Luminous Efficiency of a III-V LED Device

optoex06.in : Luminous Efficiency of a III-V LED Device

Requires: Blaze/LED
Minimum Versions: Atlas 5.28.1.R

This example demonstrates how the radiative recombination models in Atlas can be used to analyze the luminous efficiency and of LED devices.

The objective of this example is to forward bias the LED such that radiative recombination occurs in the active layer of the device. Then by extracting the integrated radiative recombination rate and total integrated recombination rate, the luminous efficiency of the device can be measured. Also, the integrated radiative recombination rate as a function of bias voltage can be extracted to estimate the luminous power versus bias voltage.

Mesh Generation

In the first section of the input file, a mesh covering an area of 4 microns by 4 microns is defined. The mesh uses the "diag.flip" option since it is symmetric about the center in "X". Mesh spacings are specified to try to resolve the heterojunctions in the "Y" direction and the doping profiles in the "X" direction. An eliminate statement is used to reduce the number of grid points used, in the direction of the substrate. Also, the value of the "space.mult" parameter has been set to 2.0 to provide faster simulation. For better accuracy (and longer simulation time) this value should be set to a smaller number.

Region and Electrode Specification

The device is composed of five regions: a GaAs "p" contact region, an AlGaAs cladding layer, the GaAs active region, a second AlGaAs cladding layer and an n-type GaAs substrate. In this example the "grad" parameters were used to grade the heterojunctions between the first and second layers and the fourth and fifth layers. The heterojunctions at both sides of the active layer are modeled as abrupt.

A "p" contact electrode is defined at the center of the top of the device. The "p" contact is narrower than the device to confine the photo-luminescence to a narrow region in the active layer. The substrate electrode extends across the device at the bottom.

Doping Profile Specification

The doping profiles in the device are specified in the third section of the input file. Here the first three layers are lightly doped "P" type, while the last two layers are heavily doped "N" type. In addition a "P+" type profile is defined around the "p" contact.

Material Models

In the fourth section of the file, a set of material and model parameters is specified. For analyzing luminous efficiency, all principal recombination mechanisms are selected. In addition, a radiative recombination constant for all the materials is set. In the solution part of the file the individual contributions of the various mechanisms to estimate luminous efficiency will be extracted.

Output

In the fifth section of the file, the various output parameters that will be written to the solution file during the solution process are chosen. These parameters will allow the user to look at the conduction band edge energy, the valence band edge energy, the total recombination rate, the SRH recombination rate, the Auger recombination rate and the radiative recombination rate.

Initial Solution

In the sixth section of the file, an initial solution with no carriers is done. This usually provides a good initial guess for the full, 2 carrier solutions to follow.

Bias Ramp

In the seventh section of the file the bias voltage across the LED is ramped from 0.0 V to 2.0 V in the forward bias direction. In this section a full two carriers solution will be done so that recombination data can be observed. In the first line of this section of the file, an output log file is specified. Current-voltage data will be written to this file. In the solve statement, a luminous wavelength l.wave is set so that luminous power can be extracted during the ramp. In addition, the saving of an output file is set in the solve statement so that snapshots of the device can be displayed at each step of the bias ramp.

Extraction

In the last section of the input file, the integrated total and radiative components of the recombination rate in the device are extracted. The luminous efficiency can be estimated by the ratio of these values.

Results

From these results, the luminous efficiency can be calculated. The luminous efficiency is defined at the ratio of the radiative recombination rate to the total recombination rate. The extracted values of these rates should be printed at the end of the run, giving a ratio a value of 70%.

To view the results, first start up TonyPlot with the solution file . This file corresponds to the device structure after the completed voltage ramp. Once TonyPlot has started you should choose to display a contour plot of the radiative recombination rate. In this plot we can see the relative luminescence in the active layer compared to other regions of the device. In this case most of the radiative recombination should be confined to the active layer of the device.

The log file contains all the terminal characteristics. This file contains values of the calculated luminescent power as a function of device bias voltage. Once the file is loaded into TonyPlot you should select to display "anode bias" along the x axis and "Luminescent power" along the y axis. In this figure it is seen that the device "turns-on" at about 1.4 Volts.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.