Kinetic and Strain-Induced Self-Organization of SiGe ...

18 CHAPTER 2. MOLECULAR BEAM EPITAXY (MBE)
Figure 2.2: Schematic illustration of surface processes occurring during MBEgrowth
[12].
� Source: MBE surface-kinetics.jpg
temperature of the region of their origin (source temperature). This initial temperature Ti is
usually higher than the substrate temperature TS; hence they have to loose energy via energy
exchange with the substrate until thermodynamic equilibrium at TS is reached. When the
impinging species stays adsorbed due to a lack of energy for desorption (re-evaporation into
vacuum) the particles diffuse along the surface in order to find an energetically favored lattice-
site, such as a kink-position, where one half of the bonds is occupied. Other possibilities for
reactions on the surface are inter-diffusion (two atoms exchange sites) or nucleation processes
that appear, when migrating atoms aggregate and form a new island on a flat part of the
substrate surface. [55]
2.1.3 Growth-Modes
As mentioned at the end of the last section, atoms preferentially become incorporated at kink-
sites. Incorporation itself has been experimentally documented [59] and can be understood
as a two-step condensation process in which the chemisorbed state is reached via a precursor
physisorbed phase [60]. According to the model in Ref. [61, 62, 63] the atom as physisorbed
species is allowed to diffuse over the surface with a rate constant kdiff to find an energetically
favored site. The interaction potentials as seen by an atom approaching the surface are
schematically depicted in Fig. 2.3 [55, 64]. It is evident from Fig. 2.3 that the physisorbed