Group-III nitride materials, gallium nitride (GaN), aluminum nitride (AlN) and indium nitride (InN) have direct band gaps with band gap energies ranging from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN) wavelengths and covering the entire spectral range from 0.7 eV to 6.2 eV upon alloying. The invention of the GaN-based blue LEDs, for which the Nobel prize in Physics was awarded in 2014, has opened up avenues for exploration of IIINitride material and device technologies and has inspired generations of researchers in the semiconductor field. Group-III nitrides have also been demonstrated to be among the most promising semiconductors for next generation of efficient high-power, high-temperature and high-frequency electronic devices. The need to build a sustainable and efficient energy system motivates the development of vertical GaN transistors and diodes for applications with power ratings of 50-150 kW, e.g., in electric vehicles and industrial inverters. The key is to grow GaN layers with low concentration of defects (impurities and dislocations), which enables an expansion in both voltage and current ratings and reduction of cost. Despite intense investigations and impressive advances in the field, defects are still a major problem hindering exploiting the full potential of GaN in power electronics. This Licentiate thesis focuses on the development of two different epitaxial approaches in MOCVD for reducing dislocation densities in GaN with controlled doping for power device applications: i) growth of planar GaN layers trough NWs reformation, which can be further exploited as templates for a subsequent growth of thick drift layers and ii) homoepitaxial GaN growth. Special attention is put on understanding homoepitaxial growth under different nucleation schemes and thermal stability of GaN. We have established conditions in homoepitaxy to deliver state-of-the-art GaN material with low impurity levels combined with a reasonable growth rate suitable for growth of thick drift layers. The results are summarized in two papers: In Paper I we investigate GaN layers with different thicknesses on reformed GaN NW templates and highlight this approach as an alternative to the expensive GaN HVPE substrates. The sapphire used as a substrate limits to some extent the reduction of threading dislocations, however, the resulting GaN material presents smooth surfaces and thermal conductivity close to the bulk value, which suggests the potential of this approach to be integrated in GaN development as an active material for power devices on various substrates. In Paper II extensive study of homoepitaxial GaN growth by hot-wall MOCVD is presented together with results on the thermal stability of GaN under typical conditions used in our growth reactor. Understanding the evolution of GaN surface under different gas compositions and temperatures allows us to predict optimum homoepitaxial conditions. Analysis in the framework of Ga supersaturation of epilayers simultaneously grown on GaN templates and on GaN HVPE substrates reveals that residual strain and screw dislocation densities affect GaN nucleation and growth and lead to distinctively different morphologies on GaN templates and native substrates, respectively. The established comprehensive picture provides guidance for designing strategies for growth conditions optimization in homoepitaxy. We demonstrate homoepitaxial GaN-on-GaN grown under optimum growth conditions with state-of-the-art smooth surface with an rms value of 0.021 nm and an average TDD of 1.4·106 cm-2 which provide good basis for augmenting power device structures.Future work will be focused on GaN NWs reformation on different substrates, p- and n-type doping of homoepitaxial GaN with impurity control and the fabrication of pn power diode device structures for further processing and assessment by C3NiT partners. / <p>Funding agencies: The Swedish Research Council (VR) under Grant No. 2016-00889, The Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program, Grant No. 2016-05190, The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009-00971, The Swedish Foundation for Strategic Research (SSF), under Grant No. EM16-0024</p>

The devolopment of group-III nitride semiconductor technology continues to expand rapidly over the last two decades. The indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN) compounds and their alloys are direct bandgap semiconductors with a wide bandgap range, spanning from infrared(IR) to deep-ultraviolet (UV), enabling their utilization in optoelectronic industry. The GaN-based light-emitting diode (LED) is already the commercial solution for efficient and energy saving lighting. Additionally, the physical properties of these materials such as the high critical electric field, the high saturation carrier velocity and the high thermal conductivity, make them promising candidates for replacing silicon (Si), and other wide-bandgap semiconductors such as silicon carbide (SiC) in power devices. More importantly, the polarization-induced two-dimensional electron gas (2DEG), forming at the interfaces of these semiconductors, led to the fabrication of the GaN-based high electron mobility transistor (HEMT). This device is suitable for high power (HP) switching, power amplifiers and high frequency (HF) applications in the millimeter-wave range up to THz frequencies. As such, HEMTs are suitable for 5G communication systems, radars, satellites and a plethora of other related applications. Achieving the efficient GaN blue LED (Nobel Prize in Physics 2014), came as a result of (partially) solving several material issues of which, p-type GaN was of crucial importance. Since 1992, a lot of effort is being dedicated on the understanding and overcoming of the limitations hindering efficient p-type conductivity and low hole mobility in metal-organic chemical vapor deposition (MOCVD) grown p-GaN. The limitations arise from the fact that magnesium (Mg) is the only efficient p-type dopant for GaN so far and only a very small percentage ∼2% of the incorporated Mg is active at room temperature. More limitations come from its solubility in GaN and the crystal quality deterioration and formation of inversion domains (IDs) at high doping levels. Free-hole concentrations in the low 1018 cm-3 range with mobilities at ∼10 cm2V-1s-1 demonstrate the state-of-art in MOCVD grown p-GaN, still leaving a wide window for improvement. Another intensively investigated topic is related to the aluminum gallium nitride (AlGaN)/GaN HEMTs. High electron density and mobility of the 2DEG in the range of 1013 cm-2 and ∼2400 cm2V-1s-1 respectively, are reported. Interface engineering, addition of interlayers and backbarriers are only some of the modifications introduced at the basic AlGaN/GaN HEMT structure in order to achieve the aforementioned values. Nevertheless, fundamental phenomena can still be revealed by special characterization techniques and provide a deeper understanding on the causal factors of theHEMT’s macroscopic properties. The main research results presented in this licentiate thesis are organized in three papers: In paper I we perform an in-depth investigation of the Mg-doped GaN growth by hot-wall MOCVD. We strive for exploiting any possible advantages of the hot-wall MOCVD on the growth of high-quality p-GaN relevant for use in HP devices. Additionally, we aim to gain a comprehensive understanding of the growth process and its limiting factors. The effects of growth conditions on the Mg, hydrogen (H) and carbon (C) incorporation in GaN are approached from the gallium (Ga)-supersaturation point of view. Control of the bis(cyclopentadienyl) magnesium (Cp2Mg)/trimethylgallium(TMGa) ratio, the V/III ratio and the growth temperature, resulted in high quality p-GaN growth on AlN/4H-SiC templates, showing state-of-the-art electrical properties. In paper II, we manage to increase the free-hole concentrations in as-grown GaN:Mg in two different ways, either by growing the GaN:Mg layer on a GaN/AlN/4H-SiC template, or by modifying the gas environment of the growth. It is shown that using a GaN/AlN/4H-SiC template results in higher carrier concentration and large improvement of the as-grown p-GaN resistivity. More importantly, the high amount of hydrogen (H2) flow during GaN:Mg growth, results in higher amount of non-passivated Mg in the as-grown layers allowing for high free-hole concentration and significantly lower resistivity in the as-grown p-GaN. Paper III focuses on the effect of aluminum (Al)-content variation in the barrier layer of AlGaN/GaN HEMTs. The THz-optical Hall effect (OHE) measurements revealed a peak of the 2DEG mobility followed by a decrease above certain value of Al%. We correlate this effect with the electron effective mass (meff) variation and draw conclusions about the mobility limiting mechanisms. In the low-Al regime, the mobility decreases because of the increase in meff while, in the high-Al regime, the mobility is limited by the lower carrier scattering time. / <p>Funding agencies: The Swedish Governmental Agency for Innovation Systems (VINNOVA) under the Competence Center Program Grant No.2016−05190, Linköping University, Chalmers University of technology, Ericsson, Epiluvac, FMV, Gotmic, Hexagem, Hitachi Energy, On Semiconductor, Saab, SweGaN, UMS, the Swedish Research Council VR under Award No. 2016 − 00889, the Swedish Foundation for Strategic Research under Grants No. RIF14 − 055 and No. EM16 − 0024, and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No.2009 − 00971.</p>

The need for electronic devices with new functionalities has caused research to move in a way to design and utilize materials with high-performance thermoelectricity, widely used in batteries, sensors, and electronic devices. Two-dimensional materials (2D) with unique structures and remarkable properties have been identified to fabricate oxide heteroepitaxy. The growth of heteroepitaxy has been focused on the growth of high-quality films on single crystalline substrates.   The preferred orientation and the crystallization of the materials with thin or two-dimensional structures require an understanding of epitaxy. In epitaxial growth, using a specific, well- defined substrate with lattice constants close to that film is decisive in controlling the film orientation with high epitaxial quality. The electrical, optical, magnetic, and structural properties of the film are strongly determined by the texture and its epitaxial alignment.  The majority of studies report epitaxial growth on Si and sapphire with different crystallographic orientations. The family of NaCl-structured materials covers a variety of nitrides and oxides broadly used in science and technology that have been epitaxially grown on monocrystalline Si and sapphire (Al2O3).   In this thesis, the structure and optical properties of NiO are investigated as functions of oxygen content on Si(100) and c-Al2O3 using pulsed dc reactive magnetron sputtering. It is found that NiO with cubic structure is a single phase with predominant orientation along (111) on both substrates. It is fiber textured on Si(100), while twin domain epitaxy is achieved on c-Al2O3.  The growth of two cases of metal oxide and nitride films (NiO and CrN) with rock-salt (NaCl) structure is also demonstrated on r-plane sapphire. It is revealed that the NaCl-structured materials NiO and CrN grow with a tilted orientation relative to the substrate. This characterization and analysis of the epitaxy, crystallography, and growth modes yield a single and identical epitaxial relationship of these two cubic materials on r-plane sapphire, in contrast to earlier studies on NaCl-structured materials on r-plane sapphire, indicating several different orientation relationships. The results advance the understanding of growth modes and unusual epitaxial relationships of two cases of metal oxide and nitride films with rock-salt (NaCl) structure broadly used in science and technology on r-plane sapphire.

Recent experiments have shown that the magnon velocity over nanoscale distancesin certain antiferromagnetic materials, with NiO in particular, far exceeds the previous theoretical maximum. Antiferromagnetic insulators are excellent candidatesfor spintronic nanodevices due to their exceptionally low energy dissipation, whichcould benefit the future speed at which information is stored. These magnons,which have since been dubbed ”superluminous-like magnons”, are classically notexpected, and it is hypothesized that the presence of a damping term in the equation of motion of the magnetic moment accounts for this anomalous behaviour.In this work, spin dynamics simulations are done using the UppASD software package in order to verify the existence of these superluminous-like magnons, where themagnon velocity in NiO is determined through a variety of ways. Analyzing simulated magnon spectra around high-symmetry points where the dispersion is linearallows for an extraction of magnon velocities, which shows no abnormal behaviourfor bulk NiO, as well as for large wavelength (low energy) magnons. Other ways todetermine the magnon velocity have been performed by studying the propagationof magnons that are excited through various methods. These studies also show thatthe magnon velocity does not far exceed the previous theoretical limit. While thesemagnons propagate slightly faster than they would in bulk, it is shown that thesemagnons very rapidly decelerate to their known bulk speeds.

The structural and functional development of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is essential to understand in order to enable pharmaceutical testing, disease modeling, and ultimately therapeutic use. Recent developments in the field of bioengineering have led to improvements in the efficiency and efficacy of growth methods that allow hiPSC- CMs to be studied in greater detail. However, engineered cardiac tissue still has not achieved a level of maturation necessary for the majority of biomedical applications. Thus, new technologies and methods are necessary to realize the long-term benefits of engineered cardiac tissue. To better understand the development of the tissue, further characterization of the structure and function of these cardiomyocytes is required. In this work, we describe advances using a method not commonly applied to these materials, Small Angle X-ray Scattering (SAXS). SAXS was used to characterize the structural development of hiPSC- CMs on a 3D multicellular platform in their early stages of maturation. The myofilament lattice spacing was found to monotonically decrease as the tissue matured from its initial state post-seeding at a rate between 0.75 and 1 nm per day between days 3 and 10 of maturation. With 49 total samples across three different batches of tissue, the p value for correlation between the lattice plane spacing and maturation time was p<0.05, indicating a statistically significant correlation. In tests of the tissue response to fixation with varying doses of KCl relaxation buffer, results showed a general trend of decreased myofilament spacing with increasing KCl concentration. However, in the concentrations between 60mM and 120mM, a characteristic increase in spacing is observed. Beat force was also measured prior to measuring myofilament spacing and this resulted in a graphically suggestive correlation. However, ANOVA analysis results in a p value of 0.35 which is statistically insignificant. Finally, methods were tested to monitor the myofilament lattice spacing in contracting tissue and found no evidence of contraction-based changes in the myofilament lattice. / 2024-01-15T00:00:00Z

The classical Landau-Lifshitz-Gilbert (LLG) equations are of crucialimportance in micro-magnetism, but a true quantum-mechanical descriptionwas not found until 2013. However, very few realistic quantumsystems have been modeled using it. This project describes the quantum LLG dynamics of a dimer system,accounting for the Heisenberg exchange and Dzyaloshinskii-Moriyainteractions, as well as local dephasing as an open system effect.Equations of motion are derived using an appropriate Hamiltonian, Wieser’s non-linear master equation and a two-qubit parametrization,then solved numerically. The non-locality and entanglenment of thesystem were then investigated using the CHSH inequality and concurrence. The solutions for the dimer system show oscillations in the Blochvector components aligned with the external magnetic field, and inthe anti-ferromagnetic case, both CHSH inequality violation and entanglementwere initially found, but underwent ”sudden death” anddisentanglement as the evolution continued, due to dephasing. Analysisof the kT-Bz parameter space reveals combinations which produceentanglement without violation of the Clauser, Horne, Shimony, Holt (CHSH) inequality, and regions of Bz where increasing kT increasesentanglement. This set of solutions to Wieser’s quantum LLG equation suggests thatthe disentangling effect of dephasing and other open-system effects willbe obstacles for future practical efforts in quantum communication.

ELNES-spectra of the L3 and L2-edge of bcc-iron are simulated using a Bloch wave method for the calculation of the DDSCS within a 1st-order Born approximation of the inelastic electron scattering process in order to study the influence of non-dipole terms on the EMCD-signal. The necessary electronic structure information is obtained from a DFT-calculation using WIEN2k.Two different different ways of extracting the EMCD-signal are considered: the pEMCD-signal, which is extracted from the pure imaginary part of the MDFF, and the eEMCD-signal, extracted via an EMCD difference method.A non-negligible contribution of the 13 cross term to the eEMCD-signal is found. Furthermore it is shown that the double difference method and the single left-right difference method cancel out the contributions of the 01-term to the eEMCD-signal. The pEMCD-signal is found to be influenced by non-dipole terms only for large scattering angles. Conclusive quantitative results on the influence of non-dipole contributions to the eEMCD-signal remain to be found, however, since it is uncovered that the eEMCD-signal is strongly disturbed by the choice of the post-edge normalization range due to the inaccurate description of the post-edge region in the present simulation. Furthermore a not anticipated "apparent anisotropy" of the real part of the MDFF is found, whose cause is presently unknown.As a byproduct of these investigations deeper insight is gained on a reason, why the double difference method is superior to the other extraction methods. It practically eliminates the effect of non-dipole terms 01 and 12.Lastly two effects are encountered that might pave the way to a deeper understanding of why the L2-edge is experimentally often observed to be weakened or suppressed relative to the L3-edge in comparison with simulations.

No description available.

III-V semiconductor quantum wells (QW) were grown by atmospheric pressure organometallic chemical vapor deposition, and characterized using low-temperature photoluminescence (PL), and photoluminescence excitation (PLE) spectroscopies. QWs composed of three material systems were studied: Al$\sb{x}$Ga$\sb{1-x}$As/GaAs, in which the constituent layers are essentially lattice matched, pseudomorphic GaAs/In$\sb{x}$Ga$\sb{1-x}$As, in which the In$\sb{x}$Ga$\sb{1-x}$As well layer is under biaxial compression, and pseudomorphic Al$\sb{x}$Ga$\sb{1-x}$As/GaAs$\sb{1-y}$P$\sb{y}$ in which the GaAs$\sb{1-y}$P$\sb{y}$ well layer is under biaxial tension. For each of the material systems, QW samples exhibited sharp, intense PL and PLE peaks corresponding to quantized ground-state excitonic transitions, whose energies agreed well with those predicted by a simple one-dimensional square-well model (that included strain effects if appropriate) for QW widths as narrow as 10 A. Saturation effects, including a reduction of energy broadening, and a decrease in the separation between heavy- and light-hole exciton energies, were observed in very narrow ($\approx$20 A or less) Al$\sb{x}$Ga$\sb{1-x}$As/GaAs and GaAs/In$\sb{x}$Ga$\sb{1-x}$As QWs. For GaAs/In$\sb{x}$Ga$\sb{1-x}$As QW samples, effects related to strain-induced layer roughening were observed as x was increased. The feasability of tailoring the relative energies of heavy- and light-hole excitons was demonstrated in various Al$\sb{0.35}$Ga$\sb{0.65}$As/GaAs$\sb{1-y}$P$\sb{y}$ QWs. The critical layer thickness for misfit dislocation formation was investigated, and for both strained material systems it was found that relaxation was sluggish and/or inhomogeneous. Finally, the effects of varying growth conditions, including growth temperature, AsH$\sb3$ partial pressure, growth interruption time, were studied.

Complexity, a fundamental concept in physics, encompasses phenomena spanning atomic to cosmic scales. The natural emergence of complexity can be explained by self-organized criticality. In this work, two complexity measures in magnetic systems are explored. The multiscale structural complexity (MSC) and spin temperature both capture complexity but are fundamentally different in nature and hence behave differently when subject to various temperature profiles. The MSC is extended to incorporate time correlations and compared to the time-averaged static MSC for examining spin glasses and bcc Fe at different temperatures. The spin glass transition temperature is determined with an accuracy of 1 K using the time-extended MSC, outperforming similar estimates based on the heat capacity in terms of accuracy, computational cost, and efficiency. Future work includes the optimization of coarse-graining scales in spin glasses, the investigation of transient magnetization dynamics, and the influence and loss of information of averaging magnetic unit cells before computing complexities.