The search for heat regenerators is currently very important due to the amount of wasted heat produced in different human activities. Thermoelectric materials have emerged as a possible solution to the world’s demand and reuse of energy. Recent advances have included the development of materials with tailored phonon properties, including localized "rattling" oscillator modes. In addition a number of interesting physical properties have emerged in rattling systems. This dissertation reports a study of several such systems, experimentally and computationally. Experiments performed include XRD, electron micro-probe, electrical and thermal conductivity, Seebeck coefficient measurements, dc magnetization, dc susceptibility and NMR. In the computational side several ab-initio models have been considered to understand the structural, vibrational and magnetic properties observed in these compounds. Among the studied compounds, the Fe-Al-Zn materials showed interesting magnetic properties combined with anomalous vibrational behavior in a chain geometry. Computational results indicated that the moment is affected by Fe antisites, but also the neighbor configuration contributes to it. Al-V-La is an example of a classical Einstein oscillator material. These properties are related to the existence of loose atoms inside the material. A purely computational study on these materials denoted the existence of two weakly bonded sites. The clathrate structural results from first-principles considerations elucidated the preferred structural configurations in several clathrates. This included Ba-Cu-Ge clathrates, where it was confirmed that the compound follows the Zintl electron counting balance. Also the bonding inside these materials was studied to address the binding of the local-oscillator atoms within the material. For Ba-Ga-Sn clathrates an unusual dimorphism was studied, with both of the two different types of structures investigated. For type-I Ba8Ga16Sn30 the preferred configuration was obtained from NMR lineshape simulations and energy considerations. For the type-VIII Ba8Ga16Sn30 the experimental thermoelectric properties were analyzed in conjunction with computational modeling. Finally in Ba-Al-Ge clathrates the local environments, preferred configuration and vacancy formation were clarified. This included an extensive experimental and computational study on Ba8AlxGe46-x-y2(box)y systems. The different local Al environments were elucidated, with the location of vacancies influencing the surroundings. Also the correlation between the Al substitution and number of vacancies was studied.

Two classes of materials are investigated using ab intio methods based on density functional theory. The structural properties, electronic structure and thermodynamic properties of binary and ternary transition metal carbides are discussed in details. In addition, two actinide compounds will be presented. A new actinide monoxide, ThO, is predicted to be stable under pressure, and the weakly correlated UN is investigated as regards to its magnetic properties and electronic structure. The atomic and electronic structures of various types of single defects in TiC such as vacancies, interstitial defects, and antisite defects are investigated systematically. Both the C-poor and C-rich off-stoichiometric Ti1-cCc composition (0.49≤c≤0.51) have been studied. For the electronic structure, the difference of density of states (dDOS) is introduced to characterize the changes produced by the defects. Concerning the atomic structures, both interstitial defects and antisites are shown to induce the formation of C dumbbells or Ti dumbbells. To date, the Ti self-diffusion mechanism in TiC has not been fully understood, and particularly the Ti diffusion is much less studied in comparison with the C diffusion. Therefore, the self-diffusion of Ti in sub-stoichiometric TiC is studied, and the formation energies, migration barriers for Ti interstitials, dumbbells and dumbbell-vacancy clusters are reported. Some of the calculated activation energies are close to the experimental values, and the migration of Ti dumbbell terminated by C vacancies gives the lowest activation energy, which is in good agreement with the experimental data. These studies must be continued to obtain a full description (including phonon contributions, prefactors, etc.) of all the feasible diffusion mechanisms in TiC. The focus is then shifted from the light transition metal carbides to the heavy transition metal carbides. Various structures of Ru2C under ambient conditions are explored by using an unbiased swarm structure searching algorithm. The structures with R3m (one formula unit) and R-3m symmetry (two formula units) have been found to be lower in energy than the P-3m1 structure, and also to be dynamically stable at ambient conditions. The R-3m structure is characterized by emergence of the Ru-Ru metallic bonding, which has a crucial role in diminishing the hardness of this material. The study of correlation and relativistic effects in Ta2AlC is also presented. We have shown that going from a scalar relativity to a fully relativistic description does not have a significant effect on the computed electronic and mechanical properties of Ta2AlC. In addition, the calculations show that the structural and mechanical properties of Ta2AlC are strongly dependent on other details of theoretical treatment, such as the value of the Hubbard U parameter. The comparison between our results and experimental data point to that Ta2AlC is a weakly correlated system, which originates from that the 5d band is relatively wide in comparison with that of the 3d band. The existence of a rock salt Thorium monoxide (ThO) under high pressure is theoretically predicted. A chemical reaction between Th and ThO2 can produce a novel compound thorium monoxide under sufficient external pressure. To determine the pressure range where this reaction can be observed, we have identified two extreme boundaries by means of different theoretical approaches. The first one is given by a fully relativity DFT code in local density approximation (LDA). The second one is given by a scalar relativistic DFT code in generalized gradient approximation (GGA). It is found that ThO is energetically favored between 14 and 26 GPa. The f orbitals are filled at the expense of s and d electrons states of Th metal, under the action of pressure. The d-p hybridization leads to the stability of metallic ThO. Dynamical stability is also investigated by computing the phonon dispersions for the considered structures at high pressure. The electronic structure and magnetic properties of a promising nuclear fuel material, uranium mononitride (UN), are studied by means of density functional theory (DFT) and several extensions, such as dynamical mean-field theory (DMFT), disordered local moment (DLM) approach, and the GW method. The role of the relativistic corrections is analyzed for different levels of approximation. The importance of correlation effects is assessed through a detailed comparison between calculated electronic structure and measured photoemission spectrum, which helps to clarify the dual itinerant/localized nature of the 5f states of U in UN. Important effects are also observed for the 2p states of nitrogen, which are positioned at much lower energies that are difficult to be well treated in the conventional electronic structure calculations. / <p>QC 20141219</p>

Oxide interfaces have attracted considerable attention in recent years due to emerging novel properties that do not exist in the corresponding parent compounds. Furthermore, modern atomic-scale growth and probe techniques enable the formation and study of new artificial interface states distinct from the bulk state. A central issue in controlling the novel behavior in oxide heterostructures is to understand how various physical variables (spin, charge, lattice and/or orbital hybridization) interact with each other. In particular, density function theory (DFT) has provided significant insight into underlying physics of materials at the atomic level, giving quantitative results consistent with experiment. In this dissertation using density functional theory methods, we explore the electronic, magnetic and structural properties developed near the interface in SrTiO3/LaAlO3, EuO/LaAlO3, Fe/PbTiO3/Pt, Fe//BaTiO3/Pt and Cs/SrTiO3 heterostructures. We study the interplay between physical interactions, and quantify parameters that determine physical properties of hetetrostructures. These theoretical studies help understanding how physical variables couple with each other and how they determine new properties at oxide interfaces. / text

In this Thesis, I have investigated multifunctional nanostructured Ti-Si-C thin films synthesized by magnetron sputtering in the substrate-temperature range from room temperature to 900 °C. The studies cover high-temperature growth of Ti3SiC2 and Ti4SiC3, low-temperature growth of Ti-Si-C nanocomposites, and Ti-Si-C-based multi¬layers, as well as their electrical, mechanical, and thermal-stability properties. Ti3SiC2 and Ti4SiC3 were synthesized homoepitaxially onto bulk Ti3SiC2 from individual sputtering targets and heteroepitaxially onto Al2O3(0001) substrates from a Ti3SiC2 target at substrate temperatures of 700 – 900 °C. In the latter case, the film composition exhibits excess C compared to the nominal target composition due to differences between species in angular and energy distribution and gas-phase scattering processes. Ti buffering is shown to compensate for this excess C. The electrical-resistivity values of Ti3SiC2 and Ti4SiC3 thin films were measured to 21-32 uOhmcm and ~50 uOhmcm, respectively. The good conductivity is because the presence of Si layers enhances the relative strength of the metallic Ti-Ti bonds. The higher density of Si layers in Ti3SiC2 than in Ti4SiC3 explains why Ti3SiC2 is the better conductor of the two. Ti3SiC2 thin films are shown to be thermally stable up to 1000 – 1100 °C. Annealing at higher temperature results in decomposition of Ti3SiC2 by Si out-diffusion to the surface with subsequent evaporation. Above 1200 °C, TiCx layers recrystallized. Nanocomposites comprising nanocrystalline (nc-)TiC in an amorphous (a-)SiC matrix phase were deposited at substrate temperatures in the range 100 – 300 °C. These nc-TiC/a-SiC films exhibit low contact resistance in electrical contacts and a ductile deformation behavior due to rotation and gliding of nc-TiC grains in the matrix. The ductile mechanical properties of nc-TiC/a-SiC are actually more similar to those of Ti3SiC2, which is very ductile due to kinking and delamination, than to those of the brittle TiC. Epitaxial TiC/SiC multilayers deposited at ~550 °C were shown to contain cubic SiC layers up to a thickness of ~2 nm. Thicker SiC layers gives a-SiC due to the corresponding increase in interfacial strain energy leading to loss of coherent-layer growth. Nanoindentation of epitaxial Ti3SiC2/TiC0.67 nanolaminates showed inhibition of kink-band formation in Ti3SiC2, as the lamination with the less ductile TiC effectively hindered this mechanism. / Materialteknik har alltid varit en central del av människans historia, och en förutsättning för utvecklingen av civilisationen. Dess betydelse märks inte minst på hur vi uppkallat historiska perioder efter vilka material som använts: stenåldern, bronsåldern och järnåldern (kiselåldern?). Modern materialvetenskap däremot handlar inte bara om att tillverka och utveckla material, utan även om att förstå sambandet mellan tillverknings¬processen, materialets struktur och dess egenskaper – samt hur denna förståelse kan användas för att designa material. I min avhandling sammanstrålar tre begrepp inom materialvetenskap, (multi-)funktionalitet, nanoteknik (nanostruktur) och tunna filmer. Inom materialvetenskap och materialteknik skiljer man på begreppen strukturmaterial, som väljs ut för sin förmåga att bära en last (t.ex. byggmaterial) och funktionella material, där det intressanta är materialets funktion, t.ex. elektriska, magnetiska, optiska eller vissa mekaniska egenskaper. Multifunktionella material är material som är utvalda eller designade för att ha flera funktioner – exempelvis god elektrisk ledningsförmåga, nötningsmot-stånd och korrosionsmotstånd. Nanoteknik handlar om material (strukturer, maskiner, etc…) där åtminstone någon dimension är på nanometerskalan (nanometer = miljarddels meter). Men det räcker inte med att enbart vara liten – nanoteknik betyder att man får nya funktioner tack vare storleken. I samhällsdebatten beskrivs nanoteknik ofta utifrån visioner om möjliga framtida kvantdatorer, molekylfabriker, medicinska ”cell-robotar”, och så vidare; det finns också negativa visioner som den om självkopierande nanorobotar som tar över världen och ut-rotar allt liv. Men om man ignorerar dessa långsiktiga och/eller långsökta visioner, så är det viktigt att inse att nanotekniken finns i våra vardagsliv redan idag, och det är framför allt som materialteknik som nanotekniken har lämnat snackstadiet och blivit verkstad. Många kommersiella produkter idag innehåller nanostrukturerade material, det vill säga material där nya funktioner uppnås genom att designa materialets struktur på nanonivå. Anledningen att man ofta vill belägga en yta med ett lager av något annat är att ytbeläggningen förändrar – förhoppningsvis till det bättre! – egenskaperna hos det belagda objektet. Det är därför man målar huset eller lackerar köksbordet. Med tunna filmer menar man ytbeläggningar tunnare än någon eller några mikrometer (miljondels meter). Antireflexbehandlingen på glasögon och teflonet i stekpannan är några exempel från vardagen. Processen jag använt kallas sputtring (egentligen heter det katodförstoftning på svenska, men ingen använder det ordet!), och äger rum i en vakuumkammare där trycket kan vara så lågt som en biljondel av atmosfärtrycket. Där placerar man det material man vill göra en tunnfilm av. Sedan släpper man in en gas, oftast en ädelgas som argon, som får bilda ett plasma, det vill säga en gas som mest består av laddade partiklar (joner). Argonjonerna accelereras med hög energi och får bombardera materialet; då slås atomer av ämnet ut och sprids i vakuumkammaren. De kan sedan kondensera på den yta man vill belägga och bilda en tunnfilm. En stor fördel med denna ”biljard på atomnivå” är att man har väldigt stora möjligheter att styra hur filmen bildas och växer, med andra ord går det att designa filmens struktur och i förlängningen dess egenskaper. Det material jag studerat är titankiselkarbid, alltså ett ternärt material – det består av tre grundämnen (titan, kisel och kol). Varför ett så krångligt val – hade det inte varit mycket enklare att bara använda ett eller två grundämnen? Visst hade det varit enklare – men också tråkigare! Det blir visserligen mer komplicerat av att lägga till fler grundämnen, men flexibiliteten och designmöjligheterna ökar i motsvarande grad. I titankiselkarbid¬systemet kan jag tillverka en rad olika typer nanostrukturerade material, där de viktigaste kanske är Ti3SiC2, vars fascinerande struktur påminner om ett laminatgolv på nanonivå, och nanokompositer, med små titankarbidkristaller inbakade i amorft material. Båda dessa har unika egenskaper tack vare sin nanostruktur – de är hyfsade elektriska ledare, lagom hårda utan att vara för hårda, inte spröda, korrosionsbeständiga, och så vidare. Kort sagt, de är Multifunktionella nanostrukturerade tunna filmer av titankiselkarbid!

This thesis concerns the materials science of carbon-based fullerene-like structures as a basis for the improvement of the applicability of FL-CNx protective thin films. In particular, structural origins of mechanical properties of FL-CNx coatings and water adsorption on their surface were investigated, both of which are critical parameters for their application as, e.g., computer hard disk protective coatings. Also, prospective FL-CPx structures were investigated by first-principles modeling. I present an introduction to theoretical methods used to study the effects of nitrogen and phosphorus as dopant elements. The modeling results include pure phosphorus clusters, mixed carbon-phosphorus clusters, and growth of fullerenelike phospho-carbide structures. Finally, I present some implications for the synthesis of FL-CPx thin films and the extension of the research to other dopant elements including sulphur, arsenic, and germanium.

<p>This thesis concerns the materials science of carbon-based fullerene-like structures as a basis for the improvement of the applicability of FL-CN_x protective thin films. In particular, structural origins of mechanical properties of FL-CN_x coatings and water adsorption on their surface were investigated, both of which are critical parameters for their application as, e.g., computer hard disk protective coatings. Also, prospective FL-CP_x structures were investigated by first-principles modeling. I present an introduction to theoretical methods used to study the effects of nitrogen and phosphorus as dopant elements. The modeling results include pure phosphorus clusters, mixed carbon-phosphorus clusters, and growth of fullerenelike phospho-carbide structures. Finally, I present some implications for the synthesis of FL-CP_x thin films and the extension of the research to other dopant elements including sulphur, arsenic, and germanium.</p>

Silicon Carbide (SiC) is an important wide band gap semiconductor with outstanding electronic properties. With figures of merit far better than silicon, SiC is believed to replace and outcompete silicon in many applications using high frequencies, high voltage and high temperatures. With the introduction of seeded sublimation technique, a realisation of substrates with large diameter and high quality became possible. Recent progress in the bulk growth using high temperature chemical vapour deposition (HTCVD) has shown excellent results with high purity substrates with semi insulating (SI) properties. The availability of high quality SI substrates allows the fabrication of microwave devices with low rf losses such as the Metal Schottky Field Effect Transistor (MESFET). With the introduction of the hot-wall CVD technique, thick low doped n-type epitaxial layers have been grown for high power devices (&gt; 4 kV) such as the PiN diode. The main contribution of the present work relates to the investigation of growth of MESFET structures. The goal has been to demonstrate the ability to grow MESFET structures using the hot-wall CVD technique. The challenge with abrupt interfaces and controlled doping has been investigated. A comprehensive investigation has been made on how nitrogen and aluminum dopant atoms incorporate into the SiC lattice using the hot-wall CVD technique. Fundamental research of MESFET structures has been combined with growth of device structures for both Swedish and European groups as well as industries. The research has been focused towards the understanding of dopant incorporation, characterization of doped epitaxial layers, the growth of device structures, the modelling of temperature distribution in a hot-wall susceptor and the development of growth systems for future up scaling. In paper 1 we present how the nitrogen dopant is incorporated into the SiC lattice. The influence of several different growth parameters on the nitrogen incorporation is presented. Equilibrium thermodynamical calculations have been performed to give a further insight into the incorporation mechanism. The investigation shows that the N2 molecule itself does not contribute directly to the nitrogen incorporation, however, molecules like the HCN and HNC are more likely. In paper 2 the incorporation of the aluminum dopant into the SiC lattice is investigated in a similar way as the nitrogen incorporation in paper 1. The results show that the aluminum incorporation in SiC is mainly controlled by the carbon coverage on the SiC surface. The investigation shows that it is difficult to obtain high aluminum doping on carbon face whereas the silicon face is sensitive to changes of the growth parameters. High growth rate resulted in a diffusion controlled incorporation. In Paper 3 we present the results from the growth of MESFET structures as well as characterization of the structures and final device properties. Knowledge taken from paper 1 and 2 was used to improve the abruptness of the grown structures. Paper 4 presents the results obtained by low temperature photoluminescence (LTPL) on separately grown 4H-SiC epitaxial layers. Doping calibration curves for nitrogen in the doping range from 1⋅1014 to 2⋅1019 cm-3 are presented. A discussion concerning the Mott transition is also presented. Paper 5 presents the results of the use of simulation to investigate the heating of a hot-wall CVD reactor. New susceptor and coil design are tested. The simulation has been verified with experimental heating tests which show excellent agreement. The new design has a temperature variation of less than 0.5 % over more than 70% of the total susceptor length in addition to a decreased power input of 15 %. In the final two papers, paper 6 and 7, we present work of growth of AlN on SiC. Thin films were grown and characterized with different techniques concerning crystal quality and thickness. The use of infrared reflectance and the features of the AlN reststrahl reflectance band allowed us to determine the thickness of AlN films as thin as 250 Å.

This thesis presents results from the development of quantum dot (QD) based infrared photodetectors (IPs). The studies include epitaxial growth of QDs, investigations of the structural, optical and electronic properties of QD-based material as well as characterisation of the resulting components. Metal-organic vapour phase epitaxy is used for growth of self-assembled indium arsenide (InAs) QDs on gallium arsenide (GaAs) substrates. Through characterisation by atomic force microscopy, the correlation between size distribution and density of quantum dots and different growth parameters, such as temperature, InAs deposition time and V/III-ratio (ratio between group V and group III species) is achieved. The V/III-ratio is identified as the most important parameter in finding the right growth conditions for QDs. A route towards optimisation of the dot size distribution through successive variations of the growth parameters is presented. The QD layers are inserted in In0.15Ga0.85As/GaAs quantum wells (QWs), forming so-called dots-in-a-well (DWELL) structures. These structures are used to fabricate IPs, primarily for detection in the long wavelength infrared region (LWIR, 8-14 μm). The electron energy level schemes of the DWELL structures are revealed by a combination of different experimental techniques. From Fourier transform photoluminescence (FTPL) and FTPL excitation (FTPLE) measurements the energy level schemes of the DWELL structures are deduced. Additional information on the energy level schemes is obtained from tunneling capacitance measurements and the polarization dependence studies of the interband transitions. From tunneling capacitance measurements, the QD electron energy level separation is confirmed to be 40-50 meV and from the polarization dependence measurements, the heavy hole character of the upper hole states are revealed. Further characterisation of the IPs, by interband and intersubband photocurrent measurements as well as dark current measurements, is performed. By comparing the deduced energy level scheme of the DWELL structure and the results of the intersubband photocurrent measurements, the origin of the photocurrent is determined. The main intersubband transition contributing to the photocurrent is identified as the QD ground state to a QW excited state transition. Optical pumping is employed to gain information on the origin of an additional photocurrent peak observed only at temperatures below 60 K. By pumping resonantly with transitions associated with certain quantum dot energy levels, this photocurrent peak is identified as an intersubband transition emanating from the quantum dot excited state. Furthermore, the detector response is increased by a factor of 10, when using simultaneous optical pumping into the quantum dots states, due to the increasing electron population created by the pumping. In this way, the potentially achievable responsivity of the detector is predicted to be 250 mA/W. Significant variations of photocurrent and dark currents are observed, when bias and temperature are used as variable parameters. The strong bias and temperature dependence of the photocurrent is attributed to the escape route from the final state in the QW, which is limited by tunneling through the triangular barrier. Also the significant bias and temperature dependence of the dark current could be explained in terms of the strong variation of the escape probability from different energy states in the DWELL structure, as revealed by interband photocurrent measurements. These results are important for the future optimisation of the DWELL IP. Tuning of the detection wavelength within the LWIR region is achieved by means of a varying bias across the DWELL structure. By positioning the InAs quantum dot layer asymmetrically in a 8 nm wide In0.15Ga0.85As/GaAs quantum well, a step-wise shift in the detection wavelength from 8.4 to 10.3 μm could be achieved by varying the magnitude and polarity of the applied bias. These tuning properties could be essential for applications such as odulators and dual-colour infrared detection. / On the day of the defence date the status on article IV was: Accepted.

Starting from silicon on insulator (SOI) material, with a top silicon layer thickness of 100 nm, silicon nanowires were fabricated in a top down approach using electron beam (e-beam) lithography and subsequent eactive ion etching (RIE) and oxidation. Nanowires as narrow as 30 nm could be achieved. Further size reduction was done using electrochemical etching and/or oxidation. The nanowires were contacted creating drain, source and back gate contacts and characterized showing similar behavior as Schottky Barrier Metal Oxide Semiconductor Field Effect Transistors (SB-MOSFETs). As an alternative, by thinning the top silicon layer down nanoribbons, ~ 1 μm wide, with a thickness down to 45 nm could be produced using standard optical lithography showing similar behavior as the nanowires. The conduction mechanism for these devices is through electrons in an inversion current layer for positive back gate voltages and through holes in accumulation mode for negative back gate voltages. When the threshold voltage is extrapolated for the nanowires and the nanoribbons it scales with inverse width and thickness respectively, attributed to charged surface and/or interface states affecting more narrow/thinner devices essentially due to increased surface to volume ratio. Nanowires were functionalized with 3-aminopropyl triethoxysilane (APTES) molecules creating amino groups on the surface reactive to pH buffer solutions. By exposing the nanowires to buffer solutions of different pH value the conduction mechanism changed due to the surface becoming more or less negative. Threshold voltage shifts from pH = 3 to pH = 9 were seen to scale with inverse width again attributed to the larger surface to volume ratio for more narrow devices. Simulations confirm this behavior and further show that a charge change of a few elementary charges on the nanowire surface can alter the conductance significantly. Upon addition of the buffer solutions the channel is seen to be quenched for small drain bias attributed to negative surface charges screening the electron current. However, as the drain bias is increased the channel is restored. Computer simulations confirmed this behavior and further showed that the restoration of the channel was due to an avalanche process. A biomolecule detection experiment was set up using the specific binding of biotin to streptavidin. By functionalizing the nanoribbon with biotin molecules the current can be logged and as streptavidin molecules are added the current decreases (increases) if the nanoribbon is run in inversion (accumulation) mode due to the negative charge of the streptavidin molecule, delivered upon binding to biotin. A sensitivity significantly below the picomolar range was observed, corresponding to less than 20 streptavidin molecules attaching to the nanoribbon surface, assuming a homogeneous binding to the biotinylated surface. By decreasing the nanoribbon thickness the response was increased, a behavior attributed to the larger surface to volume ratio of these devices. The response was seen to be larger in the accumulation mode whereas close to the lower oxide in inversion mode. Computer simulations showed that this was due to the hole current running closer to the functionalized surface in accumulation mode and opposite in inversion mode. This was further investigated for different nanoribbon thicknesses and the response was shown to increase with inverse nanoribbon thickness again attributed to the larger surface to volume ratio. The nanoribbon has the advantage of simpler fabrication using standard optical lithography in comparison with e-beam lithography and it may provide a useful scheme for a practical biomolecule sensor. / QC 20100719

<p>The present work describes the use of atomistic computer simulations in the area of Condensed Matter Physics, and specifically its application to the study of two problems: the dynamics of the melting phase transition and the properties of materials at extreme high pressures and temperatures, problems which defy experimental measurements and purely analytical calculations.</p><p>Both classical Molecular Dynamics (using semi–empirical interaction potentials) and first–principles (<em>ab initio</em>) Molecular Dynamics techniques has been applied in this study to the calculation of melting curves in a wide range of pressures for elements such as Xe and H₂, the study of the elastic constants of Fe at the conditions of the Earth’s inner core, and the characterization of diffusion and defects formation in a generic Lennard–Jones crystal at the limit of superheating, including the role they play in the triggering of the melting process itself.</p>