  We have studied the effect of deposition rate and layer thickness on the
properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor
deposition on 4H-SiC substrates. The MgB2 film deposition rate depends linearly
on the concentration of B2H6 in the inlet gas mixture. We found that the
superconducting and normal-state properties of the MgB2 films are determined by
the film thickness, not by the deposition rate. When the film thickness was
increased, the transition temperature, Tc, increased and the residual
resistivity, rho0, decreased. Above about 300 nm, a Tc of 41.8 K, a rho0 of
0.28 mikroOhm.cm, and a residual resistance ratio RRR of over 30 were obtained.
These values represent the best MgB2 properties reported thus far.
