Optimization of fabrication conditions for multilayer structures with La-doped YBCO groundplane

We have examined and optimized the fabrication conditions for multilayer structures needed in high-T_c single flux quantum circuits. La_0.2Y_0.9Ba_1.9Cu₃O_x (La-YBCO) was chosen as a material for both the groundplane (GP) and base-electrode. The GP-La-YBCO was deposited at 740°C by a sputtering method and the optimized 300 nm thick film exhibited a typical average roughness (Ra) of 2 nm. (La,Sr)₂AlTaO₆ (LSAT) isolation layer sputter-deposited under the same conditions as the GP-La-YBCO showed good crystallinity and a relatively flat surface (Ra∼2 nm). For the deposition of La-YBCO base electrode, a slightly lower substrate temperature (720°C) resulted in a smooth film surface (Ra∼2 nm), while higher substrate temperatures led to appearance of precipitates and hollows on the La-YBCO and columnar morphology of the LSAT layer which was revealed by cross-sectional SEM observation. Interface-engineered ramp-edge junctions fabricated on LSAT/GP-La-YBCO multilayer structures exhibited RCSJ-like I-V characteristics with typical I_cR_n products of 2 mV at 4.2 K. Excellent isolation between the GP and the base electrode better than 10⁹ Ω/cm² was also confirmed.