@phdthesis{Mahmoodinezhad2022, author = {Mahmoodinezhad, Ali}, title = {Atomic layer deposition and characterization of metal oxide thin films}, doi = {10.26127/BTUOpen-6134}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-61343}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {This thesis describes low temperature growth of wide band gap metal oxide thin films deposited by thermal (T-) and plasma-enhanced (PE-) atomic layer deposition (ALD) techniques in which high quality materials are grown with atomic level precision. Metal oxides are extensively investigated due to their exceptional physical and chemical properties, including relatively wide band gap, high dielectric constant and high thermal stability. This variety of properties results in a wide range of different applications. Thin films of indium oxide (InOx), gallium oxide (GaOx), zinc oxide (ZnOx), and quaternary InOx/GaOx/ZnOx (IGZO), in addition to the well-known aluminum oxide (AlOx), and the catalyst cerium oxide (CeOx), have proven to be superior candidates for many applications; from microelectronics and optoelectronics to gas sensor devices. The demanding requirements of low-temperature deposition processes for thermal sensitive substrates, which include high layer homogeneity and conformality over large areas, makes ALD a pioneer deposition technique. Although many oxides have been grown by TALD and PEALD, the deposition of wide band gap oxides at low temperatures are rarely reported and/or being investigated. In this work, the deposition method of the individual binary oxide films and combining the respective binary processes into the developed super-cycle growth of quaternary compound have been investigated at relatively low-temperatures by TALD and PEALD. Besides, the growth characteristics and chemical properties of the deposited films were evaluated by in-situ and ex-situ characterization techniques such as spectroscopic ellipsometry (SE) and X-ray photoelectron spectroscopy (XPS), where the influence of ALD process parameters on the growth mechanism and films composition are discussed in detail for any potential applications.}, subject = {Thermal atomic layer deposition; Plasma-enhanced atomic layer deposition; Indium gallium zinc oxide; Aluminum oxide; Cerium oxide; Thermische Atomlagenabscheidung; Plasmaunterst{\"u}tzte Atomlagenabscheidung; IGZO; AlOx; CeOx; Tieftemperatur; Atomlagenabscheidung; Beschichtung; Metallschicht; D{\"u}nne Schicht; Aluminiumoxide; Ceroxide; Indiumoxide}, language = {en} } @phdthesis{Reichmann2022, author = {Reichmann, Felix}, title = {Germanium, Zinn und (Zink-) Galliumoxid f{\"u}r fortschrittliche Mikro- und Optoelektronik : Einblicke in die elektronische Struktur der Oberfl{\"a}che mit Photoemissionstechniken}, doi = {10.26127/BTUOpen-6208}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-62080}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {Historically, Ge is one of the oldest materials in the semiconductor industry and its (001) surface has been the subject of extensive investigations by photoelectron spectroscopy. I am going to challenge the predominant attribution of a semi-conducting nature of the Ge(001) surface in this thesis. My investigations reveal the presence of a Ge(001) surface state above the Fermi-level, occupied at room temperature. Employing time- and temperature-dependent angle-resolved photoelectron spectroscopy, I will demonstrate that the presence of this surface state is evidence for the conducting nature of the surface at room temperature. Sparked by the remarkable properties of the GeSn-alloy and a trend towards Ge-Sn-related multiquantum well fabrication, I investigate the surface electronic structure of Ge(001) after adsorption and incorporation of Sn. With an in-depth analysis of surface core-level shifts, I will extend the growth model of the Sn wetting layer formation by also detailing structural changes in the subsurface region. At the same time, the modifications of the electronic structure will be detailed, observing the removal of the Ge(001) surface states, the creation of a new, Sn-related surface state and the initial stages of the Schottky barrier formation. β-Ga₂O₃ a transparent semi-conducting oxide that has sparked a lot of interest over the last decade, because it offers an ultra-wide band gap and high break down voltage. However, due to its monoclinic crystal structure, device fabrication is rather challenging and researchers are already looking into alternative materials. One of these candidates is ϵ-Ga₂O₃ and this work presents a combined study by photoelectron spectroscopy and ab initio calculations of its electronic structure. (Hard) X-rays reveal the impact of photoelectron recoil and the absence of a band bending to the surface, while the dispersion of experimentally determined valence states compares favorably with the calculations based on hybrid density-functional theory. Another alternative to β-Ga₂O₃ be ZnGa₂O₄ and I will present an investigation on the electronic structure of its (100) surface. Due to the novelty of ZnGa₂O₄ single-crystals, I am first going to explore the preparation of a clean and well-ordered surface by standard in-situ sputtering and annealing. I will show that already low annealing temperatures induce Zn-deficiency, leading to non-stoichiometric surfaces, further exacerbated by sputtering. By changing the sputtering parameters and the annealing conditions, the preparation of a surface with sufficient quality for subsequent investigations will be demonstrated. The results by photoemission techniques compare favorably with the expectations from theory and allowing the first fundamental insights into the surface electronic structure.}, subject = {Angle-resolved photoemission spectroscopy; X-ray photoelectron spectroscopy; Germanium-tin; Zinc gallium oxide; Surface science; ARPES; Gallium-Zink-Oxid; XPS; Halbleiteroberfl{\"a}che; Bandstruktur; Elektronenstruktur; Germaniumverbindungen; Galliumverbindungen; ARPES; R{\"o}ntgen-Photoelektronenspektroskopie}, language = {en} } @phdthesis{Akhtar2022, author = {Akhtar, Fatima}, title = {Graphene synthesis under Si-CMOS compatible conditions}, doi = {10.26127/BTUOpen-5927}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-59270}, school = {BTU Cottbus - Senftenberg}, year = {2022}, abstract = {Due to the unique electronic band structure, graphene has opened the great potential to extend the functionality of a large variety of graphene-based devices in health and environment, energy storage, or various microelectronic applications, to mention a few. At this point, the implementation of graphene into Silicon (Si) semiconductor technology is strongly dependent on several key challenges. Among them, high-quality and wafer-scale graphene synthesis on CMOS compatible substrates is of the highest importance. Though large-area graphene can be achieved on substrates like copper, platinum, silicon carbide, or single-crystal Ni, however, high growth temperatures, unavailability of large scale, or contamination issues are the main drawbacks of their usage. In this PhD work, 8-inch scale graphene synthesis is attempted on alternative substrates such as epitaxial Germanium on Si and polycrystalline Nickel on Si. To achieve the growth of the highest quality of graphene, this work focuses on the investigations of various nucleation and growth mechanisms, substrate-graphene interfaces, effects of different substrate orientations, and detailed microscopic and macroscopic characterization of the grown films. Finally, it should also be stressed that the experiments in this work were carried out in a standard BiCMOS pilot-line, making this study unique, as its results might directly pave the way to further graphene integration and graphene-based device prototyping in mainstream Si technologies.}, subject = {Graphene; Chemical vapor deposition; Growth; Oxidation; Germanium; Nickel; Graphen; CVD; Wachstum; Oxidation; Graphen; Keimbildung; Wachstum; CVD-Verfahren; Nickel; Germanium}, language = {en} } @phdthesis{Yang2020, author = {Yang, Penghui}, title = {Impact of alkali treatments on the surface and interface properties of Chalcopyrite thin-film solar cell absorbers}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-51772}, school = {BTU Cottbus - Senftenberg}, year = {2020}, abstract = {This thesis focuses on the investigation and characterization of the surfaces and interfaces of chalcopyrite-based Cu(In,Ga)Se2 (CIGSe) thin film solar cells using various x-ray and electron spectroscopies. In particular, the impact of alkali post deposition treatments (PDT) on the chemical and electronic surface and interface structure of CdS/CIGSe absorbers is studied. The structure of "real world" CdS/CIGSe interfaces and how they are impacted by different alkali PDTs was investigated by a combination of different x-ray spectroscopies. The interface formation is characterized by studying sample sets with different CdS thicknesses. The chemical environment for indium and cadmium is revealed by deriving the modified Auger parameter α'(In) and α'(Cd) using the kinetic energy of most prominent Auger line together with the binding energy of the chosen core level. A more complex situation is found for CdS/CIGSe samples that underwent NaF+KF PDT, where a K-In-Se compound is initially present on top of the chalcopyrite absorber. The conversion of the K-In-Se type species into a Cd-In-(O,OH,S,Se) interface compound is recorded at short CBD-CdS deposition times. It appears the majority of K that is present at the surface of the NaF+KF PDT CIGSe absorber is dissolved in the CBD and partially re-deposited as K-O type species. The Cd/S ratio clearly deviates from the stoichiometry expected for CdS, and a Cd(O,OH,S)-like compound is likely formed. The electronic structure of CdS/CIGSe interface is similarly more complex for the NaF+KF PDT compared to the NaF PDT case, where only Cd(O,OH,S) buffer was formed. In an attempt to shed more light into this complex situation, the impact of evaporated alkali metals (K, Rb, Cs) on the surface structure of CIGSe was studied in-system by synchrotron-based hard x- ray photoelectron spectroscopy (HAXPES), aiming at understanding the underlying mechanism of the interfacial effect of alkalis on the performance of CIGSe devices. In the case of K deposition, two K species are observed by x-ray absorption near-edge structure (XANES) and HAXPES, one of which species disappears at high annealing temperature. Furthermore, three new In contributions (In-O and K-In-Se, metallic In species) can be observed after K evaporation. The evolution of chemical contribution supports the formation of a K-In-Se and Cu-poor CIGSe (1:3:5) bilayer structure that is similar to what was reported for "real world" NaF+KF PDTs. Deposition of heavy alkali metals (Rb, Cs) induced the formation of alkali selenide phases after alkali evaporation and during low temperature annealing. Similar chemical changes as seen for the K composition (i.e. presence of metallic In, In-O, and alkali-O) are observed. However, detailed analysis of the Alk/Se ratio and composition provide direct evidence for the formation of a Alk-(In)-Se and (Cu,Alk)(In, Ga)Se2 bilayer. The insights from these studies promise to provide crucial aid to fully exploit alkali pre-treatments in scientific and industrial CIGSe production, and will deliberate use of this means of surface/interface tailoring to push efficiencies even further.}, subject = {Alkali PDT; Surface; Interface; Grenzfl{\"a}chen; Alkali-Nachbehandlungen; Oberfl{\"a}chen; CIGSe; Solarzelle; Oberfl{\"a}che; Alkalien; Grenzfl{\"a}che}, language = {en} } @phdthesis{Franck2025, author = {Franck, Max}, title = {Synthesis of hexagonal boron nitride on CMOS-compatible substrates via chemical vapor deposition}, doi = {10.26127/BTUOpen-7192}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-71925}, school = {BTU Cottbus - Senftenberg}, year = {2025}, abstract = {About 20 years ago, the discovery of an easy method to exfoliate single atomic layers of graphene opened the door for the investigation of 2D materials. They often exhibit superior electrical and mechanical properties compared to their 3D counterparts, and in addition, entirely new physical properties emerge at these ultra-low thickness scales, making 2D materials a highly interesting topic of materials research. A large variety of 2D materials with various electrical properties have since been discovered: graphene is a conductor with an extremely high charge carrier mobility, several transition metal dichalcogenides are direct bandgap semiconductors with the bandgap value being dependent on the chemical composition, and hexagonal boron nitride (hBN) is an insulator. This means that, in principle, all the building blocks for 2D microelectronic devices are available, which would mark the ultimate miniaturization. In practice, this vision is far from being realized, as large-scale fabrication methods for 2D materials and their heterostructures are still under development. Hexagonal boron nitride has a large range of applications, as it plays a central role in 2D microelectronics: not only are dielectrics a crucial part of microelectronic devices, but hBN is also necessary to shield other 2D materials from contact with the 3D environment, which causes deterioration of their intrinsic properties. Further potential applications are deep UV optoelectronics, thermal neutron detection, and single-photon emitters for quantum communication. While synthesis of single-crystalline hBN monolayers and high-quality multilayers has been demonstrated on a 2" wafer scale, it relies on catalytic transition metals, such as Cu, as the growth substrates. Obviously, one layer does not a device make, and thus transfer of the grown hBN films is usually employed to fabricate full devices. Unfortunately, this leaves behind metal contaminations at concentrations that are incompatible with the stringent purity requirements in CMOS manufacturing lines. In order to facilitate integration of 2D materials into established CMOS technology, which is deemed the most promising route for their large-scale adoption, synthesis of hBN on CMOS-compatible substrates, such as Si, Ge, and dielectrics, is desirable. At the beginning of my work, almost no literature on this topic was available, especially not for Ge and Si substrates. In this thesis, a growth process for hBN on Ge(001) via high-vacuum chemical vapor deposition is developed. The influence of various process parameters on the morphology, structure, and crystalline quality of the grown films is studied, and the growth process is optimized towards ultra-thin, smooth, and high-quality hBN. Additionally, the efficacy of standard characterization methods is evaluated and alternatives are explored. Furthermore, the optimized growth process is applied to Si(001) and graphene growth substrates, also yielding high-quality hBN films.}, subject = {Chemical vapor deposition; Hexagonal boron nitride; 2D materials; Thin film deposition; Chemische Gasphasenabscheidung; Hexagonales Bornitrid; 2D-Materialien; D{\"u}nnschichtabscheidung; Zweidimensionales Material; D{\"u}nnschichttechnik; Bornitrid; Hexagonaler Kristall; CVD-Verfahren}, language = {en} } @phdthesis{Ganie2025, author = {Ganie, Umar Bashir}, title = {Thermal analysis of lithium niobate tantalate bulk mixed crystals}, doi = {10.26127/BTUOpen-6981}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-69813}, school = {BTU Cottbus - Senftenberg}, year = {2025}, abstract = {This study comprehensively investigated the thermal properties and phase behavior of lithium niobate-tantalate (𝐿𝑖𝑁𝑏𝑥𝑇𝑎1-𝑥𝑂3, LNT) solid solutions. The research used experimental techniques to explore the variation of the ferroelectric Curie temperature (Tc) as a function of composition. LNT single crystals were grown using the Czochralski method. The elemental composition of niobium (Nb) and tantalum (Ta) in these crystals was determined using X-ray fluorescence (XRF) analysis. Small, compositionally homogeneous samples were then selected for differential scanning calorimetry (DSC) measurements to determine specific heat capacities (Cp). The DSC measurements revealed a linear decrease in Tc with increasing Ta concentration in the LNT solid solution crystals. Additionally, the ferroelectric transition width was observed to be narrower in mixed crystals compared to pure LT. Differential Thermal Analysis (DTA) and crystal growth experiments were performed further to understand the phase behavior of LNT solid solutions. The heats of fusion for the end members, LN and LT, were measured using DTA, yielding values of 103 kJ/mol at 1531 K for LN and 289 kJ/mol at 1913 K for LT. These values were used as input parameters in a thermodynamic solution model implemented in the Factsage software. The solution model enabled the calculation of a phase diagram for LNT solid solutions, which was further optimized in the Calphad Factsage. Thermodynamic parameters for the Gibbs free energy of mixing of the solid solution were also generated. The resulting phase diagram showed good agreement with the experimental data. Additionally, the temperature-dependent thermal conductivity of pure LN, LT, and LNT solid solutions and selected doped LN and LT crystals (Mg, Zn) was investigated. Measurements were conducted across a temperature range from 300 K to 1300 K. The findings indicated that thermal conductivity increases with temperature, especially above 800 K, with a more pronounced effect in tantalum-rich solutions. The interplay of the Nb/Ta ratio and doping effects was particularly significant at high temperatures. These insights into the thermal conductivity of LNT and doped LN and LT crystals are crucial for optimizing growth conditions. Understanding thermal conductivity helps ensure homogeneous crystallization during the growth process, which remains a challenge in the production of LNT single crystals.}, subject = {Thermal conductivity; Lithium niobate tantalate; Mixed crystals; Crystal growth; Thermal analysis; Phase diagram; Mischkristalle; Kristallz{\"u}chtung; Thermische Analyse; Phasendiagramm; W{\"a}rmeleitf{\"a}higkeit; Tantalate; Lithium; Niob; Mischkristall; Phasendiagramm; Temperaturabh{\"a}ngigkeit; Phasengleichgewicht}, language = {en} }