Thin nanocrystalline hcp Co-based films are grown by chemical vapor deposition on SiO2. Tetrakis (trimethylphosphine)cobalt(0), ((CH3)(3)P)(4)Co, is a single-source precursor that forms Co films that incorporate both P and C when the substrate temperature ranges from 225 to 325 degrees C, and feature crystallites sized < 15 nm. The P within the deposited film is mostly elemental with some being phosphidic, and the C exists mostly in the carbidic form along with some being graphitic. The Co is correspondingly predominantly metallic. Upon annealing to 400 degrees C for 3 h, some extent of both Co2P crystallite precipitation and minimal allotropic transformation to fcc Co is observed, and the amount of carbidic C is reduced relative to graphitic C, but the nanocrystalline microstructure of the film is preserved, still primarily composed of hcp Co-based crystallites. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3264480]