Abstract: Gas phase nucleation conditions are controlled and/or mitigated during material deposition in semiconductor manufacturing processes. According to an example embodiment of the present invention, reaction by-product gases are monitored (e.g., 140, 160) and used to detect reactant gas conditions that promote gas phase nucleation. In some applications, an optical detection approach (e.g., 140, 142) is used to detect the presence of the reaction by-product gases, and relative amounts of the gases are used as an indicator of a ratio of reactant gases (e.g., 310, 340); the supply of reactant gases and/or other deposition conditions are correspondingly controlled (e.g., 130-138, via 160).