Growth mechanism of superconducting MgB2 films prepared by various methods

The growth mechanisms of MgB2 films obtained by different methods on variou s substrates are comparably studied by transport measurements and scanning electron microscopy observations. The analyzed films include those prepared by ex situ postanneal with T-c0 similar to 38.8 K and those from in situ a nneal with T-c0 similar to 24 K. It is clearly observed that the films obta ined by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB2 grains at the film surface , indicating that this approach may not be suitable to obtain epitaxial fil ms. A significant oxygen contamination was also present in films obtained f rom pulsed-laser-deposition-grown precursors, which drag the T-c0 down to 2 4 K.