Bunching of Surface Steps and Facet Formation on Analumina Surface
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The behavior of steps on the basal surface of alumina under conditions of evaporative mass transport has been investigated. Steps on a clean surface of alumina tend to be uniformly spaced indicating a repulsive interaction between the steps. The presence of foreign particles causes step bunching that leads to the formation of a new facet. The lower energy of the macrofacet may provide a driving force for the bunching process.
I. INTRODUCTION
The (0001) surface of ␣-alumina is widely used as a substrate for many different kinds of materials. The quality of the substrate influences the quality of the film that is deposited, and hence, considerable effort has been directed toward the study of the structure of the basal surface of alumina.1–7 The quality of the substrate is determined by the presence of steps/ledges and kinks, and the structure of such a surface is often described in terms of the terrace–ledge–kink (TLK) model. The presence of steps on the surface can strongly affect the quality of the film. The influence of surface steps on the growth of a wide variety of oxides on MgO and Al2O3 has been demonstrated.8–11 Surface steps can also influence the processing of the ceramics themselves, particularly sintering and evaporation from the surface. The importance of the evaporation process has been clearly brought out in previous studies on the dewetting of silicates on a ceramic substrate.12 This study is an investigation of the steps on the basal surface of alumina under conditions where vapor-phase transport is dominant. The influence of impurities on the surface and their role on the step motion is investigated. II. BACKGROUND A. Surface structure and terrace–ledge– kink model
A surface can be defined as a boundary between two macroscopic objects with different phases. Surfaces can influence the electrical, optical, mechanical, thermal or chemical behavior of materials. For crystalline materials, the behavior of different surfaces can differ greatly if the
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B. Steps and evaporation from surfaces
The role of ledges/steps on the process of crystal growth/evaporation has been discussed for a variety of situations.14,15 It has been proposed that evaporation takes place in a series of elementary steps starting with the detachment of species from ledge sites, diffusion of the species to the terraces, followed by the transfer of species from the terrace to the vapor phase.14 The reverse holds true for the case of crystal growth. Evaporation thus leads to motion of ledges on the crystal surface and needs a constant supply of ledge sites to continue. Various sources have been proposed for the formation of ledges, crystal edges15 and dislocations14 being the predominant ones. C. Step bunching due to impurities
The motion of surface steps due to evaporation from the surface has been observed experimentally and studied theoretically.16 –20 Surface-step patterns produced by crystal growth/evaporation have been observed on a wide
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