A photoemission study was performed on the chemisorption of samarium on three different kinds of silicon substrates, i.e., crystal Si(111), amorphous Si (a-Si), and hydrogenated amorphous Si (a-Si:H), to understand completely the chemical and physical phenomena underlying the interface formation. The core-level spectra clearly indicate the existence of three interface-formation regimes. Two chemically reacted shifts are observed and identified as disilicide phase and solid solution, respectively, and a critical coverage corresponding to the onset of strong chemical reaction is also shown. Moreover, the Si atoms show no sign of surface-segregation behavior, and a heterogeneous interface is indicated by the coexistence of two reacted species. Analogous growth behavior is shown on the three different substates, suggesting that morphological and surface structure differences in the reacted film play a second role in the interfacial development. However, the Schottky-barrier-lowering phenomenon is only observed in the crystal substrate;Combining the results of three complementary experiments (i.e., valence band, resonance, and work function studies), we conclude that the three interface-formation stages are the weakly-absorbed, chemically- and metallic-reacted phases, respectively, and the nucleation and growth processes occurring in Sm thin film formation are sublayer plus island (or Stranski-Krastanov) growth mode. Furthermore, only trivalent Sm is involved in the chemical reactions. ftn*DOS Report IS-T 1399. This work was performed under Contract No. W-7405-Eng-82 with the U.S. Department of Energy.