This article dives deeper into the band offset definitions. We outline the calculations used for default band offset, and how it is related to material properties in the database. Then we show how the users can manually change this offset as needed. Finally, we explain how this manual offset can be calculated using the band offset ratios (CBO/VBO) available in literature.
Default band offset
This section describes the default band offset calculation, without strain applied. The offset values for Valence and Conduction bands rely on the affinity difference between the adjacent semiconductors, which are calculated using their bandgap and work function. This is based on the following equations:
Below is a visual outline of these parameters and their names:
Band offset override and offset after strain
In this calculation, band gap and work functions are taken from the electrical material database. Eg is fixed for a given material, so default band offset can be changed by changing default \(\phi_S\):
By changing \(\phi_S\), the band offsets will be changed in both CHARGE and MQW. In the section below we explain another way to change band offsets just for MQW.
Band offset override for MQW only
The user has the option to manually override default offsets in MQW, previously determined from the database. This assumes one barrier layer as zero reference. If strain is included it will be accounted afterwards. This manual override can be set in the Valence Band Offset column shown below:
If strain is included, the final band offsets will be slightly different than what the user has set. As mentioned, this is because strain is applied after the manual band overrides. The final band diagram, accounting for both the override and strain, can be plotted using the band diagram button.
Calculating the band offsets for manual override
Estimates of band offset ratios x (CBO/VBO=x) can often be found in literature for different materials. Using this ratio VBO can be found in the following way:
where Eg1 and Eg2 are the band gaps of the two materials. Then VBO value can be set in the MQW layer table after enabling "override band offset” option, or it can be set by adjusting the work function for both CHARGE and MQW.
We use this approach in our laser examples: