Metal-substrate misfit

The mechanism of growth of metal M on a foreign metallic substrate S is determined by the two most important parameters (1) the binding energy between M and M and M and S and (2) the crystallographic misfit between M and S. 

Figure 3. Different metal growth types depending on the metal ad-soibate-metal substrate interaction and the lattice misfit (a) Adsorbate-adsorbate interaction, is stronger than adsorbate—...

A great variety of structures are formed after deposition of one (or several) metals on the surface of another [1]. The deposited metals may form alloys with each other or they may form islands with some microstructure [7,8] with the substrate in the first or deeper layers [1-6]. Alloy formation at the surface may be observed even in those cases where there is phase separation in the bulk [9-11]. If the size mismatch between the deposited and substrate atoms is large, misfit dislocation structures may be formed [12-14]. 

The primary difficulty inherent in this issue is the small niunber of materials with suitable crystal structures and lattice constants. Some transition metals and ceramics, such as Ni, Cu, Fe, and cBN (Table 5, Ch. 3), are the few isostructural materials with sufficiently similar lattice constants (mismatch <5%). In addition, the extremely high surface energies of diamond (ranging from 5.3 to 9.2 J m for the principle low index planes) and the existence of interfacial misfit and strain energies between diamond films and non-diamond substrates constitute the primary obstacles in forming oriented two-dimensional diamond nuclei. Earlier attempts to grow heteroepitaxial diamond on the transition metals were not successful. The reasons may be related to the high solubility/ mobility of C in/on the metals (for example, Fe, Co, or the... 

Three different growth modes (Volmer-Weber, Frank-van der Merwe, and Stranski-Krastanov) can be distinguished, depending on the vertical binding energy between a metal adatom, Meads> on foreign substrate, S, and on the crystallographic Me-S misfit, as schematically illustrated in Fig. 3. 

The increase of the substrate temperature up to 100 °C results in an appearance of the new surface phase ((2/3) 3x(2/3) 3)-R30° (Fig. lb). This surface phase looks to be a thin epitaxial magnesium silicide film with the misfit 1.9 % with a silicon lattice [5]. According to our EELS data this surface phase is characterized by surface (hux = 9.8 eV) and bulk (hux, = 13.6 eV) plasmons, while for thick magnesium silicide films the surface (hoy = 10.3 eV) and bulk (htav= 14.6 eV) plasmons are typical [8]. The observed difference could be caused by tension of surface phase lattice. At further adsorption the metallic magnesium grows atop the silicide surface phase. 

Vi3uii is the molar volume of the unit cell of the bulk and Vupp that of the UPD modification. When the bulk metal and the UPD modification have the same closely packed structure, the package density in the UPD layer is similar to the bulk, which leaves similar values of molar volumes. Deviations from complete epitaxy, tensions, misfits, etc., influence the energy of interaction between substrate A and UPD layer but have less influence on the entropy. 

Thus, tensile stress seems to be related to the appearance and flow of dislocations. These can be due to the effect of misfit between the lattice of the depositing metal and that of the substrate, and hence account for the appearance of extrinsic stress. However, there are at least two intrinsic reasons for their appearance as well. 

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