Improved microwave properties of the YBa2Cu3O7-x superconducting films on large-area sapphire substrates

YBa
₂
Cu
₃
O
_7-x
(YBCO) thin films have been prepared on 2 inch in diam. (1102) sapphire substrates buffered with CeO
₂
of mixed (001)/(111) orientation. The thickness of the YBCO films was typically about 250 nm. The YBCO thin films exhibited very smooth surface (peak-to peak roughness of less than 20 nm) free of cracks and outgrowths. The critical temperatures of these films were 88 - 89 K and the critical curent densities were (2-3) 10
⁶
A/cm
²
at 77 K and zero field. The microwave surface resistance Rs of the YBCO films was measured at 18.7 GHz. Values of R
_s
- 1.4 mΩ were obtained at 77 K and R
_s
<70 μΩ below 20 K. Such low R
_s
values are comparable to the lowest reported values for thicker YBCO films grown epitaxially on strucually closely matched substrates. Such properties, including low Rs values, were obtained for the 4-5 months old YBCO films as well, that allows considering these films as stable with time. The possibility of obtaining films of similar high quality on large area sapphire substrates seems to be very promissing for the development of various microwave cryoelectronic devices. The fact that epitaxial high temperature superconducting YBCO thin films may exhibit microwave surface resistances far below that of copper makes these films to be very promissing for applications in the field of microwave electronics [1,2].