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Mat. Res. Soc. Symp. Proc. Vol. 399 01996 Materials Research Society

distance on any single time step. The forces are computed as the negative gradient of the energy determined from the familiar Lennard-Jones potential. In the case of homoepitaxy, all of the atoms in the model are of the same chemical species, and therefore only one type of interatomic interaction is considered. In the case of heteroepitaxy, we deposit one species B onto a substrate of a different species A. In this case, three different potential functions are used to represent interatomic interactions of the forms A-A, B-B and A-B. The computational cell is two dimensional (XY plane) and the equilibrium crystal structure corresponds to a triangular lattice. The cell is periodic in the X-direction and open in the +Ydirection. Several atomic layers are initially placed at the bottom of the cell (starting at Y=O) to serve as substrate, and then new particles are deposited from above along the negative Y-direction (Fig. 1). The sample is maintained at a constant temperature via the thermostating method described in [4]. Once the simulation is initialized, particle positions are recorded as a function of time and used to track the evolution of the growing films. In addition to the positions, the atomic level stresses are also recorded for each atom. The stress, in dynamic simulations, is comprised of a static term reflecting the interatomic forces between particles and a kinetic term which depends on the velocity of the particles [5],

G[ = -

i

-2

Fi

2. j~i

+ MiVitVij

where, cr.ac is the stress tensor at the position of the ith atom, Qi is th atomic volume of the ith atom, FiX is the aXth component of the force on atom i due to atom j, r.. is the pth component of the vector distance from atom i to atom j, Mi is the mass of the ithsatom and Vio is the ofh component of the velocity of the ith atom. RESULTS Homoepitaxy In order to have a reference against which to gauge the behavior of heteroepitaxial films, several simulations were performed for homoepitaxial deposition over a wide range of substrate temperatures. The predominant microstructural features in these films are voids and vacancies, as illustrated in Fig. 2 for a typical low temperature microstructure. Void formation results from a pinching off of vertical depressions in the film surface by bridges which form as a result of shadowing (by the attachment of depositing atoms to the walls of the surface depression) [2]. Since the surface roughness of the film decreases with increasing temperature, the density and depth of the surface depressions available for void formation is reduced, and therefore the density of voids also decreases with increasing temperature (Fig. 3). As described in Ref. 2, a similar reduction in void volume is seen as the kinetic energy of the depositing species in increased. All of the films grown under the deposition conditions described above are crystalline and maintain the crystallographic orientation of the substrate. Nonetheless, occasionally edge dislocation