Al/Au and Cu/Au bilayer-metal contacts to YBa₂Cu ₃O_7-x thin films

Electrical contact resistivities of both Al and Cu single layer contacts as well as Cu/Au and Al/Au bilayer metal contacts to YBa₂Cu ₃O_7-x (YBCO) thin film have been studied. It was found that aluminum and copper make poor electrical contacts to YBCO due to interface reactions. These contacts have large contact resistivity (10^-1-1 Ω cm²), orders of magnitude higher than that of a gold/YBCO contact (10^-6 Ω cm²). When an ultrathin Au interlayer (10-30 Å) was inserted between an Al or Cu overlayer and a YBCO film, interface reaction was greatly reduced. The contact resistivities of Al/Au and Cu/Au bilayer-metal contacts dropped by five orders of magnitude when Au interlayer thickness was increased from 0 to 10 Å. With the gold interlayer thickness of 15 Å, the Al/Au and Cu/Au bilayer-metal contacts reached a minimum contact resistivity, approaching that of a thick single-layer Au metal contact.