In last two years, the proposal to create artificial graphene in standard semiconducting 2D systems via surface patterning has emerged. This way, an alternative system would be created, allowing us to study phenomena related to Dirac-type particles in a fully carbon free system. The main idea of the concept assumes the creation of an additional potential in a quantum well by nanopatterning of the specimen surface or by using local electrodes. The additionally introduced modulation can transform the conventional (i.e. parabolic) energy dispersion into separated minibands with possible appearance of Dirac cones. In the theoretical part, we introduce four basic criteria that estimate appropriate technological parameters and the required experimental conditions. Experimentally, we study the cyclotron resonance of prepared heterostructures AlGaAs/GaAs with induced hexagonal potential via the etching lateral holes. The observed multi-mode resonance response is discussed with respect to the expected appearance of Dirac cones.

Both LiCoO2 and LiNi1/3Mn1/3Co1/3O2 layered cathode materials are investigated in our studies. P3 phase of CoO2, the end member of the LixCoO2, is found in both chemically and electrochemically delithiated materials. Delithiated LixCoO2 specimens decompose into fine Co3O4 and LiCoO2 particles starting at around 200 °C. This decomposing reaction is proved by in-situ X-ray diffraction and in-situ transmission electron microscopy investigations. The structures of pristine and cycled LiNi1/3Mn1/3Co1/3O2 are investigated by electron diffraction. Single and polycrystalline crystals are found in this material. The partial substitution of Co by Ni and Mn in LiNi1/3Mn1/3Co1/3O2 opens up the possibility of different cation configurations in the crystal lattice. Both 3Rm symmetry and superlattices are identified in this material. The number of particles with superlattices in pristine material (40%) is much bigger than cycled material at discharge state (10%).

Afin de faire face à la demande croissante de mémoires de plus en plus performantes dans les systèmes informatiques, de nouvelles technologies se sont développées. Parmi elles, les mémoires résistives à changement de phase (ou PCM pour Phase-Change Memory) ont des propriétés et une maturité suffisante pour développer les nouvelles mémoires SCM (pour Storage Class Memory) comme en témoigne la récente commercialisation des produits Optane par la firme INTEL®. Néanmoins, la consommation énergétique des PCM lors de leur programmation reste élevée, ce qui limite leurs performances. L’intégration de super-réseaux (GeTe)2/(Sb2Te3)m dans des mémoires dites iPCM (pour interfacial Phase-Change Memory) est une des voies les plus prometteuse pour permettre une diminution significative des courants de programmation. Cependant, le mécanisme de transition des iPCM et la structure du matériau dans ses deux états de résistances sont encore méconnus. Dans ce contexte, l’objectif de cette thèse est d’élaborer des super-réseaux (GeTe)2/(Sb2Te3)m (m=1,2,4 et 8) cristallins, de déterminer leur structure puis de les intégrer dans des dispositifs mémoires. La pulvérisation cathodique alternée des matériaux GeTe et Sb2Te3 dans un équipement industriel de dépôt est utilisée pour effectuer l’épitaxie van der Waals de ces super-réseaux. Une optimisation du procédé par l’ajout d’une cible de Te en co-pulvérisation avec la cible de Sb2Te3 montre l’obtention de super-réseaux stœchiométriques présentant la périodicité souhaitée, ainsi qu’une orientation des plans cristallins (0 0 l) parallèle à la surface du substrat. Une description de l’ordre atomique local des super-réseaux ainsi optimisés est ensuite menée par l’étude d’images HAADF-STEM couplée à des simulations. Celle-ci révèle un phénomène d’inter-diffusion entre les couches de GeTe et de Sb2Te3 déposées aboutissant à la formation locale de GexSbyTez rhomboédriques, des mesures quantitatives de l’occupation des plans atomiques en Ge/Sb confirment aussi le phénomène. De plus, un modèle de structure à longue distance de ces super-réseaux considérant un empilement aléatoire de blocs cristallins permet la simulation des courbes de diffraction obtenues expérimentalement. Enfin, les premières intégrations des super-réseaux (GeTe)2/(Sb2Te3)m dans des dispositifs mémoires mettent en évidence une réduction importante des courants de programmation jusqu’à 4 fois inférieurs à une PCM et avec une endurance dépassant les 10 millions cycles. / In order to satisfy the demand for more and more efficient memory in computer systems, new technologies have been developed. Among the latter resistive phase-change memories (PCM) exhibit capacities and sufficient maturity to achieve the so-called new SCM (for Storage Class Memory) devices as evidenced by the recent commercialization of Optane products by INTEL®. Nevertheless, PCM still require strong electrical consumption limiting their performance. Integration of (GeTe)2/(Sb2Te3)m superlattices in so-called iPCM (for interfacial Phase Change Memory) was shown to permit a significant decrease in programming currents. However, the switching mechanism of this memory and the structure of the material in its two resistance states are still under debate. The aim of this thesis is therefore to deposit crystalline (GeTe)2/(Sb2Te3)m (m=1,2,4 et 8) superlattices, to determine their structure and to integrate them into memory devices. GeTe and Sb2Te3 materials are alternately deposited by means of sputtering in an industrial deposition tool to perform van der Waals epitaxy of these superlattices. Stoichiometric superlattices with the desired periodicity and with an orientation of the (0 0 l) crystalline planes parallel to the surface of the substrate are obtained by innovative co-sputtering of Sb2Te3 and Te targets during Sb2Te3 deposition. A description of the local atomic order of superlattices is then carried out by studying HAADF-STEM images coupled to simulations. Intermixing between GeTe and Sb2Te3 deposited layers is thus revealed, leading to the local formation of rhombohedral GexSbyTez. Quantitative measurements of the Ge/Sb atomic plans occupation in further confirm the phenomenon. A long-range order structural model of superlattices by means of random stacking of crystalline blocks allows the simulation of experimental diffraction curves. Finally, the first integrations of (GeTe)2/(Sb2Te3)m (with m=1,2,4 et 8) superlattices in devices demonstrate a programming current up to 4 times lower than a PCM reference with an endurance exceeding 10 millions cycles.

Le champ électromagnétique térahertz (THz) se situe dans l'intervalle de fréquence entre l'infrarouge et les micro-ondes, à peu près entre 1 THz à 10 THz. Ce domaine est hautement souhaitable tant pour la recherche fondamentale que pour les applications. Pourtant des sources THz compacts et accordables ne sont pas encore disponibles. Depuis la première proposition en 1970, les superréseaux semiconducteurs, dans lequel deux couches semi-conductrices atomiques avec bande interdite différente sont disposés périodiquement, fournissent de nouvelles possibilités. De nouvelles techniques et de nouveaux dispositifs deviennent réalisables. Dans cette thèse, les oscillations de Bloch dans des mini-bandes électroniques d’un superréseau polarise et la dispersion du gain associée sont utilisées pour réaliser une source THz compacte et accordable : l’oscillateur de Bloch THz. Un premier ensemble de dispositifs utilisent des réseaux dopes spécifiquement conçus pour éviter la formation de domaine d’accumulation de charges. Ces dispositifs utilisent une surface semi-isolante ou deux surfaces métalliques permettant un guidage par plasmon de surface. Cependant, malgré la réalisation de couplage par les bords ou par un réseau diffractant en surface et des mesures directes ou avec un interféromètre a transformation de Fourrier (FTIR), l’électroluminescence a été observée dans le domaine térahertz, avec un gain qui n’a pas pu etre relie aux oscillations de Bloch. Avec des superréseaux non dope, l'émission THz des oscillations de Bloch a été détectée par spectroscopie dans le domaine temporel. La dépendance de la fréquence d’émission avec le champ électrique appliqué constitue une preuve directe des oscillations de Bloch. L’échelle de Wannier Stark des trous sous pompage optique continu a aussi été observe dans les superréseaux non dopes. Avec l’augmentation de la puissance de pompage optique, les pics du photocourant se décalent et leurs formes deviennent asymétriques. L’évolution est attribue a l’accumulation des porteurs photogénérés dans les deux couches encadrant le superréseau. En outre, pour une puissance de pompage élevée, la bistabilité du photocourant a été également observée. / Terahertz (THz) electromagnetic field, which lies in the frequency gap between the infrared and microwave, roughly between 1 THz to 10 THz, is highly desirable for both fundamental research and application. Yet tuneable compact THz sources are still not available. On the other hand, ever since first proposed in 1970, semiconductor superlattice provides new playground for various new technique and devices of tremendous research and application interest. In this thesis, an innovative theme, relying on Bloch oscillations in a dc biased semiconductor superlattice, is explored to realize tunable compact THz source THz Bloch oscillator. For doped superlattice Bloch oscillator, we designed quantum cascade super-superlattice structure to realize Bloch oscillations whilst prohibit electrical domain formation. The designed structures were processed into various waveguide and grating devices for electroluminescence detection using Fourier transform infrared spectroscopy (FTIR). The Bloch gain of semi-insulating surface plasmon waveguide device was also measured using THz time domain spectroscopy. Even though the electroluminescence and gain at THz regime were observed, no direct evidence of Bloch emission was confirmed. For undoped superlattice, the THz emission from Bloch oscillations was observed by time domain spectroscopy. At last, the photocurrent corresponding to heavy hole and Wannier Stark Ladder (WSL) states transitions in undoped superlattice was studied. Under CW laser pumping, the photocurrent as function of the applied voltage showed multiple WSL peaks, which indicated laser induced and controllable negative differential conductance (NDC). With increasing pumping power, the nonlinear NDC regime and bistable states were investigated as well.

L’électronique de spin tient une place primordiale dans les technologies de l’information. Un exemple flagrant est le disque dur magnétique à haute densité de stockage intégré aujourd’hui dans la plupart des ordinateurs personnels. D’un point de vue fondamental, les opérations de base comme l’injection, la propagation et la détection de l’information de spin restent néanmoins complexes à réaliser. Des plateformes adaptées à la réalisation de ces tâches élémentaires sont très recherchées. Dans ce contexte, les nanomatériaux carbonés sont très prometteurs. Au cours de ma thèse, je me suis intéressé au transport de spin dans des nanotubes de carbone multi-parois présentant des effets de super-réseaux (Moiré 1D). J’ai également étudié la croissance de barrières moléculaires conformationnelles afin d’optimiser l’injection et la détection de l’information de spin. Je présenterai tout d’abord les caractérisations électriques des dispositifs mettant en évidence des effets de Moirés, identifiés grâce aux simulations effectuées par l’équipe de Jean-Christophe Charlier. Je montrerai ensuite comment la croissance de la couche moléculaire influence le transport en formant une barrière d’injection. Finalement, je présenterai les expériences de magnéto-transport dans ces dispositifs hybrides. La magnétorésistance observée semble indiquer un transport de spin efficace sur des distances au moins de l’ordre du micromètre. Je discuterai particulièrement de l’amplitude, du signe et de la dépendance en tension de la magnétorésistance dans le cadre de modèles standards de transport de spin / Spin electronics holds a key role in information technology. A glaring example is the high-density magnetic hard disk storage built into most personal computers. From a fundamental point of view, basic operations such as injection, propagation and detection of spin information remain nevertheless complex. Platforms adapted to the realization of these basic tasks are highly sought after. In this context, carbon nanomaterials are very promising. During my thesis, I was interested in the transport of spin in multi-wall carbon nanotubes presenting super-lattice effects (Moiré 1D). I have also studied the growth of conformational molecular barriers in order to optimize injection and detection of spin information. I will present first the electrical characterizations of the devices highlighting the effects of Moirés, identified thanks to the simulations carried out by the team of Jean-Christophe Charlier. I will then show how the growth of the molecular layer influences transport by forming an injection barrier. Finally, I will present the experiments of magneto-transport in these hybrid devices. The observed magnetoresistance seems to indicate efficient spin transport over distances of at least a micrometer. I will discuss in particular the amplitude, the sign and the voltage dependence of the magnetoresistance in the framework of standard models of spin transport

Optoelectronic devices operating in the mid-wave (3-5 Μm) and long-wave (8-12 Μm) infrared (IR) regions of the electromagnetic spectrum are of a great interest for academic and industrial applications. Due to the lack of atmospheric absorption, devices operating within these spectral bands are particularly useful for spectroscopy, imaging, and dynamic scene projection. Advanced IR imaging systems have created an intense need for laboratory-based infrared scene projector (IRSP) systems which can be used for accurate simulation of real-world phenomena occurring in the IR. These IRSP systems allow for reliable, reproducible, safe, and cost-effective calibration of IR detector arrays. The current state-of-the-art technology utilized for the emitter source of IRSP systems is thermal pixel arrays (TPAs) which are based on thin film resistor technology. Thermal pixel array technology has fundamental limitations related to response time and maximum simulated apparent temperature, making them unsuitable for emulation of very hot (> 700 K) and rapidly evolving scenes. Additionally, there exists a need for dual wavelength emitter arrays for IRSP systems dedicated to calibration of dual wavelength detector arrays. This need is currently met by combining the spectral output from two separate IRSP systems. This configuration requires precise alignment of the output from both systems and results in the maximum radiance being limited to approximately half that of the capability of a given emitter array due to the optics used to combine the outputs. The high switching speed inherent to IR light-emitting diodes (LEDs) and the potential for high power output makes them an appealing candidate to replace the thermal pixel arrays used for IRSP systems. To this end, research has been carried out to develop and improve the device performance of IR LEDs based on InAs/GaSb type-II superlattices (T2SLs). A common method employed to achieve high brightness from LEDs is to incorporate multiple active regions, coupled by tunnel junctions. Tunnel junctions must provide adequate barriers to prevent carrier leakage, while at the same time remain low in tunneling resistance to prevent unwanted heating. The performance of two tunnel junction designs are compared in otherwise identical four stage InAs/GaSb superlattice LED (SLED) devices for application in IRSP systems. This research culminated in the development of a 48 Μm pitch, 512$times512 individually addressable mid-wave IR LED array based on a sixteen stage, InAs/GaSb T2SL device design. This array was hybridized to a read-in integrated circuit and exhibited a pixel yield greater than 95 %. Projections based on single element emitter results predict this array will be able to achieve a peak apparent temperature of 1350 K within the entire 3-5 Μm band. These results demonstrate the feasibility of emitter arrays intended for IRSP systems based on InAs/GaSb SLED devices. Additionally, a dual wavelength 48 Μm pitch, 8x8 emitter array based on InAs/GaSb T2SL LEDs was developed and demonstrated. This design incorporates two separate, 16 stage InAs/GaSb SL active regions with varying InAs layer thicknesses built into a single vertical heterostructure. The device architecture is a three terminal device allowing for independent control of the intensity of each emission region. Each emitter region creates a contiguous pixel, capable of being planarized and mated to drive electronics.

The effect of compositional modulation on structural and magnetic properties of magnetic heterostructures was explored. The systems under focus were ferromagnetic superlattices Fe81Ni19/Co, metal-insulator multilayers Al2O3/Ni81Fe19, nanoparticles and artificial multilayered pillars. The heterostuctures were grown by magnetron sputtering in a state-of-the-art ultra-high vacuum system. The structural characterization was done by X-ray diffraction and reflectivity, as well as by transmission electron microscopy. Magneto-optical Kerr effect, SQUID and XMCD magnetometry and magnetic force microscopy were used for magnetic characterization. The bilayer thickness, ratio of the constituents and the interface quality influence the magnetic properties (magnetic moments and anisotropy) of metallic heterostructures. In particular, magnetic moments in bcc Fe81Ni19/Co superlattices were found to scale with the interface density thus, implying different magnetic moments at the interfaces as compared to the interior part of the layers. The easy direction of magnetization can be rotated from in-plane to out-of-plane, by increasing the bilayer thicknesses, keeping other parameters unchanged. Consequently, the anisotropy strength is strongly dependent on the repeat distance. Stripe domains appear in the films that possess an out-of-plane magnetization. The average domain period was found to be dependent on the applied in-plane magnetic field and on the total thickness of the films. The structural and magnetic properties of Al2O3/Ni81Fe19 multilayers depend strongly on the individual layers thicknesses. By increasing the amount of the magnetic deposits one can change the obtained film structure, from superparamagnetic nanoparticles to ferromagnetic multilayers. By increasing the oxide layer thickness the magnetic behavior of the nanoparticles can be altered from ferromagnetic, via spin glass like, to a superparamagnetic character.

A novel hybrid cooling scheme is proposed to remove non-uniform heat flux in real time from the microprocessor. It consists of a liquid cooled microchannel heat sink to remove the lower background heat flux and superlattice coolers to dissipate the high heat flux present at the hotspots. Superlattice coolers (SLC) are solid-state devices, which work on thermoelectric effect, and provide localized cooling for hotspots. SLCs offer some unique advantage over conventional cooling solutions. They are CMOS compatible and can be easily fabricated in any shape or size. They are more reliable as they don't contain any moving parts. They can remove high heat flux from localized regions and provide faster time response. Experimental devices are fabricated to characterize the steady-state, as well as transient performance, of the hybrid cooling scheme. Performance of the hybrid cooling scheme has been examined under various operating conditions. Effects of various geometric parameters have also been thoroughly studied. Heat flux in excess of 300 W/cm² has been successfully dissipated from localized hotspots. Maximum cooling at the hotspot is observed to be more than 6 K. Parasitic heat transfer to the superlattice cooler drastically affects its performance. Thermal resistance between ground electrode and heat sink, as well as thermal resistance between ground electrode and superlattice cooler, affect the parasitic heat transfer from to the superlattice cooler. Two different test devices are fabricated specifically to examine the effect of both thermal resistances. An electro-thermal model is developed to study the thermal coupling between two superlattice coolers. Thermal coupling significantly affects the performance of an array of superlattice coolers. Several operating parameters (activation current, location of ground electrode, choice of working fluid) affect thermal coupling between superlattice coolers, which has been computationally as well as experimentally studied. Transient response of the superlattice cooler has also been examined through experiments and computational modeling. Response time of the superlattice cooler has been reported to be less than 35 µs.

<p>Negative differential conductivity (NDC) is a nonlinear property of electronic transport for high electric field strength found in materials and devices such as semiconductor superlattices, bulk GaAs and Gunn diodes. In spatially extended systems, NDC can cause rich dynamics such as static and mobile field domains and moving charge fronts. In this thesis, these phenomena are studied theoretically and numerically for semiconductor superlattices. Two classes of models are considered: a discrete model based on sequential resonant tunneling between neighboring quantum wells is used to described charge transport in weakly-coupled superlattices, and a continuum model based on the miniband transport is used to describe charge transport strongly-coupled superlattices.</p> <p>The superlattice is a spatially extended nonlinear system consisting a periodic arrangement of quantum wells (e.g., GaAs) and barriers (e.g., AlAs). Using a discrete model and only considering one spatial dimension, we find that the boundary condition at the injecting contact has a great influence on the dynamical behavior for both fixed voltage and transient response. Static or moving field domains are usually inevitable in this system. In order to suppress field domains, we add a side shunting layer parallel to the growth direction of the superlattice. In this case, the model includes both vertical and lateral spatial degrees of freedom. We first study a shunted weakly-coupled superlattice for a wide range of material parameters. The field domains are found to be suppressed for superlattices with small lateral size and good connection between the shunt and the quantum wells of the superlattice. As the lateral size of the superlattice increases, the uniform field configuration loses its stability to either static or dynamic field domains, regardless of shunt properties. A lower quality shunt generally leads to regular and chaotic current oscillations and complex spatio-temporal dynamics in the field profile. Bifurcations separating static and dynamic behaviors are characterized and found to be dependent on the shunt properties. Then we adopt the model to study the shunted strongly-coupled superlattice with the continuum model. Key structural parameters associated with both the shunt layer and SL are identified for which the shunt layer stabilizes a uniform electric field profile. These results support the possibility to realize a SL-based THz oscillator with a carefully designed structure.</p> <p>Another important behavior of the static field domains in the weakly-coupled superlattice is bistability, i.e., two possible states (i.e., electric field configurations) for a single voltage. Noise can drive the system from one of these states (the metastable state) to the other one (the globally stable state). The process of escape from the metastable state can be viewed as a stochastic first-passage process in a high-dimensional system that possesses complex stability eigenvalues and for which a global potential energy function does not exist. This process is simulated using a stochastic differential equation system which incorporates shot noise. The mean switching time &tau; is fitted to an exponential expression <italic>e</italic><super>(Vth-V)<super>&alpha;</super>/D</super>, where V_th denotes the voltage at the end of the current branch. The exponent &alpha; in the fitting curve deviates from 1.5 which is predicted for a generic one dimensional system. We develop an algorithm to determine an effective locally valid potential. Principal component analysis is applied to find the most probable path for switching from the metastable current state.</p> / Dissertation

Photonic band gap materials, commonly referred to as photonic crystals (PCs), have been a topic of great interest for almost two decades due to their promise of unprecedented control over the propagation and generation of light. We report investigations of the optical properties of a new PC structure based upon a triangular lattice in which adjacent [i, j] rows of holes possess different properties, creating a superlattice (SL) periodicity. Symmetry arguments predicted and quot;band folding and quot; and band splitting behaviors, both of which are direct consequences of the new basis that converts the Brillouin zone from hexagonal (six-fold) to rectangular (two-fold). Plane wave expansion and finite-difference time-domain (FDTD) numerical calculations were used to explore the effects of the new structure on the photonic dispersion relationship of the SL PC. Electron beam lithography and inductively coupled plasma dry etching were used to fabricate 1 mm2 PC areas (lattice constant, a =358 nm and 480 nm) with hole radius ratios ranging from 1.0 (triangular) to 0.585 (r2/r1 = 73.26 nm/125.26 nm) on Silicon-on-insulator wafers. The effects of modifying structural parameters (such as hole size, lattice constant, and SL strength) were measured using the coupled resonant band technique, confirming the SL symmetry arguments and corroborating the band structure calculations. Analysis of the dispersion contours of the static SL (SSL) PC predicted both giant refraction (change in beam propagation angle of 110 for an 8 change in incident angle) and superprism behavior (change in beam propagation angle of 108 for a 12% change in normalized frequency) in these structures. Dynamic control of these refraction effects was also investigated by incorporating electro-optic and nonlinear materials into the SSL PC structure. Wave vector analyses on these structures predicted a change in beam propagation angle and gt;96 when the refractive index inside of the holes of the structure changed from n=1.5 to 1.7. Through this investigation, the first successful measurement of the band folding effect in multidimensional PCs as well as the first explicit measurement of the dielectric band of a 2D PC were reported. In addition, the SL PCs impact on new opto-electronic devices was explored.