  Epitaxial MgB2 thick films were grown on Al2O3 substrates at 600 oC by using
the hybrid physical chemical vapor deposition (HPCVD) technique. In order to
obtain a high magnesium vapor pressure around the substrates, we used a special
susceptor having a susceptor cap and achieved a very high growth rate of 0.17
um/min. Hexgonal-shaped columnar structures were observed by cross-sectional
and planar view transmission electron microscope (TEM) images. For the
1.7-um-thick film, the Tc was observed to be 40.5 K with a Jc of 1.5 x 10^6
A/cm^2 at 30 K. The vortex pinning mechanism by intercolumnar boundaries will
be discussed.
