Codeposition of silicon and cobalt on heated silicon substrates in ratios several times the stoichiometry is found to result in epitaxial columns of CoSi 2 surrounded by a matrix of epitaxial silicon. For (111)-oriented wafers, nearly cylindrical columns are formed, where both columns and surrounding silicon are defect free, as deduced from transmission electron microscopy. Independent control of the column diameter and separation is possible, and diameters of 27–135 nm have been demonstrated. On (100) wafers, columns are less regular in shape and show a strong tendency to form {111} interfaces with the surrounding silicon. On (110) wafers, columns are elongated parallel to a 〈110〉 direction, and planar twins are observed in the silicon.