Hi,
I briefly discussed this with my colleague. Since CHARGE was designed to solve Possion and Drift-Diffusion equation in semiconductors, it can be more challenging to solve them in insulators (although as semiconductors). The electric fields will be returned everywhere in the material, but CHARGE needs at least one active semiconductor region to be able to work. So, I will recommend you to keep the SiO2 cladding as an insulator and just make the waveguide a semiconductor. Let me know if that helps.
The error message is also generic, so it can be due to several reasons. But let's try the first step and see if there is any improvement.
Regards,
Amrita
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