You can try to increase the number of iteration, and local mesh constraints. Do not use so small the edge length for the whol region. If you have thin layers, please make sure the thickness is mandatory otherwise you can use a larger thickness. Ait holes might be the main issue: air as insulator does not involve the simulation since no charge there. You can diagnose the issues by first disabling all the holes and see if the Ge layer is working. Then gradually increase the number of holes.