Not that I know of. They may have tinkered with it, and it could be a future application for RPCVD, but the current product range is grown on GaN wafers.
Technically, glass is a generic term applied to a wide range of materials, referring to the amorphous state of a normally crystalline material. We think of a glass material as being transparent, but it doesn't have to be. The use of amorphous silica (SiOx) glass has been of interest in the electronics industry because it would be a far lower cost substrate than traditional silicon wafers, which have to be sliced from a grown Si crystal and then polished to prepare the surface for epitaxy. However, there is a big difference between the two. Si is a semiconductor material whereas amorphous SiOx is and insulator. So the whole design approach of the chip has to be different. I imagine glass substrate would be useful in certain applications, but not in others.
I'm not sure if there is such a thing as amorphous GaN, but otherwise there could be another glass material that is compatible with GaN. Thermal properties (thermal conductivity and thermal expansion coefficient) are perticularly important.
The lower operating temperature of RPCVD, enables certain types and combinations of epitaxy that are otherwise impossible - or at least very difficult - with other methods. Epitaxial deposition on glass substrate, could fall into this category.
Perhaps @DRCDG you may have more insights on this subject. Cheers.
All IMO.
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