Growth mechanism of superconducting MgB_2 films prepared by various methods

The growth mechanisms of MgB_2 films obtained by different methods on various 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 ˜38.8 K and those from in situ anneal with T _c0 ˜24 K. It is clearly observed that the films obtained by the high-temperature reaction of e-beam evaporated B with Mg vapor are formed by the nucleation of independent MgB_2 grains at the film surface, indicating that this approach may not be suitable to obtain epitaxial films. A significant oxygen contamination was also present in films obtained from pulsed-laser-deposition-grown precursors, which drag the T _c0 down to 24 K.