  Superconducting thin films have been prepared on Si-substrates, using pulsed
laser deposition from a target composed of a mixture of Mg and MgB2 powders.
The films were deposited at room temperature and post-annealed at 600 degrees
C. The zero resistance transition temperatures were 12 K, with an onset
transition temperature of 27 K. Special care has been taken to avoid oxidation
of Mg in the laser plasma and deposited film, by optimizing the background
pressure of Ar gas in the deposition chamber. For this the optical emission in
the visible range from the plasma has been used as indicator. Preventing Mg
from oxidation was found to be essential to obtain superconducting films.
