In its broadest sense, there are many means through which a deposited layer may be added to a substrate, including spin-coating and sputtering. For the manufacture of effective semiconductor device structures however, it is essential to control the thickness, morphology and crystal quality of the deposition. For this reason, the discussion of thin film growth shall focus on epitaxial growth. Epitaxy takes its name from the Greek words epi, (“above”) and taxis, (“in ordered manner”) and describes a process whereby ordered layers may be formed on an existing substrate from incident atomic or molecular species.29
the idealised case in this mode of growth, one layer is completed before another is started. The surface coverage, is analogous to Langmuir surface coverage, with the modification that allowed values may extend such that > 1 to describe coverage extending beyond monolayer thickness.
Frank-van der Merwe (FdM) growth does not occur when the aggregation of the deposited material is more energetically favourable, however. In this instance, an island growth mechanism described as being Volmer-Weber (VM) growth dominates, allowing complete coverage of the substrate only as the 3D islands broaden and coalesce.
Under the conditions employed in epitaxial growth, the evolution of the surface morphology is thermodynamically driven. As a result, the consideration of the relative surface energies of the two materials ( A and B, where A < B) in combination with the interfacial energy, *, produces a qualitative picture of the mechanism likely to dominate. Considering the case of deposition of material A on B, with A + * < B, and the growth will favour the formation of 2D layers. If the reverse situation is the case however, with deposition of B on a substrate of material A, then B + * > A and 3D island growth is preferred.
Where strain is present in the system, the interfacial energy may increase with deposition thickness, causing a transition between the growth modes, as in the Stranski-Krastanov (SK) mechanism.
It is important to make the distinction between initial (VM) island growth, where the formation of islands is driven by energetic factors, and growth that proceeds with island-like structures, such as that in the latter stage of SK growth. In the second
scenario, film morphology is dominated by the existing roughness resulting from the formation of islands.
Whilst not strictly a mode of growth in the same way as those described above, step- flow growth is nevertheless a phenomenon to take into consideration. Briefly, the ability of species to migrate across the surface is a result of sufficient thermal energy allowing atoms to move through formation and cleavage of bonds and contributes to the prevention of surface roughening.31 The distance over which this may occur, or diffusion length, allows species to find a local energy minimum within that range. Step edges provide a sink for diffusing species thus, where species have sufficient energy but do not desorb, growth will preferentially evolve at these sites. In the simplest case, an adatom with a sufficient diffusion length will be able to reach either the step edge of that terrace or the next step edge down. The result is a situation where step edges migrate across the surface, resulting in no net change in roughness (Figure 1.15).
Figure 1.15: During step-flow growth, surface roughness does not change significantly, as species have diffusion lengths sufficient to reach the more energetically favourable step edge sites. This results in step edges migrating, rather than being eliminated, causing a lack of observed RHEED oscillations as a function of time. RHEED is discussed more thoroughly in Chapter 2.
When using an epitaxial method for film growth, there are many factors which must be taken into account, depending on the proposed film thickness. The first of these
of the film to be grown.32 The most important of these is the lattice constant associated with each. During the first few monolayers of layer-by-layer growth, the deposited material adopts the same lattice parameters of the substrate, introducing an inherent strain in the growing layer. Such growth is referred to as being pseudomorphic and persists while elastic strain energy may be accommodated. Should the growth proceed past the critical thickness, the layer may relax by the formation of dislocations as the energy requirement for the accommodation of strain exceeds the energetic cost of forming dislocations. For the growth of films with a low density of defects, lattice mismatch is a prime concern. Where lattice mismatch is present between the intended film and the substrate, buffer layers of intermediate lattice parameter may be utilised. In the simplest case, the strain in films of the material under investigation is reduced by the introduction of layers of another intermediate material, the lattice constants of which are between those of the substrate and the film to be grown thereon. Successive buffer layers result in a staggered reduction of the lattice constant, hence a reduction in the strain of the final grown film. An alternate method is to grow a single buffer layer of, for example, a ternary system with graduated composition.