Si(001) Molecular Beam Epitaxy: Enhanced Diffusion or Bonding?
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SI(001) MOLECULAR BEAM EPITAXY: ENHANCED DIFFUSION OR BONDING? S. CLARKE, M.R. WILBY, and D.D. VVEDENSKY The Blackett Laboratory and Semiconductor Interdisciplinary Research Centre,Imperial College, Prince Consort Road, London, SW7 2BZ, United Kingdom ABSTRACT
Through application of a lattice model of the Si(001) surface, implemented in a Monte Carlo growth simulation we investigate the structural evolution of the Si(001) surface during molecular beam epitaxy. Particular emphasis is placed upon identifying the role of both enhanced diffusion and directional bonding. INTRODUCTION
Homoepitaxial growth on the Si(001) surface provides an ideal system for studying fundamental aspects of molecular beam epitaxy (MBE). Absent are complications associated with binary systems, such as GaAs, but present is a growth mode very strongly dependent upon local changes induced by the reconstruction. The Si(001) surface exhibits a 2x1 reconstruction comprising ordered rows of paired atoms (dimers). When a surface is prepared it inevitably includes some misorientation from the desired plane, manifesting in steps on the surface. A monatomic a step forms a domain boundary between 2x1 and lx2 reconstructions, the direction of dimer formation rotating through 900 at the step. In contrast a surface comprising biatomic steps forms a single domain surface. Both cases lead to two types of steps, those with the upper terrace dimer axis normal to the step (type-A) and those in which the dimer axis is parallel (type-B) (Fig. 1). These steps are labeled
SA
and S, or
DA and DO for single or double height steps, respectively. Similarly we label domains in which the Steps Descending in [110) Direction
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sit: UN8 UN8
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Type-A Terrace
Type-B Terrace Ss Step
Type-B Terrace
SAStep
Figure 1. A schematic illustration of a monatomically stepped Si(001)
surface. Mat. Res. Soc. Symp. Proc. Vol. 159. ©1990 Materials Research Society
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dimer axis is normal to the steps as type-A and parallel as type-B. A prominent feature of Si(001) homoepitaxy is the formation of elongated clusters on the surface with the elongation in the direction perpendicular to the dimer axis of the atoms comprising the cluster. Revealed by the scanning tunneling microscope (STM) [1], elongated clusters have been ascribed to either enhanced diffusion parallel to the dimer axis of the under-layer [1] or to a difference in residence times at the SAand S. steps forming the edges of these clusters [1,2]. In this paper we investigate the origin of elongated cluster formation, testing each hypothesis in a simulation of Si(001) MBE. We then analyse the implications of our findings for step stability and propagation at high temperatures. THE SOLID-ON-SOLID MODEL OF Si(O01) MBE
We adopt as the basis of our model a simple cubic lattice in which neither overhangs nor vacancies are permitted, the solid-on-solid criterion. In this framework we include two kinetic processes, the deposition and migration of surface adatoms, the long residence time of Si a
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