Semiconducting Y–Ba–Cu–O thin films sputtered on MgO and SiOx/Si substrates: Morphological, electrical and optical properties for infrared sensing applications
Résumé
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 330 K, 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 850 nm wavelength. The response maximum at 40 kHz modulation frequency was followed by a slow decrease up to 3 MHz. The − 3 dB bandwidth in the 12 to 85 kHz range indicated a device, two to three orders of magnitude faster than the usual pyroelectric detectors.