The MQW solver has the option of including exciton effects in the absorption calculation. Exciton effects are relevant for electro-absorption modulator simulations but are not typically used for laser models. For more information on how these effects are accounted for by the MQW solver, see the MQW Solver Physics page.
Assumptions for the Exciton Model
When the exciton model is activated, it is assumed that the quantum wells are fully depleted (valence band full, conduction band empty) so the cden and cden type carrier density properties are deactivated. This also means that the temperature property no longer affects the absorption calculation and is only used to modify the material properties. The entire Brillouin zone is used for the transverse wave vectors as well, so the brillouin zone ratio property is deactivated.
By default, the MQW solver will calculate the DC permittivity for the exciton based on the quantum-mechanical average of the permittivities of the layer materials. A custom DC permittivity can be used by selecting enable custom eps dc.
Activating the Exciton Model
The exciton properties are found in the Configuration tab of the Edit MQW Gain Solver window. To activate the exciton model:
- Select enable excitons.
- (Optional) Include the angular dependence of the exciton wave functions by selecting enable exciton angular dependence. Including exciton angular dependence results in a more accurate but longer simulation. Typically, this has only a small effect at room temperature, but it can be important at lower temperatures.
- (Optional) Enter a custom DC permittivity by selecting enable custom eps dc and setting the eps dc property to the custom DC permittivity.
- Set max num cb subbands to the maximum number of conduction subbands to use for exciton mixing. Increasing this value results in a more accurate but longer simulation. In general, around 2-4 subbands per coupled well should be used.
- Set max num vb subbands to the maximum number of valence subbands to use for exciton mixing. Increasing this value results in a more accurate but longer simulation. In general, around 2-4 subbands per coupled well should be used. This does not include spin, so the actual number of subbands is 2x this value.
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