Surface segregation and incorporation of arsenic in Si molecular beam epitaxy has been investigated. It is shown that segregation is kinetically suppressed at low temperatures and extremely high As concentrations can be achieved. Effects of As on critical thickness and surface morphology are discussed. Electrical properties in both as-grown materials and annealed materials are studied and correlated with atomic structure. (C) 2002 Elsevier Science B.V. All rights reserved.