Details

Autor(en) / Beteiligte

• Dong, Zi-Wen

Titel

High transition temperature superconducting thin film devices

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

1995

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• As a key element of superconductive electronics, the fabrication of high-T$\sb{c}$ Josephson junctions is an important issue. In this dissertation, a number of approaches were explored for the fabrication of high-T$\sb{c}$ superconducting ultra-thin films and multilayer thin films by using pulsed laser deposition and reactive molecular beam epitaxy with an ozone source. With these films, several types of high-T$\sb{c}$ Josephson junctions were fabricated. The step-edge grain boundary junctions had relatively low $I\sb{c}R\sb{n}$ products and poor reproducibility. The junctions' characteristics were largely dependent on the quality of steps that were etched in the substrate. The YBCO/STO/YBCO multilayer edge junctions with thick barriers ($>$4 nm) were RSJ-like with $I\sb{c}R\sb{n}$ products of 0.1 mV at 77 K and 1-6 mV at 4.2 K, while the junctions with thinner barriers ($<$2 nm) resulted in less reproducible characteristics. Since the minimum resistance measured (about 0.1 $\Omega$-$\rm\mu$m$\sp2$) was only an order of magnitude higher than the Sharvin resistance and the $J\sb{c}$ was extremely nonuniform, it appeared that these thin barriers were shorted by pinholes. One of the most promising types of high-T$\sb{c}$ junctions is the step-edge SNS junction. We have developed a technique to eliminate the excess conductance in YBCO/Au/YBCO junctions for obtaining a high $I\sb{c}R\sb{n}$ product, 0.2 mV at 77 K and 7.8 mV at 4.2 K. dc-SQUID had a flux noise of 3 $\times$ 10$\sp{-5} \Phi\sb{o}/\sqrt{Hz}$ at 77 K and a 1/f noise knee at about 1 Hz. Junctions with hysteretic I-V characteristics at temperatures up to 80 K could be fabricated. This may allow for the fabrication of relaxation oscillation SQUIDs operating at 77 K. Bicrystal junctions with a hole-overdoped Ca$\sb{x}Sm\sb{1-x}Ba\sb2Cu\sb3O\sb{y}$ superconducting electrode were fabricated using MBE and studied in correlation with different doping levels. Three terminal devices, suFETs utilizing an ultra-thin film channel and JoFETs with a bicrystal junction in the channel were systematically investigated as a function of the applied electric field on the gate. A shift of about 1 K in $T\sb{c}$ was observed in the suFETs for a hole depletion of 5 $\times$ 10$\sp{13}$ e/cm$\sp2$. By applying an electric field of 5 $\times$ 10$\sp5$ V/cm on the channel with the bicrystal junction, a surface carrier density of about 10$\sp{12}$ e/cm$\sp2$ could be induced, which resulted in an increase in the junction critical current of 23% at 20 K and 11% at 4.2 K. The electric field necessary to produce this effect with a junction in the channel was about 100 times smaller than the electric field necessary to observe similar shifts in $I\sb{c}$ in homogeneous ultra-thin films. Although the electrodes were hole-overdoped, the sign of the field effects indicated that the grain boundary junction existed in a hole-depleted region.