Assured epitaxial CoSi2 phase formation on (001) Si-on-insulator substrates using CoSi/Ti bimetallic source materials
We report an assured way of forming epitaxial CoSi2 films on (001) SOI Si substrates when Co is inexhaustible. This is to use bimetallic CoSi/Ti source materials. For the more commonly used Co/Ti source material, epitaxial CoSi2 film forms before the Si layer is fully consumed. After the SOI Si has been fully consumed, however, pre-formed epitaxial CoSi2 reverted back to polycrystalline CoSi, leading to a dramatic sheet resistance increase. This phase reversal phenomenon arises because in using atomic Co as source material, the Gibbs free energy reduction in forming the CoSi and CoSi2 phases is not too different and consequently CoSi becomes the energetically favorable end phase when Co is still available after all Si atoms have been consumed. In using CoSi as the Co source material, the only energetically favorable reaction is to form CoSi2.