SpeCLED 2008 — Spreading of Current in Light Emitting Diodes, Software for 3D Modeling of Current Spreading and Temperature Distribution in LED chip

1. Software overview

Attention customers in Japan. SimSciD Corp. has become an official distributor of SimuLED package.

Fig. 1. 2D distribution of Internal Quantum Efficiency

The device-engineer oriented software package SpeCLED (Spreading of Current in Light-Emittind Diodes) is intended for simulation of current spreading and heat transfer in planar and vertical light-emitting diode chips with complex contact electrode configuration. The package enables simulations of the integral parameters of the device, forward current, output emission power, wall-plug efficiency, integral emission spectrum, etc. as a function of the forward voltage applied. In addition, the distributions of the current density and temperature over the chip are computed and their effects on the integral device parameters are predicted by the code.

The SpeCLED package is provided with a friendly graphical user interface (GUI) aimed at minimization of the user efforts necessary for doing simulations. Using the GUI, a researcher can specify and control the LED chip design, generate and refine computational grid, change the default materials properties, run and monitor computations, and save/export the simulation results. Internal visualization tool, SimuLEDView, is available in the package to view the simulation results.

Fig. 2. Specification of the die geometry

Every LED chip is considered in the SpeCLED package as that fabricated by planar technology operations. This allows the layer-by-layer input of the actual 3D chip geometry, which is operative, pictorial, and easy-to-learn. Functionality of the layers specified by user serves as the basis for automatic generation of the boundary conditions on all internal interfaces of the chip. A prismatic grid, unstructured/structured in plane, is generated either automatically or manually. User is capable of the grid refining, if necessary, using the options provided within the GUI. Different blocks in the grid are recognized automatically and their properties are identified from the description of the constituent layers. A complex structure of contact layers, including non-uniform doping and composition or insertion of a short-period superlattice region, may be allowed for by special script. Anisotropic carrier mobility is supported in SpeCLED in order to simulate the difference in the in-plane and normal conductivities of the short-period superlattice region.

Fig. 3. Automatic generation of the computational grid

In order to make simulations more efficient, the SpeCLED package uses a hybrid approach that distinguishes between quasi-neutral and active regions in the LED chip. 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 bias U applied to the active layer. These characteristics may be either defined manually via special parameterized functions or imported from a set of external files. In particular, the j-U characteristics may be obtained from 1D simulations by the SiLENSe software tool (www.semitech.us/products/SiLENSe/). The current spreading in thin semi-transparent metal electrodes is considered in the SpeCLED code self-consistently. The heat transfer analysis coupled with the current-spreading problem and accounting for the heat-sink via a used-defined heat-transfer coefficient 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 impurity ionization and, hence, on the materials electric conductivity is considered in simulations.

Fig. 4. Vectors of electric current in selected horizontal cross-section

Progress in simulation is visualized in a solution-monitor window providing information on the current stage of a computational run and plotting the residuals on current and temperature to control the iteration convergence. The computation is stopped automatically when required solution accuracy is achieved. The user can also stop the computation and save the intermediate results.

The computed distributions of the current density and temperature allow determination of the integral parameters of the LED chip - total electric current through the diode, total optical power emitted from the active region, total dissipated power, and integral emission spectrum. A number of additional integral parameters characterizing the LED performance are provided by SpeCLED as well.

Fig. 5. Ñurrent-voltage (I-V) characteristic of the LED

The results of the computation can be stored in ASCII files (*.cgs) and then viewed by the visualization tool SimuLEDView supplied within the SpeCLED package. The tool enables reviewing of 2D distributions of a number of physical fields in the active region plane and in different horizontal cross-sections of the die. Also SimuLEDView summarizes the LED integral characteristics, like emission spectrum, total electric current, output power, dissipation power, etc. The SimuLEDView tool allows export of the 2D distributions in a bmp-image format and of 1D distributions extracted for selected directions in a text-table format.

2. SpeCLED of version 2.1

The basic SpeCLED package (version 2.1) allows simulation of the current spreading in a planar or vertical LED chip coupled with the analysis of heat transfer. The code implements the physical models of electrical and thermal processes, based on the following assumptions:

• Every LED chip is considered as a stack of epitaxial layers consequently formed on a substrate where some planar-technology operations are applied, like metal film deposition and mesa etching.
• A planar (one-side electrode configuration) die is presented as a stack of n-contact layer with etched mesa, active region, p-contact layer, n-electrode on the top surface of the mesa, p-electrode on the top surface of p-contact layer, n- and p-pads on the electrodes.
• A vertical (two-side electrode configuration) die is presented as a stack of n-electrode on the bottom surface of the die, n-contact layer, active region, p-contact layer, p-electrode on the top surface of p-contact layer, n- and p-pads on the electrodes.
• Active region of the LED is considered as in-plane distributed non-linear resistor with known temperature-dependent characteristics relating the local normal current density with the bias applied to the active layer. These characteristics may be either manually defined via special parameterized functions or imported from a set of external files. In particular, they can be generated by using the SiLENSe simulator.
• Current spreading in the quasi-neutral regions of the LED chip is simulated within a 3D approach assuming the drift mechanism of the carrier transport to dominate.
• Current spreading in thin (semitransparent) metal electrodes is accounted for self-consistently in the simulations. A constant electric potential is assumed to be held at the contact pads.
• Temperature effects on the electric conductivities of the quasi-neutral regions and on the local internal quantum efficiency of the active region are allowed for.
• The heat-sink is modeled by user-specified heat-transfer coefficients assigned to the chip surfaces contacting with the heat-sink.
• Anisotropic mobility is supported in SpeCLED in order to imitate the difference in the in-plane and normal conductivity of, e. g. a short-period superlattice material.

Fig. 6. Power-current (L-I) characteristic of the LED

The input of necessary data, building up of the chip geometry, grid generation and refinement, running and monitoring of simulation, and visualization of the results can be done via Graphical User Interface (GUI) and SimuLEDView visualization tool.

The SpeCLED code is supplied with the user manual and description of physical mechanisms underlying operation of the code.

3. Compatibility

The SpeCLED package can import the input data generated by the SiLENSe version 2.0 or higher, 1D simulator of band diagram, carrier transport, and light emission in an LED heterostructure. More info about the SiLENSe code is available at SiLENSe page.

Files with results of computations generated by SpeCLED package can be used as input data for RATRO 1.2, a 3D software tool designed to model the light propagation in the LED chip.

4. Support

Hot-line support is provided for customers. The support includes free of charge supply of updated versions released during the license period and technical consulting on SpeCLED operation.

5. Key Publications

• M.V. Bogdanov, K.A. Bulashevich, I.Yu. Evstratov, S.Yu. Karpov
  Current spreading, heat transfer, and light extraction in multipixel LED array
  phys. stat. solidi (c) 5, No. 6, 2070–2072 (2008)
• K.A. Bulashevich, I.Yu. Evstratov, V.F. Mymrin, S.Yu. Karpov
  Current spreading and thermal effects in blue LED dice
  phys. stat. solidi (c) 4, No. 1, 45–48 (2007).