Specific contact resistance of Ti/Al/Pt/Au ohmic contacts to phosphorus-doped ZnO thin films

Abstract

The carrier concentration dependence of Ti/Al/Pt/Au ohmic contact resistance on P-doped n-type ZnO thin films is reported. Ti (200 Å)/Al (800 Å)/Pt (400 Å)/Au (800 Å) was deposited by electron-beam evaporation on ZnO thin films grown by pulsed laser deposition on (0001) sapphire substrates using a ZnO:P0.02 source. Postgrowth annealing from 30 to 600 °C resulted in carrier concentrations of 7.5×1015 cm−3–1.5×1020 cm−3 in the ZnO. After metal deposition, the specific contact resistances were measured at temperatures in the range 30–100 °C prior to alloying annealing at 200 °C and at 30–200 °C after this anneal. The lowest specific contact resistance of 8.7×10−7 Ω cm2 for nonalloyed ohmic contacts was achieved in the sample with carrier concentration of 1.5×1020 cm−3 when measured at 30 °C. In the annealed samples, minimum specific contact resistances of 3.9×10−7 Ω cm2 and 2.2×10−8 Ω cm2 were obtained in samples with carrier concentrations of 6.0×1019 cm−3 measured at 30 °C and 2.4×1018 cm−3 measured at 200 °C, respectively. Auger electron spectroscopy detected Ti–O interfacial reaction and intermixing between Al and Pt at 200 °C.