Nonuniform Current Injection Research Articles

Dimension dependence of current injection path in GaInN/GaN multi-quantum-shell (MQS) nanowire-based light-emitting diode arrays

Abstract To light emitting diodes (LEDs), solving the common non-uniform current injection and efficiency degradation issues in (0001) plane micro-LED is essential. Herein, we investigated the light emission characteristics of various mesa sizes and different p-electrode areas toward the realization of coaxial GaInN/GaN multi-quantum-shell (MQS) nanowires (NWs)-based micro-LEDs. As the mesa area was reduced, the current leakage decreases, and further reduction of the area showed a possibility of realizing micro-LED with less current leakage. The large leakage path is mainly associated with the defective MQS structure on the (0001) plane area of each NW. Therefore, more NWs involved in an LED chip will induce higher reverse leakage. The current density-light output density characteristics showed considerably increased electroluminescence (EL) intensity as the mesa area decreased, owing to the promoted current injection into the efficient NW sidewalls under high current density. The samples with a mesa area of 50 × 50 µm2 showed 1.68 times higher light output density than an area of 100 × 100 µm2 under a current density of 1000 A/cm2. In particular, the emission from (1-101) and (10-10) planes did not exhibit an apparent peak shift caused by the quantum-confined Stark effect. Furthermore, by enlarging the p-electrode area, current can be uniformly injected into the entire chip with a trade-off of effective injection to the sidewall of each NW. High performance of the MQS NW-based micro-LED can be expected because of the mitigated efficiency degradation with a reducing mesa area and an optimal dimension of p-electrode.

Feasibility of Direct Current stimulation through hair using a dry electrode: potential for transcranial Direct Current Stimulation (tDCS) application.

Conventional transcranial direct current stimulation (tDCS) protocols typically deliver 2 mA for 20-30 minutes. The most common administration uses a wet electrode approach which dries out in ~60 minutes at room temperature. This restricts its application to limited duration electrode-scalp contact use cases unless additional conductive media (saline, gel, or paste) is re-applied. This problem is further compounded by the subject's hair which not only presents administration challenges (interferes with electrode attachment and adhesion) but also acts as a conduit of current flow into the scalp resulting in current hotspots. This non-uniform current injection results in increased skin sensation. The aim of this study was to determine suitability of a commercially available hydrogel for DC delivery through hair. Experiments involved both non-clinical testing on an agar block and clinical testing on subjects' forearms. Electrodes were positioned on the posterior side of the forearm that has hair for the clinical testing. Typical dose as used in tDCS was delivered and pain scores were collected. Results indicate suitable current delivery performance and all subjects tolerated delivery with pain scores ranging between 0-6. Our study paves the way for future testing on the scalp for tDCS application.Clinical Relevance-This study demonstrates the possibility of delivering tDCS through hair via dry electrodes. Specific use cases that cannot use a traditional wet electrode approach stand to benefit from the results of our work.

Aperture diameter dependences in GaN-based vertical-cavity surface-emitting lasers with nano-height cylindrical waveguide formed by BCl3 dry etching

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.

Negative magnetoresistance due to conductivity fluctuations in films of the topological semimetal Cd3As2

Recently discovered Dirac and Weyl semimetals display unusual magnetoresistance phenomena, including a large, non-saturating, linear transverse magnetoresistance and a negative longitudinal magnetoresistance. The latter is often considered as evidence of fermions having a defined chirality. Classical mechanisms, due to disorder or non-uniform current injection, can however, also produce negative longitudinal magnetoresistance. Here, we report on magnetotransport measurements performed on epitaxial thin films of Cd3As2, a three-dimensional Dirac semimetal. Quasi-linear positive transverse magnetoresistance and negative longitudinal magnetoresistance are observed. By evaluating films of different thickness and by correlating the temperature dependence of the carrier density and mobility with the magnetoresistance characteristics, we demonstrate that both the quasi-linear positive and the negative magnetoresistance are caused by conductivity fluctuations. Chiral anomaly is not needed to explain the observed features.

Non-uniform space charge limited current injection into a nano contact solid.

We have developed a two-dimensional (2D) non-uniform model to study the space charge limited (SCL) current injection into a trap-filled solid of nano-contact, such as organic materials and dielectrics. Assuming a solid of length D with a contact of width W, the enhancement over the well-known 1D uniform model is calculated as a function of W/D for different material properties, such as the dielectric constant (ε) and the trap distribution. The non-uniform current density profile due to edge effect is predicted. The findings reported here are different from the prior uniform 2D models, which are significant for small W/D when the size of the contact reaching nanometer scale, i.e. W = 50 nm for D = 1 μm. This model will be useful for the characterization of carrier mobility and properties of traps, which are critical to many novel devices (with small nano-contact) operating in the space charge limited condition reporting in novel device and its applications. Empirical formulas are given for future comparison with experimental results.

Narrow linewidth two-electrode 1560 nm laterally coupled distributed feedback lasers with third-order surface etched gratings.

We report on the design and characterization of a re-growth free InGaAsP/InP multiple quantum well two-electrode laterally coupled distributed feedback (LC-DFB) lasers. Third-order surface etched gratings have been defined on the ridge sidewalls along the laser cavity by means of stepper lithography. The lasers oscillate in single-mode around 1560 nm with high side mode suppression ratios (>52 dB), a wavelength tuning (≥ 3nm), an output power (≥ 6 mW), and narrow linewidth (<170 kHz) under various current injection ranges at room temperature. A minimum linewidth of 94 kHz has been recorded for 1500 µm-long two-electrode LC-DFB laser while providing non-uniform current injection through the two electrodes. The effect of the width of the inter-electrode gap on these different performance measures is also studied.

Light extraction in planar light-emitting diode with nonuniform current injection: model and simulation.

We develop an analytical and numerical model for performing simulation of light extraction through the planar output interface of the light-emitting diodes (LEDs) with nonuniform current injection. Spatial nonuniformity of injected current is a peculiar feature of the LEDs in which top metal electrode is patterned as a mesh in order to enhance the output power of light extracted through the top surface. Basic features of the model are the bi-plane computation domain, related to other areas of numerical grid (NG) cells in these two planes, representation of light-generating layer by an ensemble of point light sources, numerical "collection" of light photons from the area limited by acceptance circle and adjustment of NG-cell areas in the computation procedure by the angle-tuned aperture function. The developed model and procedure are used to simulate spatial distributions of the output optical power as well as the total output power at different mesh pitches. The proposed model and simulation strategy can be very efficient in evaluation of the output optical performance of LEDs with periodical or symmetrical configuration of the electrodes.

3-D Simulation of Current Spreading in Semiconductor Light-Emitting and Laser Diodes Using Nonlinear Boundary Method

We describe and demonstrate the use of the nonlinear boundary method for 3-D simulation of semiconductor light-emitting and laser diodes. This method takes advantage of a dominant nonlinear behavior of the geometrically thin active layer in practical optoelectronic devices. This allows one to reduce the modeling only to the active layer coupled to the optical cavity. The reduction is achieved by expressing the nonuniform current injection from bulk regions into the layer using a surface integral, and does not involve any approximations. Using a device meshed with tetrahedral Delaunay meshes, we compare the performance of the boundary method against the more common method based on volume discretization, and show its advantages and limitations.

Operation of quantum-dot semiconductor optical amplifiers under nonuniform current injection

The influence of nonuniform current injection along the active region, on the linear operation of a quantum-dot semiconductor optical amplifier (QD-SOA) is investigated. For this purpose, we have utilized some functions to generate various nonuniform current injection profiles. These profiles have been considered in our numerical calculations, where the rate equation model is employed to construct different characteristics of the QD-SOA. We have found that the gain, as well as the crosstalk, of a QD-SOA is closely associated to the variance of the carrier density along the cavity. Simulation results show that nonuniform current injection can be used as a technique for gain enhancement as well as crosstalk suppression.

Spin transfer by nonuniform current injection into a nanomagnet

We have used nanofabrication techniques to incorporate an ∼20–30nm diameter nanoaperture within a 150×250nm2 elliptical magnetic multilayer to enable the localized injection of spin-polarized currents into a thin film nanomagnet. This results in very low spin transfer currents being required for at least partial nanomagnet reversal as well as for onset of dynamic precession. Micromagnetic simulations using Landau-Liftshitz-Gilbert equation with a spin-torque term indicate that reversal occurs via domain nucleation at the injection site followed by domain wall propagation away from the aperture, with the nanomagnet ending in one of several different states depending upon the current amplitude.

Performance simulation and design optimization gain-clamped semiconductor optical amplifiers based on distributed Bragg reflectors

Performance of the gain-clamped semiconductor optical amplifier (GC-SOA) based on distributed Bragg reflector (DBR) structures is investigated by a comprehensive broad-band time-domain traveling wave model. Critical factors, e.g., the material gain profile, the waveguide grating structures, the longitudinal variation of the optical field, and the carrier density as well as the broad-band spontaneous noise emission are considered in the model. The effects of operation parameters (e.g., the electrical bias current and the input optical power) and design parameters (e.g., the normalized coupling coefficient /spl kappa/L and the lasing wavelength position /spl lambda//sub L/) on device performance are examined in detail. Based on the results of the performance simulation, guidelines for design optimization are discussed and, in particular, unbalanced grating configurations combined with nonuniform current injection schemes are proposed to reduce the noise figure without much sacrifice on other device performance.

Higher‐Order Transverse Modes in Possible Nitride VCSELs

Excitation of various radiation modes during a possible room-temperature continuous-wave operation of nitride vertical-cavity surface-emitting lasers (VCSELs) is investigated and compared using the effective frequency optical model. Nitride VCSELs of a traditional design with two ring contacts have been found to exhibit unavoidable multimode operation which follows from an extremely non-uniform current injection into their active regions. To enhance desired single-fundamental-mode operation, current injection into active regions should be at least uniform, which may be realized using e.g. a special design of nitride VCSELs with an additional semi-transparent contact, two tunnel junctions and a double multiple-quantum-well active region.

Numerical modeling of undercut ridge VCSELs designed for CW operation at 1.3 μm: design optimization

Optimized undercut ridge long-wavelength vertical-cavity surface-emitting semiconductor lasers (LW-VCSEL's) are investigated and the influence of thermal effects on the light versus current characteristics is analyzed by thermal-electric, optical, and electronic modeling. The model includes nonuniform current injection, carrier diffusion, stimulated emission, distributed heat sources, and active material band structure calculations. Device parameters such as thermal resistance, threshold current, and external quantum efficiency are evaluated, in an attempt to provide an understanding of the current-funneling mechanism and the thermal limitations of such devices. Simulated power versus current characteristics exhibit the typical thermal roll-over in CW operation.

Frequency noise and modulation of a four-section DBR laser

A theoretical model is presented to describe the tuning, frequency noise, and modulation characteristics of tunable DBR lasers with two active and two passive sections. Analytical expressions are derived for the intensity and frequency modulation responses, frequency noise spectrum, spectral linewidth, effective linewidth enhancement factor, and spontaneous emission rate. It is shown that a nearly flat red-shifted FM response over several gigahertz can be obtained by nonuniform current injection and modulation of the lower carrier density section in the active region. Increasing the tuning current through the passive sections enhances the FM efficiency. However, a higher FM efficiency is generally accompanied by a larger spectral linewidth. The predictions of our theoretical model are in good agreement with the available experimental data.

A quasi-three-dimensional large-signal dynamic model of distributed feedback lasers

A simple, but powerful, quasi-three-dimensional large-signal dynamic model of distributed feedback semiconductor laser is presented. The transient response of lasers is analyzed by using a combined beam propagation method and time-domain algorithm that is capable of including the longitudinal and lateral variation of the optical-mode and carrier density profiles. In addition, the spontaneous emission noise, the nonuniform current injection resulting from the variation of Fermi-voltage as well as that of the refractive index distribution are also taken into consideration. Using this model, the influence of nonuniform waveguide structure on the static and dynamic responses of distributed feedback lasers is analyzed. In addition, a novel double tapered waveguide distributed feedback laser is proposed for stable single-mode operation at high power.

Effects of nonuniform photon distribution on high speed response in semiconductor distributed feedback lasers

The high speed performance of distributed feedback (DFB) lasers, with nonuniform longitudinal photon density distributions, has been analyzed. It was found that the relaxation resonance frequency and damping rate of the lasers depended on the distribution of traveling electric fields in the lasers. A two section DFB laser was used to demonstrate the effects of spatial hole burning on the high speed response of the DFB laser. By adjusting the photon density distribution in the laser through nonuniform current injection, the resonance frequency and damping rate of the two section DFB laser can be changed even if the output optical power is kept constant.

Spectral linewidth and FM response for a four-section DBR laser

Theoretical expressions were presented for the spectral linewidth, effective linewidth enhancement factor and FM response of a DBR laser with two active and two passive sections. It was shown that a nearly flat red-shifted FM response over several gigahertz can be obtained by nonuniform current injection and modulation of the lower-carrier-density section in the active region. However, a high FM efficiency is also accompanied by a relatively large spectral linewidth. Wavelength tuning by using the injection currents through the passive sections enhances the red-shifted FM efficiency but also broadens the spectral linewidth. In practice, linewidth broadening could be suppressed by using a large-bandgap material as the passive waveguide in order to reduce the optical confinement factor and, consequently, the tuning efficiency.

Reduction of the frequency chirp of two section distributed feedback laser by nonuniform current injection

The effect of nonuniform current injection on the frequency chirp of a two section distributed feedback (DFB) laser, which is under large signal modulation, is measured while the optical intensity and optical modulation depth are fixed. By adjusting the injection current distribution in the laser, the frequency chirp can be reduced since the effects of carrier density fluctuations in the two sections on the chirp partially compensate each other. The range of the injection current ratio of the two sections is found in which the chirp is at its minimum. The reduction of the frequency chirp is predicted by a computer simulation for two section DFB lasers which accounts for the longitudinal spatial hole burning.

A tractable large-signal dynamic model-application to strongly coupled distributed feedback lasers

The modulation characteristics of DFB semiconductor lasers have been studied using a transfer matrix method combined with an appropriate rate equation analysis. The model takes into account longitudinal mode spatial hole burning, as well as the nonuniform current injection resulting from the axially varying Fermi voltage, and can be used for the efficient simulation of static, small-signal, and large-signal dynamic properties. The program is applied to the interpretation of experimental data from a strongly coupled InGaAsP/InP DFB laser. The experimental high-frequency properties of this device are well described by the simulations. >

Effects of nonuniform current injection on the spectral dynamics of multisection distributed feedback lasers

The effects of nonuniform current injection on the linewidth enhancement factor (α) and the adiabatic chirp in a multielectrode distributed feedback laser are measured through a FM/AM modulation technique. This technique measures the adiabatic chirp due solely to carrier density effects and excludes the contributions of current induced temperature changes. The effects are shown to be a function of the pumping ratio between two active sections in the laser. The adiabatic chirp is enhanced by as much as a factor of 3 and the α parameter is reduced from a value of approximately 4 to 2.