Polarization Field Effects Research Articles

Growth and photoluminescence studies of Al-rich AlN∕AlxGa1−xN quantum wells

A set of AlN∕AlxGa1−xN (x∼0.65) quantum wells (QWs) with well width Lw varying from 1to3nm has been grown by metal organic chemical vapor deposition. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and linewidth. These results have shown that these AlN∕AlGaN QW structures exhibit polarization fields of ∼4MV∕cm. Due to effects of quantum confinement and polarization fields, AlN∕AlGaN QWs with Lw between 2 and 2.5nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet optoelectronic device applications are also discussed.

Analytical Control of Molecular Excitations Including Strong Field Polarization Effects

An analytical scheme is presented for designing a laser pulse to excite H2 from one specified vibrational-rotational state to another. The scheme is based on an adiabatic two-state approximation in a Floquet picture. By continuously and smoothly changing the laser frequency, we explicitly harness the dynamic Stark shifts and maintain resonance between the dressed diabatic states during laser-molecule interaction. The explicit time-dependent solution of the Schrödinger equation confirms the validity and efficacy of the analytically designed laser pulses. The scheme depends on the molecular polarizability to achieve its control objectives.

Photocapacitance characteristics of (In,Ga)N/GaN MQW structures

AbstractPhotocapacitance techniques have been used to study the electronic properties of (In,Ga)N/GaN quantum wells (QWs). Negative differential capacitance (NDC) features were observed at room temperature for the first time in capacitance‐voltage (C‐V) experiments under optical excitation. A detailed analysis of the results seems to indicate that charge accumulation in quantum dot‐like structures embedded in the QWs could be responsible of such behaviour. Polarization field effects present in these structures are detected through C‐V hysteresis, allowing to estimate built‐in electric fields as high as 1.9 MV/cm in 14% In QWs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization field effects at liquid-crystal-droplet–polymer interfaces

The influence of confinement (droplet size) and liquid crystal orientational order (smectic-A and nematic) on the interfacial polarization field effects [Maxwell-Wagner-Sillars (MWS) effect] existing in liquid-crystal-droplets-polymer systems is investigated by broadband dielectric spectroscopy and a forward transmittance measurement technique. A relaxation process observed in the low frequency domain of the dielectric spectrum has been associated with a MWS effect for both micron-size and submicron-size droplets. Using electro-optical measurements and numerical simulations of the field inside droplets, it is shown that a depolarization field takes place in the same frequency range as that determined by dielectric spectroscopy. Differential scanning calorimetry measurements allowed to estimate the phase-separated liquid crystal [4,4'-octylcyanobiphenyl (8CB)] fraction, which was found in the range of 55% for both micron-size and submicron-size droplets. X-ray diffraction experiments showed that smectic 8CB confined to micron-size cavities adopt bulklike properties, i.e., a partial bilayer structure, whereas in submicron-size droplets the layer spacing of the smectic phase is increased due to the strong bending deformations induced by the high curvature of the cavity walls.

Study of polarization field in GaN-based blue LEDs on Si and sapphire substrate by electroluminescence

The electroluminescence spectra of GaN-based light-emitting diodes on Si and sapphire substrate have been investigated as a function of the current density. The experimental results on dependence of the electroluminescence peak wavelength on operating current density can be explained in terms of the competition mechanism between polarization field, carrier screening, band-filling and thermal effect. The polarization field in the active layer of InGaN/GaN multiple quantum wells was estimated to be 10 5 V/cm by means of carriers screening the polarization field.

Al rich AlN/AlGaN Quantum Wells

ABSTRACTTwo sets of AlN/AlxGa1−xN quantum wells (QW) have been grown by metalorganic chemical vapor deposition (MOCVD). The first set consists of five samples of AlN/AlxGa1−xN QWs with (x ∼ 0.65) with well width, Lw, varying from 1 to 3 nm. The second set consists of four samples of AlN/AlxGa1−xN with (Lw = 1.5 nm) with Al composition, x, varying from 0.70 to 0.85. Low temperature photoluminescence (PL) spectroscopy has been employed to study the Lw dependence of the PL spectral peak position, emission efficiency, and line width. Our results have shown that these AlN/AlGaN QW structures exhibit polarization fields of ∼ 4 MV/cm. Due to effects of quantum confinement and polarization fields, AlN/AlGaN QWs with Lw between 2 and 2.5 nm exhibit the highest quantum efficiency. The dependence of the emission linewidth on Lw yielded a linear relationship. The implications of our results on deep ultraviolet (UV) optoelectronic device applications are also discussed.

Trapping and hopping of bipolarons in DNA: Su-Schrieffer-Heeger model calculations

With the Su-Schrieffer-Heeger model involving the effects of solvent polarization and external electric field, we show that bipolaron maybe more stable than two polarons when a dication induced into a DNA stack. Under the high electric field, the dication can move quite a long distance through the DNA by a series of hopping process, partially losing its configuration instantaneously due to the nonadiabatic effects.

Design and Preparation of AlN/GaN Quantum Wells for Quantum Cascade Laser Applications

An envelope function framework was used to calculate the conduction band structure of AlN/GaN quantum wells for application to quantum cascade lasers. We took into account the piezo- and spontaneous polarization fields in the nitride quantum wells. The design of the quantum cascade structures in the AlN/GaN system becomes much simpler if we utilize the effect of polarization fields. [(AlN)1/(GaN)n1]m/(AlN)n2 quantum wells were prepared using hot-wall epitaxy for mid-infrared quantum cascade laser applications, and X-ray diffraction and transmission electron microscopy (TEM) measurements were performed on them. The X-ray diffraction measurements of the quantum wells were in good agreement with the theoretical pattern, and the coherent growth of quantum wells on the GaN buffer layers was ascertained by X-ray reciprocal mapping. The existence of a single atomic layer of AlN was observed by TEM measurements.

Polarization, piezoelectric constants, and elastic constants of ZnO, MgO, and CdO

We report first principles density functional calculations of the spontaneous polarizations, piezoelectric constants and elastic constants for the II-VI wurtzite structure ZnO, MgO and CdO. Using our self interaction corrected implementation of density functional theory we obtain polarization values of we obtain polarization values of -0.05, -0.17 and -0.10 C/m$^2$, and piezoelectric constants, $e_{33}$ ($e_{31}$) of 1.34 (-0.57), 2.26 (-0.38) and 1.67 (-0.48) C/m$^2$ for structurally relaxed ZnO, MgO and CdO respectively. These properties are consistently larger in magnitude than the corresponding GaN, AlN and InN analogues. Therefore we predict that larger internal fields will be attainable in ZnO-based polarization field effect transistors than in equivalent GaN-based devices.

Quantum control of molecular motion including electronic polarization effects with a two-stage toolkit

A method for incorporating strong electric field polarization effects into optimal control calculations is presented. A Born-Oppenheimer-type separation, referred to as the electric-nuclear Born-Oppenheimer (ENBO) approximation, is introduced in which variations of both the nuclear geometry and the external electric field are assumed to be slow compared with the speed at which the electronic degrees of freedom respond to these changes. This assumption permits the generation of a potential energy surface that depends not only on the relative geometry of the nuclei but also on the electric field strength and on the orientation of the molecule with respect to the electric field. The range of validity of the ENBO approximation is discussed in the paper. A two-stage toolkit implementation is presented to incorporate the polarization effects and reduce the cost of the optimal control dynamics calculations. As an illustration of the method, it is applied to optimal control of vibrational excitation in a hydrogen molecule aligned along the field direction. Ab initio configuration interaction calculations with a large orbital basis set are used to compute the H-H interaction potential in the presence of the electric field. The significant computational cost reduction afforded by the toolkit implementation is demonstrated.

Polar Substituent Effects in the Bicyclo[1.1.1]pentane Ring System: Acidities of 3-Substituted Bicyclo[1.1.1]pentane-1-carboxylic Acids

Experimental gas-phase acidities are reported for a series of 3-substituted (X) bicyclo [1.1.1]pent-1-yl carboxylic acids (1, Y = COOH). A comparison with available calculated data (MP2/6-311++G**// B3LYP/6-311+G**) reveals good agreement. The relative substituent effects are shown to be adequately described by a much lower level of theory (B3LYP/6-31+G*). Various correlations are presented which clearly point to polar field effects as being the origin of the relative acidities.

Effects of coil dimensions and field polarization on RF heating inside a head phantom

Deterioration of radiofrequency (RF) inhomogeneity with increasing static magnetic field in magnetic resonance imaging (MRI) is one of the fundamental challenges preventing their clinical rendition and posing safety hazards. Variation in RF coil designs could help redistribute RF energy absorption over the imaged object. This work is intended to determine experimentally the difference in RF heating produced within a human head phantom by in situ measurement of RF inhomogeneity as a function of coil design utilized at 8 T. The heating patterns of 1/4 wavelength (long) and 1/8 wavelength 11-cm (short) transverse electromagnetic (TEM) coils loaded with a homogeneous human head phantom at 340 MHz were evaluated. In addition, different transmit/receive (T/R) configurations were used in search for the possibility of "hot-spot" formation. Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast for RF powers corresponding to a specific absorption rate (SAR) of 4.0 W/kg for 10 min. Numerical simulations were performed to study the general RF power deposition patterns in phantoms at 340 MHz including the effects of field polarization. The temperature increases varied from 0 to 0.8 degrees C for the long RF coil, while the short RF coil produced a maximum temperature change of 0.5 degrees C. Similar to ultra high-field electromagnetic simulations, these measurements revealed low peripheral and high deep-tissue heating at 8 T. The findings indicated that the largest temperature changes for both cases were less than 1 degrees C. While these results showed an increase in localized heating due to RF pulses at 8 T, they highlight that RF inhomogeneity could be redistributed using different RF coil designs through which the hot spots could be made cooler.

THE PERFORMANCE OF POLARIZATION DIVERSITY SCHEMES IN OUTDOOR MICRO CELLS

The application of polarization diversity reception at the mobile terminal in micro cells at 2 GHz is presented in this paper. Raytracing tool is used to study effects of electric field polarization on the received power in outdoor environments. The performance of diversity schemes with vertical/ horizontal polarization and +45◦/−45◦ slanted polarization are compared in different line-of-sight (LOS) and nonlineof-sight (NLOS) environments. Based on the evaluation of cross polarization discrimination (XPD) parameters, it is clarified that different environments will affect XPD values in micro cells. Then, the vertical/horizontal polarization diversity and +45◦/− 45◦ slanted polarization diversity are chosen to compare with space diversity. Several different combining techniques of polarization and space diversity schemes are also compared in different environments. It is found that dual-polarized antennas for mobile terminal are a promising alternative for two spaced antennas.

Polarization field effects on the recombination dynamics in low-In-content InGaN multi-quantum wells

The interplay of polarization fields and free carrier screening in In x Ga 1− x N/GaN ( 0.03 < x < 0.07 ) multiple quantum wells is studied by combining photoluminescence (time-integrated and time-resolved) and cathodoluminescence studies, in an excitation density range from 10 8 to 10 12 cm −2 of generated e–h pairs. For such low In content, the quantum-confined Stark effect is verified to rule the recombination dynamics, while effects of carrier localization in potential fluctuations have a minor role. Efficient field screening is demonstrated in CL steady-state high-injection conditions and in PL time-resolved experiments at the maximum excitation density. Under recovered nearly flat band conditions, quantum confinement effects are revealed and a high and possibly composition-dependent bowing parameter is extrapolated. Information on radiative and non-radiative rates for carrier recombination in the wells is obtained, both from steady-state and from time-resolved experiments, modelling the carrier dynamics in the framework of a theoretical rate equation model, which calculates electronic states and recombination rates in the nanostructure by coupling complete self-consistent solutions of Schrödinger and Poisson equations.

Surface complexation of carbonate on goethite: IR spectroscopy, structure and charge distribution.

The adsorption of carbonate on goethite has been evaluated, focussing on the relation between the structure of the surface complex and corresponding adsorption characteristics, like pH dependency and proton co-adsorption. The surface structure of adsorbed CO3(-2) has been assessed with (1) a reinterpretation of IR spectroscopy data, (2) determination of the charge distribution within the carbonate complex using surface complexation modeling, and (3) evaluation of the proton co-adsorption of various oxyanions, including carbonate, in relation with structural differences. Carbonate adsorption leads to a degeneration of the nu3 IR vibration. Currently, the magnitude of the Deltanu3 band splitting is used as a criterion for metal coordination. However, the interpretation is not unambiguous, since the magnitude of Deltanu3 is influenced by polarization and additional field effects, due to, e.g., H bonding. Our evaluation shows that for goethite the magnitude of band splitting Deltanu3 falls within the range of values that is representative for bidentate complex formation, despite contrarily assignments made in literature. Determination of the charge distribution (CD), derived by modeling available carbonate adsorption data, shows that a very large part (2/3) of the carbonate charge resides in the surface. Interpretation of this result with a bond valence and a ligand charge analysis strongly favors the bidentate surface complexation option for adsorbed carbonate. This option is also supported by the proton co-adsorption of carbonate. The H co-adsorption is very high, which corresponds closely to an oxyanion surface complex in which 2/3 of the ligands are common with the surface. The high H co-adsorption is in conflict with the monodentate option for adsorbed CO3(-2). The study shows that the H co-adsorption of CO3(-2) is almost equal to the experimental H co-adsorption obtained for SeO3(-2) adsorption, which can be rationalized supposing for both XO3(-2) complexes the same ligand distribution in the interface, i.e., bidentate complex formation.

Vertical transport in group III‐nitride heterostructures and application in AlN/GaN resonant tunneling diodes

We present a systematic study of vertical transport in AlGaN/GaN heterostructures. The influence of barrier thickness and Al-concentration on the current across AlGaN barriers is investigated experimentally and compared to model calculations. The effect of polarization fields on the electronic properties of single- and double barrier heterostructures is discussed and experimental results are reviewed. AlN/GaN double barrier RTD structures are fabricated under optimized growth conditions. Experimental analysis of their electronic properties reveals indications for resonant tunneling in the low forward bias regime. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Coulomb-explosion imaging ofCH2+: Target-polarization effects and bond-angle distribution

The effect of target polarization fields on the bond-angle distribution following the foil-induced Coulomb explosion of $\mathrm{CH}_{2}{}^{+}$ has been measured. Incorporating a detailed model description of the polarization effects and other target effects into a Monte Carlo simulation of the experiment, a good description of the various observables is obtained. In particular, the bond-angle distribution is found to agree with existing ab initio calculations.

Effects of polarization field on formation of two-dimensional electron gas in (0 0 0 1) and [formula omitted] plane AlGaN/GaN heterostructures

Photoluminescence (PL) and temperature-dependent Hall effect measurements were carried out in (0 0 0 1) and (1 1 2 ̄ 0) AlGaN/GaN heterostructures grown on sapphire substrates by metalorganic chemical vapor deposition. There are strong spontaneous and piezoelectric electric fields (SPF) along the growth orientation of the (0 0 0 1) AlGaN/GaN heterostructures. At the same time there are no corresponding SPF along that of the (1 1 2 0) AlGaN/GaN. A strong PL peak related to the recombination between two-dimensional electron gas (2DEG) and photoexcited holes was observed at 3.258 eV at room temperature in (0 0 0 1) AlGaN/GaN heterointerfaces while no corresponding PL peak was observed in (1 1 2 ̄ 0) . The existence of a 2DEG was observed in (0 0 0 1) AlGaN/GaN multi-layers with a mobility saturated at 6000 cm 2/V s below 80 K, whereas a much lower mobility was measured in (1 1 2 ̄ 0) . These results indicated that the SPF was the main element to cause the high mobility and high sheet-electron-density 2DEG in AlGaN/GaN heterostructures.

Electroreflectance studies of Stark shifts and polarization-induced electric fields in InGaN/GaN single quantum wells

To observe the effects of polarization fields and screening, we have performed contacted electroreflectance (CER) measurements on In0.07Ga0.93N/GaN single quantum well light emitting diodes for different reverse bias voltages. Room-temperature CER spectra exhibited three features which are at lower energy than the GaN band gap and are associated with the quantum well. The position of the lowest-energy experimental peak, attributed to the ground-state quantum well transition, exhibited a limited Stark shift except at large reverse bias when a redshift in the peak energy was observed. Realistic band models of the quantum well samples were constructed using self-consistent Schrödinger–Poisson solutions, taking polarization and screening effects in the quantum well fully into account. The model predicts an initial blueshift in transition energy as reverse bias voltage is increased, due to the cancellation of the polarization electric field by the depletion region field and the associated shift due to the quantum-confined Stark effect. A redshift is predicted to occur as the applied field is further increased past the flatband voltage. While the data and the model are in reasonable agreement for voltages past the flatband voltage, they disagree for smaller values of reverse bias, when charge is stored in the quantum well, and no blueshift is observed experimentally. To eliminate the blueshift and screen the electric field, we speculate that electrons in the quantum well are trapped in localized states.

Design of GaN/AlGaN quantum wells for maximal intersubband absorption in 1.3< λ

The design of rectangular GaN/AlGaN quantum wells for maximal intersubband absorption in the near infrared wavelength range 1.3–2 μm, on the 0→1 electronic transition is considered, taking into account the effects of internal polarization fields and nonparabolicity. The nonlinear optimization method based on solving a system of nonlinear equations is employed in finding the structural parameters which give the maximal dipole matrix element while keeping the absorption peak at the desired wavelength.