High-Quality C-Axis Oriented HTSC Films for Microwave Devices
Final rept. Nov 97-Jan 98
SWISS FEDERAL INST OF TECHNOLOGY LAUSANNE (SWITZERLAND)
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C-axis oriented HTSC films were grown by LPE on various substrates. The conditions for improving the surface flatness were systematically studied. For the achievement of interstep distances yo approx. 10 micrometers between monosteps of 12 Angstroms, as required for Tunnel devices, following requirements were estimated undercooling delta-T 0.17K, misfit 0.08, substrate misorientation 0.02 deg. The defect structure and morphology of LPE films was investigated. For YBCO on 110 NGO, at growth temperature, the epitaxial strain induced by misfit can be released by misfit dislocations MD already in the first few monolayers. During coolingoxidation, the formation of 110 twins partially relax the strain. The remaining effective strain is accommodated by crack formation. The orientational relationship between twinningcracking related to the misfit was studied and critical thicknesses for the formation of MD and of cracks were estimated. High-quality LPE films could not be obtained on LSAT substrates, whereas same growth conditions yielded good-quality LPE films on NGO. This might be due to the striations present in LSAT which may influence nucleation and growth behavior. Solid-solutions of substrates and of HTSC allow to reduce misfit problems during growth but still the coolingoxidation problem and lead to twinning and cracking.
- Electricity and Magnetism