Epitaxial scandium nitride films (225 nm thick) were grown on (I I I)-oriented silicon substrates by molecular beam epitaxy (MBE), using ammonia as a reactive nitrogen source. Film microstructure was investigated using X-ray diffraction (XRD). The (I 1 1) omega-scan FWHM of 0.551 degrees obtained for films grown at 850 degrees C is the lowest reported so far for ScN thin films. The principal orientation of ScN with respect to Si is (I I I)(ScN)//(1 11)(Si) and [1 (1) over bar0](ScN)//[0 (1) over bar1](Si), representing a 60 degrees in-plane rotation of the ScN layer with respect to the Si substrate. However, some twinning is also present in the films; the orientation of the twinned component is (I I I)(ScN)//(111)(Si) and [1 (1) over bar0](ScN)//[1 (1) over bar0](Si), representing a 'cube-on-cube' orientation. The volume percentage of these twins in the films decreases with increasing film growth temperature. (c) 2005 ElsevierB.V. All rights reserved.