STR Belgrade d.o.o.

STR provides dedicated software and consulting services for simulation and optimization of crystal growth techniques and for modelling of semiconductor based devices. The scope of STR's expertise includes CVD, sublimation growth, crystal growth from the melt, and modeling of advanced semiconductor devices, such as light emitting diodes,laser diodes, heterojunction bipolar transistors, high-electron mobility transistors, and Schottky diodes.Research projects conducted by STR make our company capable of solving a wide range of practical problems in the semiconductor technology.

-Softwares for Semiconductor Epitaxy and Deposition Simulation
Virtual Reactor, CVDSim

-Software for Stress and Relaxation Engineering of MOCVD
STREEM-AlGaN edition, STREEM-InGaN edition

-Softwares for LED, LD, FET and Solar Cell Semiconductor Device Simulation
SimuLED[SiLENSe,SpeCLED,RATRO], SimuLamp, FETIS, PVCell, BESST

STREEM-AlGaN Edition

STREEM AlGaN is a specialized software tool for self-consistent modeling of the evolution of epitaxial stress, bow, and dislocation dynamics during the growth and cooling of (0001) III-Nitride heterostructures by MOCVD on silicon, sapphire, SiC, GaN and AlN wafers. It includes simulation of the following phenomena:

-Evolution of heterostructure curvature at the heating, growth, and cooling stages of the growth process;
-Effect of process parameters on stress evolution and dislocation dynamics;
-Crack formation during the growth and cooling of the structure;
-Influence of the recipe on the through-wafer temperature drop and its contribution to the structure bow;
-Processing of in-situ curvature data to retrieve stress state in the particular layers
To predict relaxation of compressively stressed (Al)GaN layers, a model has been developed, attributing relaxation to nucleation and inclination of threading dislocations depending on the process conditions and stress state. As a result of the modeling, the user can analyze the stress, curvature, bow, effective lattice parameter, density and inclination angle of threading dislocations in the epitaxial stack. By adjusting the recipe, including the temperature, thickness and composition of the layers, sequence and durations of the particular stages of the process, one can follow the respective changes in the above characteristics and establish correlations between the recipe and properties of the heterostructure.

STREEM-InGaN Edition

STREEM-InGaN is a specialized software tool for modeling the characteristics of(0001) III-N device heterostructures grown by MOCVD from conventional metal organic precursors(TMIn,TMGa/TEGa,TMAl) and ammonia, diulted in H2/N2 carrier gases. STREEM-InGaN focuses on an InGaN- based active region which implies a sequence of quantum wells and barriers as other stages in-between. Layers grwon prior to and after the active reigon can be added into the simulations as well. The software is aimed at understanding and control of the structure properties by adjusting the process recipe. In particular, the following issues can be addressed

-influence of the process parameters on indium incorporation into the quantum wells;
-predictions of the actual composition profile in the active region of the heterostructure, including delayed indium incorporation into the QWs and indium tails in the cap layers or barriers. Due to indium surface segregation, the actual composition profile normally deviates from the nominal one built up from the steady-state solutions obtained for every individual epilayer at the respective growth conditions;
-consistent computations of indium incorporation and elastic energy allows the users to follow and adjust the strain distribution in the active region by both modifying the operating parameters for the particular layers and adding strain-relief layers underneath the quantum wells. The actual composition and strain profiles determine the distribution of the polarization charges in the structure that can be accounted for in subsequent modeling of device operation with the SiLENSe software
-stress relaxation via formation of V-shaped dislocation half-loops, annihilation of the threading dislocations, and evolution of the strain, threading dislocation density, and indium composition profile can be studied with the STREEM-InGaN software, depending on the particular parameters in the recipe

SimuLED Package

The SimuLED software is based on a multi-scale approach to LED and LD modeling. The simulator can be used for design and optimization of devices based on group-III nitrides and other wurtzite materials as well as hybrid structures. The drift-diffusion model is applied to the carrier transport, recombination, and light emission in the heterostructure, while the current spreading in an LED/LD die is simulated using the 3D hybrid approach. This makes computations very fast while reflecting the essential physics of processes occurring in the LEDs/LDs. The self-heating and current spreading in the die are simulated self-consistently which is especially important for optimization of high power devices and micro LEDs.
The SimuLED package consists of three compatible software tools: SiLENSe, SpeCLED, and RATRO

-carrier transport in the heterostructure and LED spectrum;
-current spreading/crowding in LED die;
-heat transfer in LED die;
-light extraction from the LED die.

SiLENSe LED and LD edition

SiLENSe is software tool for 1D simulation of the active region of light-emitting diodes (LEDs) and laser diodes (LDs) made of cubic III-V compounds (AlGaInAs, AlGaInP, InGaAsP, AlGaInSb, and GaInAsSb) and wurtzite III-nitrides (AlInGaN and ZnMgO). It can be used by both device and epitaxy engineers. Carrier transport model implemented in the software allows simulation of polar, semipolar, and nonpolar structures and accounts for specific features of nitride heterostructures including polarization effects, high density of threading dislocations and Auger recombination. The last one is responsible for the droop of internal quantum efficiency observed in nitride LEDs at moderate and high current densities. SiLENSe provides distribution of critical parameters over the LED heterostructure, including partial (electron and hole) currents, electric field and potential, rate of carrier recombination, and carrier concentrations. The program is capable of calculations for graded-composition heterostructures.
Note that one more channel of non-radiative recombination – the surface recombination at the sidewalls of the LED mesa – is simulated at the chip level, see micro-LEDs

-Band diagram of an LED at various biases;
-Distribution of electron and hole concentrations in the device structure;
-Electric field distribution;
-Radiative and non-radiative recombination rates;
-Dependence of the current density on the p-n junction bias (I-V curve);
-Internal quantum efficiency (IQE) dependence on the current density;
-Wave functions of electrons and holes in quantum wells;
-Emission and gain spectra of individual quantum wells and the whole structure;
-Waveguide modes (TE and TM) of an edge-emitting laser diode*;
-Threshold current and power-current characteristic of an edge-emitting laser diode*.
* these options are available in Laser Edition only.

Basic design of 815×875 μm2 blue LED die

SpeCLED
-3D modeling of Current Spreading and Temperature Distribution in LED chip

For modeling of micro-LEDs with SpeCLED see dedicated page
SpeCLED (Spreading of Current in Light-Emitting Diodes) is a software for modeling of the current spreading and heat transfer in planar and vertical LED chips. As a part of the task, it simulates distribution of the current density, internal quantum efficiency (IQE), and the temperature over the active region. SpeCLED also computes a number of integral characteristics of the device such as the forward voltage, output emission power, wall-plug efficiency, etc. as a function of the forward current.
LED chip is considered in the SpeCLED as one fabricated by planar technology operations. This allows the layer-by-layer input of the actual 3D chip geometry, which makes it much easier to set up a simulation case. A prismatic grid, unstructured/structured in plane, is generated either automatically or manually. User can change the grid refining, if necessary. Different blocks in the grid are recognized automatically and their properties are identified from the description of the constituent layers. A complex structure of semiconductor layers, including non-uniform
doping and composition, may be specified via user scripts.
A 3D model is employed to simulate the current spreading in the quasi-neutral regions where carrier drift dominates over diffusion. The active region is considered as an in-plane distributed non-linear resistor with known temperature-dependent j-U characteristics relating the local normal current density j with the p-n junction bias U applied to the active region. These characteristics may be either defined manually or imported from external files. So, the j-U characteristics can be taken from 1D simulations by SiLENSe.

The current spreading in metal electrodes/pads and semitransparent ITO layers is considered in SpeCLED self-consistently. The heat transfer analysis coupled with the current-spreading problem provides the temperature distribution inside the LED chip. The heat generation inside the chip is found with account of the current density distribution obtained from the current-spreading problem. The temperature effect on the active region characteristics, as well as on the impurity ionization in thick semiconductor layers is considered in simulations.Files with results of computations generated by SpeCLED package can be used as input data for RATRO, a 3D ray-tracing simulator of the light extraction from the LED chip.

RATRO(3D Ray Tracing)

RATRO (RAy-TRacing SimulatOr of Light Propagation) is designed for modeling the light extraction from LED chips. It involves ray-tracing simulation of the light propagation from the active region, absorption and extraction from the LED die through the n- and p-contact layers and the wafer, providing the integral extraction efficiency and the radiation patterns of the emitted light.Distribution of the light emission from the active region is calculated in SpeCLED and stored in the file imported into RATRO™ along with the heterostructure geometry. Patterned and ordinary surfaces of contact layers, electrodes, and wafer are supported.The computed light propagation gives the light extraction coefficients, fractions of light extraction through all chip surfaces, and energy loss in each chip region.RATRO shares the same graphical user interface (GUI) with SpeCLED to specify the chip geometry, while all input parameters specific for light extraction simulation are specified in RATRO tab. The results of the computation can be viewed in SimuLEDView viewer supplied within RATRO. The visualization tool provides information on the integral light extraction parameters, 3D distributions of light intensity in the near-field, 2D distributions of light intensity for the near- and far-field regions, and radiation patterns.The RATRO package provides ray-tracing simulation of the light propagation from the active region, absorption and extraction from the LED die through the n- and p-contact layers and the wafer, providing the integral extraction efficiency and the radiation patterns of the emitted light. The code implements the physical models of optical processes, based on the following assumptionsThe input of necessary data generated by SpeCLED, specification of optical parameters, running and monitoring of simulation, and visualization of the results is done via Graphical User Interface (GUI) and SimuLEDView visualization tool, respectively. RATRO is supplied with the user manual and description of physical model.

-The emission from the active region is symmetrical so that the total light intensity and radiation pattern are identical for both sides (top and bottom) of the active region.
-Angular emission distribution from the active region can be specified as (I) uniform, (II) Lambertian, or (III) custom (user-defined table).
-The active region emits monochromatic radiation. The effect of the radiation wavelength is accounted implicitly via the refraction coefficients assigned for each material.
-The light transmission and reflection in the metal electrodes can be calculated from known material parameters or specified as user-defined transmission and reflection coefficients.

FETIS(Field Effect Transistor Integrated Simulator)

FETIS software is developed for modeling of group-III nitride-based high electron-mobility field-effect transistors (HEMTs). The module includes a 1D simulator of the band diagram and potential distribution across the device heterostructure and a graphical shell providing comfortable operation with the code and visualization of modeling results. Both quasi-classical and accurate quantum-mechanical consideration of the carrier confinement in a HEMT structure, based on the self-consistent solution of the Poisson and Schrodinger equations, are available in the FETIS package. The code allows predicting such important HEMT characteristics and parameters as the carrier concentration profile, the sheet carrier concentration, the number and energetic position of two-dimensional electron/hole gas subbands, etc., as well as their variation with the gate bias.The physical models implemented into the FETIS account for specific features of the nitride materials – strong piezoeffect, existence of spontaneous electric polarization, and low efficiency of acceptor activation due to high ionization energy. There is also possible to include in the device heterostructure layers of semiconductor materials different from group-III nitrides, for instance ZnO, MgO or their alloys of a certain composition.

-Band diagram
-Energy levels and wave functions of localized carrier states
-Carrier concentration and sheet density HEMT operation analysis within the gradual channel approximation Distribution of the channel potential, electric field, and electron concentration and drift velocity along the channel
-Current-voltage characteristics
-Spontaneous polarization and piezoeffect are taken into account
-Carrier density is calculated with account for the quantum nature of the carriers
-Electroneutrality condition on the bottom heterostructure surface
-Band diagram is calculated by the Poisson equation
Two types of boundary conditions on the top contact
-Schottky barrier on the gate contact
-Band bending because of the surface trap levels on the intercontact surface

BESST(Bandgap Engineering Superlattice Simulation)

Bandgap Engineering Superlattice Simulation Tool (BESST) is a package for simulation of optoelectronic devices based on group-III nitride superlattices.Short-period superlattices (SPSLs) serve as important elements of a device heterostructure design, aimed at solving either technological or some design problems. In the case of III-nitrides, the SPSLs are used either for reduction of dislocation density in epitaxial materials, enhancement of Mg acceptor activation, increase of hole injection efficiency or even as n- and p-emitters and active regions in light emitting diodes and laser diodes. To employ the advantages of SPSLs, it is necessary to know their electric and optical properties as a function of SPSL parameters—thicknesses and compositions of the constituent layers. The BESST package allows calculating of individual SPSL properties as well as modeling of band diagram and carrier transport in a device structure consisted of a sequence of different p- and n-doped SPSL regions. The light emission spectra from such a device are also predicted.

Calculation of key electrical properties of individual SLs involved in the structure
-Electronic structure: energy levels and wave functions of localized carrier states, parameters of minibands.
-Carrier concentration and impurity ionization.
-Electric field distribution
-Conductivity of SL
Simulation of the whole device at given bias by self-consistent solution of Poisson and Schrodinger equations coupled with carrier transport equations.
-Band diagram.
-Energy levels and wave functions of carrier states in each QW.
-Carrier concentrations and impurity ionization.
-Carrier fluxes and recombination rates.
-Emission spectrum.
-Current-voltage (I-V) characteristic can be calculated by series of computations.

PVCELL(Semiconductor Photo Voltaic Cell)

Designed with engineers in mind, sotware provides fast and efficient quasi-1D simulations of simple p-n junction and tandem solar cells made of various inorganic semiconductor materials. Materials systems include Si, Ge, conventional III-V binary compounds and alloys, III-nitride and II-oxide wurtzite semiconductors and alloys.

Single computations at a given bias provide following characteristics:

-Current density, power, conversion efficiency;
-Band diagram, electric potential, electric field;
-Carrier and ionized impurity concentrations;
-Generation rate, recombination rates (different channels);
-Partial electron and hole current densities.

Single or multiple tunnel junctions can be simulated using theoretical predictions, semi-empirical models, or using measurement data.Series computations with varying bias can be used to obtain I-V characteristic, conversion efficiency, short-circuit current, open-circuit voltage, and fill factor. Series computations with varying excitation wavelength can be used to find spectral dependencies of IQE and EQE