A major drawback in GaN technology is the lack of large-size, single-crystalline GaN substrates of high quality, i.e. less than the 10⁷ dislocations per cm^-2 at present, due to thermal-expansion and lattice-parameter mismatch between GaN-based film and foreign substrates like α-Al₂O₃ (sapphire) or SiC. High-pressure (>100 MPa), solvothermal techniques are well established and currently providing the largest crystals of α-SiO₂ and ZnO from supercritical (SC) aqueous solutions. In the case of GaN, SC ammonia (NH₃) is employed as solvent. We have produced single-crystalline GaN films of < 200 mm in thickness on 1 cm² large, HVPE-grown (0001) GaN substrates employing NH₄Cl as mineralizer for solubility enhancement. System pressures < 170 MPa were applied. The average growth speed ranging 5 to 30 mm/day over longer growth runs is observed for the film growth on the (000-1) face. The maximum growth speed was achieved for the film grown at a supersaturation from a combined Ga/GaN precursor. The surface morphology of the grown film is not affected by the nature of the precursor, i.e. Ga metal or GaN