane X-ray diffraction and temperature dependent magnetotransport. A change in crystalline quality is observed with an increase in Si overpressure. For a lower silicon to osmium flux ration (JSi/JOs=1.5) both OsSi2 and Os2Si3 occur, whereas with a much larger Si overpressure (JSi/JOs>4), crystalline quality is greatly increased and only a single phase, Os2Si3, is present. The out-of-plane X-ray diffraction data show that the film grows along its [4 0 2] direction, with a good crystal quality as evidenced by the small FWHM in the rocking curve. The in-plane X-ray diffraction data show growth twins with perpendicular orientation to each other.

Dresden, Techn. Univ., Diss., 2000.

Kernpunkt der Dissertationen ist aufbauend auf langjährigen Arbeiten zur Silicumchemie die Untersuchung analoger Hochtemperaturreaktionen in dem quaternären System Ge-Cl-H-Metall. Als Metalle wurden die germanidbildenden Metalle Ni, Cu und Fe und die primär chloridbildenden Metalle La, Mg und Ca in die Untersuchungen einbezogen. Zunächst werden die Eigenschaften der binären, folgend der ternären und schließlich der quaternären Systeme untersucht. Die Experimente beinhalten Umsetzungen im Strömungsreaktor, isotherme und anisotherme DSC-Messungen mit kinetischer und thermodynamischer Zielstellung teilweise in Kopplung mit FT-IR und MS sowie die entsprechenden Charakterisierungsmethoden (XRD, REM-EDX, chemische Analyse, NMR, FT-IR). Versuchsplanung und Interpretation der Messergebnisse werden begleitet durch umfangreiche thermodynamische Berechnungen. Es resultieren neuartige Ergebnisse zur Reaktivität in den stofflichen Systemen.

The main focus of the present work was the Ge-rich part of the binary Ba – Ge system, in which by inspecting the behavior of the clathrate-I Ba8Ge43 under pressure, several new phases were found. The new phases in this system have the following compositions: BaGe3 (with two modifications), BaGe5, BaGe5.5 and BaGe6, therefore they are quite close in composition range: 75% - ~85% at. Ge. Concerning the conditions required for the synthesis of each phase, several combinations of temperature and pressure were employed in order to find a stability range. It was possible to establish such a formation range for all phases. In some cases two phases were found for a given conditions and in many other cases three or more phases were found to coexist. Thus, the stability range of pressure and temperature for single phase formation turned out to be very narrow. By inspecting of some structural features, for instance the interatomic distances, it is found that the average of the Ge – Ge distances change in line with the composition, i.e. the shorter contacts belong to BaGe6 while the longer distances are present in BaGe3 (both modification). An opposite trend is observed for the calculated density of each phase (neglecting the tI32 form of BaGe3): the lower density is found for BaGe3 and the denser compound is found to be BaGe6. Of course this is not coincidence, since due to the Ge content, BaGe6 has the largest molar mass. Similarly, by examining the density as a function of the interatomic distance. In such case, the denser compound is characterized by shorter Ge – Ge contacts, while the less dense phase holds the longest Ge – Ge contacts. This is in agreement with the building motifs within each crystal structure: columns in BaGe3 (open framework) passing through layers in BaGe5, ending in a three-dimensional network (closed framework) in BaGe6.

The main focus of the present work was the Ge-rich part of the binary Ba – Ge system, in which by inspecting the behavior of the clathrate-I Ba8Ge43 under pressure, several new phases were found. The new phases in this system have the following compositions: BaGe3 (with two modifications), BaGe5, BaGe5.5 and BaGe6, therefore they are quite close in composition range: 75% - ~85% at. Ge. Concerning the conditions required for the synthesis of each phase, several combinations of temperature and pressure were employed in order to find a stability range. It was possible to establish such a formation range for all phases. In some cases two phases were found for a given conditions and in many other cases three or more phases were found to coexist. Thus, the stability range of pressure and temperature for single phase formation turned out to be very narrow. By inspecting of some structural features, for instance the interatomic distances, it is found that the average of the Ge – Ge distances change in line with the composition, i.e. the shorter contacts belong to BaGe6 while the longer distances are present in BaGe3 (both modification). An opposite trend is observed for the calculated density of each phase (neglecting the tI32 form of BaGe3): the lower density is found for BaGe3 and the denser compound is found to be BaGe6. Of course this is not coincidence, since due to the Ge content, BaGe6 has the largest molar mass. Similarly, by examining the density as a function of the interatomic distance. In such case, the denser compound is characterized by shorter Ge – Ge contacts, while the less dense phase holds the longest Ge – Ge contacts. This is in agreement with the building motifs within each crystal structure: columns in BaGe3 (open framework) passing through layers in BaGe5, ending in a three-dimensional network (closed framework) in BaGe6.

In dieser Dissertation wird die Darstellung tetrelreicher Seltenerdmetall–Verbindungen in den Systemen SE:Tt (SE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) und die Charakterisierung ihrer Eigenschaften beschrieben. Diese Verbindungen, welche mittels der Hochdruck–Hochtemperatur–Methode dargestellt wurden, zeigen neuartige Verknüpfungsmuster in der Tetrel–Partialstruktur. Neben der Charakterisierung der Verbindungen hinsichtlich der thermischen Stabilität und der physikalischen Eigenschaften bei Normaldruck wurde bei den Germanium–reichen Seltenerdmetall–Verbindungen eine Untersuchung der Veränderungen der Kristallstruktur bei Variation des Drucks oder der Temperatur vorgenommen. Die dargestellten Seltenerdmetall–Trisilicide SESi3 (SE = Gd, Ho, Lu) kristallisieren tetragonal isotyp zu YbSi3. LuSi3 zeigt Supraleitung mit Tc = 7.0 K. In den Systemen Gd–Si und SE–Ge (SE = La, Nd, Sm, Gd, Tb) wurden die Verbindungen GdSi5 und SEGe5 (SE = La, Nd, Sm, Gd, Tb) synthetisiert. Sie kristallisieren orthorhombisch isotyp zu LaGe5. Durch in–situ Röntgenbeugungsexperimente bei erhöhten Temperaturen kann die Existenz metastabiler Germanium–ärmerer Verbindungen SE2Ge9 (SE = Nd, Sm) nachgewiesen werden. Es handelt sich um Defektvarianten der Verbindungen SEGe5 (SE = Nd, Sm). Die strukturelle Verwandtschaft zum Aristotyp SEGe5 wird anhand einer Gruppe–Untergruppe–Beziehung aufgezeigt. Eine alternative Synthesemethode zur Darstellung tetrelreicher Verbindungen mit Gerüststrukturen bei extremen Reaktionsbedingungen stellt das Spark–Plasma–Sinterverfahren (SPS) dar. Die Darstellung der Clathratphase Ba8Ga16±xGe30∓x (x = 0, 1) erfolgte mittels SPS aus den Precursoren BaGa2±x (x = 0, 0.125) und Germanium. Die Untersuchungen der thermoelektrischen Eigenschaften zeigen, dass durch Variation der nominellen Zusammensetzung sowohl n–leitende als auch p–leitende Eigenschaften erhalten werden können. / In this thesis the synthesis of tetrel–rich rare–earth metal compounds in the systems RE:Tt (RE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) and the characterization of their properties is described. These compounds, synthesized by means of high–pressure high–temperature method, show new structural motifs in the tetrel partial structure. The compounds were characterized with respect to their thermal stability and their physical properties at ambient pressure. In addition, the changes in the crystal structure of the germanium-rich rare–earth metal compounds by variation of pressure or temperature were investigated. The synthesized rare–earth trisilicides SESi3 (SE = Gd, Ho, Lu) crystallize tetragonal, isotypic to YbSi3. LuSi3 is a superconductor with Tc = 7.0 K. In the systems Gd–Si and SE–Ge (SE = La, Nd, Sm, Gd, Tb) the compounds GdSi5 and SEGe5 (SE = La, Nd, Sm, Gd, Tb) were synthesized. They crystallize orthorhombic isotypic to LaGe5. Using in-situ high–temperature X-ray experiments the metastable germanium-poorer compounds SE2Ge9 (SE = Nd, Sm) could be observed. These compounds are defect variants of the pentagermanides SEGe5 (SE = Nd, Sm). The structural relationship to the aristotype SEGe5 is given via a group-subgroup relation. An alternative synthesis route for the preparation of tetrel–rich compounds with framework structures at extreme reaction conditions is the spark plasma sintering method (SPS). The clathrate phase Ba8Ga16±xGe30∓x (x = 0, 1) was synthesized from the precursors BaGa2±x (x = 0, 0.125) and germanium by means of SPS. The investigation of the thermoelectric properties shows, that through variation of the nominal composition both n-type and p-type conduction properties can be obtained.

Es werden Fragestellungen aus zwei aktuellen Problemkreisen der elektronisch hochkorrelierten Materialien untersucht. Dem unkonventionellen supraleitenden Zustand sowie dessen Wechselspiel mit magnetischen Effekten gelten die Arbeiten am Schwere-Fermionen-System UBe13 sowie der Dotierungsreihe (UTh)Be13. Sogenanntes Nicht-Fermiflüssigkeits-Verhalten steht im Zentrum der Untersuchungen an der Schwere-Fermionen-Verbindung CeNi2Ge2. Der Schwere-Fermionen-Supraleiter U1-xThxBe13 zeigt neben einem nichtmonotonen Verlauf der Übergangstemperatur in den supraleitenden Zustand Tc(x) einen zweiten Phasenübergang Tc2 &amp;lt; Tc im Konzentrationsbereich 0,019 &amp;lt; x &amp;lt; 0,0455. Als Ursache dieses Übergangs werden sowohl eine mit dem supraleitenden Zustand koexistierende magnetische Ordnung (Spindichtewelle) als auch eine Änderung des supraleitenden Zustands selber diskutiert. Hier konnte mittels dilatometrischer Untersuchungen gezeigt werden, dass der Phasenübergang bei Tc2 eine Vorläuferstruktur im Bereich x &amp;lt; 0,019 besitzt. Die aus diesem Ergebnis folgende Zuordnung charakteristischer Linien im T-x-Diagramm von U1-xThxBe13 schließt gängige Szenarien, die sich für T &amp;lt; Tc2 ausschließlich auf die Änderung des supraleitenden Zustands beziehen, aus. Manche Schwere-Fermionen-Verbindungen zeigen bis zu tiefsten Temperaturen keinen Übergang in einen kohärenten Fermiflüssigkeits-Zustand. Als Ursache dieses Nicht-Fermi-flüssigkeits-Verhaltens wird u.a die Ausbildung kritischer Spinfluktuationen diskutiert. Diese magnetischen Fluktuationen werden in Nähe eines quantenkritischen Punkts (QKP) erwartet, für den bei T=0 als Funktion eines Kontrollparameters ein magnetischer Phasenübergang auftritt. Die Vorhersagen des Konzepts eines &amp;quot;nearly antiferromagnetic Fermi liquid&amp;quot;, für die Temperaturabhängigkeiten verschiedener Messgrößen von Systemen nahe eines QKP können an der Verbindung CeNi2Ge2 überprüft werden. Während bei nicht allzu tiefen Temperaturen in der vorliegenden Arbeit eine Übereinstimmung mit den Vorhersagen gefunden wurde, muss die Anwendbarkeit des Konzepts für CeNi2Ge2 bei tiefsten Temperaturen in Frage gestellt werden.