The MQW solver uses a 1D layer geometry, which can be defined in the Layers tab of the Edit MQW Gain Solver window. To define the layer geometry, the layer thicknesses, materials, and ordering must be specified. Before creating the layer geometry the materials of the layers should be created (see Creating Materials for the MQW Solver).
Defining the layer geometry using the MQW solver GUI involves creating the basic layer structure and (optionally) dividing the layer structure into partitions.
Creating the Basic Layer Structure
The basic layer structure of an MQW stack consists of a series of layers in order defined by their thickness and material. The current layer structure of the MQW stack can be seen in a table in the Layers tab of the Edit MQW Gain Solver window.
To create the layer structure:
- Add layers to the geometry using the Add layer button.
- Set the Material and Thickness of each layer by modifying the values in the table. The material can be selected from the materials added to the simulation and the thickness can be any positive value in increments of angstroms.
- Layers can be selected by clicking on any of the cells in that layers row in the table. Once selected, layers can be moved up or down in the stack geometry using the Move up and Move down buttons, removed with the Remove layer button, or duplicated using the Duplicate layer button.
- Extra thickness can be added to the top and bottom layers using the top layer extra thickness and bottom layer extra thickness properties. This is the same as increasing the thickness of the top and bottom layers of the stack in the layer table.
Partitioning the Layer Structure
If the barriers in the multiple quantum well structure are thick enough, for example more than 5 nm, it is a good approximation to consider quantum wells uncoupled. In this case the wave functions can be solved considering one quantum well at a time. This can significantly reduce the length of the simulation domain and make the gain simulation much faster, while producing similar results as in the coupled MQW structure. This is accomplished in the MQW solver by separating the stack into partitions, where each uncoupled section of the MQW stack is in its own partition.
If the structure of the wells (materials and layer thicknesses) and electric potential are identical, the reuse bandstructure option can be enabled. When this option is enabled, the bandstructure from the first well will be used for the other wells. This can be useful when the well structures are the same but other quantities (carrier density, for example) are different across the wells.
Alternatively, the layer structure can be defined with only a single well and these other quantities can be varied in each simulation. The results from these individual simulations can then be manually combined to get the results for the entire structure. See Using the Single Quantum Well Approximation for more information.
To partition the layer structure:
- Select enable partitions in the Layers tab of the MQW solver GUI.
- Specify the number of partitions using the num partitions property.
- Define the start and end of each partition by setting the values in the partition layer indices table. The partition layer indices should correspond to the quantum barrier layers in the MQW stack between each uncoupled quantum well structure.
- If the quantum well structures (material and layer thickness) and electric potential are identical, select reuse bandstructures.
Using the Single Quantum Well Approximation
If the quantum wells in the MQW stack have an identical structure (materials, layer thickness) and are decoupled, the quantum wells can be simulated individually to reduce the simulation time. In this case, the MQW solver layer structure should be defined using only a single quantum well.
If parameters like charge density and electric potential vary across the MQW stack, a parameter sweep can be used to obtain results for the different values of these parameters. The results from the individual simulations can be combined to obtain the results for the entire MQW structure.
The single quantum well approximation is similar to partitioning the structure and selecting the reuse bandstructure option (see Partitioning the Layer Structure), however in that case the results are automatically combined for a specific combination of simulation parameters. In the single quantum well case, the results are known for the individual quantum wells, so they can be freely recombined for different configurations.
If this option is used, the average charge density at the input to the gain simulation should be scaled accordingly to represent the same local charge density as in the full MQW structure. Similarly, the final single quantum well gain and spontaneous emission results should be scaled onto the full MQW structure.
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