The effects of Au overlayer on the thermal stability of Pt ohmic contact on p-type GaN

Thermal stabilities of Pt and Pt/Au ohmic contacts on p-type GaN were studi ed through high temperature thermal stress test at 550 degreesC. The long-t erm thermal stability of the contact resistivity for the Pt/Au contact was better than that for the Pt contat oil p-type GaN. Considering the Free ene rgy change for the formation of nitrogen and gallium molecules, the outdiff usion of N atoms continuously proceeded, but that of Ga atoms was suppresse d during the thermal stress for the Pt contacts. Thus. N vacancies, act as donor for electrons, were produced below the contact leading to rapid incre ase of contact resistivity during the thermal stress. For the Pt/Au contact , Ga atoms outdiffused through grain boundaries of Pt and dissolved to the Au overlayer producing a number of Vi:, at subsurface of p-type GaN below t he contact, resulting in the increase of net hole concentration below the P t/Au contact. Thus: the contact resistivity was not increased further under long-time thermal stress.