Nmj. Conway et al., ELECTRONIC-PROPERTIES AND DOPING OF HYDROGENATED TETRAHEDRAL AMORPHOUS-CARBON FILMS, DIAMOND AND RELATED MATERIALS, 7(2-5), 1998, pp. 477-481
Hydrogenated tetrahedral amorphous carbon (ta-C:H) films were deposite
d over a range of ion energies using a plasma beam source. The optimum
ion energy for maximum sp(3) content was found to be 92 eV per carbon
ion, and films were grown at this energy using both acetylene and met
hane as the source gas. The conductivity of the acetylene films was fo
und to be 3.9 x 10(-8) cm(-1). Upon addition of nitrogen, the conducti
vity increased, while the optical band gap remained constant up to a n
itrogen partial pressure of 10(-5) mbar, which suggested n-type doping
. At higher partial pressures, the band gap rapidly decreased. ta-C:H/
Si heterojunction measurements using both p and n-type silicon showed
that the films were n-type in character. However. there was found to b
e a large amount of nitrogen present in the acetylene gas, so a truly
undoped film could not be produced. Thus, further films were made usin
g high-purity methane as the source gas. The undoped films were then f
ound to be p-type and could be doped n-type using nitrogen. Intrinsic
behaviour was observed at a nitrogen partial pressure of 4 x 10(-6) mb
ar. (C) 1998 Elsevier Science S.A.