Adatoms mobility

Figure 4.8 Oxygen adatom mobility resulting in the growth of oxide nuclei at Mg(0001) at 295 K (a-c) separation of oxide bilayer at Mg(0001) at 295 K (d-f). (Reproduced from Ref. 41).

A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. 

A more interesting problem is that the Metropolis Monte Carlo studies used a different (physically simplified) kinetic rate law for atomic motion than the KMC work. That is, the rules governing the rate at which atoms jump from one configuration to the next were fundamentally different. This can have serious implications for such dynamic phenomena as step fluctuations, adatom mobility, etc. In this paper, we describe the physical differences between the rate laws used in the previous work, and then present results using just one of the simulation techniques, namely KMC, but comparing both kinds of rate laws. 

It is also noted that, for a given material, the transition from type 1 to type 2 response can occur as the substrate temperature Tg is raised. An example of this effect and the associated evolution of intrinsic stress in Fe films vapor-deposited on MgF2 substrates is shown in Figure 1.36. At room temperature, the Fe film thickens without a precipitous drop in the initial tensile stress because of the low mobility of adatoms at the film surface. However, as the substrate temperature is raised to 90°C or 200°C, a marked reduction in the tensile stress occurs in the film with increasing adatom mobility. For Tg = 200°C, the tensile stresses vanish at a film thickness... 

Fig. 53. Work-function change for Cr overlayers on W(110) at 100 K (open circles) and after a brief 1100 K anneal (solid circles). Chromimn coverage is determined by TPD peak area. No change was observed for the AES and LEED data. Therefore, the A4> difference is assigned to a difference in adatom mobility and two-dimensional ordering during the formation of a pseudomorphic monolayer for 0 < 0cr < 1. From [89B3].

Adatom mobility (film formation) The degree to which an adatom can move on the surface and condense at a nucleation site. The lower the mobility, the higher the nucleation density. See also Nucleation density. 

However, the increased number of adatoms at high temperatures can influence their mobility, since clusters of LJ atoms were observed to have smaller diffusion coefficients than isolated atoms. Figure 5 shows the average diffusion coeflScients of adatoms, also as a function of here the deviation from Arrheinus behavior is in the other direction. 

The rate of mass transport is the product of these two factors, the density of atoms and the diffusion coefficient per atom, as shown in Fig. 6. Over a large temperature interval up to the mass transport coefficient is almost perfectly Arrhenius in nature. The enhanced adatom concentrations at high temperatures are offset by the lower mobility of the interacting atoms. Thus, surface roughening does not appear to cause anomalies in the... 

The experiments were conducted in a cell (Fig. 4.19) at residual gas pressure of less then 10" Torr kept constant during the measurements. The surface coverage in these experiments was only lO" - 10 %. In this case, after the atomic beam was terminated, relaxation of electric conductivity has not been observed even at elevated temperatures (100 -180 C), when surface mobility of adatoms increased considerably. At larger coverages of the target surface with adatoms, or at higher surface temperatures electric conductivity relaxed to its initial value (before... 

What became evident was that interactions between adsorbed particles can also exert an influence on their surface mobility and therefore the residence time at a particular site. The mean residence time of an isolated oxygen adatom at the Ru(0001) surface varies from 60 to 220 ms when a second oxygen adatom is located two lattice constants a0 apart from the first but only 13 ms when the... 

It is clear that following NO dissociation, the formation of the (2 x 1)0 structure involves facile oxygen mobility the formation of the well-formed (2 x 3)N structure is more restricted due to the less mobile nitrogen adatoms, however, and with increasing temperature ordering occurs. Associated with the development of both structures is the diffusion of copper atoms from surface steps to form the new structures. 

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