Suchergebnisse

We study the spin Hall magnetoresistance (SMR) in Pt grown in situ on CoFe2O4 (CFO) ferrimagnetic insulating films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO, but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, the surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are responsible for such intriguing features. In addition, a contribution from the ordinary magnetoresistance of Pt is identified, which is distinguishable only due to the low resistivity of the in situ grown Pt. ; This work is supported by the European Union under the NMP project (263104-HINTS) and the European Research Council (257654-SPINTROS), by the Spanish MINECO (MAT2012-37638, MAT2015-65159-R, MAT2014-56063- C2-1R, and SEV-2015-0496), by the Basque Government (PC2015-1-01), and by the Catalan Government (2014 SGR 734). M. I. and E. S. acknowledge the Basque Government and the Spanish MECD, respectively, for a Ph.D. fellowship (BFI-2011-106 and FPU14/03102). J. F. acknowledges stimulating discussions with Xavier Martí. ; Peer reviewed

We perform systematic investigations of transport through graphene on hexagonal boron nitride (hBN) substrates, together with confocal Raman measurements and a targeted theoretical analysis, to identify the dominant source of disorder in this system. Low-temperature transport measurements on many devices reveal a clear correlation between the carrier mobility μ and the width n* of the resistance peak around charge neutrality, demonstrating that charge scattering and density inhomogeneities originate from the same microscopic mechanism. The study of weak localization unambiguously shows that this mechanism is associated with a long-ranged disorder potential and provides clear indications that random pseudomagnetic fields due to strain are the dominant scattering source. Spatially resolved Raman spectroscopy measurements confirm the role of local strain fluctuations, since the linewidth of the Raman 2D peak-containing information of local strain fluctuations present in graphene-correlates with the value of maximum observed mobility. The importance of strain is corroborated by a theoretical analysis of the relation between μ and n* that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with n* being determined by the scalar deformation potential and μ by the random pseudomagnetic field (consistently with the conclusion drawn from the analysis of weak localization). Throughout our study, we compare the behavior of devices on hBN substrates to that of devices on SiO2 and SrTiO3, and find that all conclusions drawn for the case of hBN are compatible with the observations made on these other materials. These observations suggest that random strain fluctuations are the dominant source of disorder for high-quality graphene on many different substrates, and not only on hexagonal boron nitride. ; A. F. M. gratefully acknowledges support by the Swiss National Science Foundation (SNF) and by the National Center of Competence in Research on Quantum Science and Technology (NCCR QSIT). F. G. acknowledges support from the Spanish Ministry of Economy (MINECO) through Grant No. FIS2011-23713 and the European Research Council (ERC) Advanced Grant (Contract No. 290846). C. S. and S. E. acknowledge experimental help from F. Buckstegge, J. Dauber, B. Terrés, F. Volmer, and M. Drögeler and financial support from Deutsche Forschungsgemeinschaft (DFG) and European Research Council (ERC) (Contract No. 280140). A. F. M., F. G., and C. S. acknowledge funding from the European Union (EU) under the Graphene Flagship.

Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40-60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given. The study of materials under extreme conditions can reveal interesting physics in diverse areas such as condensed matter and geophysics. Here, the authors investigate experimentally and theoretically the high pressure-high temperature phase diagram of niobium revealing a previously unobserved phase transition from body-centered cubic to orthorhombic phase. ; Funding Agencies|Spanish Ministerio de Ciencia, Innovacion y Universidades [MAT2016-75586-C4-1-P, PGC2018-097520-A-100, RED2018-102612-T]; Generalitat ValencianaGeneralitat Valenciana [Prometeo/2018/123 EFIMAT]; Spanish governmentSpanish Government [RyC-2014-15643]; US Department of EnergyUnited States Department of Energy (DOE) [DE-AC52-07NA27344]

A three-dimensional (3D) topological insulator (TI) is a unique quantum phase of matter with exotic physical properties and promising spintronic applications. However, surface spin current in a common 3D TI remains difficult to control and the out-of-plane spin texture is largely unexplored. Here, by means of surface spin photocurrent in Bi2Te3 TI devices driven by circular polarized light, we identify the subtle effect of the spin texture of the topological surface state including the hexagonal warping term on the surface current. By exploring the out-of-plane spin texture, we demonstrate spin injection from GaAs to TI and its significant contribution to the surface current, which can be manipulated by an external magnetic field. These discoveries pave the way to not only intriguing new physics but also enriched spin functionalities by integrating TI with conventional semiconductors, such that spin-enabled optoelectronic devices may be fabricated in such hybrid structures. ; Funding Agencies|Linkoping University; Swedish Research Council [621-2011-4254, 2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Swedish Foundation for Strategic Research [EM11-0002]; Key Program of Natural Science Foundation of China [61334004]; Creative Research Group Project of Natural Science Foundation of China [61321492]; Natural Science Foundation of China [61404153]; Shanghai Pujiang Program [14PJ1410600]

Point defect research in semiconductors has gained remarkable new momentum due to the identification of special point defects that can implement qubits and single photon emitters with unique characteristics. Indeed, these implementations are among the few alternatives for quantum technologies that may operate even at room temperature, and therefore discoveries and characterization of novel point defects may highly facilitate future solid state quantum technologies. First principles calculations play an important role in point defect research, since they provide a direct, extended insight into the formation of the defect states. In the last decades, considerable efforts have been made to calculate spin-dependent properties of point defects from first principles. The developed methods have already demonstrated their essential role in quantitative understanding of the physics and application of point defect qubits. Here, we review and discuss accuracy aspects of these novel ab initio methods and report on their most relevant applications for existing point defect qubits in semiconductors. We pay attention to the advantages and limitations of the methodological solutions and highlight additional developments that are expected in the near future. Moreover, we discuss the opportunity of a systematic search for potential point defect qubits, as well as the possible development of predictive spin dynamic simulations facilitated by ab initio calculations of spin-dependent quantities. ; Funding Agencies|Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Knut & Alice Wallenberg Foundation New States of Matter 2014-2019 (COTXS); Ministry of Education and Science of the Russian Federation [14.Y26.31.0005, K2-2017-080, 211]; National Research Development and Innovation Office of Hungary (NKFIH) within the Quantum Technology National Excellence Program [2017-1.2.1-NKP-2017-00001]; EU QuantERA project Q-Magine [127889]; EU H2020 ASTERIQS project; EU QuantERA project Nanospin [127902]

Organic electrochemical transistors (OECTs) have shown great promise in a variety of applications ranging from digital logic circuits to biosensors and artificial synapses for neuromorphic computing. The working mechanism of OECTs relies on the mixed transport of ionic and electronic charge carriers, extending throughout the bulk of the organic channel. This attribute renders OECTs fundamentally different from conventional field effect transistors and endows them with unique features, including large gate-to-channel capacitance, low operating voltage, and high transconductance. Owing to the complexity of the mixed ion-electron coupling and transport processes, the OECT device physics is sophisticated and yet to be fully unraveled. Here, we give an account of the one- and two-dimensional drift-diffusion models that have been developed to describe the mixed transport of ions and electrons by finite-element methods and identify key device parameters to be tuned for the next developments in the field. ; Funding Agencies|Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [SE13-0045]; Swedish Research CouncilSwedish Research Council [2016-03979]; AForsk [18-313, 19-310]; Olle Engkvists Stiftelse [204-0256]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Thermoelectric properties of chromium nitride (CrN)-based films grown on c-plane sapphire by dc reactive magnetron sputtering were investigated. In this work, aluminum doping was introduced in CrN (degenerate n-type semiconductor) by co-deposition. Under the present deposition conditions, over-stoichiometry in nitrogen (CrN1+delta) rock-salt structure is obtained. A p-type conduction is observed with nitrogen-rich CrN combined with aluminum doping. The Cr0.96Al0.04N1.17 film exhibited a high Seebeck coefficient and a sufficient power factor at 300 degrees C. These results are a starting point for designing p-type/n-type thermoelectric materials based on chromium nitride films, which are cheap and routinely grown on the industrial scale. (C) 2018 The Japan Society of Applied Physics ; Funding Agencies|European Research Council under the European Communitys Seventh Framework Programme (FP)/ERC [335383]; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders 5 program; Swedish Research Council (VR) [621-2012-4430, 2016-03365]; Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; M.ERA-NET project MC2 - French ANR program [ANR-13-MERA-0002-02]; CTEC project [1305-00002B]

Divacancy spins implement qubits with outstanding characteristics and capabilities in an industrial semiconductor host. On the other hand, there are still numerous open questions about the physics of these important defects, for instance, spin relaxation has not been thoroughly studied yet. Here, we carry out a theoretical study on environmental spin-induced spin relaxation processes of divacancy qubits in the 4H polytype of silicon carbide (4H-SiC). We reveal all the relevant magnetic field values where the longitudinal spin relaxation time T-1 drops resonantly due to the coupling to either nuclear spins or electron spins. We quantitatively analyze the dependence of the T-1 time on the concentration of point defect spins and the applied magnetic field and provide an analytical expression. We demonstrate that dipolar spin relaxation plays a significant role both in as-grown and ion-implanted samples and it often limits the coherence time of divacancy qubits in 4H-SiC. ; Funding Agencies|Knut and Alice Wallenberg Foundation through the WBSQD2 project [2018.0071]; Swedish Government Strategic Research Area SeRC; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; MTA Premium Postdoctoral Research Program; Hungarian NKFIH grants of the National Excellence Program of Quantum-coherent materials projectNational Research, Development & Innovation Office (NRDIO) - Hungary [KKP129866]; NKFIH through the National Quantum Technology ProgramNational Research, Development & Innovation Office (NRDIO) - Hungary [2017-1.2.1-NKP-2017-00001]; Quantum Information National Laboratory - Ministry of Innovation and Technology of Hungary; Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR 2016-04068, 2018-05973]; EU H2020 project QuanTELCO [862721]

Fano resonance is a fundamental physical process that strongly affects the electronic transport, optical, and vibronic properties of matter. Here, we provide the first experimental demonstration of its profound effect on spin properties in semiconductor nanostructures. We show that electron spin generation in InAs/GaAs quantum-dot structures is completely quenched upon spin injection from adjacent InGaAs wetting layers at the Fano resonance due to coupling of light-hole excitons and the heavy-hole continuum of the interband optical transitions, mediated by an anisotropic exchange interaction. Using a master equation approach, we show that such quenching of spin generation is robust and independent of Fano parameters. This work therefore identifies spin-dependent Fano resonance as a universal spin loss channel in quantum-dot systems with an inherent symmetry-breaking effect. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-05091, 2020-04530]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

beta-Ga2O3 is a wide bandgap semiconductor that is attractive for various applications, including power electronics and transparent conductive electrodes. Its properties can be strongly affected by transition metal impurities commonly present during the growth such as Cr. In this Letter, we determine the electronic structure of Cr3+ by performing a correlative study of magneto-photoluminescence (magneto-PL) and electron paramagnetic resonance. We unambiguously prove that the so-called R-1 and R-2 PL lines at around 1.79eV originate from an internal transition between the first excited state (E-2) and the (4)A(2) ground state of Cr3+. The center is concluded to have monoclinic local symmetry and exhibits a large zero-field splitting (similar to 147 mu eV) of the ground state, which can be directly measured from the fine structure of the R1 transition. Furthermore, g-values of the first excited state are accurately determined as g(a) = 1.7, g(b) = 1.5, and g(c*) = 2.1. Our results advance our understanding of the electronic structure of Cr in beta-Ga2O3 and provide a spectroscopic signature of this common residual impurity. (C) 2021 Author(s). ; Funding Agencies|Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) - Department of the Defense, Defense Threat Reduction Agency [HDTRA1-20-2-0002]; NSF DMRNational Science Foundation (NSF) [1856662]

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions. ; Funding Agencies|Spanish Ministry of Science, Innovation and Universities; Spanish Research Agency (AEI); European Fund for Regional Development (FEDER) [MAT2016-75586-C4-1/2-P]; Generalitat Valenciana [Prometeo/2018/123]; Spanish Mineco Project [FIS2017-83295-P]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Ministry of Science and High Education of the Russian Federation [K2-2019-001]; "Juan de la Cierva" fellowship [FJCI-2016-27921]; "Ramon y Cajal" fellowship [RYC-2015-17482]

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. ; 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

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. ; 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.