We report here the improvements in the electrical characteristics of Au/SixNy/GaAs structures with NH3 plasma treatment of GaAs prior to PECVD of a SixNy dielectric film followed by annealing at 450 degreesC. These structures were characterized by secondary ion mass spectrometry (SIMS) depth profiling. The depth profiles of the three samples discussed in the present report show that the unpassivated sample has a broad interface consisting of both Ga-O and As-O species. After passivation, the interface becomes quite sharp, but the presence of both the oxides is still observed. However, after annealing, although the interface thickness increases marginally due to diffusion further into the substrate, it is observed that the interface becomes N-rich, which has also been supported by preliminary FTIR data. Looking at the electrical properties, it is seen that the unpassivated sample gives poor device characteristics which are attributed to the presence of wide oxide layers as shown by the SIMS data and consequently Fermi level pinning due to high interface state density, while the NH3 passivated samples show better device characteristics. This is attributed to the fact that the interface state density reduces considerably as the interface thickness reduces. Post-deposition annealing, showed marked improvement in device characteristics with decrease in frequency dispersion, conductance and hence interface state density as revealed from C-V and G-V measurements.