Mg3Sb2-based thermoelectric materials attract attention for applications near room temperature. Here, Mg-Bi films were synthesized using magnetron sputtering at deposition temperatures from room temperature to 400 °C. Single-phase Mg3Bi2 thin films were grown on c-plane-oriented sapphire and Si(100) substrates at a low deposition temperature of 200 °C. The Mg3Bi2 films grew epitaxially on c-sapphire and fiber-textured on Si(100). The orientation relationships for the Mg3Bi2 film with respect to the c-sapphire substrate are (0001) Mg3Bi2‖(0001) Al2O3 and [112⎯⎯2¯0] Mg3Bi2‖[112⎯⎯2¯0] Al2O3. The observed epitaxy is consistent with the relatively high work of separation, calculated by the density functional theory, of 6.92 J m−2 for the Mg3Bi2 (0001)/Al2O3 (0001) interface. Mg3Bi2 films exhibited an in-plane electrical resistivity of 34 μΩ m and a Seebeck coefficient of +82.5 μV K−1, yielding a thermoelectric power factor of 200 μW m−1 K−2 near room temperature.This work was supported financially by the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), the Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program (No. KAW-2020.0196), the Swedish Research Council (VR) under Project Nos. 2016-03365 and 2021-03826, the National Key Research and Development Program of China under Grant No. 2018YFB0703600, the National Natural Science Foundation of China under Grant No. 51872133, the Guangdong Innovative and Entrepreneurial Research Team Program under Grant No. 2016ZT06G587, and the Tencent Foundation through the XPLORER PRIZE, Guangdong Provincial Key Laboratory Program (No. 2021B1212040001) from the Department of Science and Technology of Guangdong Province. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at National Supercomputer Centre (NSC) partially funded by the Swedish Research Council through Grant Agreement No. 2018-05973.
We report structure, vibrational properties, and weak antilocalization-induced quantum correction to magnetoconductivity in single-crystal Bi2GeTe4. Surface band-structure calculations show a single Dirac cone corresponding to topological surface states in Bi2GeTe4. An estimated phase coherence length, lΦ ~ to 143 nm and prefactor α~-1.54 from Hikami-Larkin-Nagaoka fitting of magnetoconductivity describe the quantum correction to conductivity. An anomalous temperature dependence of A1g Raman modes confirms enhanced electron-phonon interactions. Our results establish that electrons of the topological state can interact with the phonons involving the vibrations of Bi-Te in Bi2GeTe4. ; Funding Agencies|CSIRCouncil of Scientific & Industrial Research (CSIR) - India; SERBDepartment of Science & Technology (India)Science Engineering Research Board (SERB), India [CRG/2018/002197]; DST India grant [DST/INT/SWD/VR/P-18/2019]; Swedish Research Council (VR)Swedish Research Council [2016-03365, 2018-07070]; Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program [KAW 2020.0196]; Electron Microscopy Laboratory at Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Energy AgencySwedish Energy AgencyMaterials & Energy Research Center (MERC) [46519-1]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [RIF 14-0074]
We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn2GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method, we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically ordered phases of Mn2GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro-and antiferromagnetic configurations of Mn2GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn2GaC, implying the importance of orbital-dependent localization of the Mn 3d states. We find that Mn2GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by Hunds exchange interaction, and not the Coulomb interaction. ; Funding Agencies|Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish e-Science Research Centre (SeRC); Swedish Research Council (VR)Swedish Research Council [2019-05600]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [EM16-0004]; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; state assignment of Minobrnauki of Russia [AAAA-A18-1180201900985]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]
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.
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
Different theoretical methodologies are employed to investigate the effect of hydrostatic pressure and anisotropic stress and strain on the superconducting transition temperature ( T-c) of MgB2. This is done both by studying Kohn anomalies in the phonon dispersions alone and by explicit calculation of the electron-phonon coupling. It is found that increasing pressure suppresses T-c in all cases, whereas isotropic and anisotropic strain enhances the superconductivity. In contrast to trialed epitaxial growth that is limited in the amount of achievable lattice strain, we propose a different path by co-deposition with ternary diborides that thermodynamically avoid mixing with MgB2. This is suggested to promote columnar growth that can introduce strain in all directions. ; Funding Agencies|Knut and Alice Wallenberg (KAW) FoundationKnut & Alice Wallenberg Foundation [KAW 2015.0043]; Swedish Research Council (VR) through International Career GrantSwedish Research Council [2014-6336, 2019-05403]; Marie Sklodowska Curie Actions [INCA 600398]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Foundation for Strategic Research through the Future Research Leaders 6 program [FFL 15-0290]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Office of the Permanent Secretary, Ministry of Higher Education, Science, Research and Innovation: Research Grant for New Scholar [RGNS 63-013]; Swedish National Infrastructure for Computing (SNIC); National Supercomputer Centre (NSC)
A variety of bulk high-entropy alloy superconductors have been recently discovered; however, for thin films, only the TaNbHfZrTi highentropy alloy system has been investigated for its superconducting properties. Here, (TiZrNbTa)1-xWx and (TiZrNbTa)1-xVx superconducting films have been produced by DC magnetron sputtering at different growth temperatures. The phase formation and superconducting behavior of these films depend on the content of alloying x and deposition temperature. A single body-centered cubic (bcc) phase can be formed in the low x range with enough driving energy for crystallinity, but phase transition between amorphous or two bcc structures is observed when increasing x. The highest superconducting transition temperature Tc reaches 8.0 K for the TiZrNbTa film. The superconducting transition temperature Tc of these films deposited at the same temperature decreases monotonically as a function of x. Increasing deposition temperature to 400 °C can enhance Tc for these films while retaining nearly equivalent compositions. Our experimental observations suggest that Tc of superconducting high entropy alloys relate to the atomic radii difference and electronegativity difference of involved elements beyond the valence electron number. ; Funding: Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; VINNOVA Competence Centre FunMat-II [2016-05156]; Knut and Alice Wallenberg Foundation through the Wallenberg Academy Fellows program [KAW-2020.0196]; Swedish Research Council [2021-03826]
Energy level alignment (ELA) at donor-acceptor heterojunctions is of vital importance yet largely undetermined in organic solar cells. Here, authors determine the heterojunction ELA with (mono) layer-by-layer precision to understand the co-existence of efficient charge. Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1-2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems. ; Funding Agencies|Swedish Research Council [2016-05498, 2016-05990, 2020-04538, 2018-06048, 2018-07152]; Swedish Energy Agency [45411-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Wallenberg Wood Science Center (WWSC); Stiftelsen for Strategisk Forskning through a Future Research Leader program [FFL18-0322]; Swedish Governmental Agency for Innovation Systems [2018-04969]; Formas [2019-02496]
The experimental study of hydrogen-bonds and their symmetrization under extreme conditions is predominantly driven by diffraction methods, despite challenges of localising or probing the hydrogen subsystems directly. Until recently, H-bond symmetrization has been addressed in terms of either nuclear quantum effects, spin crossovers or direct structural transitions; often leading to contradictory interpretations when combined. Here, we present high-resolution in-situ 1H-NMR experiments in diamond anvil cells investigating a range of systems containing linear O-H ⋯ O units at pressure ranges of up to 90 GPa covering their respective H-bond symmetrization. We found pronounced minima in the pressure dependence of the NMR resonance line-widths associated with a maximum in hydrogen mobility, precursor to a localisation of hydrogen atoms. These minima, independent of the chemical environment of the O-H ⋯ O unit, can be found in a narrow range of oxygen oxygen distances between 2.44 and 2.45 Å, leading to an average critical oxygen-oxygen distance of Å. ; Funding: German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) [DU 393/13-1, DU 393/9-2, STE 1105/13-1, ME 5206/3-1, DU 954/111]; Federal Ministry of Education and Research, Germany (BMBF) [05K19WC1]; Center for High Pressure Science and Technology Advance Research, Beijing, P.R. China; Swedish Research Council (VR) [2019-05600]; Alexander von Humboldt Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]
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]
For the high-power (HP) electronic applications the existing Si-based devices have reached the performance limits governed by the material properties. Hence the device innovation itself is unable to enhance the overall performance. GaN, a semiconductor with wide bandgap, high critical breakdown field, and high electronic saturation velocity is regarded as an alternative of Si. The material properties of GaN make it very suitable for fast-switching HP electronic devices and contribute to the fast growing of GaN technology. The state-of-the-art GaN devices operating up to 650 V have recently become commercially available. Further goal is to reach higher breakdown voltage which can be done via device engineering and material growth optimization. AlxGa1−xN is an ultrawide-bandgap (UWBG) semiconductor which is considered as a natural choice for next generation in the development of GaN-based HP electronic devices. This material attracts particular interest due to the possibility for bandgap tuning from 3.4 eV to 6 eV which allows nonlinear increase of avalanche breakdown field. Furthermore, both n- and p-type conductivity can be achieved on this material permitting variety of device design with reduced energy losses during operation. β−Ga2O3 is also a promising material for HP electronics because of its ultra-wide bandgap (4.8 eV) and a huge value of Baliga's figure of merit (FOM) exceeding by far that of GaN. More interesting feature making this material attractive is the availability of low-cost natural substrates, and then the possibility to obtain high crystal quality of device structures. For the HP electronic devices thermal conductivity is one of the key parameters determining the device's performance. The initial studies have shown that the thermal conductivity of AlxGa1−xN and β−Ga2O3 is quite low comparing with that of GaN. This is one of the biggest challenges slowing the development of these materials for HP device applications. Nevertheless, AlxGa1−xN- and β−Ga2O3-based field-effect transistors and Schottky-barrier diodes have been demonstrated showing performances superior to that of GaN. To optimize and maintain good performance and reliability, heat generated in the device active regions has to be effectively dissipated. Therefore the thermal conductivity of the materials in the device structures needs to be systematically studied and accurately determined. This information is critically important for the thermal management of the devices. Transient thermoreflectance (TTR) is a contactless nondestructive method for measuring of the thermal conductivity of materials. TTR, which is based on a pump-probe technique, has shown its potential in evaluation of the thermal conductivity in bulk crystals as well as in thin layers in hetero-epitaxial structures. The method requires an analysis of experimental data based on the fit of thermoreflectance transients with the solution of the one-dimensional heat transport equations by a least-square minimization of the fitting parameters. Such a procedure allows to extract not only the thermal conductivity of the constituent materials in the structures, but also the thermal boundary resistance at different hetero-interfaces. The main research results of the graduate studies presented in this licentiate thesis are summarized in three scientific papers. Paper I. In this paper thermal conductivity of β−Ga2O3 and high Al-content AlxGa1−xN thin layers was studied. For β−Ga2O3 the the effects of Sn doping and phonon-bondary scattering on the reduction of thermal conductivity were discussed. For the AlxGa1−xN we studied the effect of Al-Ga alloying which gives rise to phonon-alloy scattering. It was found that this scattering process accounts for low thermal conductivity of this material. Finally, a comparison for the thermal conductivity of the two materials was made. Paper II. In this paper the effect of layer thickness on the thermal conductivity of AlxGa1−xN layers grown by HVPE were investigated. Due to Al alloying the thermal conductivity of this material is degraded and reduced by more than one order of magnitude. On top of that we also observed further reduction of thermal conductivity when the layer thickness goes thinner. The mechanism of this phenomenon has been revealed by studying the phonon transport properties in bulk crystal and thin layer. Paper III. This study emphasizes the role of defects in GaN and AlxGa1−xN to the thermal conductivity of these materials. The dislocations, impurities, free carries, and random alloying have been separately studied and discussed. Thermal conductivity of samples containing these defects with various concentrations was measured and the results were interpreted by a theoretical model based on relaxation time approximation (RTA). ; Additional funding agencies: the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009 − 00971
The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity kappa. However, the kappa of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in kappa is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating kappa based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates kappa near the phase transition. Our findings elucidate the influence of structural phase transitions on kappa and provide guidance for design of better thermoelectric materials. ; Funding Agencies|Science Foundation IrelandScience Foundation IrelandEuropean Commission [15/IA/3160, 13/RC/2077]; European Regional Development FundEuropean Commission; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas SeRC
Conducting polymers, such as the p-doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n-doped conducting polymers are also needed. Despite major efforts, no n-type equivalents to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n-type conductive ink. BBL:PEI thin films yield an n-type electrical conductivity reaching 8Scm(-1), along with excellent thermal, ambient, and solvent stability. This printable n-type mixed ion-electron conductor has several technological implications for realizing high-performance organic electronic devices, as demonstrated for organic thermoelectric generators with record high power output and n-type organic electrochemical transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance. The development of n-type conductive polymer inks is critical for the development of next-generation opto-electronic devices that rely on efficient hole and electron transport. Here, the authors report an alcohol-based, high performance and stable n-type conductive ink for printed electronics. ; Funding Agencies|Knut and Alice Wallenberg foundationKnut & Alice Wallenberg Foundation; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-03979, 2020-03243]; AForsk [18-313, 19-310]; Olle Engkvists Stiftelse [204-0256]; VINNOVAVinnova [2020-05223]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; National Research Foundation of KoreaNational Research Foundation of Korea [NRF2020M3H4A3081814, 2019R1A6A1A11044070]; National Science FoundationNational Science Foundation (NSF) [DMR-2003518]
Semiconductor quantum dots (QDs) acting as single-photon-emitters are potential building blocks for various applications in future quantum information technology. For such applications, a thorough understanding and precise control of charge states and capture/recombination dynamics of the QDs are vital. In this work, we study the dynamics of QDs spontaneously formed in GaNAsP nanowires, belonging to the dilute nitride material system. By using a random population model modified for these highly mismatched materials, we analyze the results from photoluminescence and photon correlation experiments and show a general trend of disparity in positive and negative trion populations and also a strong dependence of the capture/recombination dynamics and QD charge states on its surroundings. Specifically, we show that the presence of hole-trap defects in the proximity to some QDs facilitates formation of negative trions, which also causes a dramatic reduction of the neutral exciton lifetime. These findings underline the importance of proper understanding of the QD capture and recombination processes and demonstrate the possibility to use highly mismatched materials and defects for charge engineering of QDs. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2019-04312]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]
A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principles calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (also known as point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution(. )A four-parameter semi-quantum model is applied to determine all 16 pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, epsilon(infinity) = 4.05 +/- 0.05. The Lyddane-Sachs-Teller relation then gives access to the static dielectric constant, epsilon(DC) = 10.55 +/- 0.07. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped n-type sample a free electron density of n = (2.81 +/- 0.01) x 10(17) cm(-3), a mobility of mu = (112 +/- 3) cm(2)/(Vs), and an effective mass parameter of (0.208 +/- 0.006)m(e). Density and mobility parameters compare very well with the results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the Gamma point in In2O3 in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In2O3, and our model validates the static dielectric constant obtained from the Lyddane-Sachs-Teller relation. Published under an exclusive license by AIP Publishing. ; Funding Agencies|National Science Foundation (NSF)National Science Foundation (NSF) [NSF DMR 1755479, NSF DMR 1808715]; Nebraska Materials Research Science and Engineering Center, GraFOx, a Leibniz-Science Campus - Leibniz Association-Germany [DMR 1420645]; Air Force Office of Scientific Research (AFOSR)United States Department of DefenseAir Force Office of Scientific Research (AFOSR) [FA9550-18-1-0360, FA9550-19-S-0003]; Swedish Research Council VRSwedish Research Council [2016-00889]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [RIF14-055, EM16-0024]; Swedish Governmental Agency for Innovation Systems VINNOVA under the Competence Center Program [2016-05190]; Knut and Alice Wallenbergs Foundation supported grant "Wide-bandgap semiconductors for next generation quantum components"; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University; Faculty Grant SFO Mat LiU [2009-00971]; University of Nebraska Foundation; J. A. Woollam Foundation
