Growth of epitaxial MgB₂ thick films with columnar grains by using hybrid physical-chemical vapor deposition

Epitaxial MgB₂ thick films were grown on c-cut Al ₂O₃ substrates at 600 °C by using a hybrid physical-chemical vapor deposition (HPCVD) technique. In order to obtain a high magnesium vapor pressure around the substrates at low temperatures, we used a specially designed susceptor having a susceptor cap and achieved a very high growth rate of 0.17 μm/min at low temperatures. The X-ray diffraction patterns showed epitaxial growth of MgB₂ thick films. Hexagonal-shaped columnar grains with an average diameter of 250 nm were observed by using cross-sectional and planar view of transmission electron microscopy (TEM) images. For the 1.7-μm-thick film, T_c was observed to be 40.5 K with a J_c of 1.5 × 10⁶ A/cm² at 30 K for a self field. As a plausible explanation for the high J_c, the vortex pinning mechanism caused by planar arrays of intercolumnar boundaries will be considered.