Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G.  Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication.  This licentiate thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4_ offaxis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN, and have demonstrated high quality N-polar AlGaN/GaN/AlN heterostructures. / <p>Additional funding agencies: Chalmers University of technology; ABB; Ericsson; Epiluvac; FMV; Gotmic; Saab; SweGaN; UMS; Swedish Foundation for Strategic Research under Grants No. FL12-0181, No. RIF14-055, and No. EM16-0024; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009- 00971.</p>

Formamidinium lead iodide perovskite (FAPI) holds promise to be used in high–efficiency solar cells. A comprehensive understanding of the immediate surface properties of this material can provide insights into its interaction with other materials and guide future engineering of its interface with other constituents of the solar cell. In this thesis, the orientations of the formamidinium cations on flat FAI and PbI2 terminated (001) surfaces of the of α–FAPI were studied with classical and ab initio molecular dynamics simulations at a temperature of 300 K. The cations on the FAI terminated surface displayed a preference to lie flat on the surface, while the cations near the PbI2 terminated surface displayed a preference to be oriented in such a way that the molecular planes were perpendicular to the surface. Average near edge X–ray fine structure (NEXAFS) spectra at the carbon and nitrogen K–edges were simulated from the trajectories of the FAI terminated structure and compared with existing experimental spectra obtained from a clean FAPI surface under ultrahigh vacuum. By comparing the experimental and calculated NEXAFS spectra it was observed that the distribution of formamidinium orientations which contribute to the experimental NEXAFS spectra is different from the one seen in the outermost FAI layer from the molecular dynamics simulation. This observation can be explained by the finite probing depth of the X–ray or the fact that the surface is not perfectly FAI terminated in the experiment. The more uniform distribution of formamidinium orientations one layer below the surface resulted in simulated NEXAFS spectra which were more similar to the experimental ones.

The magneto-optical Kerr effect (MOKE) describes the change in polarization of light reflected from a magnetic surface. This change is proportional to the magnetization and depends on the wavelength of light. Because of these properties, MOKE has found use as a tool to probe magnetism in matter. The aim of the project is to develop an experimental setup to measure the wavelength dependence of MOKE. This is of interest as it can be used to determine the wavelength that gives the largest change in polarization to optimize measurements and provide information about the electronic structure of the sample. Initially, the experimental setup used a laser with tunable wavelength. However, it turned out to be pulsed, which made it incompatible with the rest of the setup. Therefore, three lasers with fixed wavelengths were instead used. The setup was evaluated by measuring the Kerr rotation and hysteresis curves for three Ni samples with varying thicknesses and a TbCo sample, at three different wavelengths. The primary finding was that the setup is not suitable for spectroscopic measurements due to the inability to compare values obtained at different wavelengths. To facilitate spectroscopic measurements, a new light source with tunable wavelength is required. The setup is capable of measuring hysteresis curves.

Transition Metal Nitrides (TMN) are of considerable importance for the industry and have gathered a great deal of interest in the scientific community, mostly due to their unique physical and mechanical properties. To increase the understanding of what enables them to have such extraordinary properties requires the study of lattice dynamics and their phonon dispersion. In this thesis, the transition metal nitride, TaN, is studied extensively along with preliminary results for NbN. The primary tool for this investigation is simulations. Computational methods, such as ab initio Molecular Dynamics (AIMD) and the Temperature Dependent Effective Potential (TDEP) method are used to generate phonon spectra and to compute the lattice thermal conductivity. The results indicate that TaN crystal structure stabilizes dynamically at much lower temperatures than previously established with other methods. The average linear thermal expansion coefficient of TaN is a = 9.0 * 10-6 K-1, which is consistent with other TMN. The phonon-phonon lattice thermal conductivity of TaN follows a similar behaviour as for other TMN. Preliminary result for NbN suggests a behaviour at lower temperatures that are similar to that observed for TaN. However, further investigations are required to pinpoint TaN and NbN transition temperatures more exactly and include effects, such as electron-phonon scattering and isotope effects for a better estimation of the lattice thermal conductivity.

Spin filtering properties of novel GaNAs alloys are reported in this thesis. Spin-dependent recombination (SDR) in GaNAs via a deep paramagnetic defect center is intensively studied.  By using the optical orientation photoluminescence (PL) technique, GaNAs is shown to be able to spin filter electrons injected from GaAs, which is a useful functional property for integratition with future electronic devices.  The spin filtering ability is found to degrade in narrow GaNAs quantum well (QW) structures which is attributed to (i) acceleration of band-to-band recombination competing with the SDR process and to (ii) faster electron spin relaxation in the narrow QWs.  Ga interstitial-related defect centers have been found to be responsible for the SDR process by using the optically detected magnetic resonance (ODMR) technique. The defects are found to be the dominant grown-in defects in GaNAs, commonly formed during both MBE and MOCVD growths.  Methods to control the concentration of the Ga interstitials by varying doping, growth parameters and post-growth treatments are also examined.

The effects of strain on the magnetic phases in perovskites are of interest in the highly active research field of multiferroics. A Monte Carlo program is written to investigate the influence of strain on the low– temperature magnetic phase diagram of the manganese perovskites, RMnO3, where R is a cation in the lanthanide series. A Metropolis simulation scheme is implemented together with parallel tempering to perform computations in a two–dimensional geometry using a conventional nearest–neighbor and next–nearest–neighbor Heisenberg Hamiltonian, extended to include spin–lattice couplings and single–ion anisotropies. The latter two are important to account for structural distortions such as octahedral tilting and the Jahn–Teller effect. It is shown that even weak single–ion anisotropies render incommensurability in the otherwise structurally commensurate E–type ordering, and that the Dzyaloshinskii–Moriya interaction, in combination with single–ion anisotropies, is crucial for the stabilization of previously experimentally observed incommensurate spin spirals. Simulations performed to account for strain in the crystallographic ab–plane show that tensile strain may improve stability of E–type ordering for R elements with small atomic radii and that compressive strain drives the magnetic ordering toward the incommensurate spiral states. / Spänningsinverkan på de magnetiska faserna i perovskiter är av intresse inom den just nu högaktiva forskningen om multiferroiska material. Ett Monte Carlo-program har skrivits för att undersöka effekterna av spän- ning på de magnetiska lågtemperaturfaserna i multiferroiska manganitpe- rovskiter, RMnO3, där R är en katjon i lantanoidserien. En kombination av Metropolisalgoritmen och parallelltemperering har använts för att utföra beräkningar i tvådimensionell geometri med en konventionell Heisenberghamiltonian, utökad till att även inkludera spinn–gitterkopplingar och enkeljonsanisotropier. De senare har visats vara viktiga för att ta i beaktande den strukturella distortion i materialet som följer av t.ex. syreoktahederförskjutning och Jahn–Tellereffekten. Det visas att även svaga anisotropier orsakar inkommensurabilitet i den i övrigt kommensurabla E–typsfasen, och att Dzyaloshinskii-Moriyainteraktionen, i kombination med anisotropitermerna, är avgörande för att kunna stabilisera de sedan tidigare experimentellt bekräftade inkommensurabla spinnspiralsfaserna. Simuleringar som modellerar spänning i materialets kristallografiska ab–plan visar att dragspänning kan förbättra stabiliteten hos E–typsfasen för R–atomer med liten radie och att tryckspänning leder den magnetiska ordningen mot inkommensurabla spiraltillstånd.

Thin films are structures with thicknesses ranging from the atomic scale to the mesoscale that are used to alter the properties of a surface and/or serve as functional layers in devices. Thin metal films deposited from the vapor phase on weakly-interacting substrates, including oxides (TiO2, ZnO, SiO2 etc.) and two-dimensional (2D) materials (graphene, MoS2, etc), are relevant for a wide array of technological applications, such as optical devices, nanoelectronic components, sensors, and catalytic devices. The weak interaction between deposit and surface in these film/substrate combinations leads to three-dimensional (3D) metal-layer morphological evolution in an uncontrolled manner; which often constitutes an important challenge toward integrating metal layers in key enabling devices. Thus there is a need for efficient growth manipulation strategies, such that metal films with controlled 3D and 2D microstructures and morphologies can be synthesized. Surfactants, i.e., minority metal, non-metal, and gaseous species which are deployed to the growing surface together with film-forming species, have been shown to enable growth manipulation in a multitude of homo- and heteroepitaxial metal/metal and semiconductor/semiconductor systems. This work explores the viability of N2 and O2 surfactants to manipulate growth in model weakly-interacting Ag/SiO2 and Au/SiO2 systems. Au and Ag are deposited by direct current (DC) magnetron sputtering on Si substrates covered with a 500 nm thick thermally grown SiO2 layer. Gaseous N2 and O2 surfactants are introduced to the sputtering atmosphere either continuously during deposition or at well-defined points during growth, such that specific film-formation stages as targeted. Using a combination of in situ/real-time diagnostic tools and ex situ characterization techniques, it is shown that O2 and N2 cause Ag and Au, respectively, to grow flatter, i.e., 2D growth morphology is promoted. Moreover, by deploying surfactants selectively during early or late film growth stages and studying their effect on film morphological evolution, it is concluded that N2 and O2 effectively suppress the rate of island coalescence promoting formation of flatter films. The overall results of this study are the first step toward establishing an atomic-scale understanding of the effect of surfactants on morphological evolution of metal films on weakly-interacting substrates. The knowledge generated herein is relevant for designing growth manipulation strategies in a wide range of technologically important film/substrate systems.

In the European XFEL, electrons bunches are accelerated up to 20 GeV and thenenter undulators where coherent X-rays are produced which can be used for imaging atva molecular level. Electrons may stray from the path and hit the permanent magnets inthe undulators. It is well known that ionizing radiation can affect the magnetic characteristics of permanent magnets. The undulators are therefore equipped with a type of semiconductor dosimeters, RADFETs, so that the potential damage from ionizing radiation to the magnets can be measured and corrected for. It is also known that heat will be generated from air-coils in the accelerator which can change the ambient temperature around the dosimeters up to 25 K. All semiconductor technology is highly susceptible totemperature. This report investigates the fading characteristics of the RADFET under different temperatures and times after irradiation. It also investigates the dose responseunder dierent temperatures and estimates the magnitude of errors in measured dose which can be expected if temperatures are not accounted for. It is seen that a delta T of a few K can have a large impact on RADFETs' ability to both record and retain dose. A strong time dependence is also seen. The fading is the largest during irradiation andthen slows down exponentially, stabilizing after 1 to 2 months. An increase from 20 deg C to 26 deg C will increase the fading by 2 Gy/h during irradiation, and 0.015 Gy/h weeks afterirradiation. It is estimated that dose measurements at XFEL can have errors of up to 14% if long-term fading is not accounted for. A model for estimating long-term fading as a function of temperature is proposed.

Recent experimental and theoretical studies have found evidence of coupled interactions between magnons and phonons. The aim of this study is to construct a model of coupled magnons and phonons, as well as analysing their frequency spectrum. The model is derived by quantizing spin and lattice degrees of freedom, and the frequency spectrum is derived by solving the equations of motion. We found that both the strength and the composition of the coupled interactions affect the frequencies of magnons and phonons, with emphasis on the magnons. Their frequencies are imaginary close to the center of the Brillouin zone, which opens questions for future research.

This project focused on the study of self-assembling systems that can be inuenced by an external magnetic field, following the PhD research of Hauke Carstensen. My role was to study the behavior of beads and to optimize the tunable parameters so that the main force driving the dynamics of the system is the magnetic dipolar interaction between beads. To make sure that no other force plays an important role, we checked a number of things, the most problematic of which is flocculation in the colloid, which may happen if some beads get stuck to each other; to prevent them from aggregating we have to make sure that they have a large zeta potential, which will result in an electrically repulsive force between beads and will thus increase the stability of the colloid. We also have to make sure that other forces in the sample do not exceed the magnitude of magnetic forces between particles; examples of such forces can be the drag experienced while moving in the viscous ferrofluid, the gravity force or the random thermal movement of the molecules in the fluid. In order to study these efects, I measured the zeta potential of the magnetic and non-magnetic beads and later I added a surfactant compound (SDS) to our sample in order to increase said potential.