This paper describes an investigation of the instabilities
of ohmic contacts on p-type GaN submitted to long
term high temperature stress. Transfer Length Method
(TLM) was used to separately analyze the contribution
of contact resistivity and sheet resistance on contact degradation.
Before treatment, contacts showed linear behaviour,
indicating good ohmic properties. On the other
hand, thermal ageing at 250 ◦ C induced the degradation
of electrical characteristics: in particular, (i) an increase
of the contact resistivity was detected after stress, as
well as (ii) a rectifying behaviour in current-voltage (IV)
characteristics of contact appeared. The degradation
process has been ascribed to the diffusion of an atomics
species from PECVD-deposited passivation layer to
p-type ohmic contact: the interaction of this species with
acceptor dopant can lower acceptor concentration, thus
broadening the Schottky barrier at the interface and inducing
the observed non-linearity of the contacts. The
square root time dependence of degradation of the electrical
characteristics agreed with the hypothesis of diffusion.
After 160 hours of aging, the passivation layer
was removed and the structures were submitted to a new
thermal treatment. This further treatment was found to
induce the recovery of the electrical characteristics of the
contacts, due to the fact that passivation acts like (i) a
source of atomic species responsible of diffusion and/or
(ii) a blocking layer for these species: the removal of
passivation stopped the diffusion process and make outgassing
process possible