Surfactant Mediated Heteroepitaxy versus Homoepitaxy: Kinetics for Group-IV Adatoms on As-Passivated Si(111) and Ge(111)

Abstract

Using ab initio calculations we have determined the paths and activation energies for diffusion of group-IV atoms (Si, Ge, and Sn) on top of the As layer on As-passivated Si(111), and for exchange with an As atom. The kinetics of Si, Ge, and Sn adatoms is substantially different: Si adatoms are readily incorporated under the As layer. Ge adatoms diffuse far on top of the As layer and can reach existing steps. We show for the first time that the ratio between diffusion and exchange barriers depends strongly on the strain of the growing Ge film. Sn atoms remain on top of the As layer. DOI: 10.1103/PhysRevLett.88.046101 PACS numbers: 68.35.Fx, 68.55. -a, 71.15. -m In recent years progress in information technology has been fueled by the advances in electronic materials. Typically, multicomponent films with increasing requirements on the control of growth on nanoscale dimensions are used. For many applications (transistors, lasers) the growth of atomically flat layered heterostructures is crucial. Because of lattice mismatch this is not easily realized. An important example is the Ge͞Si heterostructure whose lattice constants differ by ഠ4%. It was shown [1 -5] that the introduction of surfactant layers modifies the growth mode of Ge on Si drastically. Whereas on the clean Si(111) surface Ge grows in the Stranski-Krastanov mode with large three-dimensional islands, with an overlayer of As, Sb, or Bi one finds layer-by-layer-growth on Si(111) and Si(001). A MOSFET structure with p-type Ge as the active layer has successfully been grown on a Si(111) substrate using an Sb surfactant layer [6]. Growth on the more common Si(001) substrates was not successful [7] because the appearance of "V-shaped defects" due to misfit dislocations destroys the Ge layers. A full microscopic explanation of the surfactant effect is still missing, but several aspects have been clarified: Because of the additional electron, surfactant layers of group-V elements on Si or Ge modify the reconstruction of the surface. For example, the As-covered Si(111) surface shows a ͑1 3 1͒ structure [8] instead of the ͑7 3 7͒ reconstruction of pure Si(111). Group-V atoms have a reduced-surface free energy which makes them float on the surface without being incorporated into the growing crystal [1,9]. Thus, growth proceeds by incorporation of deposited Si or Ge atoms under the surfactant layer. In addition, the kinetics of deposited adatoms are changed by the presence of the surfactant layer. have argued in a series of papers that (i) due to the passivation of the surface the barrier for diffusion of adatoms should always be lower than for incorporation, (ii) surfactants not only passivate the flat surface but also the step edges, which reduces the incorporation probability of adatoms. Thus, in spite of a long diffusion length of adatoms homogeneous nucleation of islands on the terraces would be possible, as found experimentally. Experiments of Voigtländer et al. [13] on homoepitaxy on Si(111) show that the island density increases when surfactants (As ...

Structure of steps on As-passivated Si(111):Ab initiocalculations and scanning tunneling microscopy

“…Thus it can easily be imagined that the reactivity of the step edges is reduced in the same way as by the As coverage of the Si͑111͒ terraces: In our previous calculations, 37 we have shown that for Ge atoms deposited on Si͑111͒:As surfaces the barrier for incorporation under the As layer is about three times as large as the diffusion barrier on top of the As layer. This is in line with the speculation of Kaxiras [15][16][17] concerning the reduced incorporation probability at step edges due to surfactant atoms, especially for Ge adatoms in heteroepitaxy.…”

Growth mechanisms in Ge/Si(111) heteroepitaxy with and without Bi as a surfactant

“…This points to another kinetic limitation identified in SME, in particular, to a slow exchange/deexchange process by which the Ge atoms diffusing on top of a surfactant layer incorporate into the growing Ge layer below the surfactant and vice versa. 16,17 Top facets of the 3D islands show a ͱ3ϫͱ3 structure with Ϸ1 Å high undulations indicating the presence of a strain relieving dislocation network in the Ge/Si interface underlying the islands ͓Fig. 1͑c͔͒.…”

Surfactant-mediated epitaxy of Ge on Si(111): Beyond the surface

“…3 An alternative approach is the use of surfactants that change the surface energy and surface kinetics, thus suppressing the three-dimensional island formation and establishing a layer by layer growth mode. [4][5][6][7][8][9][10][11][12] Surfactant mediated epitaxy (SME) also has the advantage that atomically sharp interfaces can be achieved. In addition, the defect structure of Ge films grown on Si(111) can be drastically improved by employing SME: The lattice mismatch of 4.2 % is accomodated by the formation of a periodic network of interfacial misfit dislocations.…”