Low-temperature crystal growth techniques can deposit silicon films with impurity concentration orders of magnitude above their bulk solubility limits. First-principles calculations were performed of the energies (relative to the bulk) of single substitutional carbon, germanium, boron, and arsenic atoms at several positions within a thin (100) slab of silicon reconstructed as c(4×2). The energies of these impurities were found to be at least 0.2 eV lower than in the bulk, corresponding to surface enrichments of 1000 or greater at a temperature of 500 °C. General trends can be explained using the concepts of hybridization energy and lone pairs. The large surface reconstruction strain gives rise to this complex potential energy surface, and favors long-ranged order among impurities near the surface. As a result, we expect a complex dependence of trapped impurity concentrations on growth rate and temperature, with a high sensitivity to these parameters when the exchange rate of the impurity with neigboring sites is comparable to the monolayer deposition rate.