In Ansys Lumerical Multiphysics™, you can use either a semiconductor material type or an alloy material type to create a constant fraction alloy, which is an alloy where the mole fractions of its base materials are constant in space.
This article describes the differences between these two methods, how they may impact your simulation results, and the pros and cons of each method.
Creating a constant fraction alloy
We briefly outline below the two ways of creating a constant fraction alloy. For further information on creating materials and material properties, see the Material Properties section of the CHARGE Product Reference Manual.
Create as a semiconductor material
- Select a semiconductor material in the Materials Explorer.
- Specify a mole fraction in the GUI and click “create semi”.
Create as an alloy material
- Create an alloy material or select an existing one in the Materials Explorer.
- Add the material to the Object Tree using “create new”.
- Specify the alloy mole fraction in the geometry object.
Parameter handling differences
The material parameters of an alloy material are determined by combining the parameters for the base materials. Depending on whether you use a semiconductor or an alloy material, how the software handles combining the parameters of the base materials is different.
Material parameters for the base materials can either be a simple scalar value, or an equation that describes how the parameter changes with temperature, electric field, and other variables, known as a model. These models are characterized by coefficients, and parameters often do not vary linearly with the model coefficients.
The diagram below illustrates the differences in combining base material parameters for the constant fraction alloy, using parameter “c” in \(\mathrm{Al}_{0.2}\mathrm{Ga}_{0.8}\mathrm{As}\) as an example.
Seen from the figures above, the main difference between using a semiconductor material and an alloy material is that the semiconductor material type first constructs a combined model with interpolated coefficients, whereas the alloy material interpolates the results from the models during the simulation.
Therefore, in general, even if the composition of the constant fraction alloy material is the same between these two approaches, the results are not expected to be the same.
Based on the difference in interpolation procedures above, the two approaches to constant fraction alloys are only equal if:
- Material parameters are simple scalars – no model for material parameters
- Material parameter models vary linear with model coefficients
- Material parameter variation due to model is negligible
One way to create a semiconductor and an alloy material that matches each other exactly is to turn off all models, such as Slotboom bandgap narrowing model, high field mobility model, and temperature dependence of the band gap, prior to adding the material to the Object Tree using “create semi” and “create new”.
Pros and cons
These two different approaches to creating a constant fraction alloy material each have their own advantages and disadvantages.
Semiconductor material
Pros
- The interpolated material parameters are directly accessible in the material properties window in the object tree, making them easier to tweak.
- Faster compared to the alloy material type, as the interpolation is only done once before the simulation.
Cons
- The accuracy of interpolating potentially nonlinear model coefficients, as opposed to interpolating the final model results, may be questionable in some cases. However, you can tweak the final material properties to correct for this as needed.
- Cannot accommodate spatially varying mole fractions (graded alloys).
Alloy material
Pros
- The interpolation of the final model results during the simulation, as opposed to the interpolation of potentially nonlinear model coefficients before the simulation, is straightforward and physically sound.
- Alloy materials have the ability accommodate spatially varying mole fractions (graded alloys).
Cons
- The final interpolated material properties are not directly accessible in the material properties in the object tree. To tweak final properties, you must do so through the base materials or the interpolation parameters.
- Usually somewhat slower than the semiconductor material type, due to the “on the fly” interpolation.
See Also