Light Hole Research Articles

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots.

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Targeted search for gravitational waves from highly spinning light compact binaries

Abstract Searches for gravitational waves from compact-binary mergers, which to date have reported ∼100 observations, have previously ignored binaries whose components are both consistent with the mass of neutron stars (1M⊙ to 2M⊙) and have high dimensionless spin >0.05. While previous searches targeted sources that are representative of observed neutron star binaries in the galaxy, it is already known that neutron stars can regularly be spun up to a dimensionless spin of ∼0.4, and in principle reach up to ∼0.7 before breakup would occur. Furthermore, there may be primordial black hole binaries or exotic formation mechanisms to produce light black holes. In these cases, it is possible for the binary constituent to be spun up beyond that achievable by a neutron star. A single detection of this type of source would reveal a novel formation channel for compact-binaries. To determine if there is evidence for any such sources, we use PyCBC to conduct a targeted search of LIGO and Virgo data for light compact objects with high spin. Our analysis detects previously known observations GW170817 and GW200115; however, we report no additional mergers. The most significant candidate, not previously known, is consistent with the noise distribution, and so we constrain the merger rate of spinning light binaries.

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Revolutionizing carbon chemistry: Solar-powered C(sp3)-N bond activation and CO2 transformation via newly designed SBE-Y cutting-edge dynamic photocatalyst.

A solvent-free sulfur-bridge-eosin-Y (SBE-Y) polymeric framework photocatalyst was prepared for the first time through an insitu thermal polymerization route using elemental sulfur (S8) as a bridge. The addition of a sulfur bridge to the polymeric framework structure resulted in an allowance of the harvesting range of eosin-Y (E-Y) for solar light. This shows that a wider range of solar light can be used by the bridge material's photocatalytic reactions. In this context, supercharged solar spectrum: enhancing light absorption and hole oxidation with sulfur bridges. This suggests that the excited electrons and holes through solar light can contribute to oxidation-reduction reactions more potently. As a result, the photocatalyst-enzyme attached artificial photosynthesis system developed using SBE-Y as a photocatalyst performs exceptionally well, resulting in high 1,4-NADH regeneration (86.81%), followed by its utilization in the exclusive production of formic acid (210.01 μmol) from CO2 and synthesis of fine chemicals with 99.9% conversion yields. The creation of more effective photocatalytic materials for environmental clean-up and other applications that depend on the solar light-driven absorption spectrum of inorganic and organic molecules could be one of the practical ramifications of this research.

Zeemanovskoe rasshcheplenie eksitonov v kvantovykh yamakh GaAs/AlGaAs v geometrii Faradeya

The Zeeman splitting in the GaAs/AlGaAs heterostructure is investigated experimentally. Numerical analysis performed for the wavefunctions of exciton states, which takes into account the bands of heavy holes, light holes, and the band split by the spin–orbit interaction, is the quantitative agreement with experimental data both for an exciton with a heavy hole and for that with a light hole. It is shown that for explaining the experimental values of the Zeeman splitting in the quantum well under investigation, it is necessary to take into account both the Coulomb interaction and the contribution from the three bands in the valence band. The effect of screening of exciton states by a 2D gas of electrons with concentration n ≈ 109 cm–2 is described. Numerical calculations are performed for a large range of quantum well widths and aluminum concentrations in barriers; the chart of the dependence of the effective g factor on these parameters is plotted for magnetic field B = 5 T.

Trap levels analysis in MWIR InAs/InAsSb T2SL photodiode

Mid-wave infrared (MWIR) photodiode with an InAs/InAsSb superlattice active layer was characterized by the photoluminescence (PL), spectral response (SR) and deep level transient spectroscopy (DLTS) measurements. Three independent methods, in combination with the theoretical calculations, were used to determine the transitions between superlattice minibands, as well as to and from the trap states. The determined effective band gap of the investigated superlattice is 251 meV at 70 K. Most of the detected trap states lie within the energy gap, around 60 and 80 meV below the first electron miniband, and near the mid-gap. Moreover, trap states located above the first electron miniband and below the first light hole miniband were also observed. It is worth emphasizing that the PL and DLTS measurements give consistent results. Additionally, the activation energy of 16 meV for the Be acceptor dopant was determined using the PL measurement.

Fe Cluster Modified Co9 S8 Heterojunction: Highly Efficient Photoelectrocatalyst for Overall Water Splitting and Flexible Zinc-Air Batteries.

Designing bifunctional low-cost photo-assisted electrocatalysts for converting solar and electric energy into hydrogen energy remains a huge challenge. Herein, a heterojunction (Fe cluster modified Co9 S8 loaded on carbon nanotubes, Co9 S8 -Fe@CNT)for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is demonstrated. Benefiting from the good electronic conductivity and spatial confinement of the carbon skeleton, as well as the electronic structure regulation of the Fe cluster, Co9 S8 -Fe@CNT exhibits excellent catalytic performance with a low overpotential of 150mV for OER and 135mV for HER at 10mAcm-2 . Upon light irradiation, holes and electrons are generated in the valence band and conduction band of the Co9 S8 , respectively. Part of the charges are transferred to the interface to facilitate the catalytic reaction, while the remaining are transferred by the electrode. When working as a bifunctional catalyst for overall water splitting, the performance can reach 1.33V at under light conditions, which is significantly better than 1.52V in a dark environment. Theoretical calculations revealed lowered Gibbs free energy (∆GH *) of the heterojunction with the effect of Fe modification of Co9 S8 . This work sheds a new light in designing novel photoelectrochemical materials to convert solar and electric energy into chemical energy.

Red Phosphorus Grafted High‐Index (116) Faceted Anatase TiO2 for Z‐Scheme Photocatalytic Pure Water Splitting

AbstractRed phosphorus (RP) is an emerging visible‐light‐responsive photocatalyst, yet the rapid charge recombination has limited photocatalytic hydrogen production activity. In this work, a Z‐scheme heterostructure with RP nanolayer coated on high‐index (116) faceted anatase TiO2 nanoparticles (TiO2@RP) is designed and fabricated via chemical vapor deposition. Compared with pristine TiO2 and RP, the optimized TiO2@RP Z‐scheme heterostructure exhibits a significantly boosted photocatalytic activity for pure water splitting, with hydrogen evolution rate reaching 12.9 µmol·h−1, under simulated solar light irradiation. The strong interfacial interaction and staggered band alignment between (116) faceted TiO2 and RP result in the formation of built‐in electric field, which can drive the directional charge migration from the conduction band (CB) of TiO2 to the valance band (VB) of RP under light irradiation, with photoelectrons and holes of high redox ability maintained at the CB of RP and the VB of TiO2, respectively. This well‐designed heterostructure greatly promotes photogenerated charge separation and migration via a direct Z‐scheme charge transfer pathway.

Spatially separated noble-metal-free CdS photocatalyst with redox surface exposure for photodegradation of PPCPs and Cr (VI)

Adequate separation of photogenerated carriers plays an important role in the photocatalytic degradation of pollutants. Here, a noble-free spatially separated photocatalyst rGO-CdS-MnOx (GCM) has been designed for removing environmental pollutants efficiently. rGO and MnOx are employed as the collectors of photoinduced electrons and holes, respectively. Under light irradiation, the electrons and holes motivated by CdS can directionally transfer to the rGO and MnOx, leading to the exposure of both reduction and oxidation surfaces, thereby enhancing the photocatalytic activity. Our results demonstrated that GCM-30 can remove 86.56 % of carbamazepine and 95.83 % of tetracycline hydrochloride within 30 min and 15 min, respectively, and 95.49 % of Cr (VI) within 21 min. Our work demonstrates that this dual functional photocatalyst may provide new strategies for the spatially separated photocatalysts for removing environmental pollutants.

Spin precession of light holes in the spin-orbit field of strained GaAs nanowires

Spin precession of light holes in the spin-orbit field of strained GaAs nanowires

Variable mechanism of electrical impedance for MCT high voltage switch under synergetic action of the mechanical and electric loads

To study the electromechanical coupling effect of Metal Oxide Semiconductor Controlled Thyristor (MCT) high voltage switch under the synergistic action of mechanical load and strong voltage in the actual use of penetrating hard targets and launching, the universal testing machine was used to simulate the overload environment. The rising edge time, falling time and impedance changes in the conduction path of MCT under different stress–strain were tested, respectively. Experimental results showed that the rise time of MCT decreased from 0.08 ms to 0.03 ms when the uniaxial compressive stress increased from 1.20 MPa to 7.3 MPa, and the rising edge time after unloading was 0.04 ms. The falling time had no obvious change with the increase of stress. The total resistance in the path decreased from 833.30 Ω to 564.22 Ω with the increase of strain. The stress–strain of each part of MCT under different uniaxial compressive stress was collected by COMSOL Multiphysics finite element software. Based on the Schrödinger equation, the potential energy operator was established. By introducing the strain Hamiltonian H ε,ν , the E(k)-k model near the minimum value of the conduction band of Si was established by using the k·p perturbation method. Combined with the physical field interface of Schrödinger equation in COMSOL Multiphysics, the valence band structure of Si material under uniaxial strain was studied on the basis of strain Hamiltonian perturbation. The results showed that the 6-degree degenerate valley (Δ6) in the conduction band was split into a 2-degree degenerate valley (Δ2) and a 4-degree degenerate valley (Δ4) due to the stress effect. The strain caused more electrons to occupy the lower Δ4 energy valley, resulting in a decrease in the effective mass of the total conductivity. Therefore, the electron mobility of Si was increased under uniaxial strain. The uniaxial compressive stress could more effectively reduce the curvature radius of valence band top, the effective mass of carriers and the interband scattering between light and heavy hole bands, which was beneficial to improve the hole mobility.

Reversal of Band-Ordering Leads to High Hole Mobility in Strained p-type Scandium Nitride.

Low hole mobility of nitride semiconductors is a significant impediment to realizing their high-efficiency device applications. Scandium nitride (ScN), an emerging rocksalt indirect band gap semiconductor, suffers from low hole mobility. Utilizing the ab initio Boltzmann transport formalism including spin-orbit coupling, here we show the dominating role of ionized impurity scattering in reducing the hole mobility in ScN thin films. We suggest a route to increase the hole mobility by reversing band ordering through strain engineering. Our calculation shows that the biaxial tensile strain in ScN lifts the split-off hole band above the heavy hole and light hole bands, leading to a lower hole-effective mass and increasing mobility. Along with the impurity scattering, the Fröhlich interaction also plays a vital role in the carrier scattering mechanism due to the polar nature of ScN. Increased hole mobility in ScN will lead to higher efficiencies in thermoelectric, plasmonics, and neuromorphic computing devices.

Semiconductor piezoresistance prediction model for mechanical sensor design

AbstractThis paper addresses semiconductor piezoresistive materials selection in MEMS engineering design. From the practical engineering point of view, it is important to understand piezoresistance properties of semiconductors even if less accuracy under feasibility design phase. However, piezoresistance is frequently analyzed based on first principle electronic band structure simulations by sophisticate physicists. Practical engineers not familiar with this simulation cannot directly apply useful information derived from the result of simulation. This paper provides practical prediction method for piezoresistance based on electronic band parameters obtained from the state‐of‐the‐art solid‐state physics. It is demonstrated that the crucial parameters which control the p‐type shear piezoresistance coefficient π44 in diamond and zinc‐blend single crystals are the inverse of square of bond length in unit cell atom, the square root of valence light hole mass, and the shear elastic compliance coefficient S44.

Comparison between optical and electrophysical data on hole concentration in zinc doped p-GaAs

Optical and electrophysical properties of Cz-grown zinc doped p-GaAs samples have been investigated. Middle-infrared reflection spectra of ten p-GaAs samples have been obtained. Galvanomagnetic Van der Pau measurements have been made on these samples also, and the values of resistivity and Hall coefficient have been calculated. All experiments have been carried out at room temperature.Reflection spectra have been processed by Kramers–Kronig relations. The spectral dependences of real and imaginary parts of complex dielectric permittivity have been obtained and loss function has been calculated. The value of characteristic wave number corresponding to high-frequency plasmon-phonon mode has been determined by loss function maximum position.The theoretical calculations have been made, and the dependence has been obtained which gave the possibility to determine heavy hole concentration value at T = 295K by the value of characteristic wave number. Then by comparison of optical and Hall data the values of light hole mobility to heavy hole mobility ratio have been determined. This mobility ratio has been shown to be equal to (1.9–2.8) which is considerably less, than predicted theoretical value based on assumption that both light and heavy holes are scattered by optical phonons. It has been suggested that scattering mechanisms of light and heavy holes might be quite different.

Tantalum Nitride‐Enabled Solar Water Splitting with Efficiency Above 10%

AbstractDesigning photoanode semiconducting materials with visible‐light absorption and minimal charge‐carrier recombination for achieving efficient solar‐to‐hydrogen (STH) conversion is challenging. Here, hybrid Ta3N5 nanorods and thin films are developed on transparent GaN/Al2O3 substrates. A Ta3N5 photoanode with a loaded cocatalyst achieves the best current density, i.e. 10.8 mA cm−2, at 1.23 V versus the reversible hydrogen electrode under simulated AM 1.5G solar illumination. In a tandem configuration with dual‐CuInSe2 photovoltaic cells, this semi‐transparent photoanode achieves a reproducible STH energy conversion efficiency of ≈12% (the highest among photocatalytic materials), and remains at more than 10% for 6.7 h of tandem device operation. Detailed transient absorption spectroscopy and theoretical analysis indicates that this high performance originates from efficient light absorption and hole utilization inside the Ta3N5 material. The results show the feasibility of suppressing dominant optical and charge‐carrier‐ recombination losses by using nanostructured visible‐light‐absorbing materials for practical STH conversion.

Prospectives for AlN electronics and optoelectronics and the important role of alternative synthesis

Future applications for emerging AlN semiconductor electronics and optoelectronics are facilitated by emerging doping technologies enabled by low temperature, non-equilibrium epitaxy. Defect and impurity compensation can be reduced by controlling the surface chemistry with reducing compensating vacancy concentrations being a key driver for lower temperature growth. Contrary to common understanding, low temperature, metal-rich vacuum processes are shown to have higher diffusion lengths than high temperature nitrogen-rich methods. This feature can be utilized to inhibit silicon-DX center formation without compromises in crystal quality. First principles calculations identify the valence split-off band as the dominant hole band contributing to impurity band formation (as opposed to the heavy and light hole bands in other nitrides). This anomalous band structure causes an impurity band to form at dopant concentrations similar to GaN even though AlN has a deeper isolated acceptor energy and results in hole mobilities that are substantially higher than possible in GaN. AlN hole concentrations of ∼4.4 × 1018 cm−3 and 0.045 Ω cm resistivity and electron concentrations of ∼6 × 1018 cm−3 and ∼0.02 Ω cm resistivity are shown and offer substantial promise for future generations of AlN bipolar electronic and optical devices.

Stable electroluminescence in ambipolar dopant-free lateral p–n junctions

Dopant-free lateral p–n junctions in the GaAs/AlGaAs material system have attracted interest due to their potential use in quantum optoelectronics (e.g., optical quantum computers or quantum repeaters) and ease of integration with other components, such as single electron pumps and spin qubits. A major obstacle to integration has been the unwanted charge accumulation at the p–n junction gap that suppresses light emission, either due to enhanced non-radiative recombination or due to inhibition of p–n current. Typically, samples must frequently be warmed to room temperature to dissipate this built-up charge and restore light emission in a subsequent cooldown. Here, we introduce a practical gate voltage protocol that clears this parasitic charge accumulation, in situ at low temperature, enabling the indefinite cryogenic operation of devices. This reset protocol enabled the optical characterization of stable, bright, dopant-free lateral p–n junctions with electroluminescence linewidths among the narrowest (<1 meV; <0.5 nm) reported in this type of device. It also enabled the unambiguous identification of the ground state of neutral free excitons (heavy and light holes) as well as charged excitons (trions). The free exciton emission energies for both photoluminescence and electroluminescence are found to be nearly identical (within 0.2 meV or 0.1 nm). The binding and dissociation energies for free and charged excitons are reported. A free exciton lifetime of 237 ps was measured by time-resolved electroluminescence, compared to 419 ps with time-resolved photoluminescence.

Resonant Tunneling Effects on the Double‐Barrier Electron Blocking Layer of a Nitride Deep‐UV Light‐Emitting Diode

The multi‐quantum‐barrier electron blocking layer (EBL) is reported to significantly improve efficiency by nearly 3 times over a single barrier in deep‐UV AlGaN light‐emitting diodes to deal with electron leakage. The improvement is usually attributed to the enhanced effective barrier height, and this article aims to explore the benefits of the tunneling effect by calculating the tunneling currents of electrons and holes through an Al0.6Ga0.4N/AlyGa1−yN double‐barrier EBL under external bias from opposite directions. The results show that the tunneling current for holes Jh is several orders of magnitude higher than that of the electrons Je as the barriers are with Al mole fraction y greater than 0.75 and thickness larger than 2 nm, which promises effective hole injection by tunneling without much electron leakage. Tunneling mechanism works better in EBL with higher and thicker barriers because the tunneling coefficients of light hole drop much slower than electrons due to its small effective mass. A proper distance between the barriers is needed to avoid electron leakage while holes tunnel through the EBL. Built‐in electric fields tilt the band to enlarge the peak‐to‐valley ratio. This work indicates that the tunneling effect substantially facilitates a multibarrier EBL to enhance carrier‐injection efficiency.

Electro-reduced copper on polymeric C3N4 for photocatalytic reduction of CO2

Coupling light absorber with an efficient cocatalyst is the most common way to fabricate composite photocatalyst. The light absorber supplies electrons and holes, while the cocatalyst domains chemical reactions. Inspired by the results from electrochemical CO2 reduction, Cu-based materials are the most promising cocatalysts for photocatalytic CO2 reduction. However, oxide-derived copper (OD-Cu) hasn't been introduced into photocatalytic systems yet, due to easily undergoing valence changes once exposed to air. Herein, we find that OD-Cu could be decorated on photocatalyst, polymeric C3N4 (PCN), by a self-designed in-situ electroreduction method. The as-prepared OD-Cu/PCN presents better visible light absorption than the one without in-situ electroreduction and prefers multi-carbon products. According to CO2RR measurements, the highest selectivity to C2H5OH is 58%. Density functional theory calculation further indicates that the unique structure between OD-Cu and PCN leads to a low energy barrier of C–C coupling due to the enhanced *CO dimerization and thus promotes the production of C2H5OH. Moreover, this optimized sample achieves a solar-to-fuel efficiency, of up to 0.94%, which is 1.69 times the pristine PCN.

Comparison between optical and electrical data on hole concentration in zinc-doped p-GaAs

The optical and electrical properties of zinc-doped Cz p-GaAs have been studied. Reflection spectra of ten p-GaAs specimens have been taken in the mid-IR region. Van der Pau galvanomagnetic, electrical resistivity and Hall coefficient measurements have been carried out for the same specimens (all the measurements were carried out at room temperature). The reflection spectra have been processed using the Kramers–Kronig relations, spectral dependences of the real and imaginary parts of the complex dielectric permeability have been calculated and loss function curves have been plotted. The loss function maximum position has been used to calculate the characteristic wavenumber corresponding to the high-frequency plasmon-phonon mode frequency. Theoretical calculations have been conducted and a calibration curve has been built up for determining heavy hole concentration in p-GaAs at T = 295 K based on known characteristic wavenumber. Further matching of the optical and Hall data has been used for determining the light to heavy hole mobility ratio. This ratio proves to be in the 1.9–2.8 range which is far lower as compared with theoretical predictions in the assumption of the same scattering mechanism for light and heavy holes (at optical phonons). It has been hypothesized that the scattering mechanisms for light and heavy holes differ.