Thermal stability of Ti/Al/Pt/Au and Ti/Au Ohmic contacts on n-type ZnCdO

The specific contact resistivity and chemical intermixing of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn0.05Cd0.95O layers grown on ZnO buffer layers on GaN/sapphire templates is reported as a function of annealing temperature in the range 200–600 8C. A minimum contact resistivity of 2.3 10 4 V cm2 was obtained at 500 8C for Ti/Al/Pt/Au and 1.6 10 4 V cm2 was obtained at 450 8C for Ti/Al. These values also correspond to the minima in transfer resistance for the contacts. The Ti/Al/Pt/Au contacts show far smoother morphologies after annealing even at 600 8C, whereas the Ti/Au contacts show a reacted appearance after 350 8C anneals. In the former case, Pt and Al outdiffusion is significant at 450 8C, whereas in the latter case the onset of Ti and Zn outdiffusion is evident at the same temperature. The improvement in contact resistance with annealing is suggested to occur through formation of TiOx phases that induce oxygen vacancies in the ZnCdO