High Thermal Stability and Low Thermal Resistance of Large Area GaN/3C-SiC/Diamond Junctions for Practical Device Processes.

Thermal management is critical in contemporary electronic systems, and integrating diamond with semiconductors offers the most promising solution to improve heat dissipation. However, developing a technique that can fully exploit the high thermal conductivity of diamond, withstand high-temperature annealing processes, and enable mass production is a significant challenge. In this study, the successful transfer of AlGaN/GaN/3C-SiC layers grown on Si to a large-size diamond substrate is demonstrated, followed by the fabrication of GaN high electron mobility transistors (HEMTs) on the diamond. Notably, no exfoliation of 3C-SiC/diamond bonding interfaces is observed even after annealing at 1100 °C, which is essential for high-quality GaN crystal growth on the diamond. The thermal boundary conductance of the 3C-SiC-diamond interface reaches ≈55 MW m-2 K-1, which is efficient for device cooling. GaN HEMTs fabricated on the diamond substrate exhibit the highest maximum drain current and the lowest surface temperature compared to those on Si and SiC substrates. Furthermore, the device thermal resistance of GaN HEMTs on the diamond substrate is significantly reduced compared to those on SiC substrates. These results indicate that the GaN/3C-SiC on diamond technique has the potential to revolutionize the development of power and radio-frequency electronics with improved thermal management capabilities.

Implementation of Nonadiabatic Molecular Dynamics for Intersystem Crossing Based on a Time-Dependent Density-Functional Tight-Binding Method.

Intersystem crossing (ISC) and internal conversion (IC) are types of nonadiabatic transitions that play important roles in a wide range of fields, including photochemistry, photophysics, and photobiology. The nonadiabatic molecular dynamics (NA-MD) method is a powerful tool for computational simulations of dynamic phenomena involving nonadiabatic transitions. In this study, we implemented the NA-MD method, which treats ISC and IC on an equal footing, where the electronic structure is treated at the level of the time-dependent (TD) density-functional tight-binding (DFTB) method, a low-cost semiempirical analog of TD density functional theory (DFT). In particular, the spin-orbit coupling calculation algorithm was implemented in the TD-DFTB framework, and the results showed trends similar to those obtained using TD-DFT. In addition, the NA-MD method successfully reproduced ultrafast ISC of 2-nitronaphthalene.

Neutral-to-ionic photoinduced phase transition of tetrathiafulvalene-p-chloranil by electronic and vibrational excitation: A real-time nuclear-electronic dynamics simulation study.

Tetrathiafulvalene-p-chloranil exhibits photoinduced phase transition (PIPT) between neutral (N) and ionic (I) phases, in which the constituent molecules are approximately charge-neutral and ionic, respectively. In addition to visible-light irradiation, which can induce both N → I and I → N PIPTs, infrared irradiation has been reported to induce the N → I PIPT. These results suggest that N → I and I → N PIPTs can be driven by electronic excitation, and the I → N PIPT can also be driven by vibrational excitation. However, the feasibility of the N → I PIPT using vibrational excitation remains an open question. In this study, we address this issue by simulating the PIPT processes using a nonadiabatic molecular dynamics approach combined with real-time electron dynamics at the level of a semiempirical quantum chemical model, density-functional tight binding. The results show the importance of vibronic interactions in the PIPT processes, thereby suggesting the possibility of N → I PIPT by vibrational excitations with infrared irradiation.

History and Recent Advances of the Japanese Society of Biofeedback Research.

This article provides an overview of the history of the Japanese Society of Biofeedback Research (JSBR) and presents some of its recent advances. Most of the research papers published in the JSBR journal (Biofeedback Kenkyu) have been written in Japanese, and therefore have had very few opportunities to reach global readers. We would like to present some of important findings previously published there. First, we present the history of the JSBR. Secondly, we will focus on paced breathing, which is instrumental in achieving relaxation in heart rate variability biofeedback (HRV-BF). We will look back on the origin of slow-paced breathing in Japan, that could be attributed to the concept of Tanden breathing (abdominal paced breathing) practiced in Zen meditation. Thirdly, we will introduce some of the current research progresses of JSBR, especially focusing on the development of a non-contact sensing technology and relaxation device. Finally, we will explain about a very recent trial, the "Suu-Haa" Relaxation Technique, which we hope may be useful for helping people cope with the SARS-CoV-2 (COVID-19) crisis.

Non-adiabatic molecular dynamics with divide-and-conquer type large-scale excited-state calculations.

Many of the non-adiabatic processes in the condensed phase are affected by the interaction with the environment, as exemplified by Marcus theory. However, non-adiabatic molecular dynamics simulations with explicitly including the environment are computationally expensive because of the extended system size, suggesting the need for an efficient scheme applicable to huge systems. In this work, time-derivative non-adiabatic coupling (TDNAC) calculation algorithms were developed in the framework of the divide-and-conquer (DC) time-dependent (TD) density-functional tight-binding (DFTB) method, which is an extension of the TD-DFTB for larger systems based on the fragmentation-based DC scheme. The developed algorithms were incorporated into a fewest-switches trajectory surface hopping (FSSH) routine. The calculated TDNAC and the FSSH results were sufficiently accurate compared to the conventional TD-DFTB results. Use of the DC-TD-DFTB provided a significant reduction in the central processing unit (CPU) time vs that of the TD-DFTB, where the CPU time remained constant irrespective of the total system size. It was also confirmed that the present method is not only efficient but also improves the numerical stability of TDNAC calculations.

Quantum mechanical molecular dynamics simulations of polaron formation in methylammonium lead iodide perovskite.

We investigated the atomistic and dynamical mechanism of polaron formation in methylammonium lead iodide perovskite (MAPbI3), which is a representative perovskite solar cell absorber, through the quantum mechanical molecular dynamics simulations. The simulations were conducted on the spatial scale of several nanometres, which can describe charge localization and the associated structural deformation, using the divide-and-conquer-type density-functional tight-binding method, which enables a quantum chemical treatment of systems comprising thousands of atoms. We found that both the structural parts of MAPbI3, namely, the inorganic framework (PbI3-) and the MA cations, involve the structural deformation associated with polaron formation. We elucidated that in the process of polaron formation, charge localization is invoked by thermal structural fluctuation, and a further structural deformation is caused by the relaxation of the charge carrier. Finally, importance of the two structural parts, PbI3- and MA, was examined from the energetical viewpoint.

Nanoscale and Real-Time Nuclear-Electronic Dynamics Simulation Study of Charge Transfer at the Donor-Acceptor Interface in Organic Photovoltaics.

Charge-transfer (CT) processes in donor-acceptor interfaces of organic photovoltaics have been challenging targets for computational chemistry owing to their nanoscale and ultrafast nature. Herein, we report real-time nuclear-electronic dynamics simulations of CT processes in a nanometer-scale donor-acceptor interface model composed of a donor poly(3-hexylthiophene-2,5-diyl) crystal and an acceptor [6,6]-phenyl-C61-butyric acid methyl ester aggregate. The simulations were realized using our original reduced-scaling computational technique, namely, patchwork-approximation-based Ehrenfest dynamics. The results illustrated the CT pathway with atomic resolution, thereby rationalizing the observed excitation-energy dependence of the quantity of CT. Further, nuclear motion, which is affected by the electronic dynamics, was observed to play a significant role in the CT process by modulating molecular orbital energies. The present study suggests that microscopic CT processes strongly depend on local structures of disordered donor-acceptor interfaces as well as coupling between nuclear and electronic dynamics.

High thermal conductivity in wafer-scale cubic silicon carbide crystals.

High thermal conductivity electronic materials are critical components for high-performance electronic and photonic devices as both active functional materials and thermal management materials. We report an isotropic high thermal conductivity exceeding 500 W m-1K-1 at room temperature in high-quality wafer-scale cubic silicon carbide (3C-SiC) crystals, which is the second highest among large crystals (only surpassed by diamond). Furthermore, the corresponding 3C-SiC thin films are found to have record-high in-plane and cross-plane thermal conductivity, even higher than diamond thin films with equivalent thicknesses. Our results resolve a long-standing puzzle that the literature values of thermal conductivity for 3C-SiC are lower than the structurally more complex 6H-SiC. We show that the observed high thermal conductivity in this work arises from the high purity and high crystal quality of 3C-SiC crystals which avoids the exceptionally strong defect-phonon scatterings. Moreover, 3C-SiC is a SiC polytype which can be epitaxially grown on Si. We show that the measured 3C-SiC-Si thermal boundary conductance is among the highest for semiconductor interfaces. These findings provide insights for fundamental phonon transport mechanisms, and suggest that 3C-SiC is an excellent wide-bandgap semiconductor for applications of next-generation power electronics as both active components and substrates.

Scalable Ehrenfest Molecular Dynamics Exploiting the Locality of Density-Functional Tight-Binding Hamiltonian.

To explore the science behind excited-state dynamics in high-complexity chemical systems, a scalable nonadiabatic molecular dynamics (MD) technique is indispensable. In this study, by treating the electronic degrees of freedom at the density-functional tight-binding level, we developed and implemented a reduced scaling and multinode-parallelizable Ehrenfest MD method. To achieve this goal, we introduced a concept called patchwork approximation (PA), where the effective Hamiltonian for real-time propagation of the electronic density matrix is partitioned into a set of local parts. Numerical results for giant icosahedral fullerenes, which comprise up to 6000 atoms, suggest that the scaling of the present PA-based method is less than quadratic, which yields a significant advantage over the conventional cubic scaling method in terms of computational time. The acceleration by the parallelization on multiple nodes was also assessed. Furthermore, the electronic and structural dynamics resulting from the perturbation by the external electric field were accurately reproduced with the PA, even when the electronic excitation was spatially delocalized.

Trajectory Surface Hopping Approach to Condensed-Phase Nonradiative Relaxation Dynamics Using Divide-and-Conquer Spin-Flip Time-Dependent Density-Functional Tight Binding.

Nonradiative relaxation of excited molecules is central to many crucial issues in photochemistry. Condensed phases are typical contexts in which such problems are considered, and the nonradiative relaxation dynamics are expected to be significantly affected by interactions with the environment, for example, a solvent. We developed a nonadiabatic molecular dynamics simulation technique that can treat the nonradiative relaxation and explicitly include the environment in the calculations without a heavy computational burden. Specifically, we combined trajectory surface hopping with Tully's fewest-switches algorithm, a tight-binding approximated version of spin-flip time-dependent density-functional theory, and divide-and-conquer (DC) spatial fragmentation scheme. Numerical results showed that this method can treat systems with thousands of atoms within reasonable computational resources, and the error arising from DC fragmentation is negligibly small. Using this method, we obtained molecular insights into the solvent dependence of the photoexcited-state dynamics of trans-azobenzene, which demonstrate the importance of the environment for condensed-phase nonradiative relaxation.

Simulating the Coupled Structural-Electronic Dynamics of Photoexcited Lead Iodide Perovskites.

Motivated by the optoelectronic applications of lead halide perovskites (LHPs), researchers have paid considerable attention to their photoexcited-state dynamics, where the coupling between the electronic and nuclear dynamics is pronounced. Here, we present simulations of the photoexcited-state dynamics of representative lead iodide perovskites, CsPbI3 and MAPbI3 (MA = CH3NH3), by adopting nonadiabatic molecular dynamics combined with the linear-response time-dependent density-functional tight-binding (LR-TD-DFTB) method, an efficient excited-state calculation framework. In the calculations, the electronic wave function and the nuclear coordinates were propagated in a mutually dependent manner. The results suggest that the excited LHPs undergo exciton dissociation, hot carrier cooling, and polaron formation on similar time scales. In particular, the decay of the carrier energy is attributed to not only the relaxation toward the band edge but also the change in orbital energy originating from the structural deformation, highlighting the importance of coupling between the electronic and nuclear degrees of freedom.

Fast Nonadiabatic Molecular Dynamics via Spin-Flip Time-Dependent Density-Functional Tight-Binding Approach: Application to Nonradiative Relaxation of Tetraphenylethylene with Locked Aromatic Rings.

Nonadiabatic dynamics around conical intersections between ground and excited states are crucial to understand excited-state phenomena in complex chemical systems. With this background in mind, we present an approach combining fewest-switches trajectory surface hopping and spin-flip (SF) time-dependent (TD) density-functional tight binding (DFTB), which is a simplified version of SF-TD density functional theory (DFT) with semiempirical parametrizations, for computationally efficient nonadiabatic molecular dynamics simulations. The estimated computational time of the SF-TD-DFTB approach is several orders of magnitude lower than that of SF-TD-DFT. In addition, the proposed method reproduces the time scales and quantum yields in photoisomerization reactions of azobenzene at a level comparable with conventional ab initio approaches, demonstrating reasonable accuracy. Finally, we report a practical application of the developed technique to explore the nonradiative relaxation processes of tetraphenylethylene and its derivative with torsionally locked aromatic rings and discuss the effect of locking the rings on the excited-state lifetime.

Charge Carrier Trapping at Surface Defects of Perovskite Solar Cell Absorbers: A First-Principles Study.

The trapping of charge carriers at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells (PSCs). For example, it is the main limiting factor for carrier lifetime. Moreover, it causes hysteresis in the current-voltage curves, which is considered to be a serious issue for PSCs' operation. In this work, types of surface defects responsible for carrier trapping are clarified by a comprehensive first-principles investigation into surface defects of tetragonal CH3NH3PbI3 (MAPbI3). Considering defect formation energetics, it is proposed that a Pb-rich condition is preferred to an I-rich one; however, a moderate condition might possibly be the best choice. Our result paves the way for improving the performance of PSCs through a rational strategy of suppressing carrier trapping at surface defects.

Detailed analysis of charge transport in amorphous organic thin layer by multiscale simulation without any adjustable parameters.

Hopping-type charge transport in an amorphous thin layer composed of organic molecules is simulated by the combined use of molecular dynamics, quantum chemical, and Monte Carlo calculations. By explicitly considering the molecular structure and the disordered intermolecular packing, we reasonably reproduce the experimental hole and electron mobilities and their applied electric field dependence (Poole-Frenkel behaviour) without using any adjustable parameters. We find that the distribution of the density-of-states originating from the amorphous nature has a significant impact on both the mobilities and Poole-Frenkel behaviour. Detailed analysis is also provided to reveal the molecular-level origin of the charge transport, including the origin of Poole-Frenkel behaviour.

Basic study on the most relaxing respiration period in children to aid the development of a respiration-leading stuffed toy.

Following natural disasters, accidents, and shocking incidents, some children experience post-traumatic stress disorder (PTSD). The respiration control method, which relaxes the body and mind, may efficiently prevent PTSD. Therefore, we developed a stuffed toy that leads children's respiration using the up-and-down movement of the abdomen to help them relax. We investigated the most appropriate respiration period for children's relaxation. Data from studies on heart rate variability (HRV) biofeedback training suggest that breathing at the respiration period at which HRV is the highest is effective for improving chronic diseases. Therefore, we measured the relationship between the respiration period and physiological indices, including HRV. The participants were 10 children aged 5-12 years. HRV was the highest at a 10-12-s respiration period in all 10 children. However, the most suitable respiration period for smooth breathing and relaxation was different from that at which HRV is the highest. Therefore, the most relaxing respiration periods for children need to be determined by indices other than HRV.