Specific contact resistance of Ti/Al/PVAu ohmic contacts to photphorus-doped ZnO thin films

The carrier concentration dependence of Ti/Al/Pt/Au ohmic contact resistance on P-doped n-type ZnO thin films is reported. Ti (200 Angstrom)/Al (800 Angstrom)/Pt (400 Angstrom)/Au (800 Angstrom) was deposited by electron-beam evaporation on ZnO thin films grown by pulsed laser deposition on (0001) sapphire substrates using a ZnO:P-0.02 source. Postgrowth annealing from 30 to 600 degreesC resulted in carrier concentrations of 7.5 X 10(15) cm(-3) 1.5 x 10(20) cm(-3) in the ZnO. After metal deposition, the specific contact resistances were measured at temperatures in the range 30-100degreesC prior to alloying annealing at 200degreesC and at 30-200degreesC after this anneal. The lowest specific contact resistance of 8.7 X 10(-7) Omega cm(2) for nonalloyed ohmic contacts was achieved in the sample with carrier concentration of 1.5 X 10(20)cm(-3) when measured at 30degreesC. In the annealed samples, minimum specific contact resistances of 3.9 x 10(-7) Omega cm(2) and 2.2 X 10(-8) Omega cm(2) were obtained in samples with carrier concentrations of 6.0 X 10(19) cm(-3) measured at 30degreesC and 2.4 X 10(18) cm(-3) measured at 200degreesC, respectively. Auger electron spectroscopy detected Ti-O interfacial reaction and intermixing between Al and Pt at 200degreesC. (C) 2004 American Vacuum Society.