Adatom concentration profile

Figure 4. Early time adatom concentration profile for 7 =0.56r A = 32a (a) concentration, C vs. position, (b) schematic illustrating lower step interaction energy upon creation of adatoms on the top terrace (i) and higher step interaction energy upon creation of adatoms on the bottom terrace (ii).

Fig. 7.138. Adatom concentration profiles, c M/c, between two steps of growth, (a) for different cathodic (upper part of the plot) and anodic (lower part of the plot) overpotentials at X = 0.31 8. (b) For different surface diffusion (sd) rates 0.1 8step < < 3.1 8step atTo mV cathodic and anodic...

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

The equilibrium adatom concentration is a well-defined thermodynamic quantity characteristic for the respective crystal face. It does not depend on the profile of the surface, particularly on the presence of steps as long as Wfree ... 

Another way to learn about diffusion is to study the evolution of the surface morphology caused by mass transport diffusion. For instance, the disappearance of adatoms due to the onset of diffusion has been studied by low-energy ion scattering (LEIS) [42]. However, this usually reveals a threshold temperature at which a certain diffusion rate is reached, and not the whole T dependence needed for independent determination of vo and m The decay of concentration profiles has been studied by scanning electron microscopy (SEM) ]43], STM [33], and photoelectron microscopy (PEEM) ]44]. The island densities forming during... 

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