We have analyzed field emission from ultrathin Si(100) films on a metal substrate by using ab initio density functional calculations incorporating scattering states. We have shown that the clean Si(100) film can screen an external electric field sufficiently due to metallic states of surface dangling bonds, and thus field emission from the clean silicon film is attributed to that from surface states. We have also analyzed effects of metal adsorption on field emission from Si(100) films. We have obtained a much larger emission current from the Si(100)2X2-Al surface at 0.5 monolayer coverage than that from clean silicon and aluminum surfaces. The minimum local barrier height clearly explains the difference between the Si(100)2X2-Al surface and the clean Al(100) surface, which cannot be explained from the small difference in the work function between the two, surfaces. We have also found that the emission current further increases with a change in the atomic position of adsorbed aluminum addimers which causes both a decrease in the minimum local barrier height and an increase in the local density of states at a surface around the Fermi level. (C) 2003 American Vacuum Society.