Single-wavelength in situ ellipsometry shows that the SiNx:H/GaAs(110) interface formed during the deposition of silicon nitride using an electron-cyclotron resonance nitrogen plasma source with silane introduced downstream is created by the initial nitridation reaction of the substrate. Angle-dependent X-ray photoelectron spectroscopy shows that the nitridation of GaAs forms a GaN layer separated from the substrate by a layer with As-N bonds, and spectroscopic ellipsometry indicates that the GaN layer has a large void fraction. Interface-state densities in the low 10(12) eV(-1) cm(-2) range measured on SiNx:H/Si/GaAs(110) capacitors did not change significantly if a Si interface control layer 0.8-2 nm thick was deposited on cleaved GaAs(110) surfaces prior to silicon nitride deposition.