A method of thinning a bulk aluminum nitride substrate includes providing a bulk aluminum nitride (AlN) substrate with at least one epitaxially grown group-III-nitride layer on a first side of the substrate, applying a slurry having a high pH to a second side of the substrate opposite the first side, chemical mechanically polishing the second side of the substrate using the slurry to remove at least a portion of the substrate, resulting in a thinned layer with a thickness less than 50 microns, and bonding the epitaxial layer to a non-native substrate. A device has at least one active zone in a layer of epitaxial Group-III-nitride material, the epitaxial Group-III-nitride layer having a defect density of less than or equal to 108/cm2.