K. A. Bulashevich

Simulation of visible and ultra-violet group-III nitride light emitting diodes

One-dimensional drift-diffusion model accounting for the unique properties of group-III nitrides is employed to simulate the carrier transport and radiative/non-radiative recombination of electrons and holes in light emitting diode heterostructures. Mixed finite-element method is used for numerical implementation of the model. The emission spectra are computed via the self-consistent solution of the Schrodinger-Poisson equations with account of complex valence band structure of nitride materials. Simulations of a number of single- and multiple-quantum well blue and ultraviolet light emitting diodes are presented and compared with available observations. Specific features of the III-nitride LED operation are considered in terms of modelling. Applicability of the drift-diffusion model to analysis of III-nitride LEDs is proved and still open questions are discussed.

Coupled modeling of current spreading, thermal effects and light extraction in III-nitride light-emitting diodes

We study by simulation the current spreading, heat transfer, and light emission in a high-power light-emitting diode (LED) with an interdigitated multipixel array (IMPA) chip design. The applied hybrid approach allows coupled modeling of various processes involved in the LED operation, providing good agreement with observations.

Simulation of light-emitting diodes for new physics understanding and device design

The paper discusses various factors affecting internal quantum efficiency (IQE) of state-of-the-art III-nitride lightemitting diodes (LEDs). A general figure of merit for LED heterostructures, namely the quality factor, is proposed on the basis of a simple recombination model, which enables comparison of overall performances of the structures either emitting light at different wavelengths or having substantially different designs. The relationships between the quality factor, maximum value of IQE, and IQE droop with the current density are revealed. Some ways for IQE improvement and reducing its droop are considered. Among them, the use of short-period superlattice (SPSL) active regions is found to be quite promising. The operation of such structures and their properties are examined in detail by simulations accounting for quantum corrections to electron and hole transport within the quantum-potential concept.

Effect of free-carrier absorption on performance of 808 nm AlGaAs-based high-power laser diodes

We examine effects of free-carrier absorption produced by non-equilibrium electrons and holes injected in the waveguide on characteristics of high-power AlGaAs-based laser diodes emitting light at 808 nm. The carrier transport in the laser heterostructures is studied theoretically, using the drift-diffusion numerical approach. On the basis of simulations, a relation between the current density and non-equilibrium carrier concentrations in the waveguides is found. The internal optical losses of the waveguide modes and their effect on the differential quantum efficiency of the laser diodes are estimated from the computed carrier concentrations. Some approaches aimed at reducing the free-carrier absorption and, thus, improving the laser performance are discussed. The theoretical predictions are compared with available data to validate the theoretical model and justify the conclusions coming from the simulations.

Simulation of hybrid ZnO∕AlGaN single-heterostructure light-emitting diode

Using simulations, we have examined specific features of a hybrid n-ZnO∕p-AlGaN light-emitting diode (LED) operation, originated from a type-II band alignment and a negative polarization charge at the ZnO∕AlGaN interface. These factors are found to improve the carrier confinement near the interface and to affect significantly the light emission spectra and internal quantum efficiency of the LED. The theoretical predictions are compared with available observations.

Optical confinement in laser diodes based on nitrides of Group III elements. Part 1: Theory and optical properties of materials

In the first part of our study, scalar wave equations for the guided TE and TM modes in laser heterostructures based on nitrides of Group III elements are derived taking into account the anisotropy of optical properties of these nitrides. The available experimental data on optical properties of multicomponent nitrides (and also sapphire and silicon carbide used typically as substrates in growing the nitride heterostructures) are analyzed. Based on this analysis, approximations of spectral dependences of permittivities for ordinary and extraordinary waves for these materials are suggested.

Correlations between Epitaxy Recipe, Characteristics, and Performance of Nitride Light Emitting Diode Structures

Metal–organic vapor phase epitaxy of III–nitride light-emitting diode (LED) structures is analyzed by coupled process-device modeling. Different recipes to grow active regions of the structures are examined. The major attention is given to the impact of process recipe on indium composition profile in the active region. Correlations between the composition profiles and device characteristics are discussed. We have focused on the analysis of LED structures with short-period superlattice active regions providing improved efficiency, reduced efficiency droop, and high emission wavelength stability with current. Performances of the structures grown with various recipes are compared to find the most promising growth procedure and evaluate its impact on the operation of the LED heterostructure.

Optical confinement in laser diodes based on nitrides of Group III elements. Part 2: Analysis of heterostructures on various substrates

Numerical simulation is used to systematically analyze the waveguide properties of laser hetero-structures based on nitrides of Group III elements and formed on substrates made of various materials (sapphire, silicon carbide, and gallium nitride); in the analysis, the birefringence effect both in the nitride structure itself and in the substrate was taken into account. The specific features of optical confinement in typical laser structures and the effect of the substrate material and metallic contacts formed on top of the p-type layers are considered. The coefficients of optical losses for waveguide modes due to free charge carriers and to leakage into the substrate were estimated; these estimates were used to determine the significance of various channels of losses.

Coupled Modeling of Current Spreading, Thermal Effects, and Light Extraction in Ill-Nitride Light-Emitting Diodes

We study by simulation the current spreading, heat transfer, and light emission in a high-power light-emitting diode (LED) with an interdigitated multipixel array (IMPA) chip design. The applied hybrid approach allows coupled modeling of various processes involved in the LED operation, providing good agreement with observations.

effect of the parameters of AlN/GaN/AlGaN and AlN/GaN/InAlN heterostructures with a two-dimensional electron gas on their electrical properties and the characteristics of transistors on their basis

The effect of the layer thickness and composition in AlGaN/AlN/GaN and InAlN/AlN/GaN transistor heterostructures with a two-dimensional electron gas on their electrical and the static parameters of test transistors fabricated from such heterostructures are experimentally and theoretically studied. It is shown that the use of an InAlN barrier layer instead of AlGaN results in a more than twofold increase in the carrier concentration in the channel, which leads to a corresponding increase in the saturation current. In situ dielectric-coating deposition on the InAlN/AlN/GaN heterostructure surface during growth process allows an increase in the maximum saturation current and breakdown voltages while retaining high transconductance.