Growth Morphology of Ag Islands on GaAs (110) at Low Coverage: Monte Carlo Simulations
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INTRODUCTION Metal-Semiconductor interfaces, which are the basis for the formation of Schottky barrier contacts on compound semiconductors, are being extensively studied. The electrical properties of these contacts can be dramatically affected by the metal overlayer morphology 1. For non reactive metal layers the growth can be classified into three modes; layer-by-layer growth, layer + island growth (Stranski-Krastanov), or 3D island growth modes. The adatomsubstrate and adatom-adatom interactions determine which of these growth modes will dominate. The relative rates of surface processes such as nucleation, growth and coalescence
determine the microstructure evolution from discrete islands to continuous film. Development of sophisticated surface characterization techniques and surface analytical tools like scanning tunneling microscopy (STM) have made it possible to extract valuable information about the growing surface, this in turn stimulated the development of theoretical models for crystal growth to gain insight into the growth process. In general, these models are assume that the adatoms on the surface migrate randomly and independently along the surface during the deposition process. Monte Carlo simulations based on these models have been used to study the atomic level origins of the film morphology during MBE growth 2,3. STM studies of the early stages of Ag growth on GaAs (110) at room temperature (RT) showed that Ag-GaAs (110) is a non reactive system forming 3D Ag islands 4. A companion paper in this volume reports the results of a nanometer scale resolution study of Ag / GaAs (110) epitaxy performed from nucleation all the way through quasi-continuous film in order to investigate; 1) the initial stages of nucleation, 2) the growth of isolated islands and 3) the late stage impingement of growing islands, both at room temperature and 250'C 5. The shape of the islands is dependent on the growth temperature. We have developed a model which incorporates the atomic processes of deposition, surface migration and adatom attachment and detachment kinetics in an attempt to isolate the factors that influence the island shapes. In this paper we report preliminary results obtained from Monte Carlo simulations carried out using a solid-on-solid model (SOS) for low coverages of Ag on GaAs (110) to explain the Ag island morphology observed at the two deposition temperatures.
101 Mat. Res. Soc. Symp. Proc. Vol. 355 ©1995Materials Research Society
SIMULATION MODEL We employ Monte Carlo simulations based on SOS model, in which the substrate is a simple cubic lattice in which neither vacancies nor overhangs are permitted. 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 migrate on the surface and interact with each other to nucleate islands. The surface migration of adatoms is modeled by a nearest-neighbor hopping probability gi
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