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Journal of Applied Physics

Issue 9 • Date May 2011

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Displaying Results 1 - 25 of 127
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  • Enhancement of piezoelectric response in (100)/(001) oriented tetragonal Pb(Zr, Ti)O3 films by controlling tetragonality and volume fraction of the (001) orientation

    Page(s): 091601 - 091601-5
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    (100)/(001)-oriented 2 μm-thick Pb(Ti, Zr)O3, films with the Zr/(Zr + Ti) ratio of 0.39 and 0.50 were prepared by metalorganic chemical vapor deposition. The volume fraction of (001)-oriented domain, Vc, was controlled by selecting the kinds of substrates with different thermal expansion coefficient. The effective piezoelectric constant increased up to 310 pm/V with decreasing the Vc and tetragonality (lattice parameter ratio of c axis to a axis) determined by the decrease of the Zr/(Zr + Ti) ratio. Observed large piezoelectric response was considered to be mainly contributed by the extrinsic effect, such as the 90o domain switching. These experimental inputs suggest the new concept for enhancement of the piezoelectric properties. View full abstract»

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  • A non-filamentary model for unipolar switching transition metal oxide resistance random access memories

    Page(s): 091602 - 091602-6
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    A model for resistance random access memory (RRAM) is proposed. The RRAM under research utilizes certain transition metal oxide (TMO) such as NiO which shows unipolar switching behavior. The existence of metal/insulator states is not explained by filaments but attributed to different Hubbard U values, which stems from an electron correlation effect. Current-voltage formulae are given both on the metal and insulator sides by putting the appropriate solutions of Hubbard model into the mesoscopic Meir-Wingreen transport equation. The RESET phenomenon is explained by a sufficient separation of Fermi levels in the electrodes and hence a Mott transition can be triggered in the anodic region due to a lack of electrons. The SET behavior originates from a tunneling current which removes the insulating region near the anode. Several experimental evidences are also presented to support this model. The model also serves as the theoretical prototype of Correlated Electron Random Access Memory (CeRAM) which is defined to be a TMO RRAM whose working mechanism is based on the strong electron correlation effects. View full abstract»

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  • Material and process optimization of correlated electron random access memories

    Page(s): 091603 - 091603-7
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    A method of making transition metal oxide materials that result in resistive switching properties stable over time and temperature is described. We have developed an ultra low temperature (≤450°C) process for carbonyl ligand modified NiO thin films based on the chemical solution deposition (CSD) for correlated electron random access memory (CeRAM) applications. CeRAMs form the general class of devices that use the electron-electron interaction as the primary mode of operation. These devices are fabricated in the conductive state (born-ON), thus, they do not require electroforming to enter the variable resistance state. Several process parameters such as film stoichiometry, thickness, annealing temperature and ambient have been investigated to optimize CeRAMs properties. We present the coordination number ‘fine tuning’ in NiO ultra thin films via carbonyl ligand doping that regulate the number of oxygen vacancies and the surface excess of metal ions. CeRAMs contrary to just standard NiO based resistive memories use the pure Mott-like charge transfer insulator in which an abrupt metal to insulator transition is the dominant mechanism without the aid of charge trapping vacancies. In our films the effect of the oxygen vacancies are canceled due to the stabilizing effect of the carbonyl based extrinsic ligand. In this paper, detailed process sequence and the extrinsic ligand doping scheme is described in some length. It is shown that complexes formed by the introduction of the extrinsic ligand promote Ni2+ ions to enter the disproportionation reaction Ni2+ + Ni2+→Ni1+ + Ni3+ which is considered to be responsible for the memory mechanism. View full abstract»

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  • Influence of Ti substitution on the electrical properties of metal-ferroelectric (BiFeO3)-insulator (HfO2)-silicon structures for nonvolatile memory applications

    Page(s): 091604 - 091604-6
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    Metal-ferroelectric (Ti-substituted BiFeO3)-insulator (HfO2)-semiconductor structures have been fabricated via the cosputtering technique. Ti4+ substitution at the Fe site was investigated through x-ray photoelectron spectra and x-ray diffraction patterns at postannealing temperatures of 500 to 700 °C. The capacitance-voltage memory windows as functions of the insulator film thickness and the dc power for Ti were measured and compared. A memory window of 3.1 V was obtained at a sweep voltage of 8 V under O2-rich conditions. The leakage current and the charge injection effect, especially gate injection, can be greatly improved by Ti substitution. The effects of the postannealing temperature and the substitution amount on the leakage current can be well explained by the defect reaction model. View full abstract»

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  • Strong growth orientation dependence of strain relaxation in epitaxial (Ba,Sr)TiO3 films and the resulting dielectric properties

    Page(s): 091605 - 091605-6
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    The growth orientation dependence of strain relaxation and the dielectric properties were investigated for (001)- and (111)-epitaxial (Ba,Sr)TiO3 films. The films were deposited on SrRuO3/SrTiO3 and SrTiO3 substrates using rf magnetron sputtering. The residual strain was found to be remarkably different between the two orientations, although these lattice mismatches are identical; the strain relaxation of the (001)-epitaxial films is significantly slower than that of the (111)-epitaxial films and is promoted only when the growth rate is very low (≤5 nm/h). The observed orientation dependence is discussed with the surface energy for both growth orientations, which influences the growth mode of the films. Due to the large contrast of the strain in the (001)- and (111)-epitaxial films, the paraelectric to ferroelectric phase transition temperature of the (001)-epitaxial films is much higher than that of unstrained bulks, while the (111)-epitaxial films show a phase transition temperature corresponding to that of unstrained bulks regardless of the growth rates. View full abstract»

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  • Improvement of barium strontium titanate solidly mounted resonator quality factor by reduction in electrode surface roughness

    Page(s): 091606 - 091606-3
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    Two barium strontium titanate solidly mounted resonators with different bottom electrode surface roughnesses were fabricated and tested. The surface roughness of the bottom electrode was reduced by changing the deposition method of the SiO2 layer in the acoustic Bragg reflector in the solidly mounted resonator. The surface roughness was reduced from 2.81 nm RMS to 1.45 nm RMS. This reduction in the surface roughness contributed to a significant increase in the quality factor of the resonator. The quality factor for the device with a smooth platinum bottom electrode was 110 to 87 over a bias range of 5 to 40 V, and for the device with a rough platinum bottom electrode it was 1 to 31 over the same bias range. The effective electromechanical coupling coefficient was 4% at 40 V and 3.4% at 40 V for the smooth and rough platinum bottom electrodes, respectively. View full abstract»

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  • Correlation between nanoscale and nanosecond resolved ferroelectric domain dynamics and local mechanical compliance

    Page(s): 091607 - 091607-6
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    The local dynamics of ferroelectric domain polarization are uniquely investigated with sub-20-nm resolved maps of switching times, growth velocities, and growth directions. This is achieved by analyzing movies of hundreds of consecutive high speed piezo force microscopy images, which record domain switching dynamics through repeatedly alternating between high speed domain imaging and the application of 20-nanosecond voltage pulses. Recurrent switching patterns are revealed, and domain wall velocities for nascent domains are uniquely reported to be up to four times faster than for mature domains with radii greater than approximately 100 nm. Switching times, speeds, and directions are also shown to correlate with local mechanical compliance, with domains preferentially nucleating and growing in compliant sample regions while clearly shunting around locations with higher stiffness. This deterministic switching behavior strongly supports a defect-mediated energy landscape which controls polarization reversal, and that can therefore be predicted, modeled, and even manipulated through composition, processing, and geometry. Such results have important implications for the practical performance of ferroelectric devices by enabling guided optimization of switching times and feature densities, while the methods employed provide a new means to investigate and correlate dynamic functionality with mechanical properties at the nanoscale. View full abstract»

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  • Device characterization of correlated electron random access memories

    Page(s): 091608 - 091608-6
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    The switching properties and characterization of correlated electron random Access Memories (CeRAMs) are described herein. High temperature retention, cycle dispersion and optimization, cycle Fatigue, and switching parameter optimization have been investigated. CeRAM’s display initially conductive or “born-ON” behavior without the need for the high electroforming voltages usually required for other transition metal oxide based resistive memories. Nonvolatile data retention at elevated temperatures up to 573 K (300 °C) in addition to a wide operating range from 4 to 423 K for CeRAM has been confirmed. CeRAMs also show exceptional read endurance with no evidence of fatigue out to 1012 cycles. Desirable scaling characteristics for high density memory application have also been shown for CeRAMs due to a widening of the read window and consistent write window as devices are scaled down. View full abstract»

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  • PbSe quantum well mid-infrared vertical external cavity surface emitting laser on Si-substrates

    Page(s): 093101 - 093101-6
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    Mid-infrared vertical external cavity surface emitting lasers based on PbSe/PbSrSe multi-quantum-well structures on Si-substrates are realized. A modular design allows growing the active region and the bottom Bragg mirror on two different Si-substrates, thus facilitating comparison between different structures. Lasing is observed from 3.3 to 5.1 μm wavelength and up to 52 °C heat sink temperature with 1.55 μm optical pumping. Simulations show that threshold powers are limited by Shockley-Read recombination with lifetimes as short as 0.1 ns. At higher temperatures, an additional threshold power increase occurs probably due to limited carrier diffusion length and carrier leakage, caused by an unfavorable band alignment. View full abstract»

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  • Design strategies for mitigating the influence of polarization effects on GaN-based multiple quantum well light-emitting diodes

    Page(s): 093102 - 093102-5
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    The optoelectronic properties of GaN-based multiple quantum well (MQW) light-emitting diodes (LEDs) are investigated using a detailed theoretical model, in which the effects of strain, well coupling, valence band mixing, and polarization effects are fully considered. By solving the conduction and valence band effective mass equations together with Poisson’s equation self-consistently, the influence of various major design parameters, such as the well width, the barrier components, and the barrier thickness, on the electronic and optical properties of GaN-based MQW LEDs is studied. Numerical results show that the emission spectra of the LEDs are very sensitive to the above design parameters due to the polarization effect that is unique for GaN-based devices. Further analysis and simulations reveal that this sensitivity can be obviously suppressed by choosing InGaN as the barrier material. View full abstract»

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  • Design of high birefringence and low confinement loss photonic crystal fibers with five rings hexagonal and octagonal symmetry air-holes in fiber cladding

    Page(s): 093103 - 093103-5
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    We present a new cladding design for high birefringence and low confinement loss photonic crystal fibers (PCFs) using a full-vector finite element method with anisotropic perfectly matched boundary layer. Six cases of PCFs are proposed for comparison. The proposed cladding in PCFs is composed of five rings of air-holes. Air-holes on the inner two rings are arranged in a hexagonal symmetry whereas, air-holes on the outer three rings are arranged in an octagonal symmetry in fused silica. Results show that suitable design air-holes on the inner two rings will significantly increase the birefringence, whereas, elliptical holes with major axis along x-axis on the outer three rings will provide strong confinement ability. The highest modal birefringence and lowest confinement loss of our proposed case five structure at the excitation wavelength of λ = 1550 nm can be achieved at a magnitude of 0.87 × 10-2 and less than 0.01 dB/km with only five rings of air-holes in fiber cladding. View full abstract»

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  • Laser-based surface acoustic wave dispersion spectroscopy for extraction of thicknesses, depth, and elastic parameters of a subsurface layer: Feasibility study on intermetallic layer structure in integrated circuit solder joint

    Page(s): 093104 - 093104-9
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    Laser beam deflection in combination with optical heterodyne diffraction is used to detect surface acoustic waves that are generated by impulsive laser light on a Sn–CuxSny–Cu–Si multilayer structure with layer thicknesses of the order of 1 μm. The acoustic phase velocity dispersion curves of the lowest two-surface acoustic wave modes are determined by spectral analysis of the experimental signals. The sensitivity of the dispersive behavior to the thickness and elastic properties of the individual layers is analyzed on the basis of the experimental data and of simulated data with noise added. The elastic parameters of the CuxSny intermetallic alloy are determined. Statistical least squares and most squares uncertainties on all best fitting material parameters are determined, giving a quantitative measure of the feasibility of parameter extraction by this method. The feasibility of solving the inverse problem of elastic depth profiling of a multilayer by the proposed wideband and multimode SAW dispersion spectroscopy method is confirmed. View full abstract»

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  • Drug injection into fat tissue with a laser based microjet injector

    Page(s): 093105 - 093105-3
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    We have investigated a new micro drug jet injector using laser pulse energy. An infrared laser beam of high energy (∼3 J/pulse) is focused inside a driving fluid in a small chamber. The pulse then induces various energy releasing processes, and generates fast microjets through a micronozzle. The elastic membrane of this system plays an important role in transferring mechanical pressure and protecting drug from heat release. In this paper, we offer the sequential images of microjet generation taken by a high speed camera as an evidence of the multiple injections via single pulse. Furthermore, we test the proposed system to penetrate soft animal tissues in order to evaluate its feasibility as an advanced transdermal drug delivery method. View full abstract»

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  • Recombination coefficients of GaN-based laser diodes

    Page(s): 093106 - 093106-6
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    We measure the charge carrier recombination coefficients of InGaN quantum wells by analyzing the dynamical properties of (Al,In)GaN laser diodes emitting in the violet spectral range. Relaxation oscillations and turn-on delays are fitted to a rate equation model including a charge carrier density dependent recombination rate. Using optical gain spectroscopy we can directly determine the injection efficiency of the devices and thereby separate the effect of charge carrier leakage from that of carrier recombination. We find a third-order recombination coefficient of (4.5±0.9)×10-31cm6s-1 which is in agreement with theoretical predictions for phonon- and alloy-disorder-assisted Auger scattering. View full abstract»

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  • Origin of the refractive index modification of femtosecond laser processed doped phosphate glass

    Page(s): 093107 - 093107-5
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    The origin of the local refractive index modification in femtosecond laser inscribed structures has been investigated with confocal microfluorescence imaging. We have identified the origin of both, positive and negative refractive index changes in a commercial Er-Yb codoped phosphate glass upon irradiation in the low repetition rate regime (1 kHz). Consistent relations among the photoluminescence behavior of the dopants (erbium and ytterbium ions), the local sign of the density change and the local modification of the refractive index by means of characteristic emission features such as the intensity and the spectral peak position have been established. Blue or redshift in the photoluminescence emission have been observed and related to a local perturbation in the crystal field caused by a modification of the mean distance among the dopant ions, and thus of the local matrix density. These conclusions are additionally supported by the spatial distribution of photoluminescence emission intensities, which have been interpreted in terms of energy transfer mechanisms underlying the overall erbium-ytterbium emission process. View full abstract»

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  • Precision, all-optical measurement of external quantum efficiency in semiconductors

    Page(s): 093108 - 093108-10
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    External quantum efficiency of semiconductor photonic devices is directly measured by wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with an independent measurement of power-dependent temperature or power-dependent photoluminescence. Time-resolved photoluminescence lifetime and power-dependent photoluminescence measurements are used to evaluate unprocessed heterostructures for critical performance parameters. The crucial importance of parasitic background absorption is discussed. View full abstract»

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  • Design of high-contrast all-optical bistable switches based on coupled nonlinear photonic crystal microcavities

    Page(s): 093109 - 093109-5
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    We present a bistable photonic crystal configuration consisting of a waveguide directly coupled to three microcavities with instantaneous Kerr nonlinearity. Coupled mode theory combined with a numerical simulation based on the finite-difference time-domain technique is employed to design and evaluate the optical configuration. We demonstrate that with the proper choice of resonant frequencies of the constitutional photonic crystal microcavities, such a configuration can function as a high contrast switch, and the contrast between the bistable states in its transmission can be as high as 300. We expect this device to be useful in the practical application of all-optical switches. View full abstract»

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  • Magneto-optical Faraday effect in nanocrystalline oxides

    Page(s): 093110 - 093110-6
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    Magneto-optical materials have widespread applications in communication and optical devices. Besides existing applications such as optical diodes, untapped potential applications could be accessed should magneto-optical properties be improved such that smaller magnetic fields can be employed. Here we present an efficient method for fabricating oxide materials that possess excellent optical and magnetic properties—they are transparent to visible light yet have high magnetic susceptibility. Combined, these properties produce large Faraday rotations; the measured Verdet constant is >-300 rad T-1 m-1 at 632.8 nm, a high value for a thick, optically transparent material. Because this Verdet constant is more than twice that of the state of the art material, these nanocrystalline oxides produce polarized light rotations with less than half the applied magnetic field necessary. They are made by densifying rare earth nanocrystalline powder into dense, large-sized bodies using an electric current activated technique (sometimes known as spark plasma sintering). The processing temperature is optimized in order to achieve sufficient density without causing excessive phase changes that would destroy light transparency. This process produces materials quickly (<20 min), which, combined with high magneto-optical properties, promises less expensive, smaller, more portable magneto-optical devices. View full abstract»

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  • Optical bistability in mesoporous silicon microcavity resonators

    Page(s): 093113 - 093113-7
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    We report on the observation of significant optical bistability in the transmission and reflection properties of mesoporous silicon microcavities when illuminated with a 150 ns pulsed laser at 532 nm. The observed optical hysteresis is shown to be transient in nature and the properties are strongly dependent on the porosity of the cavity layer. The onset and damage threshold intensity are also shown to be porosity dependent. Our modeling suggests that the observed effects are due to changes in the nonlinear refractive index where the transient lifetime increases with increasing porosity. We investigate the role of surface states on influencing the bistable process by passivating the internal porous surface with hydrosilylation chemistry. View full abstract»

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  • Ultracompact beam splitters based on plasmonic nanoslits

    Page(s): 093114 - 093114-6
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    An ultracompact plasmonic beam splitter is theoretically and numerically investigated. The splitter consists of a V-shaped nanoslit in metal films. Two groups of nanoscale metallic grooves inside the slit (A) and at the small slit opening (B) are investigated. We show that there are two energy channels guiding light out by the splitter: the optical and the plasmonic channels. Groove A is used to couple incident light into the plasmonic channel. Groove B functions as a plasmonic scatter. We demonstrate that the energy transfer through plasmonic path is dominant in the beam splitter. We find that more than four times the energy is transferred by the plasmonic channel using structures A and B. We show that the plasmonic waves scattered by B can be converted into light waves. These light waves redistribute the transmitted energy through interference with the field transmitted from the nanoslit. Therefore, different beam splitting effects are achieved by simply changing the interference conditions between the scattered waves and the transmitted waves. The impact of the width and height of groove B are also investigated. It is found that the plasmonic scattering of B is changed into light scattering with increase of the width and the height of B. These devices have potential applications in optical sampling, signal processing, and integrated optical circuits. View full abstract»

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  • Analytical single-mode model for subwavelength metallic Bragg waveguides

    Page(s): 093115 - 093115-12
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    We develop a theoretical formalism that incorporates the method of moment with the analytical eigenmode expansion to investigate the dispersion relation of light transport in subwavelength metallic Bragg waveguide (WG) with each unit cell composed of a wide and a narrow segment of metallic gap. The approach fully accounts for the light scattering at the interface between two consecutive discontinuous segments. A simple single-mode analytical model is derived for both the fundamental even and odd guided modes. The model shows that the band structure of light transport in the structure resembles that of an ordinary dielectric one-dimensional photonic crystal with appropriate physical and geometric parameters that can be analytically derived. Numerical simulations by the finite-difference time-domain method on the optical transmission spectra and band diagrams for these metallic Bragg WGs agree well with the analytical results of band diagrams. In addition, the analytical model can handle structures working in both the microwave and infrared regimes. This indicates that the simple analytical model is effective and efficient in handling various light transport problems for subwavelength metallic Bragg WGs. View full abstract»

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  • Enhanced light output power of GaN-based light-emitting diodes by nano-rough indium tin oxide film using ZnO nanoparticles

    Page(s): 093116 - 093116-5
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    We demonstrate the performance improvement of GaN-based light-emitting diodes (LEDs) using zinc oxide (ZnO) nanoparticles inserted between the p-GaN and the indium tin oxide (ITO) layers. Upon deposition of an ITO film over the dispersed ZnO nanoparticles, the ITO surface tends to attain a nano-rough morphology due to the presence of ZnO nanoparticles. The light output power of the fabricated LEDs with ZnO nanoparticles is 39% higher than that of conventional LEDs at an injection current of 20 mA. This is attributed to the improved light extraction favored by the light scattering tendency of ZnO nanoparticles and the nano-roughened ITO film. In addition, the intermediate refractive index (n ∼2) of ZnO materials between those of the p-GaN (n ∼2.5) and the ITO (n ∼1.9) results in a broader critical angle and a reduction of total internal reflection. View full abstract»

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  • The investigation on carrier distribution in InGaN/GaN multiple quantum well layers

    Page(s): 093117 - 093117-4
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    The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer. View full abstract»

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  • Structure-induced metamorphoses of eigen modes in photonic atoll resonators

    Page(s): 093118 - 093118-7
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    An optical resonator—called a photonic atoll (PA)—is presented and investigated theoretically, which consists of microstructures arranged periodically to make a closed chain loop and permits a photon trapped there to stay in it for an amazingly long time. The systematic investigation for a variety of elliptical PAs reveals that the symmetric (or circular) resonator enables us to attain the maximum lifetime. This structural modification exhibits intriguing variations of eigen-modes, as given below. The optical eigen-modes in the PAs—their degeneracy gets lifted by this deformation—show the splitting widths that strongly depend on the mode position in the photonic band. Moreover, those modes split near the bandedge display a striking anisotropy of excitations. These phenomena should be discussed taking into account the quasi-one-dimensionality of the structure as well as considering them from the group-theoretic standpoint. We have thus clarified the eigen-modes metamorphoses caused by the modification of the PA structure. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Meet Our Editors

Editor
P. James Viccaro
Argonne National Laboratory