We have used low energy cathodo- and photoluminescence spectroscopies to characterize the formation of charge states deep within the ZnSe band gap and localized near the ZnSe/GaAs interface. These deep levels are a common feature of the ZnSe/GaAs heterostructure and depend sensitively on growth conditions and post-growth annealing. Here we report in situ measurements of deep level formation near the ZnSe/GaAs interface during post-growth annealing and their dramatic dependence on the initial epitaxical growth conditions. We used 1-2 keV electron beam energies in tandem with HeCd and HeNe excitation to obtain emission spectra characteristic of the free ZnSe surface, the ZnSe bulk, and the ZnSe/GaAs(100) interface. Annealing in ultrahigh vacuum in the 300-500 degrees C range produces a new emission feature at 1.9-2.0 eV whose intensity increases exponentially with temperature, exhibiting an activation energy of 110+/-0.08 eV. This feature appears enhanced (suppressed) in structures fabricated in Se(Zn)-rich growth, and its intensity may therefore be related to the in vacancy concentration or the Ga indiffusion. These results emphasize the importance of local composition in the formation of interface deep level defects and their subsequent effects on luminescence efficiency and heterojunction stability.