Photoresponse of Patterned YBa2Cu3O7-x Thin Films
The first object of this work was to develop YBa2Cu3O7-x (YBCO) thin films. Initially, Y, BaF2 and Cu were coevaporated with the substrate temperature (TS) 400 °C and the oxygen pressure (pO2) 5 x 10-5 mbar on SrTiO3 substrates with a subsequent high temperature annealing at 890 °C for 15 min in flowing wet oxygen. These films had critical temperatures (Tc) above 90 K and critical current densities (jc) around 5 x 105 A/cm2. The critical current density increased for thinner YBCO films. This corresponded to a decrease in the density of a-axis oriented rods on the c-axis oriented bottom layer. This was also confirmed by x-ray diffraction. Laser deposition was the preferred choice of growth method during the latter application period. Films with a thickness of ~50 nm were grown on LaAlO3, NdGaO3, MgO and SrTiO3 at TS between 760 °C and 830 °C and pO2 at 0.4 mbar or 0.8 mbar. These films had Tc around 90 K, superconducting transition widths of 1-2 K (0.4 K for LaAlO3) and jc above 1 x 106 A/cm2 at 77 K.
The second task of this work was to develop a processing method suitable for linewidths down to 1 µm in YBCO films of ~50 nm thickness resulting in reproducible samples with long lifetimes and low contact resistances (.rho.c). A new patterning method was developed where the pattern was made by ion milling through an yttria-stabilized zirconia (YSZ) lift-off mask defined by electron beam lithography. The YBCO film was thus protected by a YSZ layer during processing and as a completed sample. The passivation layer provided a long lifetime of the samples. In situ laser deposited Au contacts resulted in low .rho.c <= 1 x 10-4 .OMEGA.cm2. Both wet etching and ion milling through masks defined by photolithography were also used for patterning of coevaporated and laser deposited films.
Thirdly, the photoresponse of the patterned YBCO thin films was investigated. The voltage transient of a current biased sample was studied. For films thicker than the optical penetration depth (> 100 nm), short AlGaAs laser pulses (~50 ps full width at half maximum, FWHM) with .lambda. = 0.8 µm and a laser fluence of 200 nJ/cm2 resulted in photoresponse in the nanosecond regime, characteristic to the bolometric response. For the 50 nm thick patterned laser deposited films, 17 ps (FWHM) laser pulses with .lambda. = 0.8 µm resulted in response times down to 30 ps (FWHM). High frequency resolution was obtained in amplitude modulated laser measurements at .lambda. = 0.8 µm and .lambda. = 10.6 µm (CO2 laser). The photoresponse was observed in the current bias induced resistive state and the normal state. The fast response was followed by a slower bolometric response characterized by the phonon escape from the YBCO film to the substrate and heat diffusion in the substrate. The phonon escape time was estimated to 2-4 ns from both the pulsed laser and amplitude modulated laser measurements. The heat diffusion in the substrate followed a f-n relation. The low laser fluence minimized kinetic inductance contributions to the response. An electron heating model can be used to explain the results. The electron-phonon scattering time was calculated to 1.8 ps. The responsivity at 1 GHz was 1.2 V/W and the noise equivalent power was estimated to 1.5 x 10 -9 W/(Hz1/2).