High temperature instabilities of ohmic contacts on p-GaN

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.