The SiNx/GaAs (110) interface structure of the metal-insulator-semiconductor (MIS) capacitors modified with ultrathin Si or Si/Ge (5 to 15 Angstrom) heterolayers and sulfur passivation were studied by capacitance-voltage (CV) measurements, high resolution medium energy ion scattering and extended x-ray absorption fine structure with synchrotron radiation. The results showed that Si/S multilayer passivation can improve the interface properties of the MIS capacitors made on n-type GaAs (110). The element depth profiling in a subnanometer scale and the atomic structure measurements showed that a Si or Si/Ge heterolayer on sulfur passivated GaAs (110) can substantially limit Ga(As) precipitation at the interface but cannot completely suppress the incorporation of Ga(As) into the Si heterolayer. These results can be used to interpret the observed CV,characteristics of the capacitors and to understand the limitation of the passivation approach.