Pit nucleation in compound semiconductor thin films
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T3.24.1
Pit nucleation in compound semiconductor thin films
Mathieu Bouville, Michael L. Falk and Joanna Mirecki Millunchick Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136 ABSTRACT Pit nucleation has been observed in a variety of semiconductor thin films. We present a model in which pit nucleation is considered to arise from a near-equilibrium nucleation process in which the adatom concentration plays an important role. Although pits relieve elastic energy more efficiently than islands, pit nucleation is prevented if the adatom concentration is too high. Inhomogeneities in the adatom density on the surface due to three-dimensional islands enhance pit nucleation. Thermodynamic considerations predict several different growth regimes in which pits may nucleate at different stages of growth depending on the materials system and growth conditions. However kinetics must be taken into account to make direct comparisons to experimental observations. These comparisons show good agreement given the uncertainties in quantifying experimental parameters such as the surface energy. INTRODUCTION Pits have been observed to nucleate subsequent to the nucleation of three-dimensional islands during the heteroepitaxial growth of InGaAs on GaAs [1-4]. Pits have also been observed in silicon-germanium on silicon [5-7], InGaAs on InP [8] and InSb on InAs [9]. These experimental observations form the basis of the theoretical model presented here. This analysis focuses on the role of adatoms in the nucleation of pits. Since our attention is on the effect of adatom dynamics we ignore the strain inhomogeneities caused by three-dimensional islands assuming a uniform strain across the surface and leaving a comparison of the relative importance of these two effects to future work. This emphasis in adatom dynamics arises from direct experimental observations of adatoms on GaAs surfaces, which have revealed adatom concentrations significantly higher than generally expected [10]. Other experiments found that despite a nearly uniform adatom concentration across the surface, localized island nucleation arises from small inhomogeneities in this concentration [11]. Therefore even small variations in the adatom concentration can have a significant effect on the resulting morphology. THEORETICAL MODEL Energetics of island and pit nucleation The nucleation and growth of islands and pits is driven by the ability of such features to relieve elastic energy. When a pit is large enough, finite size effects can be neglected and the difference of energy between a cluster of size p and p bulk atoms can be expressed as the energy released per unit volume plus an energetic cost associated with the surface energy
T3.24.2
Ep = - E pit p + γ pit p 2/3.
(1)
Here p denotes the number of atoms (or the number of cation-anion pairs in the case of compound semiconductors) that have been removed to form the pit; E pit is an elastic energy released per atom, and γ pit p2/3 is the increase of energy due to the creation
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