Reactors having gas distributors for depositing materials onto micro-device workpieces, systems that include such reactors, and methods for depositing materials onto micro-device workpieces are disclosed herein. In one embodiment, a reactor for depositing materials onto a micro-device workpiece includes a reaction chamber, a passageway, and a door assembly. The reaction chamber includes a gas distributor configured to provide a flow of gas(es) to a micro-device workpiece on a workpiece holder. The passageway, which has a first end open to the reaction chamber and a second end apart from the reaction chamber, is configured to provide ingression to and egression from the chamber for processing the micro-device workpiece. The door assembly is configured to open and sealably close a door at the second end of the passageway. A gas conditioning system positioned in the door is configured to maintain a desired concentration and phase of gas constituents in the passageway.