A vapor deposition system is described. The vapor deposition system includes a reaction chamber and a reactant delivery subsystem coupled with the reaction chamber. The reaction chamber is configured to retain a substrate therein. The reactant delivery subsystem includes a inlets, a pre-reaction region, and outlets. The inlets receive precursors and chalcogen precursor(s). The pre-reaction region is configured to receive the precursors from a portion of the inlets and to react at least a portion of the precursors to form modified precursor(s). The modified precursor(s) are more thermally stable than metal-containing precursor(s) of the precursors used to form the modified precursor(s). The outlets are coupled with the reaction chamber and the pre-reaction region. The outlets separately provide the modified precursor(s) and the chalcogen precursor(s) to the reaction chamber. The modified precursor(s) and the chalcogen precursor(s) react and form a chalcogen film on the substrate in the reaction chamber.