Applying effective materials for DBR layers in the VCSEL design tool

In the simulation of VCSEL devices, the distributed Bragg reflector region typically consists of many layers of periodic heterostructures. When they are simulated normally, they typically require significant simulation time and could lead to convergence issues in the electrical simulation. However, the detail of their structure is still important for optical simulations.

To circumvent this, an effective material can be used in approximating the DBR regions for its electrical and thermal properties, allowing for fast simulations without sacrificing accuracy of the results.

Assigning an effective material

In Multiphysics, you can assign an effective material to a structure group. Therefore, you must place your DBR region inside a structure group for this technique.

To assign a material, use the material tab in the structure group properties and check “group scope override”. This option replaces electrical and thermal material properties of all objects in the structure group while retaining their individual optical material properties.

You can also assign a material using script

setnamed("structure group", "group scope effective material","CustomMaterial");

The effective material you assign cannot be an alloy material, and it’s often necessary to conduct auxiliary calculations to extract the effective material properties of the DBR layer in your design.

Effective material formulation

The script attached to this article formulates the effective material for a DBR layer under the following assumptions:

• All layers in the DBR are doped with the same dopant type and have approximately constant doping concentration.
• The free carrier concentration in the DBR is approximately equal to the dopant concentration.
• Charge transport in the DBR is approximated by only drift current, with no recombination.
• Mobility in the DBR is extracted via the conductance.

To formulate the effective material under the assumption above, follow these instructions:

1. Download and run the scripts from the Knowledge Base article Automatic Layer Import for VCSEL Design Tool to set up the VCSEL geometry.
2. Download the scripts, effective_material_main.lsf, and effective_material_functions.lsfx from this article.
3. With your simulation file open, follow the instructions in the header of effective_material_main.lsf  to input the following information:
   • Project file for your VCSEL simulation
   • DBR groups
   • List of structures exempt from the effective material calculation. When extracting an effective material, we recommend extracting and overriding all layers of the DBR region, except for 1 or 2 layers closest to the active layer.
   • Effective mass matrix for the new effective material overriding the DBR region. We recommend using the material with the smallest bandgap in the DBR layers.
4. Run the script effective_material_main.lsf. The script will run simulations to determine the appropriate effective material to use.

Effective material setup is now complete, and you can view the applied effective material in the “Material” tab of the structure group.

See Also

VCSEL Design Tool - User Manual, Workflows for VCSEL Devices