A reactor, system including reactors, and methods for depositing thin films on microfeature workpieces comprising a reaction vessel having a chamber, a gas distributor attached to the reaction vessel, a workpiece holder in the chamber, and a side unit in the reaction vessel at a location relative to the gas distributor and/or the workpiece holder. The gas distributor has a plurality of primary outlets open to the chamber, and the workpiece holder has a process site aligned with the primary outlets. The side unit has a secondary outlet open to the chamber that operates independently of the primary outlets. One of the inner compartment, the side unit and/or the workpiece holder can be movable between a first position to form a small-volume cell for introducing the reactant gases to the microfeature workpiece and a second position to form a large volume space for purging the reactant gases.