The surface segregation of phosphorus in silicon at low temperatures is studied by using delta doping structures grown by molecular beam epitaxy. The samples are characterized by X-ray crystal truncation rod (CTR) scattering using synchrotron radiation as the light source. The l/e decay length of P segregation and segregation barrier energy are obtained by fitting the CTR curves within kinematical approximation of X-ray diffraction theory. The surface segregation of P is strong at a growth temperature of 450 degrees C, with a l/e decay length of 14 nm, while for growth temperatures below 350 degrees C, P segregation is negligible with a l/e decay length not larger than 4 nm. The segregation barrier energy is determined to be 0.43 eV. (c) 2005 Elsevier B.V. All rights reserved.