Morphology of Ag Islands Grown on GaAs (110) at Low Coverage: Monte Carlo Simulations
- PDF / 1,741,354 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 54 Downloads / 200 Views
elongation at 250'C deposition is due to both anisotropy in the diffusion constant and nearneighbor interaction along and directions. However the island density observed is 2 to 3 order of magnitude higher than the experimental island densities. This was attributed to the 2D nature of the growth model. In this paper we report results obtained using a 3D growth model and the results are quantitatively compared to the experimental observations. SIMULATION MODEL We employ Monte Carlo simulations based on solid-on-solid (SOS) model, in which the substrate is a simple cubic lattice in which neither vacancies nor overhangs are permitted. The surface is represented by a square of (NxN) surface unit cells with periodic boundaries imposed. Growth is initiated by random deposition of atoms onto the substrate at a rate FA, where F is the flux of atoms arriving at substrate and A is the area of the substrate. No reemission is allowed; i.e., we assume a unity sticking factor. Adatoms diffuse freely on the surface and interact with each other to nucleate islands or join the already existing islands
resulting in their growth. The surface migration of adatoms is modeled by a nearest-neighbor hopping probability given by the Arrhenius expression: f = foexp(k-ET)
(1)
where f0 is the attempt frequency (vibrational frequency) for migration (-1013 s- 1 ), T is the substrate temperature, kB is Boltzmann's constant and E is the energy barrier for the hopping process. Each hopping barrier consists of three parts; Es due to substrate-adatom interaction, En due to in-plane near-neighbor adatom-adatom interaction and EB, the step-edge barrier. For the simplest case of isotropic substrate-adatom and adatom-adatom interaction E = Es+nEn+EB, where n is the number of inplane near-neighbor bonds (n = 0,1,2,3,4). Alternatively, a directional dependence of diffusion and adatom attachment is incorporated in the model by assigning different Es and E. values for different directions. The growth of 3D islands and their shape is largely determined by the amount of interlayer transport and hence on the relative size of the barrier for the atom to diffuse over the edges of the island. In the absence of an appreciable barrier for the down-step hopping and neglecting up-step hopping, a second layer islanding does not form until the onset of firstlayer island coalescence and results in a layer-by-layer growth. However for many systems the second layer nucleation starts before the onset of coalescence of first layer islands indicating a significant additional barrier at island edges. By incorporating the step-edge barrier for the down-step hopping the growth proceeds in vertical direction resulting in 3D islands without up-step hopping. Ag forms 3D compact islands on GaAs (110) both at RT and 250'C deposition, and to simulate such island morphologies it is important to allow the up-step hopping of atoms over the island edge along with the additional barrier for the downstep hopping. Simulation procedure As shown schematically in Fig. 1, GaAs (110) surfaces
Data Loading...
