A process to deposit in situ a dual electron injector structure (DEIS) with 5-nm SiO2 between two Si-rich SiO2 (SRO) layers of ∼20 nm each has been developed. The excess silicon, as evaluated by Auger spectroscopy and Rutherford backscattering, was of the order of 15%-17%, in agreement with previously reported values under similar deposition conditions. Thin cross-sectioned samples of DEIS structures, both as-deposited and annealed at 1000 °C with Ar in an oxygen and water-moisture-free atmosphere, were examined by spatially resolved energy-loss spectroscopy (EELS) in a scanning transmission electron microscope. The analysis has shown that the excess silicon is present either as nanometer-sized silicon islands or as submicroscopic silicon oxides of varying stoichiometry resulting from intermediate oxidation states (i.e., Si+3, Si+2, and Si+1). Additionally, the thermal anneal at 1000 °C did not appear to have any effect on silicon island size of the SRO layer in contact with the silicon substrate. This suggests that the driving force of silicon clustering might not be diffusion limited, but could be related to the conditions under which the anneal was performed (in our case, in oxygen and water-moisture-free atmosphere). From the EELS analysis, the annealing procedure caused the loss of ≊30% of Si and O atoms from the top SRO layer, which could have contributed to the observed degradation of the electrical properties of the annealed DEIS structure. Ramped current tests performed by injecting electrons either from the gate or from the substrate, demonstrate extremely high breakdown voltages (VBD). The thermal anneal at 1000 °C, performed under a very low oxygen and water partial pressure, lowered the SRO film conductivity. Also, for either the annealed or as-deposited structures, a reduced injection asymmetry has been observed.