Study of silicon-on-insulator multiple-gate MOS structures including band-gap engineering and self heating effects

The progresses of electron devices integration have proceeded for more than 40 years following the well–known Moore’s law, which states that the transistors density on chip doubles every 24 months. This trend has been possible due to the downsizing of the MOSFET dimensions (scaling); however, new issues and new challenges are arising, and the conventional ”bulk” architecture is becoming inadequate in order to face them. In order to overcome the limitations related to conventional structures, the researchers community is preparing different solutions, that need to be assessed. Possible solutions currently under scrutiny are represented by: • devices incorporating materials with properties different from those of silicon, for the channel and the source/drain regions; • new architectures as Silicon–On–Insulator (SOI) transistors: the body thickness of Ultra-Thin-Body SOI devices is a new design parameter, and it permits to keep under control Short–Channel–Effects without adopting high doping level in the channel. Among the solutions proposed in order to overcome the difficulties related to scaling, we can highlight heterojunctions at the channel edge, obtained by adopting for the source/drain regions materials with band–gap different from that of the channel material. This solution allows to increase the injection velocity of the particles travelling from the source into the channel, and therefore increase the performance of the transistor in terms of provided drain current. The first part of this thesis work addresses the use of heterojunctions in SOI transistors: chapter 3 outlines the basics of the heterojunctions theory and the adoption of such approach in older technologies as the heterojunction–bipolar–transistors; moreover the modifications introduced in the Monte Carlo code in order to simulate conduction band discontinuities are described, and the simulations performed on unidimensional simplified structures in order to validate them as well. Chapter 4 presents the results obtained from the Monte Carlo simulations performed on double–gate SOI transistors featuring conduction band offsets between the source and drain regions and the channel. In particular, attention has been focused on the drain current and to internal quantities as inversion charge, potential energy and carrier velocities. Both graded and abrupt discontinuities have been considered. The scaling of devices dimensions and the adoption of innovative architectures have consequences on the power dissipation as well. In SOI technologies the channel is thermally insulated from the underlying substrate by a SiO2 buried–oxide layer; this SiO2 layer features a thermal conductivity that is two orders of magnitude lower than the silicon one, and it impedes the dissipation of the heat generated in the active region. Moreover, the thermal conductivity of thin semiconductor films is much lower than that of silicon bulk, due to phonon confinement and boundary scattering. All these aspects cause severe self–heating effects, that detrimentally impact the carrier mobility and therefore the saturation drive current for high–performance transistors; as a consequence, thermal device design is becoming a fundamental part of integrated circuit engineering. The second part of this thesis discusses the problem of self–heating in SOI transistors. Chapter 5 describes the causes of heat generation and dissipation in SOI devices, and it provides a brief overview on the methods that have been proposed in order to model these phenomena. In order to understand how this problem impacts the performance of different SOI architectures, three–dimensional electro–thermal simulations have been applied to the analysis of SHE in planar single and double–gate SOI transistors as well as FinFET, featuring the same isothermal electrical characteristics. In chapter 6 the same simulation approach is extensively employed to study the impact of SHE on the performance of a FinFET representative of the high–performance transistor of the 45 nm technology node. Its effects on the ON–current, the maximum temperatures reached inside the device and the thermal resistance associated to the device itself, as well as the dependence of SHE on the main geometrical parameters have been analyzed. Furthermore, the consequences on self–heating of technological solutions such as raised S/D extensions regions or reduction of fin height are explored as well. Finally, conclusions are drawn in chapter 7.

The gender wage gap in developing and transition countries

The aim of my dissertation is to study the gender wage gap with a specific focus on developing and transition countries. In the first chapter I present the main existing theories proposed to analyse the gender wage gap and I review the empirical literature on the gender wage gap in developing and transition countries and its main findings. Then, I discuss the overall empirical issues related to the estimation of the gender wage gap and the issues specific to developing and transition countries. The second chapter is an empirical analysis of the gender wage gap in a developing countries, the Union of Comoros, using data from the multidimensional household budget survey “Enquete integrale auprès des ménages” (EIM) run in 2004. The interest of my work is to provide a benchmark analysis for further studies on the situation of women in the Comorian labour market and to contribute to the literature on gender wage gap in Africa by making available more information on the dynamics and mechanism of the gender wage gap, given the limited interest on the topic in this area of the world. The third chapter is an applied analysis of the gender wage gap in a transition country, Poland, using data from the Labour Force Survey (LSF) collected for the years 1994 and 2004. I provide a detailed examination of how gender earning differentials have changed over the period starting from 1994 to a more advanced transition phase in 2004, when market elements have become much more important in the functioning of the Polish economy than in the earlier phase. The main contribution of my dissertation is the application of the econometrical methodology that I describe in the beginning of the second chapter. First, I run a preliminary OLS and quantile regression analysis to estimate and describe the raw and conditional wage gaps along the distribution. Second, I estimate quantile regressions separately for males and females, in order to allow for different rewards to characteristics. Third, I proceed to decompose the raw wage gap estimated at the mean through the Oaxaca-Blinder (1973) procedure. In the second chapter I run a two-steps Heckman procedure by estimating a model of participation in the labour market which shows a significant selection bias for females. Forth, I apply the Machado-Mata (2005) techniques to extend the decomposition analysis at all points of the distribution. In Poland I can also implement the Juhn, Murphy and Pierce (1991) decomposition over the period 1994-2004, to account for effects to the pay gap due to changes in overall wage dispersion beyond Oaxaca’s standard decomposition.

TCAD approaches to multidimensional simulation of advanced semiconductor devices

Technology scaling increasingly emphasizes complexity and non-ideality of the electrical behavior of semiconductor devices and boosts interest on alternatives to the conventional planar MOSFET architecture. TCAD simulation tools are fundamental to the analysis and development of new technology generations. However, the increasing device complexity is reflected in an augmented dimensionality of the problems to be solved. The trade-off between accuracy and computational cost of the simulation is especially influenced by domain discretization: mesh generation is therefore one of the most critical steps and automatic approaches are sought. Moreover, the problem size is further increased by process variations, calling for a statistical representation of the single device through an ensemble of microscopically different instances. The aim of this thesis is to present multi-disciplinary approaches to handle this increasing problem dimensionality in a numerical simulation perspective. The topic of mesh generation is tackled by presenting a new Wavelet-based Adaptive Method (WAM) for the automatic refinement of 2D and 3D domain discretizations. Multiresolution techniques and efficient signal processing algorithms are exploited to increase grid resolution in the domain regions where relevant physical phenomena take place. Moreover, the grid is dynamically adapted to follow solution changes produced by bias variations and quality criteria are imposed on the produced meshes. The further dimensionality increase due to variability in extremely scaled devices is considered with reference to two increasingly critical phenomena, namely line-edge roughness (LER) and random dopant fluctuations (RD). The impact of such phenomena on FinFET devices, which represent a promising alternative to planar CMOS technology, is estimated through 2D and 3D TCAD simulations and statistical tools, taking into account matching performance of single devices as well as basic circuit blocks such as SRAMs. Several process options are compared, including resist- and spacer-defined fin patterning as well as different doping profile definitions. Combining statistical simulations with experimental data, potentialities and shortcomings of the FinFET architecture are analyzed and useful design guidelines are provided, which boost feasibility of this technology for mainstream applications in sub-45 nm generation integrated circuits.

In Between : The Gap of quality L'influenza dell'insegnamento di Gurdjieff nel teatro di Peter Brook, Decalan Donnellan e Robert Lepage

Il Lavoro é incentrato sull' influenza dell'insegnamento di G. I Gurdjieff sul teatro del novecento in particolare sul lavoro di Peter Brook, Declan Donnellan e Robert Lepage

Photoinduced electronic transitions and leakage correlation to defects/dislocations in GaN heterostructures

III-nitride materials are very promising for high speed electronics/optical applications but still suffer in performance due to problems during high quality epitaxial growth, evolution of dislocation and defects, less understanding of fundamental physics of materials/processing of devices etc. This thesis mainly focus on GaN based heterostructures to understand the metal-semiconductor interface properties, 2DE(H)G influence on electrical and optical properties, and deep level states in GaN and InAlN, InGaN materials. The detailed electrical characterizations have been employed on Schottky diodes at GaN and InAl(Ga)N/GaN heterostructures in order to understand the metal-semiconductor interface related properties in these materials. I have observed the occurrence of Schottky barrier inhomogenity, role of dislocations in terms of leakage and creating electrically active defect states within energy gap of materials. Deep level transient spectroscopy method is employed on GaN, InAlN and InGaN materials and several defect levels have been observed related to majority and minority carriers. In fact, some defects have been found common in characteristics in ternary layers and GaN layer which indicates that those defect levels are from similar origin, most probably due to Ga/N vacancy in GaN/heterostructures. The role of structural defects, roughness has been extensively understood in terms of enhancing the reverse leakage current, suppressing the mobility in InAlN/AlN/GaN based high electron mobility transistor (HEMT) structures which are identified as key issues for GaN technology. Optical spectroscopy methods have been employed to understand materials quality, sub band and defect related transitions and compared with electrical characterizations. The observation of 2DEG sub band related absorption/emission in optical spectra have been identified and proposed for first time in nitride based polar heterostructures, which is well supported with simulation results. In addition, metal-semiconductor-metal (MSM)-InAl(Ga)N/GaN based photodetector structures have been fabricated and proposed for achieving high efficient optoelectronics devices in future.

Scaled down physical properties of semiconductor nanowires for nanoelectronics scaling up

Semiconductor nanowires (NWs) are one- or quasi one-dimensional systems whose physical properties are unique as compared to bulk materials because of their nanoscaled sizes. They bring together quantum world and semiconductor devices. NWs-based technologies may achieve an impact comparable to that of current microelectronic devices if new challenges will be faced. This thesis primarily focuses on two different, cutting-edge aspects of research over semiconductor NW arrays as pivotal components of NW-based devices. The first part deals with the characterization of electrically active defects in NWs. It has been elaborated the set-up of a general procedure which enables to employ Deep Level Transient Spectroscopy (DLTS) to probe NW arrays’ defects. This procedure has been applied to perform the characterization of a specific system, i.e. Reactive Ion Etched (RIE) silicon NW arrays-based Schottky barrier diodes. This study has allowed to shed light over how and if growth conditions introduce defects in RIE processed silicon NWs. The second part of this thesis concerns the bowing induced by electron beam and the subsequent clustering of gallium arsenide NWs. After a justified rejection of the mechanisms previously reported in literature, an original interpretation of the electron beam induced bending has been illustrated. Moreover, this thesis has successfully interpreted the formation of NW clusters in the framework of the lateral collapse of fibrillar structures. These latter are both idealized models and actual artificial structures used to study and to mimic the adhesion properties of natural surfaces in lizards and insects (Gecko effect). Our conclusion are that mechanical and surface properties of the NWs, together with the geometry of the NW arrays, play a key role in their post-growth alignment. The same parameters open, then, to the benign possibility of locally engineering NW arrays in micro- and macro-templates.

Fabrication, Electrical Characterization and Simulation of Thin Film Solar Cells: CdTe and CIGS Materials

CdTe and Cu(In,Ga)Se2 (CIGS) thin film solar cells are fabricated, electrically characterized and modelled in this thesis. We start from the fabrication of CdTe thin film devices where the R.F. magnetron sputtering system is used to deposit the CdS/CdTe based solar cells. The chlorine post-growth treatment is modified in order to uniformly cover the cell surface and reduce the probability of pinholes and shunting pathways creation which, in turn, reduces the series resistance. The deionized water etching is proposed, for the first time, as the simplest solution to optimize the effect of shunt resistance, stability and metal-semiconductor inter-diffusion at the back contact. In continue, oxygen incorporation is proposed while CdTe layer deposition. This technique has been rarely examined through R.F sputtering deposition of such devices. The above experiments are characterized electrically and optically by current-voltage characterization, scanning electron microscopy, x-ray diffraction and optical spectroscopy. Furthermore, for the first time, the degradation rate of CdTe devices over time is numerically simulated through AMPS and SCAPS simulators. It is proposed that the instability of electrical parameters is coupled with the material properties and external stresses (bias, temperature and illumination). Then, CIGS materials are simulated and characterized by several techniques such as surface photovoltage spectroscopy is used (as a novel idea) to extract the band gap of graded band gap CIGS layers, surface or bulk defect states. The surface roughness is scanned by atomic force microscopy on nanometre scale to obtain the surface topography of the film. The modified equivalent circuits are proposed and the band gap graded profiles are simulated by AMPS simulator and several graded profiles are examined in order to optimize their thickness, grading strength and electrical parameters. Furthermore, the transport mechanisms and Auger generation phenomenon are modelled in CIGS devices.

Optimization of purification procedures in organic semiconductor synthesis for improving the performances of organic field-effect transistors

The aim of the research activity focused on the investigation of the correlation between the degree of purity in terms of chemical dopants in organic small molecule semiconductors and their electrical and optoelectronic performances once introduced as active material in devices. The first step of the work was addressed to the study of the electrical performances variation of two commercial organic semiconductors after being processed by means of thermal sublimation process. In particular, the p-type 2,2′′′-Dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DH4T) semiconductor and the n-type 2,2′′′- Perfluoro-Dihexyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DFH4T) semiconductor underwent several sublimation cycles, with consequent improvement of the electrical performances in terms of charge mobility and threshold voltage, highlighting the benefits brought by this treatment to the electric properties of the discussed semiconductors in OFET devices by the removal of residual impurities. The second step consisted in the provision of a metal-free synthesis of DH4T, which was successfully prepared without organometallic reagents or catalysts in collaboration with Dr. Manuela Melucci from ISOF-CNR Institute in Bologna. Indeed the experimental work demonstrated that those compounds are responsible for the electrical degradation by intentionally doping the semiconductor obtained by metal-free method by Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) and Tributyltin chloride (Bu3SnCl), as well as with an organic impurity, like 5-hexyl-2,2':5',2''-terthiophene (HexT3) at, in different concentrations (1, 5 and 10% w/w). After completing the entire evaluation process loop, from fabricating OFET devices by vacuum sublimation with implemented intentionally-doped batches to the final electrical characterization in inherent-atmosphere conditions, commercial DH4T, metal-free DH4T and the intentionally-doped DH4T were systematically compared. Indeed, the fabrication of OFET based on doped DH4T clearly pointed out that the vacuum sublimation is still an inherent and efficient purification method for crude semiconductors, but also a reliable way to fabricate high performing devices.

Investigation of organic semiconductor/water interfaces for optobioelectronic devices

The thesis investigates the potential of photoactive organic semiconductors as a new class of materials for developing bioelectronic devices that can convert light into biological signals. The materials can be either small molecules or polymers. When these materials interact with aqueous biological fluids, they give rise to various electrochemical phenomena, including photofaradaic or photocapacitive processes, depending on whether photogenerated charges participate in redox processes or accumulate at an interface. The thesis starts by studying the behavior of the H2Pc/PTCDI molecular p/n thin-film heterojunction in contact with aqueous electrolyte. An equivalent circuit model is developed, explaining the measurements and predicting behavior in wireless mode. A systematic study on p-type polymeric thin-films is presented, comparing rr-P3HT with two low bandgap conjugated polymers: PBDB-T and PTB7. The results demonstrate that PTB7 has superior photocurrent performance due to more effective electron-transfer onto acceptor states in solution. Furthermore, the thesis addresses the issue of photovoltage generation for wireless photoelectrodes. An analytical model based on photoactivated charge-transfer across the organic-semiconductor/water interface is developed, explaining the large photovoltages observed for polymeric p-type semiconductor electrodes in water. Then, flash-precipitated nanoparticles made of the same three photoactive polymers are investigated, assessing the influence of fabrication parameters on the stability, structure, and energetics of the nanoparticles. Photocathodic current generation and consequent positive charge accumulation is also investigated. Additionally, newly developed porous P3HT thin-films are tested, showing that porosity increases both the photocurrent and the semiconductor/water interfacial capacity. Finally, the thesis demonstrates the biocompatibility of the materials in in-vitro experiments and shows safe levels of photoinduced intracellular ROS production with p-type polymeric thin-films and nanoparticles. The findings highlight the potential of photoactive organic semiconductors in the development of optobioelectronic devices, demonstrating their ability to convert light into biological signals and interface with biological fluids.

Kinetics of photogenerated carriers in nanostructured semiconductor heterojunctions for solar water splitting

This thesis aims to investigate the fundamental processes governing the performance of different types of photoelectrodes used in photoelectrochemical (PEC) applications, such as unbiased water splitting for hydrogen production. Unraveling the transport and recombination phenomena in nanostructured and surface-modified heterojunctions at a semiconductor/electrolyte interface is not trivial. To approach this task, the work presented here first focus on a hydrogen-terminated p-silicon photocathode in acetonitrile, considered as a standard reference for PEC studies. Steady-state and time-resolved excitation at long wavelength provided clear evidence of the formation of an inversion layer and revealed that the most optimal photovoltage and the longest electron-hole pair lifetime occurs when the reduction potential for the species in solution lies within the unfilled conduction band states. Understanding more complex systems is not as straight-forward and a complete characterization that combine time- and frequency-resolved techniques is needed. Intensity modulated photocurrent spectroscopy and transient absorption spectroscopy are used here on WO3/BiVO4 heterojunctions. By selectively probing the two layers of the heterojunction, the occurrence of interfacial recombination was identified. Then, the addition of Co-Fe based overlayers resulted in passivation of surface states and charge storage at the overlayer active sites, providing higher charge separation efficiency and suppression of recombination in time scales that go from picoseconds to seconds. Finally, the charge carrier kinetics of several different Cu(In,Ga)Se2 (CIGS)-based architectures used for water reduction was investigated. The efficiency of a CIGS photocathode is severely limited by charge transfer at the electrode/electrolyte interface compared to the same absorber layer used as a photovoltaic cell. A NiMo binary alloy deposited on the photocathode surface showed a remarkable enhancement in the transfer rate of electrons in solution. An external CIGS photovoltaic module assisting a NiMo dark cathode displayed optimal absorption and charge separation properties and a highly performing interface with the solution.