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The adsorption of one monolayer or less seems to be a simple case. But when the coverage exceeds a single monolayer, what will happen? It has been shown that strong chemisorption usually leads to ordered overlayers. However, adsorbates may or may not take on structures related to the substrate at high coverage. They may resemble much more the bulk adsorbate structure and this tension between substrate and adsorbate properties leads to concepts such as surface strain and surface stress determining the overall film morphology.26 Intimately

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connected with both of these is the surface energy which is related to the surface tension in the case of a liquid. Hence, several factors may influence the order of a monolayer, such as the strength of the adsorption interaction and lateral interactions, the relative strength between them, and so on. Subsequently, different modes of layer growth will be encountered. However, it will not be quantified in the present study. Interested readers may refer the literature (reference26) for a more detailed coverage.

As an example, let us consider what happens when two Si{111} surfaces are brought in contact. In this case, because of a perfect match, new bonds between the two surfaces will be formed so that the interface will be indistinguishable from the rest of the crystal. In contrast, if two unreconstructed (111) surfaces of Si and Ge are brought in contact, they will not match because of the difference of their lattice constants. Forcing them to form bonds will intensively perturb the atomic distances in the interface from those in the bulk. Whether or not such a perturbation will result in a stable interface after relaxation needs to be explored. The region of a surface or interface in which properties are distinguishable from either of the two bulk phases forming the interface is called the selvedge. The selvedge of metal surfaces in UHV often shows an oscillatory relaxation of the interplanar spacing moving from vacuum to the bulk due to imbalances in forces between surface and bulk atoms.26

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Figure 1.6 Stress and strain in pseudomorphic layers in cross-sectional view. (a) The substrate has a larger lattice spacing than the film. (b) The substrate has a smaller lattice spacing. (c) The film expands to the lattice spacing of the substrate and is under tension. (d) The film contracts upon attachment and is under compression. The net force vanishes in both cases with different directions. Reprinted from reference 26.

Two types of growth, namely homoepitaxy and heteroepitaxy, are involved in the above two cases (Figure 1.6). In the first case, the growth is simplified since the growing layer has the same atomic dimensions and lattice structure as the substrate. While in the second growth process, the dissimilar layers have to share the same structures as the substrate, which lead to

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different bonding and phenomena. Therefore, strain is introduced to determine growth characteristics and define the electrical characteristics. For example, strain can reduce the band gap of GeSi layers grown on Si and make them useful form high-speed switches manufacturing. A pseudomorphic layer is then used to describe the layer that has assumed the structure of the substrate instead of exhibiting in the bulk material.30 Figure 1.6 illustrates stress and strain.

Figure 1.7 Three types of interfaces formed between two materials: (a) sharp interface; (b) nonabrupt interface; and (c) reactive interface.

Several types of interfaces could be formed when the effect of strain relaxation is taken into consideration. As shown in Figure 1.7, the simplest situation is the sharp interface formation when there is no mixing of the two materials. However, the other two forms of nonabrupt

interface involve the mixing. If one of the materials is soluble in the other, a region of

variable composition will be created due to its diffusion into the other. If they form a new compound which stabilises between the two pure phases, a so-called reactive interface will be formed.

The type of interface not only depends on the materials, but also on the fashion in which growth is performed. The balance between kinetics and thermodynamics has an impact on the

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morphology and composition of the layered structures.26 The reaction conditions and the rate of deposition are critical in interface growth. For example, high temperature favours surface diffusion and promotes interdiffusion between layers. Another important parameter that affects layer structure is the layer thickness. The introduction of defects (misfit dislocations) at the interface, which generally occurs at the overlayer with a value greater than the critical

thickness, supports one mechanism of strain relief. Otherwise, the upper surface of the

overlayer would relax the interfacial strain through roughening. Both of them may appear together with interdiffusion in either a cooperative or competitive way.

Since the sites at a surface exhibit different strength of interaction with adsorbates and these sites are present in ordered arrays, it is expected for adsorbates to bind in well-defined sites. Interactions between adsorbates can enhance the order of the overlayer; indeed, these interactions can also lead to a range of phase transitions in the overlayer.

Since the materials involved in growth vary, growth modes can be either a simple process without stress or a rather complicated one. In the case of thin liquid layers growing on a solid substrate, only the balance of forces at the liquid-gas (lg), solid-gas (sg) and solid-liquid (sl) needs to be considered, while gravity is neglected. There are only two growth modes which are wetting (2D layer-by-layer) and nonwetting (3D island formation). Meanwhile, three growth modes, named after their original investigators, were revealed for solid-on-solid growth, as shown in Figure 1.8. Due to the strain caused by lattice mismatch, this type of growth became more complicated.

Figure 1.8 The thermodynamically controlled solid-on solid growth modes in the presence of a gas (or fluid or vacuum). (a) Frank-van der Merwe (FM) layer-by-layer growth of two lattice matched materials; (b) Stranski-Krastanov (SK) layer-plus-island growth; and (c) Volmer-Weber (VW) island growth.

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Figure 1.8a depicts Frank-van der Merwe (FM) growth in which the deposited metal grows in a layer-by-layer fashion with the second layer starting only after the completion of the first. The second mode is Stranski-Krastanov (SK) growth, which describes a situation where three-dimensional islands start forming on top of the first one or two completed monolayers (Figure 1.8b). Dislocations are not occurring at any interface of the strained layer. However, the islands continuously relax with a lattice distortion in the growth direction. The last one called Volmer-Webber (VW) growth occurs with three dimensional islands forming at all stages of metal deposition (figure 1.8c). There are dislocations at the interface between the lattice mismatched materials.30, 155, 156

Two thermodynamic properties of the materials control the growth of one of them on the surface of another. The first is the instability free energy of the film, which is the sum of the surface free energy of the growing film on the substrate ( F/S) and the interfacial energy between the film and the substrate ( I). The second is the surface free energy of the substrate ( S). The difference between the two values of these two free energies decides the growth modes. FM (Pseudomorphic) growth occurs when the instability free energy ( F/S + I) is smaller than the substrate free energy S at all stages of metal deposition. VW growth takes place when ( F/S + I) is always greater than S. SK growth combines the above two conditions at different stages. FM growth happens at least for one single monolayer when ( F/S + I) is smaller than S, then followed by VW growth (( F/S + I) > S).30

The validity of these models in predicting the growth mode also depends on the local surface equilibrium and kinetic factors. Since overlayer growth is a dynamic process, the growth mode can change with substrate temperature or crystallographic orientation of the substrate. Hence, kinetic factors sometimes could dominate thermodynamic features. If kinetic limitations prevent the growth from achieving equilibrium, it will be inappropriate to invoke the classical „thermodynamic‟ description.157, 158