We present data on interface characteristics ofSi3N4/Si/GaAsmetal–insulator–semiconductor (MIS) structures and correlate electrical properties with spectroscopic ellipsometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) observations. The interface ofSi3N4/Si/GaAsheterostructures was electrically characterized by a combination of capacitance–voltage and conductance methods. The nature of an insulator/GaAs interface and the microstructure ofSi3N4/Si/GaAsinterfaces after high temperature annealing were investigated by variable angle spectroscopic ellipsometry and high resolution TEM, respectively. The evolution of chemical species inSi3N4/Si/GaAsheterostructures was examined usingin situangle-resolved XPS. The interface trap density(Dit)of theSi3N4/SiMIS capacitor was in the2×1010 eV−1 cm−2range near the Si midgap after rapid thermal annealing at 550 °C inN2.However, this density increased to high1010 eV−1 cm−2with annealing at 800 °C. The interface characteristics ofSi3N4/Si/GaAsstructures withDitin the7×1010 eV−1 cm−2range also degraded after annealing at 750 °C inN2withDitincreasing to5×1011 eV−1 cm−2near the GaAs midgap. The spectroscopic ellipsometry results together with high resolution TEM observations appear to suggest that the degradation is due in part to the interface changing from crystalline to amorphous through chemical reaction. XPS measurements revealed that the as-deposited Si interlayer is nitridated during the initial stages of silicon nitride deposition, thus the thinned Si cannot prevent the outdiffusion of Ga and As species. We circumvented thermally induced interface degradation ofSi3N4/Si/GaAsstructures by employing a novelex situ/in situgrowth approach.