Optimization of surface morphology and electrical parameters of Tl-Ba-Ca-Cu-O thin films for high frequency devices

We prepared double-sided Tl₂Ba₂CaCu₂O₈ (Tl-2212) thin films on large area sapphire substrates (up to 3 inch) for high frequency filters, which are intended to be used in future communication systems. For high frequency operation one demands film thicknesses three or four times the penetration depth λ, but the thin film preparation process has several limits for the film thickness. Beside the crack formation on sapphire substrates due to different thermal expansion coefficients the surface morphology with parameters as roughness or the formation of precipitates depend on the film thickness. We show results of an optimization process to achieve the best suitability of the films for high frequency devices. The investigations were done in terms of SEM and AFM surface inspections as well as the measurement of electrical film parameters as critical temperature T_c, critical current density J_c, or surface resistance R_s.