Kinetics of Facet Formation During Growth and Etching of Crystals
- PDF / 467,610 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 83 Downloads / 233 Views
KINETICS OF FACET FORMATION DURING GROWTH AND ETCHING OF CRYSTALS D. G. VLACHOS, L. D. SCHMIDT, AND R. ARIS Department of Chemical Engineering and Materials Science, University of Minnesota,
Minneapolis, MN 55455 ABSTRACT Morphological instabilities of surfaces with respect to faceting in deposition of microelectronic materials and etching of semiconductors and catalysts are of great technological importance. Macroscopic theories fail in predicting correctly the time and size dependence of microstructures observed. We have examined the temporal and spatial evolution of unstable interfaces in zero net flux, growth, and etching of crystals as functions of temperature and degree of irreversibility using the Monte Carlo method. In addition to faceting, thermal or kinetic roughening transitions are also observed under some conditions. We have found that the type of transition and the surface structures depend strongly on the irreversibility of the process and the surface temperature. It is demonstrated that surface diffusion has a profound influence on the transitions and surface morphologies. INTRODUCTION Faceting is the breaking of unstable surface orientations into hill-and-valley structures, as illustrated in fig. 1. In thermal faceting, the topography of solid surfaces changes at elevated temperatures in vacuum or in non-reacting gases. Faceting of surfaces which act as catalysts for reaction among gaseous species is called catalytic faceting. Thermal faceting takes place usually over long time periods whereas catalytic faceting occurs more rapidly than thermal faceting and at lower temperatures. Catalytic faceting has been the subject of extensive experimental research, with NH 3 oxidation and HCN synthesis being the most well studied systems [1]. The low conversion efficiency of catalysts after a few months of operation and the non-sharp microelectronic interfaces formed during growth as a result of faceting require a better understanding of this phenomenon. Faceting of surfaces has been studied since the beginning of the century and has been rationalized using both equilibrium and non-equilibrium conditions. Initially, minimization of the surface free energy has been suggested as the driving force for faceting. Since catalytic faceting occurs faster than thermal faceting, it has been argued that the removal of material is the major path for faceting in which case transport will be important [1]. Other interpretations suggest that exothermic reactions can facilitate surface diffusion or result in production of energetic Pt atoms, or formation of locally hot regions on the surface, or even preferential removal of material from regions (defects) where the reaction rate is larger [1]. In this paper, we examine the faceting problem at the microscopic level using the Monte Carlo method in the grand canonical ensemble where adsorption and desorption of atoms from the crystal occur. We investigate the dynamics of faceting and patterns of surfaces in zero net flux, growth, and etching, and we compare our results with th
Data Loading...
