  The effects of Mg deficiency in MgB2 films grown by using hybrid
physical-chemical vapor deposition were investigated after vacuum annealing at
various temperatures. High-quality MgB2 films grown on c-cut Al2O3 substrates
with different superconducting transition temperatures (Tc) of 40.2 and 41 K
were used in this study. As the annealing temperature was increased from 200 to
800 C, the Mg contents in the MgB2 films systemically decreased, but the Tc's
did not change, within 0.12 K, until the annealing temperature reached 700 C.
For MgB2 films annealed at 800 C for 30 min, however, no superconductivity was
observed, and the temperature dependence of the resistivity showed a
semiconducting behavior. We also found that the residual resistivity ratio
decreased with increasing annealing temperature.
