Features of growth of non-cubic bicrystals from the melt (example of leucosapphire and neodymium gallate)

摘要

Neodymium gallate (NdGaO_3) and sapphire (Al_2O_3) are most suitable materials to produce bicrystal substrates for high-temperature superconducting (HTSC) films' microwaves Josephson junctions. Perfect bicrystal border is important for the devices, and it can be obtained in bicrystals grown directly from the melt. Unfortunately both materials have non-cubic symmetry and anisotropic thermophysical properties what complicates the bicrystal growth process. The experimental results of NdGaO_3 bicrystals growth by Czochralski method using compound bicrystal seeds with different symmetries in the plane perpendicular to the growth axis are presented. The results proved to be very unstable and depended critically upon growth conditions and on bicrystal border orientation. The bicrystals with misorientation angle 180° which correspond to the natural (110) twins are grown. When bicrystal angles are within 22-28 ° range which is optimum for bicrystal Josephson junctions it is very difficult to grow the long bicrystals: the bicrystal border goes out of the crystal quickly. It is supposed that in the last case the symmetry of compound bicrystal seed in the plane perpendicular to the growth direction can differ from the heat field symmetry in the growth system. Requirements for growth of anisotropic crystals by Czochralski method are formulated. It is shown that the requirements of seed and heat field symmetry correspondence can be satisfied more easily in Stepanov's growth method (EFG). The sapphire bicrystal ribbons suitable for manufacturing of high quality substrates for HTSC film Josephson junctions using Stepanov's method are grown.