Energy Barrier Height Research Articles

Fabrication of n-Si/n-Ga2O3 heterojunctions by surface-activated bonding and their electrical properties

We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (qϕb) at the Si/Ga2O3 interface for different reverse voltages (Vrev) were derived from temperature-dependent current density–voltage (J–V–T) characteristics. With shifting Vrev to the negative direction, qϕb gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the Vrev dependence of qϕb. The qϕb calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J–V–T characteristics, attributing to spatially inhomogeneous qϕb caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J–V–T characteristics, which was estimated to be about 6 × 1012 cm−2 eV−1.

Understanding electrocaloric cooling of ferroelectrics guided by phase‐field modeling

AbstractWith the urgent need to explore low‐cost, high‐efficiency solid‐state refrigeration technology, the electrocaloric effects of ferroelectric materials have attracted much attention in the past decades. With the development of modern computing technology, the phase‐field method is widely used to simulate the evolution of microstructure at mesoscale and predict the properties of different types of ferroelectric materials. In this article, we review the recent progress of electrocaloric effects from phenomenological Landau thermodynamics theory to phase‐field simulation by discussing the microcosmic composition, mesoscopic domain structures, macroscopic size/shape, and external stimulus of strain/stress. More importantly, in searching for new ferroelectric electrocaloric cooling materials, it is possible to find materials whose free energy barrier height changes rapidly with temperature, such materials have a faster change rate with polarization temperature in terms of ferroelectric macroscopic properties, from them could get superior electrocaloric effects. We compile a relatively comprehensive computational design on the high performance of electrocaloric effects in different types of ferroelectrics and offer a perspective on the computational design of electrocaloric refrigeration materials at the mesoscale microstructure level.

Spectroscopic signatures of plasmon-induced charge transfer in gold nanorods.

Plasmon-induced charge transfer has been studied for the development of plasmonic photodiodes and solar cells. There are two mechanisms by which a plasmonic nanoparticle can transfer charge to an adjacent material: indirect transfer following plasmon decay and direct transfer as a way of plasmon decay. Using single-particle dark-field scattering and photoluminescence imaging and spectroscopy of gold nanorods on various substrates, we identify linewidth broadening and photoluminescence quantum yield quenching as key spectroscopic signatures that are quantitatively related to plasmon-induced interfacial charge transfer. We find that dark-field scattering linewidth broadening is due to chemical interface damping through direct charge injection via plasmon decay. The photoluminescence quantum yield quenching reveals additional mechanistic insight into electron-hole recombination as well as plasmon generation and decay within the gold nanorods. Through these two spectroscopic signatures, we identify charge transfer mechanisms at TiO2 and indium doped tin oxide interfaces and uncover material parameters contributing to plasmon-induced charge transfer efficiency, such as barrier height and resonance energy.

Free Energy Profiles of Proton Transfer Reactions: Density Functional Benchmark from Biased Ab Initio Dynamics.

By coupling an enhanced sampling algorithm with an orbital-localized variant of Car-Parrinello molecular dynamics, the so-called atomic centered density matrix propagation model, we reconstruct the free energy profiles along reaction pathways using different density functional approximations (DFAs) ranging from locals to hybrids. In particular, we compare the computed free energy barrier height of proton transfer (PT) reactions to those obtained by a more traditional static approach, based on the intrinsic reaction coordinate (IRC), for two case systems, namely malonaldehyde and formic acid dimer. The obtained results show that both the IRC profiles and the potentials of mean force, derived from biased dynamic trajectories, are very sensitive to the density functional approximation applied. More precisely, we observe that, with the notable exception of M06-L, local density functionals always strongly underestimate the reaction barrier heights. More generally, we find that also the shape of the free energy profile is very sensitive to the density functional choice, thus highlighting the effect, often neglected, that the choice of DFA has also in the case of dynamics simulations.

The Mechanism of a Retro-Diels-Alder Fragmentation of Luteolin: Theoretical Studies Supported by Electrospray Ionization Tandem Mass Spectrometry Results.

The mechanisms of retro-Diels–Alder fragmentation of luteolin are studied theoretically using the Density Functional Theory method (B3LYP hybrid functional) together with the 6-311++G(d,p) basis set and supported by electrospray ionization tandem mass spectrometry (ESI-MS) results. The reaction paths leading to the formation of 1,3A− and 1,3B− fragment ions observed as the main spectral features in the ESI-MS spectrum are described and discussed, including the structures of the transition states and intermediate products. The heights of the activation energy barriers which have to be overcome along the reaction paths corresponding to 1,3-retrocyclization cleavage of the ionized luteolin are predicted to span the 69–94 kcal/mol range (depending on the initial isomeric structure) for the concerted retrocyclization mechanism and the 60–89 kcal/mol (first barrier) and 24–52 kcal/mol (second barrier) barriers for the stepwise mechanism (also depending on the initial isomeric structure). It is also demonstrated that the final fragmentation products (1,3A− and 1,3B−) are in fact represented by various isomeric systems which are not experimentally distinguishable. In addition, the absence of the spectral feature corresponding to the [M-B]− fragment ion formed by the rupture of the C-C bond connecting luteolin’s B and C rings (which does not occur during the ESI-MS experiment) is explained by much larger energy barriers predicted for such a process.

Trópos and Átropos Biindole Chiral Electroactive Monomers: A Voltammetry and HPLC Comparative Insight

AbstractA series of 2,2’‐biindole‐based inherently chiral electroactive monomers are comparatively investigated with their 3,3’ analogues as an excellent study case of two equivalent redox centres interacting through a torsional barrier. The twin peak potential splitting observed in voltammetry for the first oxidation of the biheteroaromatic core accounts for the energy barrier height: the lower the barrier, the larger the peak potential splitting, with modulation by solvent and temperature. The height of the energy barrier is determining for the electrochemical and spectroscopic features of the monomers as well as for their configurational stability and applicability for enantioselection purposes. The 3,3’ monomers, featuring large twin peak splittings in CV, are “trópos” systems with a low torsional barrier, so they cannot exist as stable enantiomers at room temperature. Instead their 2,2’ isomers, with much smaller twin peak splittings, are “átropos” systems and can be separated by enantioselective HPLC into stable enantiomers, providing powerful “inherently chiral” selectors with outstanding enantioselection properties in chiral electroanalysis and electrochemistry as well as in chiroptical spectroscopy, with fascinating reciprocal correlations.

Mechanical Stress Modulation of Resistance in MoS2 Junctions.

Strain engineering is a powerful strategy to control the physical properties of material-enabling devices with enhanced functionality and improved performance. Here, we investigate a modulation of the transport behavior of the two-dimensional MoS2 junctions under the mechanical stress induced by a tip of an atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering a tip-induced force. Analysis of the stress-induced evolution of the I-V characteristics indicates a combined effect of the tip-induced strain and strain gradient on the energy barrier height and profile. In addition, we show that the tip-generated flexoelectric effect leads to significant enhancement of the photovoltaic effect in the MoS2 junctions. A combination of the optical and mechanical stimuli facilitates reversible photomechanical tuning of resistance of the narrow-band 2D semiconductors and development of devices with an enhanced photovoltaic response.

Extraction of the spin-glass free-energy landscape from 1/f noise measurements

The $1/f$ resistance noise has been measured in thin CuMn (13.5 at.%) spin-glass films. The temperature and frequency dependence have been analyzed in terms of the landscape of free-energy barriers. This analysis provides the full barrier distribution for various thin film thicknesses between 10 nm and 80 nm. The free-energy barrier height distribution's width and energy position have been determined. Contrary to previous models with fixed shape and energy, the free-energy landscape is described by a distribution of barriers that both shifts and changes shape as the temperature is reduced. The dependence of this distribution is in contrast with recent predictions. Using the fluctuation dissipation theorem, the $1/f$ measurements connect with ${\ensuremath{\chi}}^{\ensuremath{''}}$, displaying agreement with other direct measurements of the latter.

Modeling of the Point Defect Migration across the AlN/GaN Interfaces-Ab Initio Study.

The formation and diffusion of point defects have a detrimental impact on the functionality of devices in which a high quality AlN/GaN heterointerface is required. The present paper demonstrated the heights of the migration energy barriers of native point defects throughout the AlN/GaN heterointerface, as well as the corresponding profiles of energy bands calculated by means of density functional theory. Both neutral and charged nitrogen, gallium, and aluminium vacancies were studied, as well as their complexes with a substitutional III-group element. Three diffusion mechanisms, that is, the vacancy mediated, direct interstitial, and indirect ones, in bulk AlN and GaN crystals, as well at the AlN/GaN heterointerface, were taken into account. We showed that metal vacancies migrated across the AlN/GaN interface, overcoming a lower potential barrier than that of the nitrogen vacancy. Additionally, we demonstrated the effect of the inversion of the electric field in the presence of charged point defects and at the AlN/GaN heterointerface, not reported so far. Our findings contributed to the issues of structure design, quality control, and improvement of the interfacial abruptness of the AlN/GaN heterostructures.

Assembly mechanism of surface-functionalized nanocubes.

Faceted nanoparticles can be used as building blocks to assemble nanomaterials with exceptional optical and catalytic properties. Recent studies have shown that surface functionalization of such nanoparticles with organic molecules, polymer chains, or DNA can be used to control the separation distance and orientation of particles within their assemblies. In this study, we computationally investigate the mechanism of assembly of nanocubes grafted with short-chain molecules. Our approach involves computing the interaction free energy landscape of a pair of such nanocubes via Monte Carlo simulations and using the Dijkstra algorithm to determine the minimum free energy pathway connecting key states in the landscape. We find that the assembly pathway of nanocubes is very rugged involving multiple energy barriers and metastable states. Analysis of nanocube configurations along the pathway reveals that the assembly mechanism is dominated by sliding motion of nanocubes relative to each other punctuated by their local dissociation at grafting points involving lineal separation and rolling motions. The height of energy barriers between metastable states depends on factors such as the interaction strength and surface roughness of the nanocubes and the steric repulsion from the grafts. These results imply that the observed assembly configuration of nanocubes depends not only on their globally stable minimum free energy state but also on the assembly pathway leading to this state. The free energy landscapes and assembly pathways presented in this study along with the proposed guidelines for engineering such pathways should be useful to researchers aiming to achieve uniform nanostructures from self-assembly of faceted nanoparticles.

Role of post-CCSD(T) corrections in predicting the energetics and kinetics of the OH˙ + O3 reaction.

The present work investigates the OH˙ +O3 reaction by means of chemical kinetics and quantum chemical calculations. To estimate the reaction barrier height and reaction energy, we have included various corrections, like zero point energy corrections, contribution from full triple excitations and partial quadratic excitations at the coupled-cluster level, core corrections, diagonal Born-Oppenheimer corrections and spin-orbit coupling corrections. The reaction barrier and reaction energy were estimated to be 1.64 kcal mol-1 and -39.97 kcal mol-1, respectively, which are in good agreement with the experimental results. Using this barrier height, we have estimated the rate constants using transition state theory augmented with zero curvature tunneling (TST/ZCT) in the temperature range of 220-450 K and the thus obtained rate constants were compared with available experimental results.

Interactions between conducting surfaces in salt solutions.

In this work, we simulate interactions between two perfectly conducting surfaces, immersed in a salt solution. We demonstrate that these forces are quantitatively different from those between (equally charged) non-conducting surfaces. There is, for instance, a significant repulsion between net neutral surfaces. On the other hand, there are also qualitative similarities, with behaviours found with non-conducting surfaces. For instance, there is a non-monotonic dependence of the free energy barrier height, on the salt concentration, and the minimum essentially coincides with a flat profile of the apparent surface charge density (i.e. the effective net surface charge density, some distance away from the surface, when accounting for ion neutralization), outside the so-called Stern layer. These conditions can be described as "perfect surface charge neutralization". Despite observed quantitative differences, we demonstrate that it might be possible to mimic a dispersion containing charged colloidal metal particles by a simpler model system with charged non-conducting particles, using modified particle-ion interactions.

Give or Take: Effects of Electron-Accepting/-Withdrawing Groups in Red-Fluorescent BODIPY Molecular Rotors.

Mapping microviscosity, temperature, and polarity in biosystems is an important capability that can aid in disease detection. This can be achieved using fluorescent sensors based on a green-emitting BODIPY group. However, red fluorescent sensors are desired for convenient imaging of biological samples. It is known that phenyl substituents in the position of the BODIPY core can shift the fluorescence spectra to longer wavelengths. In this research, we report how electron-withdrawing (EWG) and -donating (EDG) groups can change the spectral and sensory properties of -phenyl-substituted BODIPYs. We present a trifluoromethyl-substituted (EWG) conjugate with moderate temperature sensing properties and a methoxy-substituted (EDG) molecule that could be used as a lifetime-based polarity probe. In this study, we utilise experimental results of steady-state and time-resolved fluorescence, as well as quantum chemical calculations using density functional theory (DFT). We also explain how the energy barrier height () for non-radiative relaxation affects the probe’s sensitivity to temperature and viscosity and provide appropriate ranges for the best possible sensitivity to viscosity and temperature.

Solid‐Phase Crystallization of GeSn Thin Films on GeO2‐Coated Glass

Polycrystalline Ge thin films are promising candidates for next‐generation thin‐film transistors. However, the difficulty in Fermi‐level control due to the high density of defect‐induced acceptors has been one of the main problems for device applications. Herein, GeO2 preparation and Sn addition are combined in the advanced low‐temperature (375 °C) solid‐phase crystallization of Ge layers that have been recently developed. The GeO2 underlayer works effectively at a low Sn composition (< 4%), enlarging the grain size by a magnification of ≈5. An appropriate amount of unsubstituted Sn reduces acceptor defects and achieves a minimum hole concentration of 3 × 1016 cm−3 by combining post‐annealing (500 °C). This hole concentration is the lowest level for undoped polycrystalline Ge‐based thin films, which allows control over their Fermi level widely by impurity doping. Although the lower hole concentration provides a higher energy barrier height of the grain boundary, the hole mobility is maintained at 190 cm2 V−1 s−1 owing to the large grains (15 μm diameter). This achievement paves the way for the implementation of poly‐Ge‐based thin films in various semiconductor devices, including advanced thin‐film transistors.

Physical device simulation of dopant-free asymmetric silicon heterojunction solar cell featuring tungsten oxide as a hole-selective layer with ultrathin silicon oxide passivation layer

The dopant-related issues are amongst the major performance bottleneck in crystalline silicon solar cells that can be alleviated via implementation of dopant-free layers. This work presents the implementation of tungsten oxide (WOx) and titanium oxide (TiOx) as hole- and electron-selective films for heterostructure solar cell design whereby n-type Si wafer has been passivated with ultrathin silicon oxide (SiO2) layer. Several designs have been investigated including passivated hydrogenated amorphous silicon (i-a-Si:H) and characterized by evaluating work function, electron affinity, interfacial charge, and layer thickness. The high work function of WOx induces significant upward band bending to permit holes transportation towards anode, whereas, low electron-affinity for TiOx reduces the barrier against electrons at the cathode. Smaller band offsets have been observed against minority carriers for devices that employ passivated i-a-Si:H film. However, incorporating SiO2 significantly improves the energy barrier height against minority carriers that leads to an enhancement in electric field along with reduction in recombination. The best-performance device with an optimum SiO2 thickness of 1 nm numerically validated Voc of 751 mV, Jsc 40.2 mA/cm2, FF 79.7%, and η of 24.06%. A comparative analysis with hole-selective vanadium oxide (V2Ox) demonstrated η of 21.73% limited by the low work function of V2Ox.

Thermodynamics and kinetics of crystallization in deeply supercooled Stillinger-Weber silicon.

We study the kinetics of crystallization in deeply supercooled liquid silicon employing computer simulations and the Stillinger-Weber three-body potential. The free energy barriers to crystallization are computed using umbrella sampling Monte Carlo simulations and from unconstrained molecular dynamics simulations using a mean first passage time formulation. We focus on state points that have been described in earlier work [S. Sastry and C. A. Angell, Nat. Mater. 2, 739 (2003)] as straddling a liquid-liquid phase transition (LLPT) between two metastable liquid states. It was argued subsequently [Ricci et al., Mol. Phys. 117, 3254 (2019)] that the apparent transition is due to the loss of metastability of the liquid state with respect to the crystalline state. The presence of a barrier to crystallization for these state points is therefore of importance to ascertain, which we investigate, with due attention to ambiguities that may arise from the choice of order parameters. We find a well-defined free energy barrier to crystallization and demonstrate that both umbrella sampling and mean first passage time methods yield results that agree quantitatively. Our results thus provide strong evidence against the possibility that the liquids at state points close to the reported LLPT exhibit slow, spontaneous crystallization, but they do not address the existence of a LLPT (or lack thereof). We also compute the free energy barriers to crystallization at other state points over a broad range of temperatures and pressures and discuss the effect of changes in the microscopic structure of the metastable liquid on the free energy barrier heights.

Intercalation Behavior of Water-in-Salt Electrolytes in Nanoscale Hydrophobic Carbon Confinement

Intercalation of water-in-salt (WIS) electrolytes in nanoscale confinement is an important phenomenon relevant to the energy storage applications. Here, we use enhanced sampling molecular dynamics simulations to investigate the effects of intersurface separation on the structure and free energy underlying the intercalation of the Li bis(trifluoromethane)sulfonimide ([Li][TFSI]) water-in-salt (WIS) electrolyte confined between nanoscale hydrophobic carbon surfaces. We observe that the confinement separation and the WIS concentration governs the relative stability of the condensed and vapor phases of WIS inside the confinement. The relative height of the free energy barrier separating the condensed and vapor phases of WIS also strongly depends on the concentration of [Li][TFSI] inside the confined space. We find that this concentration dependence is majorly due to changes in line tension. The process of deintercalation of electrolyte passes through a vapor tube formation inside the confined space. The size of the critical vapor tube required for spontaneous evaporation of WIS from the confinement is found to be dependent on the intersurface separation and WIS concentration.

(Invited) Study on Impact of MOS Interface Passivation Processes on Band Alignment and Flat-Band Voltage of 4H-SiC Gate Stacks

A sufficiently large threshold voltage (Vth) is generally demanded for high-voltage power MOSFETs with 4H-SiC, however, the increase in channel doping concentration to tune Vth inevitably induces a significant reduction of carrier mobility in the inversion channel, due to Coulomb scattering at SiO2/4H-SiC interface [1]. Another serious problem is that negative Vth shifts are often observed by employing SiC surface nitridation process, usually done by a post-oxidation annealing (POA) in NO ambient at elevated temperature. Such processes introduce nitrogen atoms selectively to the surface of SiC by substituting the topmost carbon atoms [2]. Currently such nitrogen passivation process is inevitable to fabricate 4H-SiC MOS gate stacks with sufficiently low interface state density and good device reliability. Therefore the development of technique to design Vth of 4H-SiC MOSFETs is strongly demanded based on the understanding of the factors to cause Vth shift.First we systematically studied the flat-band voltage (Vfb) of SiO2/4H-SiC (0001) n-type MOS capacitors with various oxide thickness, fabricated by conventional dry oxidation followed by NO-POA. From the oxide thickness dependence of Vfb we could deduce the expected Vfb value when the effects of fixed charges are removed. As a result we found an anomalous negative shift of Vfb in several hundreds of mV irrespective of oxide thickness, which was enlarged by extending NO annealing duration [3]. This phenomenon is quite unexpected but would be explainable by assuming a dipole layer formation at the interface by SiC surface nitridation. Considering the polar Si-N bond formation with high density ~1014 cm2 at the interface, the SiC surface with Si-coordinated N atoms could introduce a dipole layer.SiO2/4H-SiC band alignment was investigated by two kinds of methods: the valence-band spectra analysis using x-ray photoelectron spectroscopy and the energy barrier height characterization of Fowler-Nordheim (F-N) current analysis. A clear trend was observed that the valence-band offset between 4H-SiC (0001) and SiO2 changes monotonically by the NO-POA durations to result in a increase of conduction-band offset at SiO2/SiC interface, which naturally explains the unexpected negative shift of Vfb observed in MOS capacitors. To our surprise, this trend appeared to the opposite direction for the gate stacks on 4H-SiC (000-1) or (1-100) fabricated with similar processes. For the MOS capacitors on (0001), we could clearly see a decreasing trend of F-N tunneling current by NO annealing duration which was attributable to the increase of conduction-band offset, while the opposite trend was observed for the capacitors on (000-1) and (1-100). Even though the defect-assisted conduction mechanisms such as Pool-Frenkel model appears to dominantly determine the temperature-dependence and electric-field-dependence of the leakage current at room temperatures or higher, obviously higher leakage current level for a given electric field was observed on (000-1) and (1-100) stacks than (0001), which should be attributable to the significant difference of the band alignment in several hundreds of mV. These results clarify that there are significant intrinsic difference of the gate leakage characteristics among SiC gate stacks on different crystal orientations, as well as the value of Vfb, explainable by the difference of band alignment.Next we examined a way to tune the Vfb of SiC gate stacks by the introduction of another dipole layer, by depositing of just a few nm-thick Al2O3 layer on top of SiO2 after NO-POA, based on the concept of oxide interface dipole layers reported for various high-k/SiO2 interfaces [4]. As a result we successfully demonstrated ~1V Vfb shift without a deterioration of interface state density only by Al2O3 deposition and post-deposition annealing. This technique is expected as a method not only to overcome the unexpected change of Vfb by nitrogen passivation process, but to achieve larger Vth without increasing the channel doping to avoid the deterioration of channel mobility.In conclusion, we found that nitrogen passivation of SiO2/4H-SiC gate stacks resulted in a significant change of band alignment to increase the conduction band offset on (0001) stack but to decrease the offset on (000-1) or (1-100) stacks, which results in a difference of gate leakage characteristics as well as Vfb. Control of the formation of dipole layers at SiO2/SiC interface and/or that formed in the multilayer dielectrics would have important roles to tune the band alignment of SiC gate stacks.[1] M. Noguchi et al., Jpn. J. Appl. Phys. 58, 031004 (2019).[2] J. Rozen et al., J. Appl. Phys. 105, 124506 (2009).[3] T. H. Kil and K. Kita, Appl. Phys. Lett. 116, 122103 (2020).[4] K. Kita and A. Toriumi, Appl. Phys. Lett. 94, 132902 (2009).

Harnessing Phase Transitions in Antiferroelectric ZrO2Using the Size Effect

AbstractThe unique nonlinear dielectric properties of antiferroelectric (AFE) oxides are promising for advancements in solid state supercapacitor, actuator, and memory technologies. AFE behavior in high‐k ZrO2is of particular technological interest, but the origin of antiferroelectricity in ZrO2remains questionable. The theory of reversible electric field‐induced phase transitions between the nonpolar P42/nmc tetragonal phase and the polarPca21orthorhombic phase is experimentally tested with local structural and electromechanical characterization of AFE ZrO2thin films. Piezoresponse force microscopy identifies signature evidence of a field‐induced phase transition. A significant size effect in AFE ZrO2is experimentally observed as film thickness is scaled down from 14.7 to 4.3 nm. The size effect is explained by modifications to the phase transition energy barrier heights ranging from 0.6 to 7.6 meV f.u−1depending on crystallite size and in‐plane compressive strain with decreasing ZrO2film thickness. Using the size effect, it is possible to double the energy storage density in ZrO2from 20 J cm−3to greater than 40 J cm−3, thus highlighting a feasible route for superior performance in AFE fluorite supercapacitors.

Thin smectic liquid crystalline fibrils of chiral rodlike particles.

Inspired by recent experimental work on virus-polymer mixtures, we study the properties of thin smectic fibrils composed of chiral rodlike particles using Monte Carlo simulations. Due to the interplay between surface energy, elastic deformation energy, and entropic effects, the fibril's layers relax into a twisted state. We focus our study on the layers' twist direction and map our results to the antiferromagnetic Ising model. In this view, the chiral interaction mimics an external field that drives the layers to have the same sense of twist. Besides, we determine the free energy difference and barrier height between an alternating and a nonalternating sequence of twisted layers composed of achiral rods and find that an alternating sequence is slightly preferred. We also see that the fibrils contract on increasing the chiral interaction strength and think that further studies on self-assembled functional materials can use our results.