RATRO — RAy-TRacing SimulatOr of Light Propagation

1. Software overview

Fig. 1. 2D distribution of light emission from active region

The device-engineer oriented software tool 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. RATRO 1.2 can only be used as a module of SpeCLED software package. It has an easy-to-learn graphical user interface (GUI) designed to specify the necessary chip parameters and carry out the simulations. GUI is aimed at minimization of the user efforts necessary for doing simulations. Using the GUI, a researcher can specify optical parameters of the die materials bulk and of all die surfaces, run the computations, and save the simulation results.

The whole chip modeled in RATRO consists of two elements: heterostructure (GaN contact layers with the active region) and the wafer. The heterostructure represents planar 3D object whose geometry is specified in SpeCLED software tool and imported by RATRO from the files generated by SpeCLED. The wafer is not considered in SpeCLED™ and is specified in RATRO graphical user interface. RATRO supports flat and shaped wafers. A flat wafer is a parallelepiped defined by its height only (the horizontal dimensions match the dimensions of the Heterostructure). A shaped wafer is specified within a generalized predefined geometry.

Fig. 2. Specification of chip parameters

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. The active region is considered as a Lambertian surface so that the angular emission distribution obeys the Lambert's cosine law. Patterned and ordinary surfaces of contact layers, electrodes, and wafer are supported.

Progress in simulation is visualized in a solution-monitor window providing information on the percentage of traced rays. The computation is stopped automatically when all rays are traced. The user can also save the intermediate results and interrupt the computation.

The computed distributions of light propagation allow determination of the light extraction coefficients, fractions of light extraction through all chip surfaces, and energy loss in each chip region.

Fig. 3. Light extraction from a LED die with heat sink at the wafer bottom

The results of the computation can be stored in ASCII files (*.cgs) and then viewed by the visualization tool SimuLEDView supplied within the RATRO tool. 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 visualizing the near-field and far-field regions, and radiation patterns. 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. RATRO 1.2

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 electrical and thermal processes, based on the following assumptions:

• Fig. 4. Light extraction from a LED die with a flip-chip structure

  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.
• The active region is considered as a Lambertian surface so that the angular emission distribution obeys the Lambert's cosine law.
• The active region emits monochromatic radiation. The effect of the radiation wavelength is accounted implicitly via the refraction coefficients assigned for each material.
• The effect of the light polarization on ray propagation is not considered.
• The diffuse fraction of the radiation reflected and refracted on the chip surfaces obeys the Lambert's cosine law.
  

• Fig. 5. Radiation pattern in the top hemisphere

  The light transmission and reflection in the metal electrodes are governed by a user-defined transmission and reflection coefficients.
• The light reflection on the GaN / wafer / environment interfaces are described by a simplified approximation ignoring the angular dependence of the transmission and reflection coefficients unless the incidence angle exceeds the angle of the total internal reflection.

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

Fig. 6. Light extraction from an LED die with a shaped wafer

3. Compatibility

The RATRO 1.2 package can import the input data generated by the SpeCLED 2.0. More info about the SpeCLED code is available at SpeCLED page (www.semitech.us/products/SpeCLED/).

4. Support

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