The structure and interface state density of thin Si and Si/Ge heterolayers on sulphur passivated GaAs (110) were studied by capacitance–voltage measurements, x-ray scattering, and x-ray absorption with synchrotron radiation. The results show that the reduction of interfacial state density by utilizing Si or Si/Ge thin heterolayers on sulphur passivated GaAs (110) correlates better with the short-range electronic structure at the interface than the commonly believed long-range atomic abruptness. ©1997 American Institute of Physics.