We show that the production of adherent polycrystalline diamond films
upon Ge substrates by chemical vapour deposition is problematical for
three reasons: (a) the melting point of Ge is close to the optimum dia
mond deposition temperature which leads to partial melting of the Ge s
urface, retarding diamond nucleation, (b) there is no Ge carbide layer
formed at the interface to bond the diamond to the Ge chemically and
(c) there is a large thermal expansion mismatch between diamond and Ge
, resulting in cracking and flaking off of the films. Barrier layers o
f silicon dioxide, nitride or amorphous Si 1 mum thick prevent the Ge
surface melting, but do not alleviate the expansion mismatch problem.
The resulting three-layer sandwich crazes upon cooling, with the delam
ination occurring at the Ge-barrier interface.