<div>Inorganic metal chalcogenide materials are of great importance in the semiconducting field for various electronic applications such as photovoltaics, thermoelectrics, sensors, and many others. Compared to traditional vacuum processing routes, solution processing provides an alternate cost-effective route to synthesize these inorganic materials through its ease of synthesis and device fabrication, higher material utilization, mild processing conditions, and opportunity for roll-to-roll manufacturing. One such versatile solution chemistry involving a mixture of amine and thiol species has evolved in the past few years as a common solvent for various precursor dissolutions including metal salts, metal oxides, elemental metals, and chalcogens.</div><div><br></div><div>The amine-thiol solvent system has been used by various researchers for the fabrication of inorganic materials, but without the complete understanding of the chemistry involved in this system, utilizing its full potential, and overcoming any inherent limitations will be difficult. So, to identify the organometallic complexes and their reaction pathways, the precursor dissolutions in amine-thiol solutions, specifically for elemental metals like Cu, In and chalcogens like Se, Te were studied using X-ray absorption, nuclear magnetic resonance, infrared, and Raman spectroscopy along with electrospray ionization mass spectrometry techniques. These analyses suggested the formation of metal thiolate complexes in the solution with the release of hydrogen gas in the case of metal dissolutions confirming irreversibility of the dissolution. Insights gained for chalcogen dissolutions confirmed the formation of different species like monoatomic or polyatomic clusters when different amine-thiol pair is used for dissolution. Results from these analyses also identified the role of each component in the dissolution which allowed for tuning of the solutions by isolating the complexes to reduce their reactivity and corrosivity for commercial applications.</div><div><br></div><div>After identifying complexes in metal dissolution for Cu and In metals, the decomposition pathway for these complexes was studied using X-ray diffraction and gas chromatography mass spectrometry techniques which confirmed the formation of phase pure metal chalcogenide material with a release of volatile byproducts like hydrogen sulfide and thiirane. This allowed for the fabrication of impurity-free thin-film Cu(In,Ga)S2 material for use in photovoltaic applications. The film fabrication with reduced carbon impurity achieved using this solvent system yielded a preliminary promising efficiency beyond 12% for heavy alkali-free, low bandgap CuInSe2 material. Along with promising devices, by utilizing the understanding of the chalcogen complexation, a new method for CuInSe2 film fabrication was developed with the addition of selenide precursors and elemental selenium which enabled first-ever fabrication of a solution-processed CuInSe2 thin film with thickness above 2 μm and absence of any secondary fine-grain layer.</div><div><br></div><div>Along with thin-film fabrication, a room temperature synthesis route for lead chalcogenide materials (PbS, PbSe, PbTe) with controlled size, shape, crystallinity, and composition of nanoparticle self-assemblies was demonstrated. Micro-assemblies formed via this route, especially the ones with hollow-core morphology were subjected to a solution-based anion and cation exchange to introduced desired foreign elements suitable for improving the thermoelectric properties of the material. Adopting from traditional hot injection and heat up synthesis routes, a versatile synthesis procedure for various binary, ternary, and quaternary metal chalcogenide (sulfide and sulfoselenide) nanoparticles from elemental metals like Cu, Zn, Sn, In, Ga, and Se was developed. This new synthesis avoids the incorporation of impurities like O, Cl, I, Br arising from a traditional metal oxide, halide, acetate, or other similar metal salt precursors giving an opportunity for truly impurity-free colloidal metal chalcogenide nanoparticle synthesis.</div>

<p>A wide range of optoelectronic applications demand semiconductors optimized for purpose.</p> <p>My research focused on data-driven identification of ABX3 Halide perovskite compositions for optimum photovoltaic absorption in solar cells.</p> <p>I trained machine learning models on previously reported datasets of halide perovskite band gaps based on first principles computations performed at different fidelities.</p> <p>Using these, I identified mixtures of candidate constituents at the A, B or X sites of the perovskite supercell which leveraged how mixed perovskite band gaps deviate from the linear interpolations predicted by Vegard's law of mixing to obtain a selection of stable perovskites with band gaps in the ideal range of 1 to 2 eV for visible light spectrum absorption.</p> <p>These models predict the perovskite band gap using the composition and inherent elemental properties as descriptors.</p> <p>This enables accurate, high fidelity prediction and screening of the much larger chemical space from which the data samples were drawn.</p> <p><br></p> <p>I utilized a recently published density functional theory (DFT) dataset of more than 1300 perovskite band gaps from four different levels of theory, added to an experimental perovskite band gap dataset of \textasciitilde{}100 points, to train random forest regression (RFR), Gaussian process regression (GPR), and Sure Independence Screening and Sparsifying Operator (SISSO) regression models, with data fidelity added as one-hot encoded features.</p> <p>I found that RFR yields the best model with a band gap root mean square error of 0.12 eV on the total dataset and 0.15 eV on the experimental points.</p> <p>SISSO provided compound features and functions for direct prediction of band gap, but errors were larger than from RFR and GPR.</p> <p>Additional insights gained from Pearson correlation and Shapley additive explanation (SHAP) analysis of learned descriptors suggest the RFR models performed best because of (a) their focus on identifying and capturing relevant feature interactions and (b) their flexibility to represent nonlinear relationships between such interactions and the band gap.</p> <p>The best model was deployed for predicting experimental band gap of 37785 hypothetical compounds.</p> <p>Based on this, we identified 1251 stable compounds with band gap predicted to be between 1 and 2 eV at experimental accuracy, successfully narrowing the candidates to about 3% of the screened compositions.</p>

In dieser Arbeit wurde die atomare Struktur von Si(100)- und Ge(100)-Oberflächen untersucht, die mit metallorganischer chemischer Gasphasenabscheidung (MOCVD) für anschließende Heteroepitaxie von III-V-Halbleitern präpariert wurden. An der III-V/IV Grenzfläche werden atomare Doppelstufen auf der Substratoberfläche benötigt, um Antiphasenunordnung in den III-V-Schichten zu vermeiden. Die MOCVD-Prozessgasumgebung beeinflusst die Domänen- und Stufenbildung der Si- und Ge(100)-Oberfläche sehr stark. Deswegen wurden in situ Reflexions-Anisotropie-Spektroskopie (RAS) und Ultrahochvakuum-(UHV)-basierte oberflächensensitive Messmethoden verwendet, um die verschiedenen Oberflächen zu charakterisieren. In situ RAS ermöglicht die Identifizierung der Oberflächenstruktur und somit Kontrolle über die Oberflächenpräparation, insbesondere der Domänenbildung auf Si- und Ge(100). Beide Oberflächen wechselwirken stark mit dem H2-Prozessgas, was zu Monohydrid-Bedeckung während der Präparation führt und sogar zu Si-Abtrag während Präparation unter hohem H2-Druck. Die Erzeugung von Leerstellen auf den Terrassen bewirkt eine kinetisch bedingte Oberflächenstruktur, basierend auf Diffusion von Leerstellen und Atomen. Dadurch kommt es zu ungewöhnlichen DA-Doppelstufen auf verkippten Si(100)-Substraten während auf exakten Substraten ein schichtweiser Abtrag stattfindet. Unter niedrigem H2-Druck bildet sich eine energetisch bedingte Domänen- und Stufenstruktur. Während das H2-Prozessgas keinen direkten Einfluss auf die Stufen- und Domänenbildung von verkippten Ge(100)-Oberflächen zeigt, ist der Einfluss von Gruppe-V-Elemente entscheidend. Die As-terminierten Ge(100)-Oberflächen bilden eindomänige Oberflächen unterschiedlicher Dimerorientierung und Stufenstruktur abhängig von Temperatur und As-Quelle. Angebot von P an Ge(100)-Oberflächen durch Heizen in Tertiärbutylphosphin führt zu einer ungeordneten, P-terminierten Ge(100)-Oberfläche, die instabiler als die Ge(100):As-Oberfläche ist. / In this work, the atomic surface structure of Si(100) and Ge(100) surfaces prepared in metalorganic chemical vapor phase deposition (MOCVD) ambient was studied with regard to subsequent heteroepitaxy of III-V semiconductors. At the III-V/IV interface, double-layer steps on the substrate surface are required to avoid anti-phase disorder in the epitaxial film. The MOCVD process gas ambient strongly influences the domain and step formation of Si and Ge(100) surfaces. Therefore, in situ reflection anisotropy spectroscopy (RAS) and ultra-high vacuum-based (UHV) surface sensitive methods were applied to investigate the different surfaces. In situ RAS enabled identification of the surface structure and the crucial process steps, leading to complete control of Si and Ge(100) surface preparation. Both surfaces strongly interact with H2 process gas which leads to monohydride termination of the surfaces during preparation and Si removal during processing in high H2 pressure ambient. The generation of vacancies on the terraces induces a kinetically driven surface structure based on diffusion of vacancies and Si atoms leading to an energetically unexpected step structure on vicinal Si(100) substrates with DA-type double-layer steps, whereas Si layer-by-layer removal occurs on substrates with large terraces. Processing in low H2 pressure ambient leads to an energetically driven step and domain structure. In contrast, H2-annealed vicinal Ge(100) surfaces show no direct influence of the H2 ambient on the step structure. At the Ge(100) surface, group-V elements strongly influence step and domain formation. Ge(100):As surfaces form single domain surfaces with different majority domain and significantly different step structures depending on temperature and As source, respectively. In contrast, exposure to P by annealing in tertiarybutylphosphine leads to a very disordered P-terminated vicinal Ge(100) surface which is less stable compared to the Ge(100):As surfaces.