What about using nanomaterials as seed/substrate for crystal growth? Design the unit cell to the exact dimensions? Or use the (imperfect) GaN crystals to grow incrementally better crystals over multiple rounds?

I work on biomolecular crystallography and nucleation is half the battle! The other half, used to be size, but thanks to microbeam beam lines, at synchrotrons like APS, we can get away with very tiny crystals for X-Ray diffraction data.

I am fascinated by the idea of using semi/synthetic materials as seeding agents. But then again, biomolecular crystals have huge unit cells compared to semiconductors. People have tried zeolites in the past, but surprisingly a random speck of dust sometimes works better than the best designed substrate.