Ferromagnetic semiconductor GaMnN is very promising for developing spintronic devices.@footnote 1@ To investigate the mechanism of Mn incorporation in GaN, we have grown digital heterostructures by alternately depositing GaN (10 to 50 monolayers) and monolayer of either Mn or MnGa using ECR plasma assisted molecular beam epitaxy. The heterostructures are grown on 6H-SiC(0001) substrate with plasma power of 30 W at 550 °C. Adsorption and desorption of the Mn and MnGa on the non-growing surface, surface reactions and reconstruction are monitored by reflection high-energy electron diffraction (RHEED) and in situ scanning tunneling microscopy (STM). All the surfaces immediately following MBE are composed of spiral hillocks, with the edges of the spirals form bilayer steps. On the terraces, a gallium rich (1x1) structure is observed with a lattice spacing of 3.2 Å. At temperatures between 500 and 550 °C, deposition of approximately 1 ML of Mn on this surface results in a domain superstructure with a periodicity of ~ 32 Å. Within the domains the surface atoms are in (@sr@3x@sr@3) geometry. By closely monitoring the surface reconstruction present during MBE, heterostructures with high crystalline quality are grown, as confirmed by high-resolution x-ray diffraction. These results and their implications for Mn doping of GaN will be presented at the meeting. This research is supported by NSF DMR-0094105. @FootnoteText@@footnote 1@T. Dietl et al., Science 287, 1019 (2000).