Large Internal Electric Fields Research Articles

THz generation by AlGaAs/GaAs heterostructured p-i-n diode

The generation of terahertz radiation by heterostructure p-i-n AlxGa1−xAs/GaAs diodes excited by femtosecond optical pulses was studied experimentally and using the Monte Carlo method. It is shown that when the reverse bias varies, the terahertz generation mechanism changes. With a positive bias on the p-i-n diode, the THz generation mechanism is due to the reflection of the photoexcited electrons from the interface. With a large internal electric field, THz generation in the p-i-n diode occurs due to the acceleration of electrons at the ballistic stage of their movement in the electric field to velocities significantly exceeding the steady state velocity (“velocity overshoot”). The subsequent sharp decrease in velocity of electrons is associated with their inter-valley transitions from the Γ-valley to the L-valley of the conduction band. At electric fields less than 22 kV/cm, the effect of electric field screening by photoexcited carriers has a significant impact on the formation of photocurrent and, accordingly, on the THz generation mechanism. As the reverse bias decreases, this effect leads to a shift in the maximum of the THz pulse toward shorter times and it begins to dominate at electric fields less than 10 kV/cm.

Synthesis of ZnO nanosheets @In2O3 hollow micro-rods heterostructures for enhanced ethanol gas sensing performance

In this paper, the ZnO nanosheets @In2O3 hollow microrods heterostructures were successfully prepared by co-precipitation and annealing methods. It was observed by SEM and TEM that the surface of the In2O3 hollow microrods was uniformly coated with porous ZnO nanosheets, which increased the specific surface area of ZnO@In2O3 composites. The In2O3 hollow microrods functions as electron transport channels and facilitate the transfer of electrons between ZnO nanosheets, consequently elevating the base resistance of sensors. The maximum sensor response of ZnO@In2O3-2 (the molar ratio of ZnO and In2O3 is 5:2) was 269.1 for 100 ppm ethanol at the optimal working temperature of 200 ºC, which is about 30.9 times higher than that of pure ZnO nanosheets (8.7) and 19.1 times higher than that of pure In2O3 (14.1). Additionally, the ZnO@In2O3-2 composites exhibit rapid response-recovery times, excellent selectivity and favorable stability. The improved gas sensing performance of ZnO@In2O3-2 heterostructures were ascribed to its heterogeneous structure building, abundant oxygen vacancies, large specific surface area and internal electric field through characterized by XPS, EPR, and BET.

Conjugated polymers S-scheme homojunction with large internal electric field and matching interface for efficient visible light photocatalytic degradation of ciprofloxacin

Given the urgent demand and broad prospects for renewable energy and sustainable environmental technologies, the semiconductor-based photocatalysis represents an increasingly attractive frontline technique. Herein, a conjugated polymers S-scheme homojunction was prepared by electrostatic self-assembling the hollow tubular g-C3N4 (PCN) and nitrogen deficient boron doped g-C3N4 nanosheets (BCNx). The photocatalytic removal of the typical antibiotic ciprofloxacin (CIP) was conducted to verify the performance of PCN/BCNx. The pathway of S-scheme charge transfer was validated through the techniques, such as DFT calculations and Kelvin probe force microscope. The PCN/BCNx S-scheme homojunction features the efficient separation of carriers without compromise their redox potentials. The internal electric field (IEF) intensity of PCN/BCN3 was 2.34 times that of PCN and 1.40 times that of BCN3. Given the credit to the unique S-scheme carriers transfer route and the enhanced IEF intensity, under visible light illumination, the tallest CIP degradation percentage (94.9%) was gained in PCN/BCN3. The rate constants (0.0251 min−1) were 2.1 times that of BCN3 and 3.8 times that of PCN, respectively. Furthermore, the reactive oxygen species for CIP degradation was clarified based on chemical trapping experiment and ESR results. The conjugated polymers S-scheme homojunction with large IEF and matching interface was synthesized in the current study for treating antibiotics polluted waters.

Low-efficiency-droop c-plane InGaN/GaN light-emitting diodes through the use of thick single quantum wells and doped barriers

Efficiency droop at high current densities is a problem for InGaN-based light-emitting diodes (LEDs), especially for conventional c-plane devices. The large internal electric fields in c-plane quantum wells (QWs) lead to an increase in the active region carrier density (n), causing the electrical efficiency droop onset to occur at low current densities. Here, we present an approach to reduce the internal electric fields (Eint) in c-plane QWs by placing doped p-type and n-type GaN barriers close to the QW. The reduced Eint also allows a thick QW active region design, which helps to lower n to further reduce the droop. The concept of using doped barriers to control Eint is explained using theory and device simulations. Following that, multiple series of thick single QW (SQW) LEDs were grown and characterized. Key parameters in the epitaxial design such as the doping levels and the relative position of the doped barriers were systematically studied and optimized. By using optimized doped barriers and a thick SQW, c-plane LEDs with a low-efficiency droop of 14% at 300 A/cm2 [with respect to the peak external quantum efficiency (EQE)] and a high peak EQE of 49% were demonstrated.

Origin of the Critical Thickness in Improper Ferroelectric Thin Films

Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO3 thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO3 surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.

The Precision Defect Engineering with Nonmetallic Element Refilling Strategy in g-C3 N4 for Enhanced Photocatalytic Hydrogen Production.

Traditional defect engineering and doping strategies are considered effective means for improving H2 evolution, but the uncontrollability of the modification process does not always lead to efficient activity. A defect-induced heteroatom refilling strategy is used here to synthesize heteroatoms introduced carbon nitride by precisely controlling the "introduction" sites on efficient N1 sites. Density functional theory calculations show that the refilling of B, P, and S sites have stronger H2 O adsorption and dissociation capacity than traditional doping, which makes it an optimal H2 production path. The large internal electric field strength of heteroatom-refilled catalysts leads to fast electron transfer and the hydrogen production of the best sample is up to 20.9mmolg-1 h-1 . This work provides a reliable and clear insight into controlled defect engineering of photocatalysts and a universal modification strategy for typical heteroatom and co-catalyst systems for H2 production.

Origin of Bismuth‐Rich Strategy in Bismuth Oxyhalide Photocatalysts

Recently, the bismuth‐rich strategy via increasing the bismuth content has been becoming one of the most appealing approaches to improve the photocatalytic performance of bismuth oxyhalides. However, insights into the mechanism behind the encouraging experiments are missing. Herein, we report the results of the theory‐led comprehensive picture of bismuth‐rich strategy in bismuth oxyhalide photocatalysts, selecting Bi5O7X (X = F, Cl, Br, I) as a prototype. First‐principle calculations revealed that the strategy enables good n‐type conductivity, large intrinsic internal electric field, high photoreduction ability and outstanding harvest of visible light, and particularly ranked the intrinsic activity of this family: Bi5O7F > Bi5O7I > Bi5O7Br > Bi5O7Cl. Designed experiments confirmed the theoretical predictions, and together, these results are expected to aid future development of advanced photocatalysts.

Novel Z‐scheme 2D/2D Bi4O5Br2/BiOCl heterojunction with enhanced photocatalytic activity for RhB degradation

AbstractBACKGROUNDWith the rapid development of industry and commerce, organic pollutants such as RhB are detrimental to human health and living environment. As a promising technology for treating the organic pollutants in wastewater, photocatalysis technology has received worldwide attention. For practical application, a novel photocatalyst with superior photocatalytic activity should be developed.RESULTSA novel Bi4O5Br2/BiOCl (BC2) heterojunction photocatalyst with 2D/2D structure was fabricated via a facile one‐pot solvothermal method. Compared with pure Bi4O5Br2 and BiOCl, the BC2 heterojunction exhibited enhanced photocatalytic performance which reached 91% photodegradation efficiency for RhB under 120 min irradiation of simulated solar‐light at pH 7. And 99% RhB was degraded by BC2 under 60 min irradiation at pH 6.CONCLUSIONAccording to experimental and characterization analytical results, the enhanced photocatalytic activity of Bi4O5Br2/BiOCl (BC2) heterojunction could be attributed to the following four points: (i) The BC2 heterojunction with excellent visible light response could effectively utilize the simulated solar‐light. (ii) The more negative surface electronegativity of BC2 favored the adsorption of RhB during the photodegradation process. (iii) The 2D/2D BC2 heterojunction had large interface region and internal electric field, thus it displayed superior carriers transfer capability and enhanced charge separation efficiency. (iv) The carriers transfer mechanism of Z‐scheme could hinder the recombination of photoinduced charge carriers and endow this photocatalyst with enough redox capacity to degrade RhB. Moreover, slightly acidic condition (pH 6) could promote degradation of RhB, due to the enhanced adsorption capacity of BC2 heterojunction. © 2022 Society of Chemical Industry (SCI).

Laser Characteristic and Strain Distribution Dependence on Embedding Layer Thickness of Quantum Dots Laser Diodes Grown on InP(311)B Substrate

Herein, 14 layer‐stacked quantum dots laser diodes (QD‐LDs) are fabricated with different thicknesses of embedding layers grown on InP(311)B substrate, and the lowest threshold current (Ith) of 21.6 mA is demonstrated in an as‐cleaved ridge‐structured QD‐LD with 13 nm‐thick embedding layers. Ith has the minimum value when the thickness is 13 nm. The factors unique to QDs grown on an InP(311)B substrate are assumed as the overlap integral of the wavefunction of electrons and holes and the decrease in gain owing to the formation of a miniband. According to the simulation results of the local strain profile in the embedding and QD layers, the strain in the QD and embedding layer is higher in the case of 7 nm thickness than in the case of 20 nm thickness. As a result of the large strain, because the thickness of the embedding layer decreases, a large internal electric field tends to be generated, causing the wavefunctions to become largely separated and localized. Therefore, the overlap integral of the wavefunction becomes smaller, and because of the decrease in the radiation recombination probability, the internal quantum efficiency also decreases, causing the characteristic of Ith on the thickness of the embedding layer.

Charge and spin transport over record distances in GaAs metallic n -type nanowires

We have investigated charge and spin transport in $n$-type metallic GaAs nanowires ($\ensuremath{\approx}{10}^{17}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ doping level) grown by hydride vapor phase epitaxy (HVPE) on Si substrates. For this doping level, charge and spin transport might appear difficult because of the expected localization of minority holes in the valence band potential fluctuations generated by statistical fluctuations of the donor concentration. In contrast with these expectations, it is found, using spatially and spectrally resolved investigation of the luminescence intensity and circular polarization under laser excitation, that (i) establishment of a charge thermodynamic equilibrium between the photoelectrons and the Fermi sea occurs over a distance from the excitation spot of $2\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. At this distance, the spin polarization is still observed, implying that photoelectrons have preserved their spin orientation and that the two spin reservoirs remain distinct. (ii) Charge can be transported over record distances larger than $20\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ at 6 K. (iii) Spatially-resolved investigations show that a photoelectron spin polarization of $20%$ can be transported over a record distance of more than $20\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$. This long distance transport occurs because of the presence of large internal electric fields of ambipolar origin, further enhanced by the spatial redistribution of the Fermi sea. These findings have potential applications for long distance spin transport in $n$-type doped nanowires.

Excellent Photocatalytic Efficiency of t-ZrO2/g-C3N4 Photocatalyst for Pollutants Degradation: Experiment and theory

The large work function difference between two semiconductors will effectively adjust the interfacial electronic distribution and catalytic performances of heterojunction. In this work, a novel t-ZrO2/g-C3N4 composite photocatalyst was prepared by integrating t-ZrO2 nanoparticle with graphitic carbon nitride (g-C3N4) film considering the large work function of tetragonal ZrO2 (t-ZrO2). With the aid of experimental measurements and density functional theory calculations, the electronic distributions of g-C3N4 film were explicitly adjusted by the supporting t-ZrO2 nanoparticle on account of the large interfacial internal electric field, which leaded to the improvement of photogenerated carriers mobility and lifetime, the enhancement of reduction ability and the increase of oxidation active sites. Results showed that the photocatalytic activity of the composite sample was increased by 9.4 and 2.9 times compared with g-C3N4 sample for Rhodamine B and tetracycline hydrochloride degradation under visible-light. This modification strategy shall be applied to devise other high-efficient catalysts by selecting semiconductor with suitable work function.

Ferroelectricity temperature characterizes of hardened KNN–CC lead-free piezoceramics

The qualitative and quantitative analyses of ferroelectricity temperature characterizes for (K0.48Na0.52)NbO3–1 mol%CuO–1 mol%Co2O3 (KNN–CC) lead-free piezoceramic has been systematically investigated. KNN–CC ceramic shows an asymmetric P–E loops at various temperatures, and remnant polarization (Pr) reaches the maximum at T ~ 80 °C. Dynamic hysteresis characterizes indicate orthorhombic (O) to monoclinic (M) phase transition occurs round 80 °C. Large internal bias electric field (Ei) is attributed to defect dipoles polarization (PD), which is difficult to orientation because of low migration rate of defects. Moreover, owing to CuO and Co2O3 additives, the space charge cluster at grain boundary confirmed by impedance spectroscopy (IS) analysis result in enlarged Ei with increasing temperature. Mechanical quality factor (Qm ~ 868) of aged KNN–CC ceramic is almost doubled that of unaged one. In short, both defects and phase structure affect the temperature stability of ferroelectricity and macroscopic electrical property for KNN lead-free piezoceramics.

Electric-field-induced optical hysteresis in single-layer WSe2

We demonstrate that the exciton energy of a monolayer of tungsten diselenide on an SiO2/Si substrate can be tuned by an applied in-plane electric field for two samples with different dielectric capping materials. The exciton energy can be either red- or blue-shifted by up to 20 meV based on the polarity of the applied electric field. We argue that a piezoelectric effect creates a large internal electric field, which is either partially aligned or partially antialigned with the external electric field. Additionally, optical hysteresis is observed on cycling of the external electric field due to trapped charges.

Mechanism of electric field-induced softening (EFIS) of alkali silicate glasses

Electric field-induced softening (EFIS) has been suggested as an innovative glass processing technique to reduce the furnace temperature required for softening of alkali silicate glasses. Although its overall mechanism was proposed previously, further investigation is required to fully understand the beginning of the EFIS process from initial polarization onwards. Accordingly, we have investigated the effect of externally applied DC voltage across glass on thermally stimulated currents during EFIS. The role of alkali ion migration has been investigated using two binary alkali (lithium and sodium) disilicate glasses, two mixed alkali lithium-sodium disilicate glasses and a sodium aluminosilicate glass of varying electrical resistivity. Comparison of thermally stimulated poling currents between single and mixed-alkali disilicate glasses has delineated the role of alkali ion migration and charge transport mechanisms. Activation energies corresponding to polarization mechanisms and charge transport processes have been calculated from experimental data. Based on these observations, the complex viscous flow response of glass during EFIS is explained as comprising of following phenomena: the formation of an alkali ion depletion layer, development of large internal electric fields, electrolysis and charge injection processes followed by dielectric breakdown, ultimately leading to thermal runaway by resistive heating.

Research Update: Relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells

The hybrid perovskite CH3NH3PbI3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe, and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior. The indirect gap is retained with dynamic disorder.

Low dislocation density InAlN/AlN/GaN heterostructures grown on GaN substrates and the effects on gate leakage characteristics

This paper reports on the electrical characterization of Ni/Au Schottky diodes fabricated on InAlN high-electron-mobility transistor (HEMT) structures grown on low dislocation density free-standing GaN substrates. InAlN HEMT structures were grown on sapphire and GaN substrates by metal-organic vapor phase epitaxy, and the effects of threading dislocation density on the leakage characteristics of Ni/Au Schottky diodes were investigated. Threading dislocation densities were determined to be 1.8 × 104 cm−2 and 1.2 × 109 cm−2 by the cathodoluminescence measurement for the HEMT structures grown on GaN and sapphire substrates, respectively. Leakage characteristics of Ni/Au Schottky diodes were compared between the two samples, and a reduction of the leakage current of about three to four orders of magnitude was observed in the forward bias region. For the high reverse bias region, however, no significant improvement was confirmed. We believe that the leakage current in the low bias region is governed by a dislocation-related Frenkel–Poole emission, and the leakage current in the high reverse bias region originates from field emission due to the large internal electric field in the InAlN barrier layer. Our results demonstrated that the reduction of dislocation density is effective in reducing leakage current in the low bias region. At the same time, it was also revealed that another approach will be needed, for instance, band modulation by impurity doping and insertion of insulating layers beneath the gate electrodes for a substantial reduction of the gate leakage current.

Visible Plasma Clouds With an Externally Excited Spherical Porous Cavity Resonator

A microwave driven resonator is as an electron/ion cloud generator for illumination and plasma source applications. A sustained porous cavity resonator (PCR) glow discharge is externally excited using a resonant frequency electromagnetic (EM) wave that excites large internal electric fields. The resonator with a Q of 300 amplifies the incident electric field by factors of about 100 causing a breakdown of the neutral gas inside the sphere. The rise time of the fields inside the sphere is Q times the wave period. A glowing plasma ball is sustained around the point where the maximum electric fields exceed the plasma-discharge threshold for the low-pressure gas inside the resonator. After the EM pump field is removed, the plasma light source is rapidly quenched. The externally driven spherical PCR was fabricated from a theoretical design of the PCR for a laboratory demonstration of the plasma ball generation system. Using both theory and experiment, basic research has been applied to understand: 1) the EMs of resonator excitation and amplification; 2) the generation of light by glow radio frequency discharge; and 3) the effect of background neutral density of the size and intensity of the glowing plasma clouds. For this study, plasma resonators were constructed for the spherical TM101 and TE101 modes and a TE011 circular-cylindrical cavity. All PCR structures provide the ability to confine a plasma into a desired spatial shape without magnetic fields.

Engineering ferroelectric tunnel junctions through potential profile shaping

We explore the influence of the top electrode materials (W, Co, Ni, Ir) on the electronic band profile in ferroelectric tunnel junctions based on super-tetragonal BiFeO3. Large variations of the transport properties are observed at room temperature. In particular, the analysis of current vs. voltage curves by a direct tunneling model indicates that the metal/ferroelectric interfacial barrier height increases with the top-electrode work function. While larger metal work functions result in larger OFF/ON ratios, they also produce a large internal electric field which results in large and potentially destructive switching voltages.

Enhanced electrical transparency by ultrathin LaAlO3 insertion at oxide metal/semiconductor heterointerfaces.

We demonstrate that the electrical conductivity of metal/semiconductor oxide heterojunctions can be increased over 7 orders of magnitude by inserting an ultrathin layer of LaAlO3. This counterintuitive result, that an interfacial barrier can be driven transparent by inserting a wide-gap insulator, arises from the large internal electric field between the two polar LaAlO3 surfaces. This field modifies the effective band offset in the device, highlighting the ability to design the electrostatic boundary conditions with atomic precision.

Geiger mode mapping: A new imaging modality for focused ion microprobes

Geiger mode detectors fabricated in silicon are used to detect incident photons with high sensitivity. They are operated with large internal electric fields so that a single electron–hole pair can trigger an avalanche breakdown which generates a signal in an external circuit. We have applied a modified version of the ion beam induced charge technique in a nuclear microprobe system to investigate the application of Geiger mode detectors to detect discrete ion impacts. Our detectors are fabricated with an architecture based on the avalanche diode structure and operated with a transient bias voltage that activates the Geiger mode. In this mode avalanche breakdown is triggered by ion impact followed by diffusion of an electron–hole pair into the sensitive volume. The avalanche breakdown is quenched by removal of the transient bias voltage which is synchronized with a beam gate. An alternative operation mode is possible at lower bias voltages where the avalanche process self-quenches and the device exhibits linear charge gain as a consequence. Incorporation of such a device into a silicon substrate potentially allows the exceptional sensitivity of Geiger mode to register an electron–hole pair from sub-10keV donor atom implants for the deterministic construction of shallow arrays of single atoms in the substrate required for emerging quantum technologies. Our characterization system incorporates a fast electrostatic ion beam switcher gated by the transient device bias, duration 800ns, with a time delay, duration 500ns, that allows for both the ion time of flight and the diffusion of the electron–hole pairs in the substrate into the sensitive region of the device following ion impact of a scanned 1MeV H microbeam. We compare images at the micron scale mapping the response of the device to ion impact operated in both Geiger mode and avalanche (linear) mode for silicon devices engineered with this ultimate-sensitivity detector structure.