The operating characteristics of non-volatile memory for modern requirement are high-density , low power consumption, fast read and write speed, and good reliability. The floating gate memory generated leakage path in the tunnel oxide during the trend of scaling down, which will result in the loss of all stored charge to the silicon substrate. As the data retention time and endurance are taken into consideration, the thickness of tunnel oxide exist a physical limit, owing to the demand of high-density capacities. RRAM is offered as an option in the next generation non-volatile memories, due to the following advantages: (1) simple structure and easy to process, and low cost ; (2) less restrictive in the scaling-down process; (3) with the multi-bit data storage features; (4) high speed operation; (5) Repeat write and read is more than one million. In the thesis, we use a simple and low-temperature process to form the silicon germanium oxide (Si-Ge-O) RRAM and silicon germanium oxide RRAM with nitrogen doping between the electrode and silicon-germanium oxide interface. By sputtering at argon and oxygen (Ar/O2), and sputtering at argon and ammonia (Ar/NH3) with silicon-germanium target to form silicon germanium oxide RRAM and silicon germanium oxide (Si-Ge-O)/silicon germanium oxnitride (Si-Ge-O-N) RRAM. By informing a SiGeON layer between the interface of electrode and silicon-germanium oxide improve the stability of write voltage and endurance reliability. In addition, both silicon and germanium are useful as materials in the optoelectronics industry and extensively studied in material science. Based on the two materials, the smiting characterizations of RRAM will be improved in the read-write stability and operation reliability.

Recently Silicon Germanium alloy (Si_1-xGe_x) is showing lots of potentials in device fabrication. Most of the structures containing Si_1-xGe_x that are fabricated at present involve Si/Si_1-xGe_x heterostructure. The fabrication process involves several high temperature anneal steps in either inert, oxidizing or nitriding ambient which results the interdiffusion of Si and Ge through the hetero-interfaces. The interdiffusion causes broadening of Si/Si1_xGex interface and changes the physical position of the heterointerface which can cause degradation of device performance. Several studies have so far been done to quantify the amount of Ge interdiffusion in heterostructures. However no study has yet been performed to model this phenomenon. Modeling the interdiffusion mechanism is important for two reasons: (1) it will facilitate to calibrate the device characteristics taking the effect of interdiffusion mechanism into calculations prior to device fabrication; and (2) to get a better insight of the actual mechanism involved in the interdiffusion process. In this study, attempt has been taken to model interdiffusion of Si and Ge in structures having Si/Si_1-xGe_x hetero-interfaces. Mathematical models are proposed to model the behavior and the models are applied to previously published results where samples were annealing in inert, oxidizing and nitriding ambient at different anneal temperatures for different anneal times. First only the contributions of vacancies in the interdiffusion mechanism are considered. This can successfully model the interdiffusion mechanism for samples annealed in inert and oxidizing ambients at low temperatures (below 1050°C). Next the contributions of interstitials along with vacancy in the interdiffusion mechanism are considered. These are able to successfully model the interdiffusion phenomenon for the samples annealed in oxidizing and nitriding ambients at high temperatures (above 1050°C). The success of the modeling is justified by getting good match between the simulated and the experimental interdiffusion profiles along with good match between the fitting parameters used in the simulations compared with previous reported values. Besides modeling the interdiffusion mechanism, for the first time, a mathematical model is proposed for vacancy injection while nitridation of silicon is done. / Thesis / Master of Applied Science (MASc)

L'alliage Si-Ge est utilisé depuis de nombreuses années dans les modules thermoélectriques dans les sondes spatiales de la NASA. Ils convertissent la chaleur résultant de la désintégration radioactive de matériaux riches en un ou plusieurs radio-isotopes en électricité. Cet alliage est performant à haute température (à partir de 700 °C), c'est pourquoi il trouve également un fort intérêt dans l'industrie automobile. De nombreuses recherches dans ce secteur s'orientent vers la thermoélectricité, notamment vers des modules fonctionnant à haute température pour permettre la réduction de consommation de carburant.La meilleure composition de l'alliage en thermoélectricité est Si0,8Ge0,2. Le facteur de mérite réduit (ZT) de ces matériaux est généralement proche de 0,75 et de 0,45 à 700 °C pour les types n et p respectivement. Le germanium étant très onéreux, la composition retenue dans cette étude est Si0,92Ge0,08. Pour améliorer les performances de la composition choisie et se rapprocher de celles de la meilleure composition, la voie retenue est l'incorporation de nanoinclusions à base de siliciures de molybdène dans le matériau, permettant la diminution de la conductivité thermique.L'alliage Si-Ge est synthétisé par mécanosynthèse, et densifié par SPS. Les dopants utilisés sont le phosphore et le bore pour les types n et p respectivement. Le taux de dopage optimal est de 0,7 %. Ainsi, les ZT obtenus à 700 °C sont égaux à 0,7 et 0,5 pour les types n et p respectivement. La nature des inclusions stables dans la matrice est déterminée par la méthode CalPhad qui permet l'obtention du diagramme ternaire Mo-Si-Ge. La phase MoSi2 apparait alors comme étant la seule phase stable dans la matrice Si0,92Ge0,08. La fraction volumique optimale de molybdène est de 1,3 % lorsque les matériaux sont densifiés à 1280 °C. Le ZT obtenu est supérieur à 1 à 700 °C pour le type n, et proche de 0,8 pour le type p. L'ajout de nanoinclusions a permis d'augmenter les performances de 43 % et de 60 % à 700 °C. / Si-Ge alloys has been used for many years in the thermoelectric modules in the NASA space probes in which they convert heat produced by the radioactive decay of a heat source into electricity. This alloy is effective at high temperature (from 700 °C), so it is also a strong interest in the automotive industry. The strong incentive in this area to reduce fuel consumption leads researchers to develop thermoelectric modules that can operate at high temperatures. The composition at which SiGe alloys are the most thermoelectrically efficient is Si0.8Ge0.2. Their figure of merit (ZT) is generally close to 0.75 and 0.45 at 700 °C for type n and p respectively. As Germanium is very expensive, this study aims to develop a Si0.92Ge0.08 alloy that can compare to the existing Si0.8Ge0.2 alloys. To get to a higher level of performance, the thermal conductivity of the chosen composition has to be decreased, which is done by incorporating molybdenum silicides in the Si0.92Ge0.08 alloys.The Si-Ge alloy was synthesized by mechanical alloying, and sintered by SPS. The dopants used are phosphorous and boron for the n and p types respectively. The optimal doping level is 0.7%. ZT obtained for Si0.92Ge0.08 base alloys at 700 °C are equal to 0.7 and 0.5 for n and p types respectively. The nature of stable inclusions in the matrix has been determined by the CALPHAD method to obtain the ternary diagram Mo-Si-Ge. Then, the MoSi2 phase appears to be the only stable phase in the matrix Si0.92Ge0.08. The optimum volume fraction of molybdenum was 1.3% when the materials are sintered at 1280 °C. Therefore, the ZT obtained is higher than 1 at 700 °C for n-type and close to 0.8 for p-type. Adding nanoinclusions has increased performance by 43% (n-type) and 60% (p-type) at 700 °C.

Utilizando métodos de primeiros princípios, baseados na Teoria do Funcional da Desidade, investigamos, de forma sistemática, problemas de interesse na física de semicondutores: (i) A liga SixGe1-x: observa-se um pequeno desvio da linearidade, para o parâmetro de rede da liga, em função da concentração, sendo que as distâncias entre átomos de Ge são as que mais variam. O comportamento de vacâncias nessa liga se mostrou intermediário entre o Ge e o Si, e a energia de formação (EF) das vacâncias variou entre 2,06 eV e 2,90 eV, dependendo da vizinhança dessa. Propusemos um modelo para a difusão de Ge nessa liga. O fato das vacâncias com mais átomos de Ge como vizinhos ter menor energia de formação, faz com que a difusão das vacâncias se dê por caminhos lembrando o Ge puro, ou seja, a tendência será de que os vizinhos dessa vacância sejam de Ge, para ligas ricas em Ge; (ii) Superfícies: estudamos a adsorção de monômeros e dímeros de Ge sobre Si (100). Verificamos que, para monômeros, podem existir diversos mínimos locais da superfície de energia potencial para a mesma posição (x, y) na superfície, cuja diferença está na inclinação dos dímeros da superfície perto deste átomo. Mostramos que isso pode ser identificado em imagens teóricas de STM (Scanning Tunelling Microscopy) e propomos que isso seja verificado experimentalmente. Este tipo de efeito também ocorre com dímeros, sendo que isso já foi observado experimentalmente, mas com uma interpretação errônea. Através do nosso estudo podemos propor uma estrutura mais condizente com as imagens experimentais. O comportamento de átomos e dímeros de Si e Ge perto de degraus também foi analisado. Devido à diferença no parâmetro de rede do Si e do Ge, mostramos que a adsorção se torna diferente quando somente dois átomos estão na superfície. Através desses resultados, propomos um modelo para explicar a reversão da rugosidade dos degraus durante o crescimento de Ge sobre Si(100); (iii) Impurezas de Mn em Si: analisamos o comportamento de impurezas de Mn no cristal de Si e sobre a superfície. No cristal, observamos que a EF de sítios intersticiais é menor do que os sítios substitucionais. Sobre a superfície, existe um sítio substitucional que possue a mesma EF de um sítio intersticial. Esse é um fato importante para o desenvolvimento de semicondutores ferromagnéticos à base de Si. Propomos um método para que esse fato seja verificado experimentalmente, através de imagens de STM. / Using ab initio methods, based on Density Functional Theory, we have systematically investigated problems of interest for semiconductor physics: (i)The SixGe1-x alloy: we observe a small bowing in the lattice parameter, as a function of concentration, and that a larger variation in interatomic distances occurs for Ge-Ge bonds. The behavior of vacancies in this alloy is intermediate between Si and Ge, with formation energies (EF) varying from 2,06 eV to 2,90 eV, depending on the neighborhood of the vacancy. We propose a model for the diffusion of Ge in this alloy. As the vacancy with four Ge atoms as first neighbors hás smaller formation energy, the diffusion of the vacancies Will take place along paths that resemble purê Ge, for Ge-rich alloys; (ii)Surfaces: we study the adsorption of Ge monomers and dimers on Si(100). For monomers, there are several local mínima of the potential energy surface, for the same (x, y) position in the surface, depending on the buckling of the surface dimers near the adatom. We show that this can be identified in theoretical STM (Scanning Tunelling Microscopy) images. This kind of effect also occurs for dimers, and hás already been observed experimentally, but with a wrong interpretation. Through our study, we propose a new structure for the experimental images. The behavior of Si and Ge atoms and dimers near steps is also analyzed. Due to the differencein the lattice parameter of Si and Ge, we show that the adsorption becomes different when only two atoms are in the surface. Through these results, we propose a model to explain the reversion of step roughness due to Ge deposition on Si(100); (iii)Mn impurities in Si: we analise the behavior of Mn impurities in bulk Si ando n the surface. For the bulk, we observe that EF is lower for intersititial than for substitutional sites. On the surface, there is a substitutional site that hás the same EF AS A INTESTITIAL ONE. This fact may be important to the development of Si based ferromagnetic semiconductors. A method is proposed to verify this fact experimentally, through STM images.

Utilizando métodos de primeiros princípios, baseados na Teoria do Funcional da Desidade, investigamos, de forma sistemática, problemas de interesse na física de semicondutores: (i) A liga SixGe1-x: observa-se um pequeno desvio da linearidade, para o parâmetro de rede da liga, em função da concentração, sendo que as distâncias entre átomos de Ge são as que mais variam. O comportamento de vacâncias nessa liga se mostrou intermediário entre o Ge e o Si, e a energia de formação (EF) das vacâncias variou entre 2,06 eV e 2,90 eV, dependendo da vizinhança dessa. Propusemos um modelo para a difusão de Ge nessa liga. O fato das vacâncias com mais átomos de Ge como vizinhos ter menor energia de formação, faz com que a difusão das vacâncias se dê por caminhos lembrando o Ge puro, ou seja, a tendência será de que os vizinhos dessa vacância sejam de Ge, para ligas ricas em Ge; (ii) Superfícies: estudamos a adsorção de monômeros e dímeros de Ge sobre Si (100). Verificamos que, para monômeros, podem existir diversos mínimos locais da superfície de energia potencial para a mesma posição (x, y) na superfície, cuja diferença está na inclinação dos dímeros da superfície perto deste átomo. Mostramos que isso pode ser identificado em imagens teóricas de STM (Scanning Tunelling Microscopy) e propomos que isso seja verificado experimentalmente. Este tipo de efeito também ocorre com dímeros, sendo que isso já foi observado experimentalmente, mas com uma interpretação errônea. Através do nosso estudo podemos propor uma estrutura mais condizente com as imagens experimentais. O comportamento de átomos e dímeros de Si e Ge perto de degraus também foi analisado. Devido à diferença no parâmetro de rede do Si e do Ge, mostramos que a adsorção se torna diferente quando somente dois átomos estão na superfície. Através desses resultados, propomos um modelo para explicar a reversão da rugosidade dos degraus durante o crescimento de Ge sobre Si(100); (iii) Impurezas de Mn em Si: analisamos o comportamento de impurezas de Mn no cristal de Si e sobre a superfície. No cristal, observamos que a EF de sítios intersticiais é menor do que os sítios substitucionais. Sobre a superfície, existe um sítio substitucional que possue a mesma EF de um sítio intersticial. Esse é um fato importante para o desenvolvimento de semicondutores ferromagnéticos à base de Si. Propomos um método para que esse fato seja verificado experimentalmente, através de imagens de STM. / Using ab initio methods, based on Density Functional Theory, we have systematically investigated problems of interest for semiconductor physics: (i)The SixGe1-x alloy: we observe a small bowing in the lattice parameter, as a function of concentration, and that a larger variation in interatomic distances occurs for Ge-Ge bonds. The behavior of vacancies in this alloy is intermediate between Si and Ge, with formation energies (EF) varying from 2,06 eV to 2,90 eV, depending on the neighborhood of the vacancy. We propose a model for the diffusion of Ge in this alloy. As the vacancy with four Ge atoms as first neighbors hás smaller formation energy, the diffusion of the vacancies Will take place along paths that resemble purê Ge, for Ge-rich alloys; (ii)Surfaces: we study the adsorption of Ge monomers and dimers on Si(100). For monomers, there are several local mínima of the potential energy surface, for the same (x, y) position in the surface, depending on the buckling of the surface dimers near the adatom. We show that this can be identified in theoretical STM (Scanning Tunelling Microscopy) images. This kind of effect also occurs for dimers, and hás already been observed experimentally, but with a wrong interpretation. Through our study, we propose a new structure for the experimental images. The behavior of Si and Ge atoms and dimers near steps is also analyzed. Due to the differencein the lattice parameter of Si and Ge, we show that the adsorption becomes different when only two atoms are in the surface. Through these results, we propose a model to explain the reversion of step roughness due to Ge deposition on Si(100); (iii)Mn impurities in Si: we analise the behavior of Mn impurities in bulk Si ando n the surface. For the bulk, we observe that EF is lower for intersititial than for substitutional sites. On the surface, there is a substitutional site that hás the same EF AS A INTESTITIAL ONE. This fact may be important to the development of Si based ferromagnetic semiconductors. A method is proposed to verify this fact experimentally, through STM images.

Thermoelectric phenomena couple thermal and electric currents, allowing for solid-state conversion of heat into electricity. For decades Radioisotope Thermoelectric Generators have supplied power to NASA satellites and deep space probes. A more accessible application to consumers is the automotive industry's aspiration to incorporate thermoelectrics into active waste heat recovery systems. Higher power demands require these new thermoelectric devices to operate at higher temperatures and higher efficiencies, justifying new materials research. Recently, clathrates have gained interest for thermoelectric applications due to the unique properties they possess.These properties are directly related to their crystal structure. Therefore, clathrates are not only of interest from the standpoint of potential thermoelectric applications but are also of scientific interest as they presents an opportunity to investigate fundamental properties of group-IV elements in novel crystal structures. Clathrates are a class of novel open-structured materials in which molecules or atoms of one species are completely enclosed within a framework comprised of another species. This work presents a systematic investigation of the electrical properties of type I clathrate alloys, specifically Si-Ge alloys, for the first time. A series of Ba8Ga16-ySixGe30-x+y clathrates with varying Si content were synthesized and their structural and transport properties were studied. Two additional series of type I clathrates were also synthesized and characterized and their properties compared to those of the Si-Ge alloys in order to develop an understanding of their structure-property relationships. The increasing Si content correlates to a dramatic increase in Seebeck coefficient even as the resistivity decreases, suggesting the complex interaction between the Ba and the Si substitution within the Ga16Ge30 framework significantly modifies the band structure.

In this study, we used ion implantation technique to synthesize semiconductor (Ge, Si) nanocrystals in SiO2 matrix. Ge and Si nanocrystals have been successfully formed by Ge and Si implantation and post annealing. Implanted samples were examined by characterization techniques such as TEM, XPS, EDS, SAD, SIMS, PL, Raman and FTIR spectroscopy and the presence of Ge and Si nanocrystals in the SiO2 matrix has been evidenced by these measurements. It was shown that implantation dose, implantation energy, annealing temperature, annealing time and annealing ambient are important parameters for the formation and evolution of semiconductor nanocrystals embedded in SiO2 matrix. The size and size distribution of Ge and Si nanocrystals were estimated successfully by fitting Raman and PL spectra obtained from Ge and Si implanted samples, respectively. It was demonstrated that Si implanted and post annealed samples exhibit two broad PL peaks at &amp / #8764 / 625 and 850 nm, even at room temperature. Origin of these peaks was investigated by temperature, excitation power and excitation wavelength dependence of PL spectrum and etch-measure experiments and it was shown that the peak observed at &amp / #8764 / 625 nm is related with defects (clusters or chain of Si located near the surface) while the other is related to the Si nanocrystals. As an expected effect of quantum size phenomenon, the peak observed at &amp / #8764 / 850 nm was found to depend on the nanocrystal size. Finally, the formation and evolution of Ge and Si nanocrystals were monitored by FTIR spectroscopy and it was shown that the deformation in SiO2 matrix caused by ion implantation tends to recover itself much quicker in the case of the Ge implantation. This is a result of effective segregation of Ge atoms at relatively low temperatures.

We have performed a first principles density functional theory (DFT) calculations to study the thermal conductivity in ZnO nanotubes, ZnO nanowires, and Si/Ge shell-core nanowires. We found the equilibrium configuration and the electric band structure of each nanostructure using DFT, the interatomic force constants and the phonon dispersion relations were calculated using DFPT as implemented in Quantum Espresso. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach using a Green's function method. All ZnO nanostructures studied exhibit semiconducting behavior, with direct bandgap at the Gamma point. The calculated values for the bandgaps were larger than the value of the bandgap of the bulk ZnO. We were able to identify phonon modes in which the motion of Zn atoms is significant when it is compared with the motion of oxygen atoms. The thermal conductivity depends on the diameter of the nanowires and nanotubes and it is dramatically affected when the nanowire or nanotube is doped with Ga. For Si/Ge nanowires, the slope and the curvature of acoustic modes in the phonon dispersion relation increases when the diameter increases. For nanowires with the same number of atoms, the slope and curvature of acoustic modes depends on the concentration of Si atoms. We were able to identify phonon modes in which the motion of core atoms is significant when it is compared with motion of atoms on the nanowire's shell. The thermal conductivity in these nanostructures depends on the nanowire's diameter and on the Si atoms concentration.

L'alliage Si-Ge est utilisé depuis de nombreuses années dans les modules thermoélectriques dans les sondes spatiales de la NASA. Ils convertissent la chaleur résultant de la désintégration radioactive de matériaux riches en un ou plusieurs radio-isotopes en électricité. Cet alliage est performant à haute température (à partir de 700 °C), c'est pourquoi il trouve également un fort intérêt dans l'industrie automobile. De nombreuses recherches dans ce secteur s'orientent vers la thermoélectricité, notamment vers des modules fonctionnant à haute température pour permettre la réduction de consommation de carburant.La meilleure composition de l'alliage en thermoélectricité est Si0,8Ge0,2. Le facteur de mérite réduit (ZT) de ces matériaux est généralement proche de 0,75 et de 0,45 à 700 °C pour les types n et p respectivement. Le germanium étant très onéreux, la composition retenue dans cette étude est Si0,92Ge0,08. Pour améliorer les performances de la composition choisie et se rapprocher de celles de la meilleure composition, la voie retenue est l'incorporation de nanoinclusions à base de siliciures de molybdène dans le matériau, permettant la diminution de la conductivité thermique.L'alliage Si-Ge est synthétisé par mécanosynthèse, et densifié par SPS. Les dopants utilisés sont le phosphore et le bore pour les types n et p respectivement. Le taux de dopage optimal est de 0,7 %. Ainsi, les ZT obtenus à 700 °C sont égaux à 0,7 et 0,5 pour les types n et p respectivement. La nature des inclusions stables dans la matrice est déterminée par la méthode CalPhad qui permet l'obtention du diagramme ternaire Mo-Si-Ge. La phase MoSi2 apparait alors comme étant la seule phase stable dans la matrice Si0,92Ge0,08. La fraction volumique optimale de molybdène est de 1,3 % lorsque les matériaux sont densifiés à 1280 °C. Le ZT obtenu est supérieur à 1 à 700 °C pour le type n, et proche de 0,8 pour le type p. L'ajout de nanoinclusions a permis d'augmenter les performances de 43 % et de 60 % à 700 °C.

Thermoelectric devices with the ability to convert rejected heat into electricity are widely used in nowadays technology. Several studies have been done to improve the efficiency of these devices. However, because of the strong correlation between thermoelectric properties (electrical conductivity, Seebeck coefficient, and thermal conductivity including lattice and electron counterpart), improving ZT has always been a challenging task. In this study, thermal conductivity of group IV-based binary and ternary alloys such as SiGe, SiSn, GeSn, and SiGeSn has been studied. Phonon Boltzmann Transport Equation has been solved in the relaxation time approximation including intrinsic and extrinsic (in the presence of boundary and interfaces in the low-dimensional material) scattering mechanisms. Full phonon dispersion based on the Adiabatic Bond Charge model has been calculated for Si, Ge, and Sn. Virtual crystal approximation has been adapted to calculate the dispersion of SiGe, SiSn, GeSn, and SiGeSn. Two approaches have been introduced to reduce the lattice thermal conductivity of the materials under study. First, alloying results in a significant reduction of thermal conductivity. But, this reduction has been limited by the mass disorder scattering in the composition range of 0.2 to 0.8. Second, nanostructuring technique has been proposed to further reduce the thermal conductivity. Our study shows that, due to the atomic mass difference which gives rise to the elastic mass scattering mechanism, SiSn has the lowest thermal conductivity among the other materials under study. SiSn achieved the thermal conductivity of 1.18 W/mK at 10 nm at the Sn composition of 0.18, which is the experimentally stable state of SiSn. The results show that SiSn alloys have the lowest conductivity (3 W/mK) of all the bulk alloys, more than two times lower than SiGe, attributed to the larger difference in mass between the two constituents. In addition, this study demonstrates that thin films offer an additional reduction in thermal conductivity, reaching around 1 W/mK in 20 nm SiSn, GeSn, and ternary SiGeSn films, which is close to the conductivity of amorphous SiO$_2$. This value is lower than the thermal conductivity of SiGe at 10 nm which is 1.43 W/mK. Having lattice thermal conductivity reduced, electron transport has been studied by solving Boltzmann Transport Equation under low electric field including elastic and inelastic scattering mechanisms. Rode's iterative method has been applied to the model for obtaining perturbation of distribution function under a low electric field. This study shows that nanostructuring and alloying can reduce $\kappa_{ph}$ without significantly changing the other parameters. This is because of the phonon characteristics in solids in which MFP of phonons is much larger than those of electrons, which gives us the possibility of phonons confinement without altering electrons transport. Thermoelectric properties of SiGe in the bulk and nanostructure form have been studied to calculate ZT in a wide range of temperatures. The results demonstrate that ZT reaches the value of 1.9 and 1.58 at the temperatures of 1200 K and 1000 K respectively, with the Ge composition of 0.2 and carrier concentration of 5$\times$10$^{19}$ cm$^{-3}$ at 10 nm thickness. This model can be applied to SiSn and other binary and ternary alloys, to calculate the improved ZT. Hence, we conclude that group IV alloys containing Sn have the potential for high-efficiency TE energy conversion.