In the transition layer of the Si(001)–SiO2interface, Si lattice strain and its distribution were directly observed by medium energy ion scattering spectroscopy for thermal and ion beam oxides. The strain was in the vertical direction, and the maximum values at theSiO2side of the transition layer were 0.96&percent; and 2.8&percent; for the thermal and ion beam oxides, respectively. ©1997 American Institute of Physics.