  Thin films of hydrogenated amorphous carbon (a-C:H) deposited by radio
frequency plasma-enhanced chemical vapour deposition (rf-PECVD) have been
studied for various applications. An interesting property of these films is
their high breakdown strength (107 Vcm-1). This property of a-C:H can be
exploited in high breakdown heterostructure diodes or as passivation layers and
insulator layers in MIS devices. Reports on the applications of a-C:H/Si diodes
exist in the literature. Diodes in which the a-C:H films have been deposited by
rf-PECVD, have been reported only once. In this article the diodes produced
reportedly failed to exhibit reproducible I-V characteristics under high
voltage stress. We have investigated the process dependence of structural and
electrical properties of rf-PECVD a-C:H films deposited at room temperature
from a CH4/Ar gas mixture (at a pressure of 100 mTorr) using a capacitively
coupled rf-PECVD. We observe a clear correlation between the dc-self bias and
the rectification ratio of a-C:H/Si heterojunction diodes. Optimised diodes
show rectification ratios upto 104 and a stable reverse breakdown voltage,
typically around 850 V. I-V and C-V measurements show no evidence of hystersis.
Scanning Electron Microscopy was carried out to determine the quality of the
films deposited. Micro-Raman analysis was used to estimate the ID/IG ratio in
the films deposited under different dc-self bias.
