Dans le cadre d'une approche bottom-up, la fabrication de nanofils par une croissance catalysée ouvre la voie à nombres d'applications: nano--transistors verticaux à grille enrobantes, heterostructures cœur--coquilles... Avec ces nouveaux objets, de nouvelles interrogations apparaissent quant à l'influence du catalyseur et de la surface sur les propriétés électroniques des nanofils. Mon travail basé sur une étude spectroscopique via des expériences de photoluminescence a mis en évidence le rôle prépondérant de la surface sur les propriétés électroniques des nanofils. La passivation des états de surface a permis d'observer la recombinaison radiative des porteurs libres d'une phase dense : le liquide électron-trou, dans des nanofils catalysés par de l'or et du cuivre. Cette phase liquide a la particularité d'être stable thermodynamiquement et sa densité est constante. Cette propriété unique dans les semiconducteurs a conduit à l'étude quantitative de l'influence de la surface via la modification du ratio surface/volume. Une méthode originale de mesure de la vitesse de recombinaison de surface (VRS) a ainsi été développée et des VRS relativement faibles ont été mesurées indiquant une excellente passivation des états de surface. Les propriétés de volume de nanofils catalysés 'or' sont très similaires à celles d'un silicium massif utilisé en micro-électronique. Enfin, l'oxydation sacrificielle du silicium a permis d'obtenir des nanofils de diamètre inférieur à 10 nm. L'oxydation progressive des nanofils a permis d'observer un décalage de la raie vers le rouge attribué à la présence de contraintes, puis l'augmentation du gap est corrélée au confinement quantique des porteurs.

In this work, we have developed two different fabrication processes for relaxed Si₁₋xGex-on-insulator (SGOI) substrates: (1) SGOI fabrication by etch-back approach, and (2) by "smart-cut" approach utilizing hydrogen implantation. Etch-back approach produces SGOI substrate with less defects in SiGe film, but the SiGe film uniformity is inferior. "Smart-cut" approach has better control on the SiGe film thickness and uniformity, and is applicable to wider Ge content range of the SiGe film. We have also fabricated strained-Si n-MOSFET’s on SGOI substrates, in which epitaxial regrowth was used to produce the surface strained Si layer on relaxed SGOI substrate, followed by large-area n-MOSFET’s fabrication on this structure. The measured electron mobility shows significant enhancement (1.7 times) over both the universal mobility and that of co-processed bulk-Si MOSFET’s. This SGOI process has a low thermal budget and thus is compatible with a wide range of Ge contents in Si₁₋xGex layer. / Singapore-MIT Alliance (SMA)

Extending electronic devices beyond the limitations of current micro-electronics manufacturing will require detailed knowledge of how to make contacts to semiconductor surfaces. In this work, we investigated several methods by which such connections to silicon surfaces could be achieved. Scanning tunneling microscopy (STM) was our main experimental tool, allowing direct imaging of the surfaces at the atomic level. First, the growth of self-forming linear nanostructures of organic molecules on silicon surfaces offers a possibility of creating devices with hybrid organic-silicon functionality. We have studied the growth of many different molecules on a variety of hydrogen-terminated silicon surfaces: H-Si(100)-2x1, H-Si(100)-3x1, and H-Si(111)-1x1. We found molecular growth patterns affected by steric crowding, by sample doping level, or by exposure to ion-pump created radicals. We formed the first contiguous "L-shaped" molecular lines, and used an external electric field to direct molecular growth. We attempted to study a novel method for nanoscale information transfer along molecular lines based on excitation energy transfer. The second part of the work focuses on the development and use of a new multiple-probe STM instrument. The design and the custom STM control software written for it are described. Connections to Si surfaces were achieved with a combination of lithographically defined metal contacts and STM tips. Two-dimensional surface conductivity of the Si(111)-7x7 surface was measured, and the effect of modifying the surface with organic molecules was investigated. A novel method, scanning tunneling fractional current imaging (STFCI), was developed to further study surface conductance. This method allowed us to determine, for the first time, that the resistance of steps on the Si(111)-7x7 surface is significantly higher than that of the surface alone.

The development of functional flexible electronics is essential to enable applications such as conformal medical imagers, wearable health monitoring systems, and flexible light-weight displays. Intensive research on thin-film transistors (TFTs) is being conducted with the goal of producing high-performance devices for improved backplane electronics. However, there are many challenges regarding the performance of devices fabricated at low temperatures that are compatible with flexible plastic substrates. Prior work has reported on the change in TFT characteristics due to mechanical strain, with especially extensive data on the effect of strain on field-effect mobility. This thesis investigates the effect of gate-bias stress and elastic strain on the long-term stability of flexible low-temperature hydrogenated amorphous silicon (a-Si:H) TFTs, as the topic has yet to be explored systematically. An emphasis was placed on bias-stress measurements over time in order to obtain information on the physical mechanisms of instability. Drain current was measured over various intervals of time to track the degradation of devices due to metastability, and results were then compared across devices of various sizes under tensile, compressive, and zero strain. Transfer characteristics of the TFTs were also measured under the different conditions, to allow for extraction of parameters that would provide insight into the instability mechanisms. In addition to parameter extraction, the degradation and recovery of TFT output current was quantitatively compared for various bias-stress times across the different levels of strain. Finally, the instability mechanisms are modelled with a Markov system to further examine the effect of strain on long-term TFT operation. From the analysis of results, it was found that shallow charge trapping in the dielectric is the main mechanism of instability for short bias stress times, and did not seem to be greatly affected by strain. For longer bias stress times of over 10000 seconds, defect creation in the a-Si:H becomes a more significant contributor to instability. Both tension and compression increased defect creation compared to TFTs with zero applied strain. Compression appeared to cause the greatest increase in the rate of defect formation, likely by weakening Si-Si bonds in the a-Si:H. Tension appeared to cause a less significant increase, possibly due to a strengthening of some proportion of the Si-Si bonds caused by the slight elongation of bond length or because the applied tension relieves intrinsic compressive stress in a-Si:H film. A longer conduction path and greater dielectric area appears to increase the bias-stress and strain-related effects. Therefore reducing device size should increase the reliability of flexible TFTs.

Compared to wafer based solar cells, thin film solar cells greatly reduce material cost and thermal budget due to low temperature process. Monolithically manufacturing allows large area fabrication and continuous processing. In this work, several photovoltaic thin films have been developed by rf-PECVD including a-Si:H and μc-Si, both intrinsic and doped on Corning 4 inch glass substrate at low temperature. The conductivity of n type and p type μc-Si at 180ºC was 17S/cm and 7.1E-2S/cm, respectively. B dopants either in a-Si:H or μc-Si films require higher plasma power to get active doping. The B2H6-to-SiH4 flow ratio for p type μc-Si lies from 0.01 to 0.025. Chamber conditions have critical effect on film quality. Repeatable and superior results require a well-established cleaning passivation procedure. Moreover, μc-Si films have been deposited from pure silane on glass substrate by modified rf-ICP-CVD. The deposition rate has been dramatically increased to 5Å/s due to little H2 dilution with crystalline fraction was around 69%, and 6.2Å/s with crystalline fraction 45%. Microstructure started to form at 150ºC with a thin incubation layer on the glass substrate, and became fully dense conical conglomerates around 300nm where conductivity and crystallinity saturated. Additionally, a-SiGe:H films have been developed by modified rf-ICP-CVD. The optical band gaps have been varied from 1.25 to 1.63eV by changing SiH4-to-GeH4 ratio. Also high temperature resulted in low bandgap. Cross-section TEM showed some microcrystllites appeared near interface region. Heterojunction solar cells on p type c-Si wafer have been fabricated using films developed in this thesis. Interference fringes in EQE disappeared on either textured substrate or cells with lift-off contacts. Maximum EQE was 87% around 700nm. I-V curves have also been studied where the interesting kink suggests a counter-diode has formed between emitter region and contacts.

In this master’s thesis, a turbine model and a common control structure for theturbocharger for SI and CI-engines is developed. To design the control structure,simulations are done on an existing diesel engine model with VGT. In order tobe able to make simulations for engines with a wastegated turbine, the model isextended to include mass flow and turbine efficiency for that configuration. Thedeveloped model has a mean absolute relative error of 3.6 % for the turbine massflow and 7.4 % for the turbine efficiency. The aim was to control the intake manifoldpressure with good transients and to use the same control structure for VGTand wastegate. By using a common structure, development and calibration timecan be reduced. The non-linearities have been reduced by using an inverted turbinemodel in the control structure, which consists of a PI-controller with feedforward.The controller can be tuned to give a fast response for CI engines and a slowerresponse but with less overshoot for SI engines, which is preferable.

Metal-oxide-semiconductor field-effect transistors (MOSFETs) are major devices inintegrated circuit, extensively used in various electronic products. In order to improve the electrical characteristics, scaling channel width and length, using high-£e gate dielectric insulator, and strained silicon may be utilized to increase the driving current and circuit speed. Nevertheless, the scaling of the channel width and length must overcome the limitation of the photolithographytechnology and cost. Once the method is employed, the MOSFETs will face a serious short-channel effect and gate leakage current. In the aspect of high-£e gate dielectric insulator, there still have problems, containing the trap states, phonon scattering, dipole-induced threshold voltage variation, needed to be solved. This dissertation focuses on the properties of MOSFETs experienced an external-mechanical strain, where the channel will be strained. Hence, the mobility, driving current, and circuit speed will increase. Our research can be divided into three topics: fabricating process-induced strained Si, external mechanical stress-induced strained Si, and the properties of strained Si MOSFETs at different temperatures. Except the electrical measurement, we also used the ISE-TCAD to simulate the electrical characteristic of MOSFETs under stress. Firstly, we apply the stress on n-MOSFETs by utilizing the nitride-capping layer. Once the lattice is strained, the mobility will increase, hence resulting in the operating speed. Secondly, the electrical characteristics under external stress is explored by introduced the external mechanical stress along the channel length of nMOSFETs. In addition to the fabricating process-induced strain, the fabricating process condition will also influence the device characteristics. As a result, we propose a new strain technology for our following research. Thirdly, the device performance of strained Si under different temperatures is investigated. Finally, we discuss the gate leakage current in strained Si depending on the ultra-thin gate oxide layer.

We reported a non-crosslinking approach to synthesize nitroxide radical polymer brushes for thin-film electrodes via surface-initiated atom transfer radical polymeization (SI-ATRP), which was effective to yield the organic radical polymer brushes with high grafting density and to attain a uniform surface. As mentioned above, the covalent bonding of nitroxide polymer brushes to the conducting substrate not only prevented the polymer dissolution into organic electrolyte solution but improved the cycle life performance of batteries. Moreover, they can be the potential application in microbatteries by using microcontact printing to produce the patterned nitroxide polymer brushes on a conducting substrate. Even though the organic radical polymer brushes provided a new approach to syn-thesize thin-film electrodes, they still existed many problems that needed to study and to figure out. We discussed the morphology and electrochemical performance about ni-troxide radical polymer in the thesis. In the measurement of surface properties, we used the contact angle, electron spectroscopy for chemical analysis (ESCA) and atomic force microscopic (AFM) to proceed. Another, in the measurement of electrochemical analysis, we used the cyclic voltammetry(CV), alternating current (AC) impedance and charge-discharge to understand the regarding mechanism in this polymer layer during the electrochemical reaction. In chapter 4, we discussed the oxidative problem in the polymer brushes. It should be well controlled during the oxidation reaction, because the oxidation level may affect the diffusion of electron that resulted the capacity better or not. In chapter5, we controlled the density of polymer brushes to construct the possible mechanism during the electro-chemical reaction, and found out the possible factors that affected the electrochemistry. In chapter 6, we applied the better results from the front chapter to the organic radical battery, and compared their electrical performance.

The existence of polymer constituent in piezoelectric fiber composites (PFCs) could lead to significant viscoelastic behaviors, affecting overall performance of PFCs. High mechanical and electrical stimuli often generate significant amount of heat, increasing temperatures of the PFCs. At elevated temperatures, most materials, especially polymers show pronounced time-dependent behaviors. Predicting time-dependent responses of the PFCs becomes important to improve reliability in using PFCs. We study overall performance of PFCs having unidirectional piezoceramic fibers, such as PZT fibers, dispersed in viscoelastic polymer matrix. Two types of PFCs are studied, which are active fiber composites (AFCs) and macro fiber composites (MFCs). AFCs and MFCs consist of unidirectional PZT fibers dispersed in epoxy placed between two interdigitated electrode and kapton layers. The AFCs have a circular fiber cross-section while the MFCs have a square fiber cross-section. Finite element (FE) models of representative volume elements (RVEs) of active PFCs, having square and circular fiber cross-sections, are generated for composites with 20, 40, and 60 percent fiber contents. Two FE micromechanical models having one fiber embedded in epoxy matrix and five fibers placed in epoxy matrix are considered. A continuum 3D piezoelectric element in ABAQUS FE is used. A general time-integral function is applied for the mechanical, electrical, and piezoelectric properties in order to incorporate the time-dependent effect and histories of loadings. The effective properties of PZT-5A/epoxy and PZT-7A/LaRC-SI piezocomposites determined from the FE micromechanical models are compared to available experimental data and analytical solutions in the literature. Furthermore, the effect of viscoelastic behaviors of the LaRC-SI matrix at an elevated temperature on the overall electro-mechanical and piezoelectric constants are examined.

DC and RF magnetron co-sputtering techniques are one of the most suitable techniques in fabrication of thin films with different compositions. In this work, Al2O3 and Si/Al2O3 thin films were fabricated by using magnetron co-sputtering technique. For Al2O3 films, the stoichiometric, optical and crystallographic analyses were performed. For Si contained Al2O3 films, the formation conditions of Si nanocrystals were investigated. To do so, these thin films were sputtered on Si (100) substrates. Post annealing was done in order to clarify the evolution of Al2O3 matrix and Si nanocrystals at different temperatures. Crystallographic properties and size of the nanocrystals were investigated by X-ray diffraction (XRD) method. The variation of the atomic concentrations and bond formations were investigated with X-ray photoelectron spectroscopy (XPS). The luminescent behaviors of Si nanocrystals and Al2O3 matrix were investigated with photoluminescence (PL) spectroscopy. Finally, the characteristic emissions from the matrix and the nanocrystals were separately identified.