Semiconducting Y–Ba–Cu–O thin films sputtered on MgO and SiOx/Si substrates: Morphological, electrical and optical properties for infrared sensing applications

Abstract

Semiconducting YBa2Cu3O6+x (x<0.5, YBCO) films were deposited by direct current (DC) sputtering at temperatures ranging from Tsub=150 to 610°C; the substrates were either MgO (001) or SiOx/Si (001). All films exhibited a granular microstructure. Films deposited at Tsub≤500°C were purely amorphous, but films deposited at Tsub≥550°C also exhibited the presence of crystallized tetragonal phase. DC conductivity of purely amorphous films followed a variable range hopping (VRH) behavior from 180 to 330K, in line with hopping between localized hole states around the Fermi level. DC conductivity of the partially crystallized films followed first a thermally activated behavior at high temperature, in line with energy transitions to extended states, then a VRH behavior at lower temperature. For purely amorphous films deposited at Tsub=150°C on SiOx/Si, ellipsometry measurements showed refractive index value n≈2.25, suggesting a low oxygen content (x≈0.1), as confirmed by the optical conductivity vs. wavelength behavior. An infrared sensing demonstrator (metal/YBCO, Tsub=150°C, on SiOx/Si) exhibited a high-pass pyroelectric response at 850nm wavelength. The response maximum at 40kHz modulation frequency was followed by a slow decrease up to 3MHz. The −3dB bandwidth in the 12 to 85kHz range indicated a device, two to three orders of magnitude faster than the usual pyroelectric detectors.