• Methane concentration and C2 radical levels critically control the transition from faceted polyhedral to ballas diamond growth at low nucleation densities.
• At higher nucleation densities, gas pressure and substrate temperature become dominant factors, yielding diverse morphologies including planar, polycrystalline, octahedral, and step-flow growth.
• In situ optical emission spectroscopy provides a powerful tool for real-time monitoring and control of plasma species to tailor diamond growth modes.
• The study offers practical guidance for optimizing MPCVD parameters to achieve desired heteroepitaxial diamond film morphologies for electronic and optical applications.