GaN thin films were deposited by reactive molecular beam epitaxy method with a modified ASTeX(R) compact electron cyclotron resonance (ECR) plasma source of nitrogen. The effect of different ECR exit apertures on the growth rate and photoluminescence properties of GaN films was investigated. An aperture with a matrix of 1.6 mm holes allowed for GaN films with good optical properties at growth rates up to 1 mu m/h. Apertures of larger diameter resulted in semi-insulating films. Room temperature photoluminescence for both undoped and Si-doped GaN films with full width at half maximum less than 100 meV has been obtained at the highest growth rates. We suggest that such an aperture effectively blocks the energetic ions from reaching the substrate thus reducing ion damage, resputtering, and increasing the Ga surface population. It also creates a higher pressure inside the ECR resonator, and therefore, provides a higher flux of active species. While further optimization will result in better GaN film quality, our results suggest that the growth rate in Ga-rich condition is limited by the microwave power available in the ASTeX compact ECR source.