Three-dimensional integrated circuits (3D-IC) have emerged as promising candidates to overcome the interconnect bottlenecks of nanometer scale designs while also helping to reduce wire delay and increase memory throughput. While this technology offers many potential advantages, it also produces large thermal mismatch stress in 3D-IC structures employing Through-Silicon-Via (TSV). The stress distribution in silicon and interconnect is affected by the via diameter and layout geometry. TSV-induced stress effects on electron/hole mobility and device performance will be studied for the widely used 6-transistor (6T) SRAM cell. Simulation results in this study show that static noise margin (SNM), Read Margin (RM) and write margin (WM) tend to increase with decreasing electron mobility or increasing hole mobility. Considering TSV-induced stress, we propose that for practical layouts of TSV-based 3D-IC, p-type substrates should be placed further away from TSVs or closer to the smaller TSVs if multiple TSVs exist. / text

Field-effect transistors (FET) based on graphene as channel has extraordinaryproperties in terms of charge mobility, charge carrier density etc. However, there aremany challenges to graphene based FET due to the fact graphene is a monolayer ofatoms in 2-dimentional space that is strongly influenced by the operating conditions.One issue is that the Dirac point, or K-point, shifts to higher gate voltage whengraphene is exposed to atmosphere. In this study graphene field-effect transistors(GFET) based on commercially available CVD graphene are electrically characterizedthrough field effect gated measurements. The Dirac point is initially unobservable andlocated at higher gate voltages (>+42 V), indicating high p-doping in graphene.Different treatments are tried to enhance the properties of GFET devices, such astransconductance, mobility and a decrease of the Dirac point to lower voltages, thatincludes current annealing, vacuum annealing, hot plate annealing, ionized water bathand UV-ozone cleaning. Vacuum annealing and annealing on a hot plate affect thegated response; they might have decreased the overall p-doping, but also introducedDirac points and non-linear features. These are thought to be explained by localp-doping of the graphene under the electrodes. Thus the Dirac point of CVDgraphene is still at higher gate voltages. Finally, the charge carrier mobility decreasedin all treatments except current – and hot plate annealing, and it is also observed that charge carrier mobilities after fabrication are lower than the manufacturer estimatesfor raw graphene on SiO2/Si substrate.

My master thesis is focused on design and realization fully automated system which will be used for the characterization of the organic FET structure, based on DPP derivatives with follow optimization of the characterization process. Program „MeasFET“ has been created at the LabVIEW surroundings that drives hardware gadgets was build-up during last year. Furthermore, initial tests were taken for optimalization process of charge carrier mobility measurements in derivatives DPP.

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Konjuguotieji polimerai kaip funkcinės medžiagos gali būti panaudoti įvairiuose prietaisuose: organiniuose šviestukuose, organiniuose lauko tranzistoriuose, organiniuose saulės elementuose. Šio darbo tikslas - nustatyti fotogeneruotų krūvininkų pernašos dėsningumus π – konjuguotuose polimeruose panaudojant naują žadinimo-zondavimo metodą pagrįstą išoriniu elektriniu lauku indukuota antrosios optinės harmonikos generacija. Pagrindinis dėmesys buvo skiriamas pernašos dinamikai. Molekulinių darinių fizikos laboratorijoje buvo įrengta matavimų schema ir įvertintas metodo tinkamumas krūvio pernašos tyrimams. Buvo atlikti krūvio pernašos matavimai trijuose skirtinguose konjuguotuosiuose polimeruose. Nustatyta, kad fotogeneruotų krūvininkų judris tuoj po sužadinimo yra daug didesnis lyginant su stacionaria judrio verte, o krūvio pernašos dinamiką lemia konjuguoto polimero struktūrinė hierarchija, krūvininkų judėjimas yra daugialypis, susidedantis iš greito judėjimo viena polimero grandine ar konjuguotais polimero grandinės segmentais ir lėto šokavimo tarp atskirų polimero grandinių Pirmą kartą detaliai išnagrinėta šviesa sugeneruotų krūvininkų pernašos dinamika konjuguotuose polimeruose. Darbo rezultatai suteikia žinių apie fundamentalius krūvininkų pernašos mechanizmus konjuguotuose polimeruose, kurios gali būti panaudotos kuriant organinius elektronikos prietaisus. / Conjugated polymers are promising candidates for applications in all kinds of organic optoelectronic devices: OLEDs, organic field-effect transistors (OFETs) and organic photovoltaic cells. The main goal of this work was to investigate transport features of photogenerated electrical charge in pi-conjugated polymers by means of novel technique based on time-resolved electric field-induced second harmonic generation (TREFISH). TREFISH measurement setup was implemented in the laboratory of Molecular compounds physics, and applicability of the method has been verified. Measurements were performed on three different model polymers: methyl substituted ladder-type poly(para-phenylene) (MeLPPP), poly(fluorene-co-benzothiadiazole) (F8BT) and poly(spirobifluorene-co-benzothiadiazole) (PSF-BT), having different morphological and chemical structure. It has been found that motion of photogenerated charge carriers in π-conjugated polymer films experiences rapid dynamics after excitation. Different time domains of charge transport were distinguished. Initial fast transport of photogenerated charge carriers corresponds to the carrier motion along the single polymer chain or conjugated segment of the polymer chain. Slowest carrier motion phase is well described by the stochastic drift, which is attributed to interchain jumps and determines the macroscopic equilibrium mobility. Thus, the equilibrium mobility value is not applicable to the transport on nanometer scale up to tens of nanometers... [to full text]

One of the major challenges in the field of organic semiconductors is to develop molecular design rules and processing routes which optimise the charge carrier mobility, whilst independently controlling the radiative and non-radiative processes. To date there has existed a seeming trade-off between charge carrier mobility and photoluminescence efficiency, which limits the development of some devices such as electrically pumped laser diodes. This thesis investigates fluorescence enhancement strategies for high-mobility polymer semiconductor systems and the mechanisms by which they currently display poor emission properties. Four independent approaches were taken and are detailed as follows. 1. Solubilising chain engineering It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole), the addition of a phenyl- initiated side chain can enhance the solid-state fluorescence quantum yield, exciton lifetime and exciton diffusion length significantly in comparison to that without phenyl-addition. 2. Energy transfer to a highly fluorescent chromophore It is shown that for the high mobility polymer poly(indacenodithiophene-co-benzothiadiazole) efficient energy transfer to a more emissive squaraine dye molecule is possible despite fast non-radiative decay short exciton diffusion lengths. This results in a significant fluorescence enhancement, which in turn facilitates an order of magnitude increase of the efficiency of polymer light emitting diodes made from this material combination. 3. Energy gap engineering The well known Energy Gap Law predicts an increase in the non-radiative rate as the optical bandgap of an organic chromophore decreases in energy. In combination with this, almost all polymer semiconductors reported to date with high charge carrier mobility have low optical bandgaps. Therefore, molecular design principles which act to increase the optical bandgap of polymer semiconductors whilst retaining a high mobility were sought out. One specific system was successfully identified and showed a significant fluorescence enhancement compared to is predecessor poly(indacenodithiophene-co-benzothiadiazole) in both the solution and the solid state. It is found that the Frenkel exciton lifetime in this new system is a factor of four larger which also results in a significantly increased exciton diffusion length. An inter-chain electronic state is also identified and discussed. 4. Hydrogen substitution For some low-bandgap material systems such as erbium chromophores, high energy vibrational modes such as the C-H stretching mode can act as non-radiative pathways. The effect of hydrogen substitution with deuterium and fluorine was therefore investigated in a series of polythiophene derivative families. It was found that in the solid state, fluorescence and exciton lifetime enhancement occurred when the backbone hydrogen atoms were replaced with fluorine. However, evidence is given that this was not owing to the initial hypothesis, and is more likely owing to structural differences which occur in these substituted material systems.

The importance of studying the physico-chemical properties of nano-dimensional materials has gained significant attention in the fields of semiconductors, pharmaceuticals, materials science, and atmospheric chemistry owing to the differences in physical properties between macro- and nano-dimensional solids. Nonetheless, direct studies of physical properties of materials at nanoscale is limited in part due to their inherent size constraints and experimental limitations. However, development of atomic force microscopy (AFM) led to the implementation of methods to characterize a wide range of physical properties, including – but not limited to – mechanical properties, electrical properties, viscoelastic properties, and surface tension. Herein, the dissertation focuses on AFM-based method development for characterization of atmospheric particles as well as understanding the relationship between structure and physical properties of organic solids at both macro- and nano-dimensions. In the atmospheric chemistry realm, the combined aerosol effect on the climate and environment has significant uncertainty in part due to lack of direct characterization of their physico-chemical properties. The difficulty in assessing the physical and chemical properties arises due to the presence of diversified aerosol sources, which in turn influences the size, morphology, phase states and chemical compositions. Sea spray aerosols (SSAs) are the second-largest source of aerosols in the atmosphere. Studying SSAs – especially in submicrometer-dimensions – requires high-resolution microscopy techniques such as AFM. AFM can be used for imaging of individual aerosols, quantifying organic volume fraction for core-shell morphologies, measuring water uptake, quantifying surface tension of individual droplets, and measuring mechanical and viscoelastic properties of materials. Herein, we employed AFM-based morphology and force spectroscopy studies to correlate the 3D morphology, phase state, and viscoelastic properties of selected single-component chemical systems found in sea spray aerosol (SSA). We established a quantitative framework toward differentiation of the solid, semisolid and liquid phase states of individual particles by utilizing both relative indentation depth (RID) and viscoelastic response distance (VRD) data obtained from the force−distance plots. Moreover, we established a semi-quantitative and quick phase assessment by measuring the aspect ratio (AR) that refers the extent of particle spreading as a result of impaction. Overall, the established AFM-based quantitative and semi-quantitative phase identification method can be utilized to assess the phases of aerosols irrespective of chemical identity. Next, we investigated the factors that may control the electrical and mechanical properties of pharmaceutical and organic semiconducting materials in nano- and macro-dimensions. Understanding the structure-property relationship of materials, especially in the nano-dimension, is necessary for proper drug design and development of organic semiconducting materials. In this context, cocrystals provide a means to modulate the physico-chemical properties of organic solids. For example, the modulation of the mechanical properties is important in the pharmaceutical industry for improving the tabletability. The mechanical properties may be affected by packing arrangement, interaction strength and type, and atomic and chemical composition. Herein, we report the influence of alkane and alkene functional groups on the mechanical properties of organic solids based on salicylic acid (SA). The approach affords both isostructural and polymorphic solids. The isostructural alkane functional solid exhibits a two-fold larger Young’s modulus (YM) compared to the cocrystal with the alkene, where the YM refers to the stiffness of the material. Here, the higher YM values are attributed to the presence of a bifurcated weak C-H···O interactions involving the alkane and neighboring SA molecules. On the other hand, in the case of alkene polymorphisms, molecular packing with column arrangement shows higher YM values compared to the herringbone arrangements. Thus, functional groups and crystal arrangements influence the stiffness of the solid organic cocrystals. Moreover, we report the modulation of mechanical properties of salicylic acid (SA) through cocrystallization by variation of propane and butane functionality with bipyridine coformers. We show that the variation of propane and butane functionality in bipyridine coformer with salicylic acid leads to synthesis of cocrystal and salt-cocrystal, respectively. The AFM nanoindentation study revealed that the Young’s modulus values follow the order salicylic acid < cocrystal << salt-cocrystal. The highest Young’s modulus values of the salt-cocrystal, among the studied systems, are attributed to the presence of strong N+–H···O– and O–H···O– interactions. On the other hand, higher Young’s modulus values of the propane-based cocrystal compared to the macro-dimensional salicylic acid are attributed to the stronger O–H ···N hydrogen bonding. Thus, homologous alkane functional groups can influence the mechanical properties of the organic solid crystals. Additionally, in situ solid-solid polymorphic phase transformation and nucleation of a metastable and elusive polymorph of SA cocrystals in combination with 4,4’-bipyridine were studied. Understanding the solid-solid phase transformations and nucleation mechanisms are important for proper control over the parameters associated with the synthesis of targeted crystalline solids with desired crystal structure. Using in situ powder X-ray diffraction (PXRD) and terahertz time domain spectroscopy (THz-TDS) data we showed that the Form II polymorph transforms to Form I over time. AFM imaging and nanoindentation techniques were utilized to follow and quantify in real-time the solid-solid polymorphic transformation of the metastable Form II to the thermodynamically stable Form I on a single crystal basis. AFM in situ single crystal data revealed that the metastable Form II has a rod-shaped morphology and relatively high elasticity (Young’s modulus), which transforms to prism-shaped nanocrystals of much smaller sizes with significantly reduced elasticity. The AFM imaging reveals that the single crystals on the order of 80-150 nm to undergo catastrophic changes in morphology that are consistent with cracking and popping owing to a release of mechanical stress during the transformation. The nucleation mechanism for the polymorphic transformation is not spatially localized and occurs over the entire crystal surface. The higher mechanical properties of the metastable Form II is due to the presence of the additional interlayer C-H···O interactions. Furthermore, we have studied the electrical properties of boron-based cocrystals. More specifically, cocrystallization of a nonconductive 2,4-difluorophenylboronic ester catechol adduct of a 4,4’-bipyridine (BEA) host with two aromatic semiconducting guests (pyrene and tetrathiafulvalene) generated conductive cocrystals with variable charge carrier mobilities. Charge carrier mobilities of the cocrystals with either pyrene or tetrathiafulvalene were measured using conducting probe AFM (CP-AFM). The incorporation of π-rich aromatic guests through face-to-face and edge-to-face π-contacts results in electrically conductive cocrystals. The cocrystal with tetrathiafulvalene as a guest shows approximately 7 times higher charge carrier mobility than the cocrystal with pyrene. Overall, the current dissertation demonstrates the AFM-based method development and applications towards materials characterization to measure the morphological, electrical, mechanical, and phase-states at both nano- and macro-dimensions. The high spatial precision of the methods developed enables us to better understand the controlling factors for materials design and processing across nano- and macro-dimensions.

Perylene-based materials, including charge-transport discotic liquid crystals and charge-transfer long-wavelength absorbing chromophores, for potential organic electronic and optoelectronic applications, were designed, synthesized and characterized. Two types of discotic liquid crystals, perylene diimides and coronene diimides, can form columnar liquid crystalline phases over a wide temperature range; many of them can have room-temperature liquid crystalline phases after cooling from isotropic liquid. Their charge transport properties were studied by space-charge limited current method; high charge carrier mobilities, with the highest being up to 6.6 cm2/Vs, were found in liquid crystalline phases of these materials under ambient conditions. Structural variables, including aromatic cores and side groups, were examined to get a certain degree of understanding of charge transport properties in these discotic liquid crystals. It was found that mesophase order can have an important effect on charge carrier mobilities. The discotic liquid crystals with high charge carrier mobilities are serious candidates for use in large-area low-cost applications such as solar cells. Long-wavelength, highly absorbing chromophores, featuring donor-substituted perylene diimides, were generated by a combination of charge-transfer process and conjugation extension. The charge-transfer chromophores are expected to lead to further investigation on their potentials as sensitizers in Grtzel solar cells.

In this research, the contents are divided into two sections. In the first section, we investigated carrier transport behavior of organic optoelectronic devices by using space charge limited current(SCLC) method. Firstly, we fabricated a hole-only device (ITO/Spiro-MeOTAD/Al) for Sprio-MeOTAD and the current density¡V voltage(J-V) characteristics of the device was measured. The J-V characteristics of the device do not match with SCLCs very well at high voltage since the number of hole injection was not enough to achieve SCLCs condition. To enhance the injection of hole carrier into the organic layer, a MoO3 buffer layer was inserted between ITO electrode and organic layer. The current density in device with MoO3 buffer layer achieved 5 times enhancement, indicating that the concentration of hole in MoO3 device is increment. Hence, we succeeded in making the J-V characteristics of the hole-only device to match with SCLCs well at high voltage, and the hole mobility of Sprio-MeOTAD estimated by SCLCs was 1.44¡Ñ10-4cm2/Vs. Li salt was also doped into Sprio-MeOTAD as an n-type dopant. We found that Li salt could form hole-traps in Sprio-MeOTAD, which reduced hole carriers in Spiro-MeOTAD. The current density of the device was decreased, and the device could not achieve SCLCs condition at high voltage. In the second section, we use two novel iridium(Ir) complexes to fabricate blue-green emitting devices by solution process. First, we obtained optimum concentration of phosphorescent emitters by controlling of the dopants concentration. Then, we adjusted the thickness of the electron injection layer, hole injection layer, and emission layer to design more suitable device structure. Finally, we succeeded in fabricating blue-green light emitting devices. The maxima luminescence was 37.7cd/m2 and maxima current efficiency was 1.68 cd/A in the Ir complex based devices.