The domain structure in epitaxial erbium silicide (ErSi2) films grown on (100)Si substrates through the solid phase reaction during annealing is mainly investigated by X-ray diffraction methods. The ErSi2 films grown on both the vicinal substrate and the on-axis substrate have a preferred orientation, i.e., the (1 (1) over bar 00) plane of the ErSi2 is parallel to the (100)Si surface, and the films consist of two types of domains, i.e., [0001]ErSi2 is parallel to either [01 (1) over bar ]Si or [011]Si. The omega-mode rocking curve measurements of the (2 (2) over bar 01)ErSi2 asymmetric reflection show that the volume fractions of two types of domains are almost equal on the on-axis surface and have a difference of about 30% on the vicinal surface. The formation of the double-domain structure is discussed on the basis of geometrical matching at interface between the (1 (1) over bar 00)ErSi2 film and the (100)Si substrate, and a growth model is also proposed. (C) 2002 Elsevier Science B.V. All rights reserved.