Doctor of Philosophy / Department of Chemistry / Sundeep Rayat / Research in our laboratory has focused on designing photoactivated DNA cleaving agents based on tetrazolethione scaffolds. The key step in the activation of these involves conversion of tetrazolethione moiety to carbodiimides upon irradiation. However, the mechanism of this reaction was not previously reported. Therefore, we undertook a study to elucidate the mechanism of photodecomposition of tetrazolethione as to identify reactive intermediates involved, that may interfere or aid with the activity of our synthesized DNA cleaving agents under physiological conditions. In Part 1 of this dissertation, we present mechanistic studies on this photodecomposition. Our results indicate the clean photoconversion of tetrazolethiones I to their respective carbodiimides IV via the expulsion of sulfur and dinitrogen. Photoirradiation in the presence of trapping agent (e.g. 1,4-cyclohexadiene) resulted into the formation of their corresponding thioureas. Thus, providing strong evidence for the intermediacy of a 1,3-biradical III, which is believed to be in its triplet spin multiplicity. Further investigations (triplet sensitization and quenching experiments) to determine the precursor of the biradical argued against the involvement of a triplet excited state (T[subscript]1). We believe that the mechanistic pathway that leads to the formation of a 1,3-triplet biradical III is a diradicaloid species II-II" generated directly from the singlet excited state of tetrazolethiones (S[subscript]1) after the expulsion of dinitrogen. Once formed, this diradicaloid species could be envisioned to undergo intersystem crossing to generate the 1,3 triplet biradical III which then undergoes desulfurization to form carbodiimides IV (Chapter 2). Bridgehead-nitrogen containing fused heterocycles are regarded as “privileged structure” in biology and have found widespread applications in pharmaceutical industry. These heterocycles have also been evaluated in electroluminescent devices and organic dyes. Part II of the dissertation present new, concise and low cost strategies to a unique class of bridgehead nitrogen-containing fused heterocyclic scaffolds which involves two sequential intramolecular cyclizations from heteroenyne-allenes in the presence of Lewis acids such as SnCl[subscript]4 and BF[subscript]3.OEt[subscript]2, and trace water. The starting heteroenyne-allenes VI can be prepared from commercially available substrates V in 4 – 5 steps following standard protocols (Chapter 3). Furthermore, we employed density functional theory to gain insights into the optoelectronic properties of select derivatives of phenanthridine-fused quinazoliniminiums (PNQs) VII and their free base in order to evaluate their scope in OLED technology. Our results show that the energies of the Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO), the HOMO-LUMO energy gaps, the ionization potentials, electron affinities and the reorganization energies can be finely tuned by varying the substituents on these chromophores. In addition, we found that the introduction of an electron donating group (NMe[subscript]2) on the PNQs and their free base increases the energies of the HOMOs and decreases the ionization potentials, relative to its unsubstituted derivative, whereas substitution by an electron withdrawing group (NO[subscript]2) decreases the energies of the LUMOs and increases the electron affinities which in turn suggests an improvement in their hole and electron creating abilities, respectively (Chapter 4).

Le silicium (Si) et le germanium (Ge) sont les matériaux de base utilisés dans les circuits intégrés. Cependant, à cause de leur gap indirect, ces matériaux ne sont pas adaptés à la fabrication de dispositifs d'émission de lumière, comme les lasers ou diodes électroluminescentes. Comparé au Si, le Ge pur possède des propriétés optiques uniques, à température ambiante son gap direct est de seulement 140 meV au-delà du gap indirect tandis qu'il est supérieur à 2 eV dans le cas du Si. Compte tenu du coefficient de dilatation thermique du Ge, deux fois plus grand que celui du Si, une croissance de Ge sur Si à hautes températures suivie d'un refroidissement à température ambiante permet de générer une contrainte en tension dans le Ge. Cependant, l'existence d'un désaccord de maille de 4,2% entre deux matériaux conduit à une croissance Stranski-Krastanov avec la formation des films rugueux et contenant de forte densité des dislocations. Nous avons mis en évidence l'existence d'une fenêtre de température de croissance, permettant de supprimer la croissance tridimensionnelle de Ge/Si. En combinant la croissance à haute température à des recuits thermiques par cycles, une contrainte de 0,30% a pu être obtenue. Le dopage de type n a été effectué en utilisant la décomposition de GaP, ce qui produit des molécules P2 ayant un coefficient de collage plus grand par rapport à celui des molécules P4. En particulier, en mettant en oeuvre la technique du co-dopage en utilisant le phosphore et l'antimoine, nous avons mis en évidence une augmentation de l'émission du gap direct du Ge à environ 150 fois, ce qui constitue l'un des meilleurs résultats obtenus jusqu'à présent. / Silicon (Si) and germanium (Ge) are the main materials used as active layers in microelectronic devices. However, due to their indirect band gap, they are not suitable for the fabrication of light emitting devices, such as lasers or electroluminescent diodes. Compared to Si, pure Ge displays unique optical properties, its direct bandgap is only 140 meV above the indirect one. As Ge has a thermal expansion coefficient twice larger than that of Si, tensile strain can be induced in the Ge layers when growing Ge on Si at high temperatures and subsequent cooling down to room temperature. However, due to the existence of a misfit as high as 4.2 % between two materials, the Ge growth on Si proceeds via the Stranski-Krastanov mode and the epitaxial Ge films exhibits a rough surface and a high density of dislocations. We have evidenced the existence of a narrow substrate temperature window, allowing suppressing the three-dimensional growth of Ge on Si. By combining high-temperature growth with cyclic annealing, we obtained a tensile strain up to 0.30 %. The n-doping in Ge was carried out using the decomposition of GaP to produce the P2 molecules, which have a higher sticking coefficient than the P4 molecules. In particular, by implementing a co-doping technique using phosphorus and antimony, we have evidenced an intensity enhancement of about 150 times of the Ge direct band gap emission. This result represents as one of the best results obtained up to now.

This master's thesis deals with the design of a differential high-gain transimpedance amplifier in TSMC's 0.18 um mixed signal process that utilizes a DC photodiode current cancellation loop and a switching automatic gain control (AGC) with a bilinear gain curve. The amplifier is designed to satisfy the demands of Optical Coherence Tomography applications where the receiver is expected to measure the envelope power of an amplitude modulated sinusoidal optical signal that incorporates a large DC component. Methods of increasing dynamic range and gain linearity through the use of DC photodiode current cancellation and bilinear gain are explored. Effects of changing DC photodiode current on the overall system response is also demonstrated.

This Thesis relates to the continually advancing field of microelectromechanical systems (MEMS). With MEMS technology, there are many different areas of concentration available for research. This Thesis addresses analysis and preliminary characterization of a cantilever-type MEMS chemical sensor for detection of chemicals and organic components operating at room temperature (20˚C and sea level pressure of 1 atm). Such sensors can be useful in a wide variety of applications. There currently exist several different types of MEMS chemical sensors. Each is based on a different detection method, e.g., capacitive, thermal, resistive, etc., and is used for specific tasks. Out of all currently available detection methods, the most common is the gravimetric method. The gravimetric sensor works by absorbing the chemical in a special material, usually a polymer, which alters the overall mass of the sensing element that can then be measured, or detected, to identify the chemical absorbed. One of the more exciting developments in the field of gravimetric chemical MEMS has been with the advancement of cantilever-type sensors. These cantilevers are small and usually on the order of only about 300 m in length. In order to utilize the gravimetric method, a cantilever is coated with a polymer that allows an analyte to bond to it and change its mass, which in turn changes the resonant frequency of the cantilever. The change in frequency can then be measured and analyzed and from it, the amount of absorbed mass can be calculated. Current research in the cantilever-type resonating sensors for the detection of hydrogen is developing measurement capabilities of 1 ppm (part-per-million). In this Thesis number of sample cantilevers were qualitatively assessed and their dimensional geometry measured. Based on these measurements, frequency data were obtained. In addition, the overall uncertainty in the resonant frequency results was calculated and the contributing factors to this uncertainty were investigated. Experimental methods that include laser vibrometry, optoelectronic laser interferometric microscopy (OELIM), and atomic force microscopy (AFM), were utilized to measure the frequency responses of the samples. The analytically predicted natural frequencies were compared to the experimental data to determine correlation subject to the uncertainty analysis. Parametric analyses involving chemical absorption processes were also conducted. Such analyses considered different parameters, e.g., damping and stiffness as well as changes in their values, to determine contributions they make to the quality of the frequency data and the effect they have on sensitivity of the MEMS cantilever-type chemical sensors. Once these parametric analyses were completed, it was possible to estimate the sensitivity of the cantilever, or the ability for the cantilever to detect frequency shifts due to absorption of the target chemical. Results of the parametric analyses of the fundamental resonant frequency were then correlated with the sensitivity results based on the chemical absorption. This Thesis correlates many results and ideas and probes problems revolving around the analysis and characterization of a MEMS cantilever-type chemical sensor.

This dissertation describes our study on different physical properties of passivated and chemically modified hybrid metal halide perovskite materials and development of highly efficient charge transport layers for perovskite solar cells. We first developed an efficient electron transport layer via modification of titanium dioxide nanostructure followed by a unique chemical treatment in order to have clean interface with fast electron injection form the absorber layer in the perovskite solar cells. We then explored monovalent cation doping of lead halide perovskites using sodium, copper and silver with similar ionic radii to lead to enhance structural and optoelectronic properties leading to higher photovoltaic performance of the resulting perovskite solar cells. We also performed thorough experimental characterizations together with modeling to further understand the chemical distribution and local structure of perovskite films upon monovalent cation doping. Then, we demonstrate a novel passivation approach in alloyed perovskite films to inhibit the ion segregation and parasitic non-radiative losses, which are key barriers against the continuous bandgap tunability and potential for high-performance of metal halide perovskites in device applications, by decorating the surfaces and grain boundaries with potassium halides. This leads to luminescence quantum yields approaching unity while maintaining high charge mobilities along with the inhibition of transient photo-induced ion migration processes even in mixed halide perovskites that otherwise show bandgap instabilities. We demonstrate a wide range of bandgaps stabilized against photo-induced ion migration, leading to solar cell power conversion efficiencies of 21.6% for a 1.56 eV absorber and 18.3% for a 1.78 eV absorber ideally suited for tandem solar cells. We then systematically compare the optoelectronic properties and moisture stability of the two developed passivation routes for alloyed perovskites with rubidium and potassium where the latter passivation route showed higher stability and loading capacity leading to achieve substantially higher photoluminescence quantum yield. Finally, we explored the possibility of singlet exciton fission between low bandgap perovskites and tetracene as the triplet sensitizer finding no significant energy transfer between the two. We then used tetracene as an efficient dopant-free hole transport layer providing clean interfaces with perovskite layer leading to high photoluminescence yield (e.g. ~18%). To enhance the poor ohmic contact between tetracene and the metal electrode, we added capping layer of a second hole transport layer which is extrinsically doped leading to 21.5% power conversion efficiency for the subsequent solar cells and stabilised power output over 550 hours continuous illumination.

Cette thèse concerne le développement de futurs appareils, systèmes et réseaux prenant en charge l’internet haute vitesse, sans fil 5éme g´enération (5G). La demande de débit très élevé nécessite une bande passante suffisante, et ainsi la bande de fréquence millimetrique (mm-wave) a beaucoup d’intérêt. Un certain nombre de technologies devront converger, coexister et interagir, et surtout, coopérer, si cette vision doit être efficace et rentable. Le concept principal de cette de 5G est l’intégration de réseaux de fibre optique et Les réseaux radio grâce à la technologie Radio-sur-Fibre (RoF) aux fréquences d’onde millimetriques, pour fournir des services à large bande passante et permettre des réseaux évolutifs et gérables sans structure d’interface très complexe et multiples protocoles superposés.Dans cette thèse, les systèmes de communication RoF à ondes millimetriques sont théoriquement étudiés et démontrés expérimentalement pour étudier les altérations du système. Le travail présenté dans cette thèse est axé sur le bruit optique représenté par le bruit de phase et d’intensité induit par la source optique et la dispersion chromatique introduite par la fibre optique. Le bruit optique est analysé et mesuré pour différentes techniques de génération optique. Deux dispositifs différents de conversion, un mélangeur et un détecteur d’enveloppe sont, appliqués pour le traitement du signal et pour décorréler la phase et le bruit d’intensité. Nous souhaitons souligner que cette étude et le modèle peuvent s’appliquer à tout type de système de génération optique hétérodyne et à toute gamme de fréquences. La corrélation entre les modes optiques en peigne à fréquence optique est examinée pour montrer l’impact de la dispersion chromatique. Cette thèse présente la distribution d’énergie des ondes millimetriques et son influence sur la portée des fibres et la façon dont l’effet de dispersion chromatique sur le réseau RoF depend des paramètres de dispersion. Ensuite, cette thèse démontre comment la décorrélation de la phase optique induite par la dispersion chromatique entraîne un bruit de partition de modes dans les réseaux de communication RoF à ondes millimétriques.Lors de la transmission de certains types de données sur le système, les résultats démontrent l’impact du bruit optique et de la dispersion chromatique sur le qualité du signal. Les résultats de simulation sont présentés et sont en très bon accord avec les résultats expérimentaux. La grandeur du vecteur d’erreur evaluée par en processus en ligne montre l’impact des altèrations du système sur les performances du système. Le débit de données et l’évolution du système présentée sont en conformité avec les normes de communication comme à ondes millimétriques. / This thesis is for the development of future devices, systems and networks supporting the 5th Generation (5G) high-speed wireless internet. The demand for very high bit rate requires a sufficient large bandwidth, and therefore Millimeter-Wave (mm-wave) frequency band has a lot of interest. Several number of technologies will need to converge, co-exist and interoperate, and most importantly, cooperate, if this vision is to be efficiently and cost-effectively realized. The main concept within this next 5G is the integration of optical fiber networks and radio networks through Radio-over-Fiber (RoF) technology at mm-wave frequencies, to provide high-bandwidth front/backhaul services and enable scalable and manageable networks without a highly complex interface structure and multiple overlaid protocols.In this thesis, the mm-wave RoF communication systems are theoretically studied and experimentally demonstrated to investigate the system impairments. The work presented in this thesis is focused on optical noise represented by phase and intensity noise induced by optical source and chromatic dispersion introduced by optical fiber. The optical noise is analyzed and measured for different optical generation techniques. Two different down-conversion stages, mixer and envelope detector, are applied for signal processing and to decorrelate phase and intensity noise. We would like to highlight that this study and the model can be applicable toany kind of optical heterodyne generation system and any frequency range. The correlation among optical modes in optical frequency comb is examined to show the impact of chromatic dispersion. This thesis also exhibits the mm-wave power distribution over fiber span and how the chromatic dispersion effect on the RoF network is modified by varying dispersion parameters. Then, this thesis demonstrates how the optical phase decorrelation induced by chromatic dispersion results in mode partition noise at mm-wave RoF communication networks.When transmitting some types of data over the system, the results demonstrate the impact of optical noise and chromatic dispersion on the signal quality. The simulation results are presented and are in very good agreement with experimental results. The error vector magnitudethrough online process shows the impact of the system impairments on the system performance. The data rate and system evolution are compliance with communication standards at mm-wave.

Efficiency droop is a critical issue for the Group III-nitride based light-emitting diodes (LEDs) to be competitive in the general lighting application. Carrier spill-over have been suggested as an origin of the efficiency droop, and an InAlN electron-blocking layer (EBL) is suggested as a replacement of the conventional AlGaN EBL for improved performance of LED. Optimum growth condition of InAlN layer was developed, and high quality InAlN layer was grown by using metalorganic chemical vapor deposition (MOCVD). A LED structure employing an InAlN EBL was grown and its efficiency droop performance was compared with a LED with an AlGaN EBL. Characterization results suggested that the InAlN EBL delivers more effective electron blocking over AlGaN EBL. Hole-injection performance of the InAlN EBL was examined by growing and testing a series of LEDs with different InAlN EBL thickness. Analysis results by using extended quantum efficiency model shows that further improvement in the performance of LED requires better hole-injection performance of the InAlN EBL. Advanced EBL structures such as strain-engineered InAlN EBL and compositionally-graded InAlN EBLs for the delivery of higher hole-injection efficiency were also grown and tested.

Jahresbericht des Instituts für Halbleiter- und Mikrosystemtechnik der Technischen Universität Dresden

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This dissertation includes two parallel topics: optical modeling of wavelength converters based on semiconductor optical amplifiers (SOA) and optical modeling for LEDs and solid state lighting. A steady-state numerical model of wavelength converters based on cross-gain SOAs is developed. In this model, a new model of the gain coefficient is applied. Each physical variable, such as the carrier density, gain coefficient, differential gain, and internal loss, spatially varies across the SOA cavity and is numerically calculated throughout the device. Increased accuracy over previous studies is achieved by including such spatial variations. This model predicts wavelength-dependent characteristics of a wavelength converter of the SOA in both large and small signal regimes. Some key performance factors of SOA wavelength converters. A hybrid method incorporating both guided wave optics and optical ray tracing is also developed to model LEDs and solid state lighting. This method can model either single wavelength or dual-wavelength LED structures with different die shapes and packages. The waveguide and diffraction optics are mainly used to model the near-field optics inside LED chips and its vicinity and to identify guided modes and leakage modes. Geometrical ray tracing is applied to model the far-field pattern and light interactions at different material interfaces, such as LED chip structures, LED package materials, and light scattering at those rough surfaces and textures. To improve LED light extraction efficiency, different LED die shapes and device structures can also be optimized using this method. New technologies for future research on SOAs and LEDs are also proposed.