Dynamic study and experimental “two-step process” of substrate step preparation for high-Tc Josephson junctions

Abstract

We report theoretical and experimental studies of the dynamics of substrate step preparation for high-Tc Josephson junctions. A maximum step edge angle of 70.8° has been calculated for SrTiO3 (STO) substrates with a Nb mask. This calculated angle agrees well with our experimental result of 66°. Step-edge angles can be predicted for different purposes using this method. We also utilized a “two-step process” to improve the surface morphology of the stepped substrate, and step-edge Josephson junctions were fabricated with good uniformity.

Different relaxation mechanisms of epitaxial strain in YBa2Cu3O7-δfilms deposited on SrTiO3and LaAlO3

“…For very thin SEYFs on STO and LAO (less than 20 nm), the surface corrugations or composition modulations will lead to modi®cation of the structures and surface morphologies as well as a variation in superconducting transition temperatures. As we recently reported, the SEYF on LAO has a higher T c than that of the SEYF on STO (Zhai and Chu 2000a). For the SRYF prepared on LAO, the grain sizes are small (usually the size of 908 domains is much smaller than that of c-axis grains).…”

“…The superconducting temperature T c is doubled in 10±15 nm La 1:9 Sr 0:1 CuO 4 thin ®lms on SrLaAlO 4 through in-layer epitaxial strain (Locqut et al 1998, Chen et al 2000. Recently, we have reported an interfacial strain in YBa 2 Cu 3 O 7¡¯( YBCO) thin ®lms on SrTiO 3 (STO) and LaAlO 3 (LAO) down to two to three unit cells and found LAO is the preferred substrate that favours higher T c when the ®lm becomes thinner (Zhai and Chu 2000a). However, because of the accumulation of extra energy, strained epitaxial YBa 2 Cu 3 O 7¡¯® lms (SEYFs) must release their excess energy one way or another when the ®lm becomes thicker; as a result, a SEYF becomes a strain-relaxed YBa 2 Cu 3 O 7¡¯® lm (SRYF).…”

“…Experimental details c-axis oriented YBCO ®lms were grown on STO (001) and LAO (001) using an inverted cylindrical dc sputtering technique as reported recently (Zhai and Chu 2000a). An optimized low-pressure oxygen anneal at a high temperature was also carried out to improve the crystalline quality (Zhai and Chu 2000b).…”

“…Experimental details c-axis oriented YBCO ®lms were grown on STO (001) and LAO (001) using an inverted cylindrical dc sputtering technique as reported recently (Zhai and Chu 2000a). An optimized low-pressure oxygen anneal at a high temperature was also carried out to improve the crystalline quality (Zhai and Chu 2000b). The critical layer thickness h c reported for SEYFs on diOE erent substrates is quite diverse, ranging from 24 A Ê on MgO to several hundred aÊ ngstroÈ ms on other substrates (Kamigaki et al 1991, Huttner et al 1995, Chu et al 1999, Wordenweber 1999.…”

Optimization of step-edge substrates for high-TC superconducting devices

“…In addition, the long-term stability of the bi-crystal Josephson junction is susceptible to the thermal shock in cooling the device from room temperature. The above problems encountered by the bi-crystal junction can be solved with the step-edge Josephson junction [5][6][7][8][9][10][11][12].…”