Si3N4 has been produced on GaAs with low interface trap densities by electron cyclotron resonance N-2-He plasma assisted nitridation of a Si layer deposited on a GaAs (100) substrate. Nitridation at 150 and 400 degrees C was monitored by x-ray photoelectron spectroscopy (XPS) and produced stoichiometric Si3N4 The nitride layer thickness, as determined from XPS as a function of photoelectron takeoff angle, initially increased rapidly with nitridation time with a transition at a thickness of 12-18 Angstrom to slower growth. Capacitance/voltage and conductance/angular frequency measurements were performed on metal-insulator-semiconductor capacitors fabricated from the nitrided samples. The results demonstrated interface trap densities with a minimum of 3.0 x 10(11) eV(-1) cm(-2) when nitrided at 150 degrees C. At 400 degrees C the nitridation produced a poor quality interface, which resulted either from the higher temperature or from nitridation of all of the Si, leaving the Si3N4 in direct contact with the GaAs.