Effect Of Phosphorus On Ge/Si(001) Island Formation
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Effect Of Phosphorus On Ge/Si(001) Island Formation Theodore I. Kamins, Gilberto Medeiros-Ribeiro1, Douglas A. A. Ohlberg, and R. Stanley Williams Hewlett-Packard Laboratories, Quantum Science Research, Palo Alto CA 94304 1 Also Laboratório Nacional de Luz Síncrotron, Campinas, SP, Brazil
ABSTRACT When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge in layers thicker than about four monolayers 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 the deposition, the surface energies change, modifying the island shapes and sizes, as well as the deposition process. When phosphine is introduced to the germane/hydrogen ambient during Ge deposition, the deposition rate decreases because of competitive adsorption. The steady-state deposition rate is not reached for thin layers. The deposited, doped layers contain three different island shapes, as do undoped layers; however, the island size for each shape is smaller for the doped layers than for the corresponding undoped layers. The intermediate-size islands are the most significant; the intermediate-size doped islands are of the same family as the undoped, multifaceted “dome” structures, but are considerably smaller. The largest doped islands appear to be related to the defective “superdomes” discussed for undoped islands. The distribution between the different island shapes depends on the phosphine partial pressure. At higher partial pressures, the smaller structures are absent. Phosphorus appears to act as a mild surfactant, suppressing small islands.
INTRODUCTION When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge in layers thicker than about three monolayers to form distinctive, three-dimensional islands. The thermodynamically stable shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces [1-2]. To control the size and shape of the island, these energies can be modified, possibly by introducing additional species onto the growing surface [3-5]. In a previous study, Cl was added to the surface in an attempt to control the island shape [6]. Although the surface Cl reduced the surface diffusion needed to form the thermodynamically stable islands, it did not appear to significantly change the island shapes. At the thickness corresponding to the transition between two different island shapes, adding gaseous HCl during the deposition (while keeping the deposition rate constant), favored retaining the low-thickness pyramidal island shape and retarded formation of the dome shape normally favored in thicker films. Thus, adding Cl during the deposition influenced the kinetics, rather than the thermodynamics.
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However, when phosphorus is added, the surface energy changes, modifying the steady-state island shapes and sizes. The deposition kinetics are also changed. When phosphine is added to the
