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Wafer level transient voltage measurement (WLTVM) to estimate the short circuit capability of AlGaN/GaN HEMT devices is suggested. Two groups of samples with similar DC and switching properties but different short circuit capabilities of 4-7 and $>{10}\mu \text{s}$ were evaluated. The extracted junction temperature and the measured saturation/linear current under repeated short circuit stresses suggest that the short circuit failure is attributed to the degradation in the drift region. The WLTVM could measure the transient potential change along the drift region during short circuit condition. The sample with lower short-circuit survivability showed a faster propagation of the high field traveling from the gate to the drain. The time the high electric field reaches the drain coincides with the time of the short circuit failure. In addition to providing the insight into the short circuit failure mechanism, wafer-level method can provide a quick and non-destructive evaluation of the short circuit capability before packaging devices.

This work reports the transient measurement of the AlGaN/GaN high electron-mobility transistor (HEMT) drift region potential in the high power state during switching. The effect of high power stress has been reported by device characterization after stress or during high power DC stress at voltages below the operating voltages to avoid device failure. However, transient measurement during high power stress is challenging, since the device can sustain the high power stress only for a few $\mu \text{s}$ before failure occurs. In this work, a test structure with voltage probe inserted in the drift region between the gate and the drain is used to perform real-time measurement of the drift region potential during the high power state at operating voltages. It is found that the high electric field formed at the gate side during the off-state stress propagates towards the drain side within a few $\mu \text{s}$ . The propagation speed of this high electric field increased with increase in drain voltage. It is proposed that trapping of hot channel electrons is causing this effect, and dynamic on-resistance measurement suggests that the traps are 0.44 eV below the conduction band.

This work investigates the effect of N2O plasma treatment on the reliability of p-GaN gate AlGaN/GaN high-electron-mobility transistors (HEMTs), specifically in the AlGaN drift region. The formation of a GaON/AlON compound layer on the AlGaN surface after N2O plasma treatment was confirmed by energy-dispersive x-ray spectroscopy mapping and x-ray photoelectron spectroscopy analysis. When a device is under highly stressed conditions, the compound layer reduces the number of negatively charged interface traps and protects the AlGaN surface by hindering the Ga-out diffusion. The high temperature reverse bias reliability test demonstrated that the N2O plasma treatment enhanced the reliability of p-GaN gate HEMTs by suppressing the degradation of the on-resistance from 18.7% to 9.0%, after being subjected to a high drain bias (VDS = 700 V) at 200 °C for 1000 s.

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX2 (X = F, Cl, Br) has been demonstrated to improve the photovoltaic performance of Sn halide perovskite solar cells, the mechanistic roles of SnX2 in the performance enhancement have not yet been studied appropriately. Here we perform a comparative study of CsSnI3 films and devices and examine how SnX2 additives affect their stability, and the results are corroborated by first-principles-based theoretical calculations. Unlike the conventional belief that the additives annihilate defects, we find that the additives effectively passivate the surface and stabilize the perovskite phase, promoting the stability of CsSnI3. Our mechanism suggests that SnBr2, which shows ca. 100 h of prolonged stability along with a high power conversion efficiency of 4.3%, is the best additive for enhancing the stability of CsSnI3.

The cesium lead triiodide (CsPbI3) perovskite is a promising candidate for stable light absorbers and red-light-emitting sources due to its outstanding stability. Phase engineering is the most impo...

Various dielectrics deposited on the surfaces of AlGaN/GaN-based metal-gate high-electron-mobility transistors (HEMTs) were investigated to understand their effects on the device characteristics. The observed increase by 30% in the two-dimensional electron gas (2DEG) sheet carrier density by the deposition of SiO2, Si3N4, or Al2O3 was in line with the improved output and transfer characteristics of the HEMT devices with the deposited dielectrics, as compared to those of the bare HEMT device without dielectric. The improvements seemed to result from the strain accumulation in AlGaN due to the difference in thermal expansion between AlGaN and the dielectric and from the effective treatment of surface electrical passivation by the deposited dielectric. Furthermore, suppression of current collapse was observed in the SiO2− and Si3N4−deposited samples, as compared to the bare HEMT device.

Ternary metal halides (A3X2I9) have attracted considerable interest because they have good stability and reduced toxicity compared with Pb-based halide perovskites. The main issue with A3X2I9 is their band gap, which is relatively large for use in a single junction solar cell (1.9–2.2 eV for the Cs3Bi2I9). This theoretical study found that the band gap of Cs3Bi2I9 can be successfully modulated by using dual metal cations, i.e., by forming Cs3BiXI9 (X: trivalent cation). Among the various trivalent atoms investigated, In and Ga showed very promising band gap modulation behaviors. Additionally, the indirect band gap of Cs3Bi2I9 can be changed into a direct band gap.

Preserving the stability of Sn-based halide perovskites is a primary concern in developing photovoltaic light-absorbing materials for lead-free perovskite solar cells. Whereas the addition of SnX

A fluorescent turn-on chemosensor for CO2 was developed using intra-molecular hydrogen bonding stabilization. The chemosensor (sodium (anthracen-9-ylmethyl)glycinate (1)) reacts with CO2 and forms chemosensor-CO2 adduct stabilized by strong intra-molecular hydrogen bonds. The stabilization consequently allows sensing CO2 with high sensitivity and the detection limit of this chemosensor for CO2 was estimated to be ∼2 ppm. In addition, the chemosensor was applied to develop an easy screening protocol to evaluate the efficiency of CO2 absorbents. The protocols discriminate the relative ability of CO2 absorbents as strong, intermediate, and weak using easily measured fluorescence changes.

Organic–inorganic perovskites are promising materials for improving the efficiency of solar cells, but there are still uncovered issues on the understanding of their electronic band structures. Using first-principles calculations, we investigate the electronic band features of organo-lead iodide perovskites and present the efficient model to predict the band gap variation based on the orbital interaction scheme. The orbital interaction between Pb and I atoms can be controlled through the structural modification such as the change in lattice constant and the deviation of I atoms from cubic symmetry sites. The increase of the lattice constant and the positional distortion of I atoms from the cubic symmetry sites lead to the increase of the band gap. With our findings, puzzling band gap variation behaviors in previous experiments and simulations can be understood, and we suggest a pathway to precisely control their band gap. Our study can serve as the design rule for band gap engineering for various kinds of...

The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼ 2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15254-15258), this work establishes the first theoretical framework for understanding dopant-distributions in over-10 nm highly doped silicon nanowires.

We investigate the role of atomic hydrogen in Ge/Si core–shell nanowires with first-principles calculations and present that the hole doping in the Ge/Si heterointerface is achievable through interstitial hydrogen mediated remote doping. This atomic hydrogen induced hole doping could generate one-dimensional hole gas in Ge/Si core–shell nanowires. Hydrogen prefers to be incorporated in the Si shell due to the lattice strain effect. As the charge transition energy level of the interstitial hydrogen in Si is lower than the valence band maximum of the Ge band, the electrons in the Ge core prefer to move toward the Si shell and become trapped by the interstitial hydrogen. This unique hydrogen energy level in the Ge/Si heterostructure between the Ge and Si valence band edges drives the electron transfer from the Ge core and induces holes states in the Ge core through remote hole doping. We also perform a quantum transport simulation and show that a high conductive hole channel in the Ge core can be generated w...

Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure and oxygen plasma treatment and outstanding improvement of Vth variation is observed. The origin of the observed positive Vth shift and reduced variation window induced by oxygen plasma treatment is investigated by computational methods. Formation energy calculations for oxygen inclusions in III–N reveal that a negatively charged VAl-ON complex in the AlGaN passivation layer can be a major source of Vth variation in AlGaN/GaN hetero-structured devices. Calculated trap energy levels are used as the parameters of a device simulation, which indicated that significant Vth variation can be induced by a small fluctuation in the AlGaN layer thickness and defect densities. Our theoretical investigation shows that normally-off AlGaN/GaN HEMTs having reliable Vth variation can be produced by oxygen inclusions accompanying a recessed-gate structure.

Organic–inorganic hybrid perovskites have unique electronic properties in which deep level defects are rarely formed. This unique defect characteristic is the source of the long carrier diffusion length. This theoretical study shows what causes this characteristic formation of shallow level defects in lead tri-halide perovskites. Comparative studies between iodides and other halides showed that deep level defect states were generated for Cl based perovskites. Longer Pb–halide bond lengths and narrower band gaps are beneficial for preventing deep level defect states. Additionally, our study shows that the formation of shallow level defects does not change even when the lattice structures of the perovskites do not reach their equilibrium structures.

We present a defect-level induced by a new rehybridization of broken-bonded GaN in open-core threading edge dislocations (TED) of heteroepitaxial GaN. The rehybridization was discovered toward sp 2-/ sp 3- from the sp 2-/ p 3- of full-core TED by density functional theory calculation. The filled non-bonding state of the nitrogen’s sp 3- bond is located in the middle of the band gap, which corresponds to the source of yellow luminescence.

Double perovskites using dual metal cations are promising candidates for Pb-free perovskites. This study shows that the electronic structures of double perovskites (A2B+B3+X6) can be significantly modulated by cation ordering changes. The bandgap of Cs2AgBiCl6 can be affected by changing octahedron alignments, and even zero gap states can be realized for the 2-dimensional BiCl6 (AgCl6) configuration. It is presented that different types of B+/B3+-site orderings in double perovskites could be the origin of bandgap dispersion. Comparative studies on the various compositions show that, among B+/B3+ cations, Tl/Bi could be promising for the suppression of ordering variation.Double perovskites using dual metal cations are promising candidates for Pb-free perovskites. This study shows that the electronic structures of double perovskites (A2B+B3+X6) can be significantly modulated by cation ordering changes. The bandgap of Cs2AgBiCl6 can be affected by changing octahedron alignments, and even zero gap states can be realized for the 2-dimensional BiCl6 (AgCl6) configuration. It is presented that different types of B+/B3+-site orderings in double perovskites could be the origin of bandgap dispersion. Comparative studies on the various compositions show that, among B+/B3+ cations, Tl/Bi could be promising for the suppression of ordering variation.

This letter studies the current collapse phenomenon during switching in p-GaN gate AlGaN/GaN high-electron-mobility transistors. It is found that channel hot electrons play a major role in increasing the current collapse and that adding a field plate significantly reduces the effect. By stressing the device with OFF-state pulses of 100 μs× 10 μs with a VGS rise/fall time of 10 ns at Vdc 400 V, compared to the ON-resistance before stress, the ON-resistance was 78 times larger after stress without field plates. With a field plate, it was only 1.8 times larger.

The effects of cations of zeolite for the adsorption of organosulfur compounds were investigated by using density functional theory calculations. The binding energies of tetrahydrothiophene (C4H8S, THT), dimethyl sulfide (C2H6S, DMS), tert-butylmercaptan (C4H10S, TBM), hydrogen sulfide (H2S), and carbonyl sulfide (COS) on the zeolite model clusters [X(HO)3SiOAl(OH)3, X = H+, Na+ and Ag+] were obtained and compared with those of H2O, CO2, and C1−C3 light hydrocarbons. Compared to the H+ and Na+ cations, the Ag+ cation induces much stronger binding of THT, DMS, TBM, and H2S over H2O suggesting great enhancements in the adsorption selectivity. The order of binding energies of these sulfur compounds is THT > DMS > TBM > H2S > COS, and it does not depend on the cation types. These results agree well with the experimental adsorption uptake and selectivity properties of AgNaY zeolites for organosulfur compounds.

We investigate the electronic band structures of Ge/Si core-shell nanowires (CSNWs) and devise a way to realize the electron quantum well at Ge core atoms with first-principles calculations. We reveal that the electronic band engineering by the quantum confinement and the lattice strain can induce the type-I/II band alignment transition, and the resulting type-I band alignment generates the electron quantum well in Ge/Si CSNWs. We also find that the type-I/II transition in Ge/Si CSNWs is highly related to the direct to indirect band gap transition through the analysis of charge density and band structures. In terms of the quantum confinement, for [100] and [111] directional Ge/Si CSNWs, the type-I/II transition can be obtained by decreasing the diameters, whereas a [110] directional CSNW preserves the type-II band alignment even at diameters as small as 1 nm. By applying a compressive strain on [110] CSNWs, the type-I band alignment can be formed. Our results suggest that Ge/Si CSNWs can have the type-I band alignment characteristics by the band structure engineering, which enables both n-type and p-type quantum-well transistors to be fabricated using Ge/Si CSNWs for high-speed logic applications.

One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic–inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.

We report herein calixarene derivatives, which could adapt to various fields of application, as novel pore generators for making nanoporous materials. The pore structure of nanoporous materials exhibits disordered pores with small mesopore diameter (2―3 nm), which is similar to the micelle-like assembled structure of the calixarene compounds. The electro-optical properties such as dielectric constants and refractive indexes of these porous thin films can easily be manipulated. The calixarene-templated nanoporous films could find a variety of potential applications, such as low-dielectric constant (k) materials and high-surface area materials for catalysis and biotechnology.

The retarded dopant diffusion in Si nanostructures is investigated using the first principles calculation. It is presented that weak dopant–dopant interaction energy (DDIE) in nanostructures is responsible for the suppressed dopant diffusion in comparison with that in bulk Si. The DDIE is significantly reduced as the diameter of the Si nanowire becomes smaller. The mechanical softening and quantum confinement found in nanostructures are the physical origin for the small interaction energy. Reduced dopant–dopant interaction slows down the diffusion process from heavily doped regions to undoped regions. Thus, we suggest that an additional annealing process is indispensable to make a desired dopant profile in the nanoscale semiconductor devices.

Sensitivity of Phosphorus dopant placement to the channel size of highly doped silicon nanowires is studied using a 10-band sp3 d5 s∗ tight-binding approach coupled to self-consistent simulations. Extending the simulation scope to realistically sized nanowires, we observed that uniform doping does not necessarily reduce the channel energy compared to surface-oriented doping when the diameter of a nanowire cross-section is smaller than 20nm, whilst uniform doping lowers the energy, making the channel more stable at larger cross-sections. This size-dependency, firmly connected to the recent experiment, is understood well in detail by investigating channel electrostatics.

We investigate the structures, NMR chemical shifts, absorption spectra, frontier molecular orbitals, and transition density matrices of pi-stacked polyfluorenes by ab initio calculations. For F1-F4, we consider two different conformations, syn and anti. The simulated 1H NMR chemical shifts are in good agreement with the previous experiment, and the significantly upfielded chemical shifts explain that the fluorene moieties are stacked on each other. It is found that the relative stability for syn and anti conformers is almost equivalent in B3LYP calculations; however, the syn conformer becomes much more stable than the anti conformer in MP2 calculations, which is consistent with the experimental finding that only the syn conformers are relevant. The vertical detachment energy, which is linearly proportional to the ionization potential, shows the same size dependence as the previous experiment. The electron attachment energy decreases exponentially as the size increases, which implies that the electron transport would be possible even for long chains such as F3 and F4. This was evident from the frontier molecular orbitals (HOMO and LUMO). Also, it is found that the syn conformers are very favorable for electron transport through the pi-stacked fluorene moieties.

A pathway to increase the threshold voltage $(V_{\rm TH})$ of p-GaN gate high-electron-mobility transistors (HEMTs) is presented. The hole depletion width in the p-GaN layer at the gate interface is one of the key controlling factors of $V_{\rm TH}$ in p-GaN gate HEMTs. In order to increase the depletion width, we devise a new device structure of p-GaN gate HEMT having a source-connected p-GaN bridge. We demonstrate that a bridged p-GaN gate HEMT structure increases the $V_{\rm TH}$ from 0.93 to 2.44 V.

The impact of gate metals on the threshold voltage (VTH) and the gate current of p-GaN gate high-electron-mobility transistors (HEMTs) is investigated by fabricating p-GaN gate HEMTs with different work function gate metals-Ni and W. p-GaN gate HEMTs incorporate a p-GaN layer under the gate electrode as the gate stack on top of the AlGaN/GaN layer. In comparison to the Ni-gate p-GaN HEMTs, the W-gate p-GaN HEMTs showed a higher VTH of 3.0 V and a lower gate current of 0.02 mA/mm at a gate bias of 10 V. Based on TCAD device simulations, we revealed that these high VTH and low gate current are attributed to the low gate metal work function and the high Schottky barrier between the p-GaN and the W gate metal.

The structures, interaction energies, and vibrational frequencies of acetone and its complexes with proton and various metal monocations/dications such as H+, Li+, Na+, K+, Rb+, Cs+, Be2+, Mg2+, Ca2+, and Zn2+ have been investigated from the ab initio calculations. The linear or bent structure for the cation-acetone complexes has been found to be deeply influenced by the amount of charge transfer. The amount of red-shift of CO stretching frequencies of acetone is almost equivalent regardless of the kind of alkali metal monocations. This behavior can be attributed to the electrostatic nature of the interactions rather than orbital interactions. This phenomenon has been supported by the highest occupied molecular orbitals of the acetone complexes investigated, charge transfer based on natural bond orbital (NBO) atomic charges, and the inversely proportional behavior of the interaction energies to the interatomic distances r(M+⋯O).

We investigated the decarboxylation of 2-phenylcyclopent-3-ene-1,1-dicarboxylic acid (1) and its derivatives from the ab initio calculations. The reaction pathway starting from the R-configuration ...