We have investigated arsenic doping of GaN films grown at high temperature (similar to 800 degreesC) by molecular beam epitaxy. Growth in an arsenic environment changes the growth mode and promotes the growth of the cubic polytype. Growth under arsenic also produces films which show strong blue emission at room temperature, whose intensity is dependent on both the As Ru,v and the Ga:N ratio. We propose a growth model based on As incorporation onto the Ga sublattice, which can explain all of our observed data