Unequal Electron Research Articles

Photovoltaic Response of Heterojunction Photodiode based on Ba0.8Sr0.2TiO3 Nanorod Film

The BST/Si photodiode has been effectively synthesized by growth BST on the p-type Si (100) substrate utilizing the hydrothermal process. Screen printing was used to prepare TiO2 film deposited on Si substrate, then immersing TiO2 film in Ba(OH)2 and Sr(OH)2 solution to fabricate BST/Si photodiode using a hydrothermal process. The BST film was studied using XRD, FESEM, and reflection spectra. The band gap was calculated for a BT film using the reflection methods. Hall measurement confirmed the n-type conductivity of BST film. Curie temperature of the BST film was observed at 87 °C according to DC measurement. The dark and illuminated (J-V) characteristics of the BST/Si photodiode have been measured under simulated AM1 conditions using a Xenon lamp. The Shockley-Read- Hall recombination caused unequal electron and hole capture rates that dominated the I-V characteristics resulting in an absence of classic superposition phenomena. The open circuit voltage (Voc) measurement is carried out to know the recombination mechanism in an open circuit. The BST/Si film showed more conductivity after increasing illuminating power density, which qualifies it for photovoltaic applications.

Particle-in-cell modeling of electron beam generated plasma

Plasmas generated using energetic electron beams are well known for their low electron temperature (T e) and plasma potential, which makes them attractive for atomic-precision plasma processing applications such as atomic layer etch and deposition. A 2-dimensional particle-in-cell model for an electron beam-generated plasma in argon confined by a constant applied magnetic field is described in this article. Plasma production primarily occurs in the path of the beam electrons in the center of the chamber. The resulting plasma spreads out in the chamber through non-ambipolar diffusion with a short-circuiting effect allowing unequal electron and ion fluxes to different regions of the bounding conductive chamber walls. The cross-field transport of the electrons (and thus the steady-state characteristics of the plasma) are strongly impacted by the magnetic field. T e is anisotropic in the electron beam region, but low and isotropic away from the plasma production zone. The plasma density increases and the plasma becomes more confined near the region of production when the magnetic field strengthens. The magnetic field reduces both electron physical and energy transport perpendicular to the magnetic field. T e is uniform along the magnetic field lines and slowly decreases perpendicular to it. Electrons are less energetic in the sheath regions where the sheath electric field repels and confines the low-energy electrons from the bulk plasma. Even though electron and ion densities are similar in the bulk plasma due to quasi-neutrality, electron and ion fluxes on the grounded chamber walls are unequal at most locations. Electron confinement by the magnetic field weakens with increasing pressure, and the plasma spread out farther from the electron beam region.

Potential‐Driven Semiconductor‐to‐Metal Transition in Monolayer Transition Metal Dichalcogenides

AbstractThe potential‐driven semiconductor‐to‐metal transition is investigated in monolayer transition metal dichalcogenides by employing a new proposed method, i.e., the fixed‐potential method (FPM). Under the same voltage, the semiconducting and metallic phases will be charged differently due to their different electronic properties. The potential‐driven phase transition process is simulated by the injection of unequal electrons in the semiconducting and metallic phases. The unequal electron injection is more consistent with the actual experimental process, although equal electron injection also can theoretically induce a phase transition. MoTe2 is chosen as a prototypical example to examine the physical mechanism. When the fixed electrode potential is above the potential of zero‐charge, excess electrons are injected into the metallic 1T’ phase instead of the semiconducting 2H phase, stabilizing the 1T’ phase. In addition, the potential‐dependent kinetics, in which the charge transfer is fluctuating, suggests that increasing the electrode potential will decrease the kinetic barrier of the 2H→1T’ transition process. The calculated relative transition voltage of 2.5 V agrees well with the experimental results, demonstrating the validity of the FPM. This study provides new insight into potential‐driven semiconductor‐to‐metal phase transitions and suggests a new theoretical approach for studies under constant voltage conditions.

Exploring the photocatalytic properties and carrier dynamics of 2D Janus XMMX' (X = S, Se; M = Ga, In; and X' = Te) materials.

Recently, two-dimensional (2D) Janus structures have been extensively explored because of their robust electron mobility and unique photocatalytic properties. In spite of the increasing interest, the origin of high photocatalytic activities and the behaviors of photoinduced carriers in this kind of materials have not been well understood. Herein, we present a step-by-step protocol based on the first-principles calculations combined with the ab initio non-adiabatic molecular dynamics (NAMD) simulations to unveil the origin of high photocatalytic activity of highly stable typical 2D Janus XMMX' structures (X = S, Se; M = Ga, In; and X' = Te). Their band structures, optical properties, exciton binding energies, carrier effective masses, solar-to-hydrogen efficiency, hot carrier relaxation and recombination times, etc. have been calculated. We find that the difference between X and X' atoms on the two surfaces of the XMMX' monolayer not only builds an out-of-plane electric field, which significantly affects the charge distributions on the valence band maxima (VBM) and the conduction band minima (CBM) and subsequently decreases the exciton binding energy, but also transforms the indirect band structures of XM into the direct ones with well suitable energy gaps for visible-light absorption as well as endows the XMMX' structures with unequal electron and hole mobility, rapid hot carrier relaxation and slow electron-hole recombination processes on a timescale of tens of nanoseconds. The current work suggests that Janus XMMX' monolayers are good photocatalytic materials for overall water splitting and provides a guide to regulate the materials' properties for efficient energy harvesting and optoelectronic applications.

Nonlinear solitary waves (oscillitons) in dusty magnetized pair plasmas

Nonlinear theory is presented for solitary waves propagating parallel to the magnetic field in a cold plasma with charged dust grains or heavy ions. The particular case of an imbalanced electron-positron plasma with a portion of the charge residing on the dust particles is investigated. The charge imbalance plays a similar role to unequal ion and electron masses, in that an “oscilliton” type solution is found, using an approximate analytic solution, as well as numerical solutions of approximate equations and the full non-relativistic equations. Circularly polarized oscillitons occur for the electron-positron plasma for a Mach number above a critical value which is <1, in contrast to the pure pair plasma which has a linearly polarized non-oscillatory nonlinear wave solution for Mach number >1.

Interplay of Coulomb interactions and disorder in three-dimensional quadratic band crossings without time-reversal symmetry and with unequal masses for conduction and valence bands

Coulomb interactions famously drive three dimensional quadratic band crossing semimetals into a non-Fermi liquid phase of matter. In a previous work, Phys. Rev. B 95, 205106 (2017), the effect of disorder on this non-Fermi liquid phase was investigated, assuming that the bandstructure was isotropic, assuming that the conduction and valence bands had the same band mass, and assuming that the disorder preserved exact time-reversal symmetry and statistical isotropy. It was shown that the non-Fermi liquid fixed point is unstable to disorder, and that a runaway flow to strong disorder occurs. In this work, we extend that analysis by relaxing the assumption of time-reversal symmetry and allowing the electron and hole masses to differ (but continuing to assume isotropy of the low energy bandstructure). We first incorporate time-reversal symmetry breaking disorder, and demonstrate that there do not appear any new fixed points. Moreover, while the system continues to flow to strong disorder, time-reversal-symmetry-breaking disorder grows asymptotically more slowly than time-reversal-symmetry-preserving disorder, which we therefore expect should dominate the strong-coupling phase. We then allow for unequal electron and hole masses. We show that whereas asymmetry in the two masses is irrelevant in the clean system, it is relevant in the presence of disorder, such that the 'effective masses' of the conduction and valence bands should become sharply distinct in the low-energy limit. We calculate the RG flow equations for the disordered interacting system with unequal band masses, and demonstrate that the problem exhibits a runaway flow to strong disorder. Along the runaway flow, time-reversal-symmetry-preserving disorder grows asymptotically more rapidly than both time-reversal-symmetry-breaking disorder and the Coulomb interaction.

The 2.35 year itch of Cygnus OB2 #9

X-ray and radio data recently acquired as part of a project to study Cyg OB2#9 are used to constrain physical models of the binary system, providing in-depth knowledge about the wind-wind collision and the thermal, and non-thermal, emission arising from the shocks. We use a three-dimensional, adaptive mesh refinement simulation (including wind acceleration, radiative cooling, and the orbital motion of the stars) to model the gas dynamics of the wind-wind collision. The simulation output is used as the basis for radiative transfer calculations considering the thermal X-ray emission and the thermal/non-thermal radio emission. To obtain good agreement with the X-ray observations, our initial mass-loss rate estimates require a down-shift by a factor of roughly 7.7 to $6.5\times10^{-7}$ and $7.5\times10^{-7}$ solar mass per year for the primary and secondary star, respectively. Furthermore, the low gas densities and high shock velocities in Cyg OB2#9 are suggestive of unequal electron and ion temperatures, and the X-ray analysis indicates that an (immediately post-shock) electron-ion temperature ratio of $\simeq 0.1$ is also required. The radio emission is dominated by (non-thermal) synchrotron emission. A parameter space exploration provides evidence against models assuming equipartition between magnetic and relativistic energy densities. However, fits of comparable quality can be attained with models having stark contrasts in the ratio of magnetic-to-relativistic energy densities. The radio models also reveal a subtle effect whereby inverse Compton cooling leads to an increase in emissivity as a result of the synchrotron characteristic frequency being significantly reduced. Finally, using the results of the radio analysis, we estimate the surface magnetic field strengths to be $\approx 0.3-52\;$G. (Abridged)

The influence of the secondary electron induced asymmetry on the electrical asymmetry effect in capacitively coupled plasmas

In geometrically symmetric capacitive radio-frequency plasmas driven by two consecutive harmonics, a dc self-bias can be generated as a function of the phase shift between the driving frequencies via the Electrical Asymmetry Effect (EAE). Recently, the Secondary Electron Asymmetry Effect (SEAE) was discovered (Lafleur et al., J. Phys. D: Appl. Phys. 46, 135201 (2013)): unequal secondary electron emission coefficients at both electrodes were found to induce an asymmetry in single-frequency capacitive plasmas. Here, we investigate the simultaneous presence of both effects, i.e., a dual-frequency plasma driven by two consecutive harmonics with different electrode materials. We find that the superposition of the EAE and the SEAE is generally non-linear, i.e., the asymmetries generated by each individual effect do not simply add up at all phases. The control ranges of the dc self-bias and the mean ion energy can be enlarged, if both effects are combined.

Observation of inhibited electron-ion coupling in strongly heated graphite

Creating non-equilibrium states of matter with highly unequal electron and lattice temperatures (Tele≠Tion) allows unsurpassed insight into the dynamic coupling between electrons and ions through time-resolved energy relaxation measurements. Recent studies on low-temperature laser-heated graphite suggest a complex energy exchange when compared to other materials. To avoid problems related to surface preparation, crystal quality and poor understanding of the energy deposition and transport mechanisms, we apply a different energy deposition mechanism, via laser-accelerated protons, to isochorically and non-radiatively heat macroscopic graphite samples up to temperatures close to the melting threshold. Using time-resolved x ray diffraction, we show clear evidence of a very small electron-ion energy transfer, yielding approximately three times longer relaxation times than previously reported. This is indicative of the existence of an energy transfer bottleneck in non-equilibrium warm dense matter.

Energy transfer rate in double-layer graphene systems: Linear regime

We theoretically investigate the energy transfer phenomenon in a double-layer graphene (DLG) system. We use the balance equation approach in linear regime and random phase approximation screening function to obtain energy transfer rates at different electron temperatures, densities and interlayer spacings. We find that the rate of energy transfer in the DLG is qualitatively similar to that obtained in the double-layer two-dimensional electron gas but its values are an order of magnitude greater. Also, at large electron temperature differences between two graphene layers, the electron density dependence of energy transfer is significantly different, particularly in case of unequal electron densities.

Modelling and Simulation of Field-effect Surface Passivation of Crystalline Silicon-based Solar Cells

An effective surface passivation plays a vital role in the performance of crystalline silicon (c-Si) solar cells. Experimental research shows that fixed charge-induced field-effect passivates the c-Si efficiently. In this work, n-type and p-type c-Si wafers symmetrically passivated by the negative-charge dielectric Al2O3 were numerically modelled with SENTAURUS TCAD. Surface recombination is traditionally modelled by employing the extended Shockley-Read-Hall (SRH) model with single energy level of interface traps. However, experiments show interface traps distribute across the silicon bandgap. Thus, we implemented the extended SRH model with the ability to describe arbitrary energy distribution of interface traps within the bandgap. The extended SRH model predicts a constant effective surface recombination velocity at low injection levels for lightly doped n-type and p-type c-Si passivated by dielectrics with either a high negative or positive charge density. However, this prediction contradicts experimental results which show a significant reduction of the effective lifetime at low injection levels if the polarity of the fixed charge is attracting the minority bulk charge carriers. One explanation is assuming the presence of a thin defect-rich layer close to the c-Si surface in which the lifetimes are degraded. However, the choice of the lifetime and depth of such damaged surface region seem rather arbitrary and its physical origin is still unclear. We show that unequal electron and hole bulk lifetime parameters can also account for the phenomenon. This proposition is physically plausible and the simulation results also show a good agreement with the experimental data from both n-type and p-type c-Si wafers passivated by Al2O3. Our modelling results predict a simple but unambiguous experiment to distinguish between these two explanations.

Universality of Zener tunneling in homojunction p-n diodes

We show that in homojunction p-n diodes made of semiconductors with unequal electron and hole effective masses, Zener tunneling is approximately universal, but not perfectly universal, as a function of effective tunneling width, where the effective tunneling width takes the effects of band curvature into account. We find that the curvature correction (CC), which is applied to obtain the approximate universality of tunneling, is by itself not universal but show that (1−CC)∕N2 is universal as a function of the tunneling width for a given material, where N is the doping density.

Spin-orbit interactions in bilayer exciton-condensate ferromagnets

Bilayer electron-hole systems with unequal electron and hole densities are expected to have exciton condensate ground states with spontaneous spin-polarization in both conduction and valence bands. In the absence of spin-orbit and electron-hole exchange interactions there is no coupling between the spin-orientations in the two quantum wells. In this article we show that Rashba spin-orbit interactions lead to unconventional magnetic anisotropies, whose strength we estimate, and to ordered states with unusual quasiparticle spectra.

Charge-dependent effects in double photoionization of He-like ions

A study is made of triple differential cross sections (TDCS) resulting from the double photoionization (DPI) of He-like ions. The angular correlations between the outgoing electrons are examined as a function of the nuclear target charge. Time-dependent close-coupling theory (TDCC) is used to solve the time-dependent Schrödinger equation for both outgoing electrons. Previous theoretical models that have calculated the TDCS for He-like ions have only included the electron–electron interaction through approximate perturbative methods. We have analysed the effects of the electron correlation and the dependence relative to the nuclear charge. We find that the differing TDCS shapes for unequal electron energy sharings observed by Otranto and Garibotti (2003 Eur. Phys. J. D 27 215, 2005 Phys. Rev. A 71 034703-1, 2003 Phys. Rev. A 67 064701) for He-like ions are not due to the decreasing relative strength of the electron–electron correlation, but rather due to the different energy regions in which each calculation is carried out. Our calculations are compared with previous experimental and theoretical work, where available.

P-Type Versus n-Type Silicon Wafers: Prospects for High-Efficiency Commercial Silicon Solar Cells

Chemical and crystallographic defects are a reality of solar-grade silicon wafers and industrial production processes. Long overlooked, phosphorus as a bulk dopant in silicon wafers is an excellent way to mitigate recombination associated with these defects. This paper details the connection between defect recombination and solar cell terminal characteristics for the specific case of unequal electron and hole lifetimes. It then looks at a detailed case study of the impact of diffusion-induced dislocations on the recombination statistics in n-type and p-type silicon wafers and the terminal characteristics of high-efficiency double-sided buried contact silicon solar cells made on both types of wafers. Several additional short case studies examine the recombination associated with other industrially relevant situations-process-induced dislocations, surface passivation, and unwanted contamination. For the defects studied here, n-type silicon wafers are more tolerant to chemical and crystallographic defects, and as such, they have exceptional potential as a wafer for high-efficiency commercial silicon solar cells

A comprehensive model for photomixing in ultrafast photoconductors

A comprehensive model for photomixing phenomenon in a dc-biased ultrafast photoconductor is proposed. The dependence of the carrier lifetime and carrier velocity on the electric field is taken into account, and the significance of carrier transport and carrier generation and recombination mechanisms in a terahertz photomixer are evaluated. A method for calculating the nonuniform induced electric field due to excess charge density resulting from unequal electron and hole recombination lifetimes is presented

Redistribution of Electronic Charge in (TMTTF)2ReO4: 13C NMR Investigation

13C NMR measurements were performed for one-dimensional organic conductors, (TMTTF)2ReO4. An intermediate charge-ordering (CO) phase has been found firstly for a TMTTF salt with a Td-symmetry counter anion: The NMR parameters indicate two inequivalent molecules with unequal electron densities below 225 K. Moreover, the spin-singlet transition associated with ReO4 anion ordering was confirmed at around 158 K by 13C NMR measurements. A drastic change of NMR parameters below 158 K also indicates a redistribution of the electronic charge at the anion ordering temperature.

Novel oxidation of RbGa 3 through substitution of gold for gallium—a reapportionment of the dodecahedral clusters

Exploratory syntheses in the rubidium–gallium–late transition metal systems have revealed a family of nonstoichiometric compounds are formed in RbGa 3− x M x systems (M=Cu, Ag and Au) that remain isostructural with RbGa 3 ( I4̄m2). Only the RbGa 3− x Au x products could be obtained in high yields, and these have been structurally defined for x=0.26(1) and 0.36(2) (saturation). Gold substitutes unequally on all three gallium positions in the structure of RbGa 3, the anion of which consists of layers of interlinked Ga 8 dodecahedral clusters plus 4-bonded gallium spacers between the layers. The effects of the oxidation of the structure by Au are well focused on a 0.19 Å elongation of a short, evidently π-bond at the 4̄ extremes of the Ga 8 dodecahedron between x=0 and x=0.36. Extended Hückel band calculations on the anion network provide insights into the distortion, which evidently reflects an unequal electron distribution of the bonding states in a deltahedron made of two types of cluster atoms.

Stability analyses of two-temperature radiative shocks: formulation, eigenfunctions, luminosity response and boundary conditions

We present a general formulation for stability analyses of radiative shocks with multiple cooling processes, longitudinal and transverse perturbations, and unequal electron and ion temperatures. Using the accretion shocks of magnetic cataclysmic variables as an illustrative application, we investigate the shock instabilities by examining the eigenfunctions of the perturbed hydrodynamic variables. We also investigate the effects of varying the condition at the lower boundary of the post-shock flow from a zero-velocity fixed wall to several alternative types of boundaries involving the perturbed hydrodynamic variables, and the variations of the emission from the post-shock flow under different modes of oscillations. We found that the stability properties for flow with a stationary-wall lower boundary are not significantly affected by perturbing the lower boundary condition, and they are determined mainly by the energy-transport processes. Moreover, there is no obvious correlation between the amplitude or phase of the luminosity response and the stability properties of the system. Stability of the system can, however, be modified in the presence of transverse perturbation. The luminosity responses are also altered by transverse perturbation.

Charge ordering in the TMTTF family of molecular conductors

Using one- and two-dimensional NMR spectroscopy applied to 13C spin-labeled (TMTTF)2AsF6 and (TMTTF)2PF6, we demonstrate the existence of an intermediate charge-ordered phase in the TMTTF family of charge-transfer salts. At ambient temperature, the spectra are characteristic of nuclei in equivalent molecules. Below a continuous charge-ordering transition temperature T(co), there is evidence for two inequivalent molecules with unequal electron densities. The absence of an associated magnetic anomaly indicates only the charge degrees of freedom are involved and the lack of evidence for a structural anomaly suggests that charge-lattice coupling is too weak to drive the transition.