725 SURFACE RECONSTRUCTION AND STRAIN RELIEF IN SIi_.,GE,. FILMS ON SI(100) QIUMING YU AND PAULETTE CLANCY School of Chemical Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT The equilibrium structure of a variety of Sij_.Ge,/Si heterostructures have been sim- ulated by Molecular Dynamics, modeled by the Stillinger-Weber potential, to investigate the effect of strain on the surfaces of SiGe thin films. It was found that the strain in SiGe/Si(100) thin films was relaxed by the segregation of Ge to the surface. Rebonding of sub-surface atoms into dimers in the presence of a vacancy or cluster of vacancies above them was observed in the ensuing surface reconstruction. For SiGe/Si, the amount of "re- bonded missing dimers" in the top two layers increased with increasing Ge composition. But for Ge/Si(100), a V-shaped twinning defect was observed in the Ge thin film. To further investigate the effect of strain on surface reconstruction, bulk Si and Ge structures were also studied. For bulk Si, several rebonded missing dimers were found at the surface, while for bulk Ge(100), the surface showed a typical 2x1 reconstruction. All these findings corroborate recent experimental studies and theoretical predictions. INTRODUCTION To explain the different Si (100) surface patterns observed, several models have been proposed. Among them, Pandey [1] proposed a so-called "rebonded missing dimer" surface reconstruction model, which by removing a dimer in the surface, and then rebonding the second-layer atoms into two dimers, the number of dangling bonds could be decreased, although at a considerable cost in local strain. Evidence for this "missing dimers" surface reconstruction model was strongly supported by scanning tunneling microscopy (STM) observations in real space [2]. Recently, Tersoff [3] suggested that the missing dimers in Ge surface serve as a strain- relief mechanism for the Ge thin films on Si(100)-2xl, though no simulations to support this calculations were provided. In contrast to Pandey and Tersoff's energy minimization predictions, we present Molec- ular Dynamics (MD) simulations of a variety of Sil.-,Ge,/Si heterostructures, as well as bulk Si and Ge for comparison, modeled by the Stillinger-Weber potential. Recently, a study by Balamane etal. [5] showed that in comparison to first principles calculations, the Stillinger-Weber potential provided a good description of the properties of bulk diamond cubic Si and the Si(100) surface. In this paper, the effect on the surface reconstruction of increasing the strain by raising the percentage of Ge present was studied. POTENTIAL MODELS AND SIMULATION METHODS The Stillinger-Weber [6] potential was used in this work to represent the interactions between Si-Si, Ge-Ge and Si-Ge. The size and energy parameters for Ge were fitted to the lattice constant and cohesive energy, respectively. The other parameters in the Stillinger-Weber potential for Ge were kept at the same values as those for Si. For Si-Ge interactions, an arithmetic average of Si-Si and Ge-Ge ',tteractions was taken for the size parameter and a geometric average for the energy parameter. The units of length, energy, mass and time are 0.20591 nm, 3.4862 x 10-19 J, 4.6459 x 10"2 kg and 7.6634 x 10'" s, respectively. Unless otherwise stated, all values reported hereafter were scaled to these Mat. Res. Soc. Symp. Proc. Vol. 280. ©1993 Materials Research Society