Surface segregation has been a major factor limiting dopant incorporation in silicon and Si1-xGex epitaxy. This paper discusses arsenic incorporation and surface segregation during mixed gas- and solid-source molecular beam epitaxy of silicon and Si1-xGex. The use of Si2H6 gas as the silicon source and careful control of growth conditions enable calculation of the surface arsenic concentration during growth from its effect on the gas-source growth rate. A wide range of surface arsenic concentrations were examined to determine the near-equilibrium relationship between the surface arsenic concentrations and the incorporated arsenic concentrations in the silicon epilayers. A new model based on 2-dimensional islanding of surface arsenic dimers is proposed to explain the observed segregation relationship and the changing surface morphology. The effects of germanium on arsenic surface segregation in Si1-xGex are also discussed qualitatively. (c) 2005 Elsevier B.V. All rights reserved.