Various reactors for growing thin films on a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants are disclosed. In one embodiment, the reactor comprises a reaction chamber. A showerhead plate divides the reaction chamber into upper and lower parts. A first precursor is directed towards the lower half of the reaction chamber and a second precursor is directed towards the upper half of the reaction chamber. The substrate is disposed within the lower half of the reaction chamber. The showerhead plate includes plurality passages such that the upper half is in communication with the lower half of the reaction chamber. In another arrangement, the upper half of the reaction chamber defines a plasma cavity in which in-situ radicals are formed. In yet another arrangement, the reaction chamber includes a shutter plate, which is configured to selectively open and close the passages in the showerhead plate. In other arrangements, the showerhead plate is arranged to modify the local flow patterns of the gases flowing through the reaction chamber.