Polarization Field Effects Research Articles

Effect of Internal Polarization Fields in InGaN/GaN Multiple-Quantum Wells on the Efficiency of Blue Light-Emitting Diodes

The author theoretically investigates the effects of internal polarization fields on the efficiency of InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs) based on numerical simulations. Internal quantum efficiency (IQE), current leakage, and carrier distributions are calculated with varying the strength of polarization fields to study the role of the polarization fields on the efficiency characteristics. Simulation results show that, as the strength of the polarization fields increases, electron current leakage from active layers to the p-GaN layer increases and hole concentration distribution at MQWs becomes more inhomogeneous. It is found that these two effects by the internal polarization fields lead to significant decrease in the IQE of InGaN blue LEDs.

Effect of Internal Polarization Fields in InGaN/GaN Multiple-Quantum Wells on the Efficiency of Blue Light-Emitting Diodes

Effect of Internal Polarization Fields in InGaN/GaN Multiple-Quantum Wells on the Efficiency of Blue Light-Emitting Diodes

Field effects induce bathochromic shifts in xanthene dyes.

There is ongoing interest in near-infrared (NIR) absorbing and emitting dyes for a variety of biomedical and materials applications. Simple and efficient synthetic procedures enable the judicious tuning of through-space polar (field) effects as well as low barrier hydrogen bonding to modulate the HOMO-LUMO gap in xanthene dyes. This affords unique NIR-absorbing xanthene chromophores.

Valleytronics, Carrier Filtering and Thermoelectricity in Bismuth: Magnetic Field Polarization Effects

AbstractValley polarization of multi‐valleyed materials is of significant interest for potential applications in electronic devices. The main challenge is removing the valley degeneracy in some controllable way. The unique properties of bismuth, including its anisotropic electronic structure and Dirac valley degeneracy, make this material an excellent system for valleytronics. It is demonstrated theoretically that the direction of an externally applied magnetic field in the binary‐bisectrix plane has a profound effect not only on the charge, but also on the thermal transport along the trigonal direction. The rotating field probes the electronic mass anisotropy and tunes the contribution from a particular Dirac valley in the electrical resistivity, Seebeck coefficient, and thermal conductivity at moderate temperatures and field strengths. It is further shown that the field polarization of the transport properties is accompanied by selective filtering of the carriers type providing further opportunities for thermoelectric transport control.

Effects of polarization field on vertical transport in GaN/AlGaN resonant tunneling diodes

Polarization-field effects on the vertical transport in GaN/AlGaN resonant tunneling diodes (RTDs) were theoretically investigated by using the transfer matrix formalism. The self-consistent model shows that the resonant peaks are shifted toward higher energies with increasing Al composition in the AlGaN barrier, and the transmission probability values are shown to decrease rapidly. In the case of the flat-band model, on the other hand, the shift of the resonant peaks is smaller than it is for the self-consistent model and the variation of transmission probability values with increasing Al composition is relatively smaller than that of the self-consistent model. The current-voltage characteristics of the self-consistent model are asymmetric while those of the flat-band model are symmetric for positive and negative current directions. The peak-to-valley ratio (PVR) of the self-consistent model is shown to be slightly smaller than that of the flat-band model for Al = 0.3.

Ferroelectricity-induced resistive switching in Pb(Zr0.52Ti0.48)O3/Pr0.7Ca0.3MnO3/Nb-doped SrTiO3 epitaxial heterostructure

We investigated the effect of a ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin film on the generation of resistive switching in a stacked Pr0.7Ca0.3MnO3 (PCMO)/Nb-doped SrTiO3 (Nb:STO) heterostructure forming a p-n junction. To promote the ferroelectric effect, the thin PZT active layer was deposited on an epitaxially grown p-type PCMO film on a lattice-matched n-type Nb:STO single crystal. It was concluded that the observed resistive switching behavior in the all-perovskite Pt/PZT/PCMO/Nb:STO heterostructure was related to the modulation of PCMO/Nb:STO p-n junction’s depletion width, which was caused either by the PZT ferroelectric polarization field effect, the electrochemical drift of oxygen ions under an electric field, or both simultaneously.

Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals

Colloidal CdSe nanocrystals (NCs) are successively overcoated with CdS monolayers (MLs), and the effective two-photon absorption (TPA) coefficient and refractive index of this series of CdS/CdSe core/shell NCs are measured by a Z-scan technique at 800 nm wavelength. The TPA and nonlinear refraction are enhanced dramatically with the CdS shell growth towards 3 MLs, but decreased with the further shell growth. The effects of surface states, local field, and intrinsic piezoelectric polarization are discussed to explain the optical nonlinearity of the colloidal core/shell NCs.

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting DZFS. At room temperature in nonpolar solvents, the most probable values are: kx = ky = 1.2 × 1011 s−1 and kz = 1.9 × 1010 s−1 for the intersystem crossing from the energetically lower and upper triplet substates, respectively, kp = 1.6 × 109 s−1 for the cleavage reaction and for the zero-field splitting DZFS = −3.4 × 1010 s−1 (0.18 cm−1).

Temperature dependence of InGaN/GaN multiple quantum well based high efficiency solar cell

In this paper, a new p–InGaN multiple quantum well–n solar cell has been investigated. In order to obtain the exact solar cell parameters such as conversion efficiency, the polarization field effects of nitride materials are taken into account. It has been found that the conversion efficiency of the p–i(MQW)–n solar cell is significantly higher than those of normal p–i(bulk)–n solar cells. The optimized conversion efficiency of about 35% is obtained for p–MQW–n solar cells at room temperature. Also, the temperature dependence of open-circuit voltage and short-circuit current and consequently conversion efficiencies of both structures are investigated, and it is observed that the increasing of temperature slightly increases the short-circuit current and decreases the open-circuit voltage and efficiency.

Electrical field and temperature effects in 2D‐2D resonant tunneling diodes based on cubic InGaN/AlGaN

AbstractThe present work reports on a study of the electron transport in a 2D‐2D resonant tunneling diode based on strained Al0.3Ga0.7N/In0.2Ga0.8N/Al0.3Ga0.7N quantum wells embedded between relaxed n‐Al0.15Ga0.85N/strained In0.2Ga0.8N emitter and collector contacts. The epilayers are assumed to be with cubic crystal structure to avoid the effects of spontaneous and piezoelectric polarization fields. The aluminum composition in both the injector and collector contacts is taken relatively weak, but large band offsets are achieved at the boundary of the preconfinement wells. The heterostructure modeled is aimed to operate at high biases and at elevated temperatures. Calculations of the band edges and quantum‐bound states were made using a self‐consistent solver of the Schrödinger and Poisson equations. Obtained results revealed: (i) a charge transfer occurs due to a resonant tunneling through a double barrier, (ii). A special attention is paid to the effects of resonant tunneling and thermal activation on the charge transfer. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Significant increase of crystalline quality and green emission by an interface modification of InGaN/GaN quantum wells

A GaN-to-InGaN interface modification by predeposition of an ultrathin In-rich InGaN incomplete layer followed by a thin triangular InGaN well layer was employed to overcome the negative effects of polarization field on light emission efficiency of InGaN/GaN quantum wells as well as to improve the crystalline quality by avoidance of a significant strain generation and enhanced surfactant effect. Further, the interface modification induced energy band structure engineering reduces the spatial separation of electrons and holes, and thus increases the carrier recombination rate. The improvement in crystalline quality, localized potential fluctuation, and energy band engineering contribute to the significant increase of green emission of the InGaN/GaN quantum wells.

The effects of polarization fields on exciton binding energy in GaN quantum dots

In this paper the exciton binding energy in wurtzite GaN quantum dot is investigated by considering the influences of polarization in details. We find that the strong built-in electric field gives rise to an obvious modification of conduction band profile of QDs and leads to remarkable electron-hole spatial separation. This effect has a significant influence on exciton states and optical properties of the QDs. The relationship between exciton states and QD size and QD confinement potential is studied. Our results represent the decrease of exciton binding energy by considering piezoelectric polarization.

Radio emission in a toy model with point-charge-like air showers

In the present paper we stress some characteristics of the electric field produced by a point-charge-like air shower developing in an atmosphere of refractive index fixed to unity and restricting ourselves to the charge excess and geosynchrotron terms. The aim of this toy model is obviously not to compete with refined theories nor full Monte Carlo simulations but to understand the main features of radio emission process as observed in recent experiments, in particular the field polarization effects. In this simplified frame, the antenna radio time pulse for vertical or inclined showers is merely given by the product of two analytical functions. The calculation time is reduced to a fraction of a second. The analytical pulse amplitudes are relatively close to those obtained with the ReAIRES code. Further works must be carried out to evaluate the limitations and the possible extensions of this toy model.

周期量级激光脉冲在不同密度的原子介质中传播行为的比较

Using the numerical solution of the full Maxwell-Bloch equations without the slowly varying envelope approximation and the rotating-wave approximation, propagation behavior of few-cycle laser pulse in a dense ladder-type three-level atomic medium is investigated and it is compared with the case in the corresponding dilute medium. It is found that, in the propagation process, time evolution of the pulse in the dense medium is obviously different from that in the dilute medium, and the difference will become more evident with increasing of the initial pulse area. When the initial pulse area is smaller, in the propagating process, the pulse shape in the dilute medium just have a small change; the pulse shape in the dense medium has considerable variation but the pulse splitting doesn't occur. When the initial pulse area is larger enough, for the dense medium case, the pulse splits into sub-pulses with different numbers and shapes at different propagation distances; while for the dilute medium case, variation of the pulse shape is still smaller in propagation process. The cause producing above difference is from two effects in the dense medium: the effect of the local-field correction (LFC) from the near dipole-dipole interaction and effect of stronger polarization field than that in the dilute medium. In which the effect of stronger polarization field plays a main role, but the role of LFC also cannot be neglected,and the stronger polarization field is due to the atomic density increasing .

Combined angular effects of spin polarization and magnetic field on magnetic element switching

We explore the combined effects of polarization direction and magnetic field direction on magnetization switching. Optimal polarization and magnetic field angles are obtained for the most effective magnetization switching. Analytical results are obtained for specific symmetric configurations based upon spin torque magnetization dynamics symmetry. Modeling results are used to understand experimental measurements for spin torque switching of a magnetic tunneling junction with canted pinning layers from nanosecond region to second region.

Local electric field enhancement and polarization effects in a surface-enhanced Raman scattering fiber sensor with chessboard nanostructure

A surface-enhanced Raman scattering fiber sensor with chessboard nanostructure on a cleaved fiber facet is studied by finite-difference time-domain method. Surface plasmons at the metal coated nanostructured fiber facet can be effectively excited and strong local electric field enhancement is obtained. Studies on the influence of light polarization demonstrate a large polarization dependence of the field enhancement factor while the polarization effects on the plasmon resonance wavelength are relatively small.

AlxGa1−xN/GaN multi-quantum-well ultraviolet detector based on p-i-n heterostructures

We report on characterization of a set of AlGaN/GaN multiple-quantum-well (MQW) photodetectors. The model structure used in the calculation is the p–i–n heterojunction with 20 AlGaN/GaN MQW structures in i-region. The MQW structures have 2nm GaN quantum well width and 15nm AlxGa1−xN barrier width. The cutoff wavelength of the MQW photodetectors can be tuned by adjusting the well width and barrier height. Including the polarization field effects, on increasing Al mole fraction, the transition energy decreases, the total noise increases, and the responsivity has a red shift, and so the detectivity decreases and has a red shift.

Local environment dependent linewidth of plasmon absorption in gold nanoshell: Effects of local field polarization

The linewidth of the plasmon absorption of a gold nanoshell was theoretically studied by using a Drude model and quasistatic approximation. Increasing the dielectric constant of the core or surrounding can broaden the absorption peak linearly. A mechanism based on the polarization direction of the local electric field in the gold shell was investigated to illuminate the effect of dielectric constant on the broadening of the plasmon absorption. The perpendicular local field enhances the scattering of the oscillatory electrons and brings a loss of electron phase coherence, which in turn results in a broadening of the plasmon absorption linewidth.

Double heterostructure ultraviolet light emitting diodes with nanometer scale compositionally inhomogeneous active regions

AbstractWe report on the fabrication and evaluation of flip‐chipped double heterostructure UVLEDs operating at 320 nm that employ a bulk (80nm) AlGaN active region containing nanoscale compositional inhomogeneities (NCI), deposited by plasma‐assisted molecular beam epitaxy. The devices were deposited on AlGaN templates grown by hydride vapor phase epitaxy and contain electron and hole injection layers grown by metalorganic chemical vapor deposition. A packaged, 300 μm x 300 μm device with 3% internal quantum efficiency has a peak output power of 0.56 mW at 90 mA DC current that corresponds to an external quantum efficiency of 0.15%. Nearly constant external quantum efficiency is observed for DC current density up to 100 A/cm2, suggesting that the NCI regions effectively suppress the deleterious effects of polarization fields at low injection currents that should inhibit radiative recombination in double heterostructure devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of internal electric fields on the transition energy of InGaN/gaN quantum well

We present a new way to meet the amount of strain relaxation in an InGaN quantum well layer grown on relaxed GaN by calculating and measuring its internal field. With perturbation theory, we also calculate the transition energy of InGaN/GaN SQWs as affected by internal fields. The newly reported experimental data by Graham et al. fit our calculations well on the assumption that the InGaN well layer suffered a 20% strain relaxation, we discuss the differences between our calculated results and the experimental data. Our calculation suggests that with the increase of indium mole fraction in the InGaN/GaN quantum well, the effect of polarization fields on the luminescence of the quantum well will increase. Moreover, our calculation also suggests that an increase in the quantum well width by only one monolayer can result in a large reduction in the transition energy.