A INTRODUCTION

The important role of the reconstructed SiGe(100) surface in epitaxial growth techniques has been well recognised and studied in the last decade. It has become apparent that the structure of epitaxially grown SiGe crystals is basically determined by the equilibrium properties of the surface layers rather than by bulk equilibrium. The atomic mobility is enhanced on and near the surface, and this establishes instant local equilibrium in the top layers [1], contrary to the mobility in the bulk, which is very slow at typical growth temperatures. Once the surface configurations are formed, they are frozen-in as further material is deposited on top. Eventually this process determines the final bulk structure of the epitaxial films. The ordering phenomenon in SiGe crystals [2,3] is attributed to this process (see Datareview 2.2 in this book).

The surface properties of the elemental solids, Si and Ge, are well understood. This includes their primary reconstructions, step formation and other inhomogeneities. The challenge in the SiGe alloy case, where we have similar structural environments, is to deal with and identify the most probable atomic configurations. Compositional equilibration is not easily characterized, either theoretically or in the laboratory. Theoretical simulations have contributed a great deal [1,4,5]. Experimental work has also made significant progress in probing the surface composition profile [6,7]. There are two fundamental driving forces that dictate this profile: the strong segregation of Ge to the surface layer, and...