We present data on interface characteristics of Si3N4/Si/GaAs metal-insulator-semiconductor (MIS) structures and correlate electrical properties with spectroscopic ellipsometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) observations. The interface of Si3N4/Si/GaAs heterostructures was electrically characterized by a combination of capacitance-voltage and conductance methods. The nature of an insulator/GaAs interface and the microstructure of Si3N4/Si/GaAs interfaces after high temperature annealing were investigated by variable angle spectroscopic ellipsometry and high resolution TEM, respectively. The evolution of chemical species in Si3N4/Si/GaAs heterostructures was examined using in situ angle-resolved XPS. The interface trap density (D-it) of the Si3N4/Si MIS capacitor was in the 2 x 10(10) eV(-1) cm(-2) range near the Si midgap after rapid thermal annealing at 550 degrees C in N-2 However, this density increased to high 10(10) eV(-1) cm-2 with annealing at 800 degrees C. The interface characteristics of Si3N4/Si/GaAs structures with D-it in the 7x10(10) eV(-1) cm(-2) range also degraded after annealing at 750 degrees C in N-2 with D-it increasing to 5 X 10(11) eV(-1) cm(-2) near 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 of Si3N4/Si/GaAs structures by employing a novel ex situ/in situ growth approach.