Influence of phosphine on Ge/Si(001) island growth by chemical vapor deposition

When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during chemical vapor deposition of Ge, the surface energies change, modifying the island shapes and sizes. Three different island shapes are found for doped layers, as for undoped layers; however, each doped island type is smaller than the corresponding undoped island type. The intermediate-size doped islands are of the same family as the undoped multifaceted “dome” structures, but are considerably smaller; they also have a narrow size distribution. The largest doped islands are related to the defective “superdomes” found for undoped islands, but are bounded by a smaller number of facets, creating pyramidal-shaped structures with their edges aligned along 〈110〉 directions. The distribution of Ge among the different island types depends on the phosphine partial pressure. Phosphorus appears to act as a mild surfactant, suppressing small islands at high PH3 partial pressures. Within the assumptions made, the segregation enthalpy is estimated to be −0.4 eV. Phosphine decreases the Ge deposition rate because of competitive adsorption; however, the steady-state surface coverage (as indicated by the Ge deposition rate) is not reached for thin layers.