N-rich growth by NH3-based molecular beam epitaxy was investigated for intermediate-temperature GaN and InGaN on c-plane GaN templates. The dependences of growth mode and surface morphology on group-V overpressure, In/Ga ratio, and temperature were explored with atomic force microscopy and high resolution x-ray diffraction. Extension to an "ultra-NH3-rich" regime of very high NH3-flows showed a decreased growth rate and increased In-content for InGaN alloys for constant group III source fluxes. Rapid modulation of NH3 overpressure, growth rate, and substrate temperature has enabled the growth of high quality, many-period InGaN/GaN superlattices, while suppressing morphological instabilities and subsequent stress relaxation. Published by Elsevier B.V.