8 GROWTH AND STRUCTURE OF FILMS

Contents

• General features
• Nucleation theories
• Post-nucleation growth Thin film structures
• Structural defects

The Growth and Structure of Thin Metallic Films

Thin metallic films formed by evaporation are characterized by properties which differ considerably from those of the bulk metal. Thin metal films on metal substrates have been studied intensely in recent years, primarily due to their unusual physical and chemical properties. These properties are determined to a large extent by the structure and morphology, and hence by the growth mode of the films, and studies on these subjects have uncovered a variety of complex atomic surface structures and growth behaviours. Recently, most of these studies have employed averaging techniques from which local atomic structures and film morphological information can only be obtained indirectly. Because of the highly local nature of the measurements scanning tunneling microscopy (STM) can provide direct information on these structural properties, similar to other microscopic techniques such as low energy electron microscopy (LEEM) or transmission electron microscopy (TEM).

Fig. 33: Large scale STM topograph of a surface covered Au film deposited at room temperature.

In particular the different nucleation and growth processes, such as two-dimensional (2D) condensation of adatoms into islands and their subsequent growth, the transition from 2D to three-dimensional (3D) growth and finally the continuing 3D growth, can be studied and understood to a large extent from systematic STM observations following different procedures for metal deposition and subsequent annealing. Cluster sizes and distributions can be investigated as a function of growth parameters, and the results can be compared to theoretical predictions. The effects of surface defects can also be directly imaged.

Formation of thin films involves three basic modes of growth:

1)      Island (or Volmer- Weber)

2)      Layer (or Frank van der Merwe)

3)      Stranski- Krastanov

Island growth: this occurs when the smallest stable clusters nucleate on the substrate and expand in 3 dimensions to form islands. Here, atoms or molecules present are more strongly bonded to one another than to the substrate. Example: The mode of growth is commonly seen in alkali halide crystals, graphite.

Layer growth: here, the smallest stable nucleus grows in two dimensions resulting in the formation of planar sheets. The atoms or molecules present in the deposit are more strongly bounded to the substrate than to each other. After the first complete monolayer is formed, it is covered with less tightly bound second layer. Example: single – crystal epitaxial growth of semiconductor films.

Stranski- Krastanov (layer plus island): this is an intermediate combination of the layer and island growth mechanism. After the formation of one or more mono layers, eventually the growth of layer becomes unfavourable and formation of islands takes place. The reason behind the transition of two to three dimensions could be speculated as the disturbance in the monotonic decrease of binding energy characteristic of layer growth. Example: due to film- substrate lattice mismatch, strain energy accumulates in the film. When released, the high energy at the deposited intermediate layer triggers the island formation.

Nucleation theories

Nucleation is an important phenomenon in crystal growth and is the precursor of the overall crystallization process. Nucleation is the process of generating the initial fragments of a new and more stable phase within a metastable motherphase, capable of developing spontaneously gross fragments of the stable phase. Nucleation is consequently a study of the initial stages of the kinetics of such transformations. Nucleation may occur spontaneously or it may be induced artificially. There are cases referred to as homogeneous and heterogeneous nucleations respectively. Growth of crystals from solutions can occur if some degree of supersaturation supercooling has been achieved first in the system. There are three steps involved in the

Crystallization process.

(i)   Achievement of supersaturation or supercooling.

(ii)   Formation of crystal nuclei.

(iii)   Successive growth of crystals to get distinct faces.

Nucleation:

• Appearance of new phases when the critical boundaries ( which separate the stable phase fields on equilibrium phase diagrams) are crossed.
• For instance, a temperature decrease can trigger vapor‐phase condensation, solidification, or solid‐state phase transformations from vapors & melts (example: formation of ice on crossing 0°C from above).
• When such transformation occurs, emergence of a new phase from the prior parent phase/ phases. Generally, the new phase comprises of different structure and composition
• This process is known as nucleation. It occurs during the initial stages of phase change.
• Nucleation is of utmost importance in thin films because it often influences the developed grain in a fixed deposition process.

Post-nucleation growth– In the post-nucleation stage particles are formed by an atom-by-atom building process.

Nucleation Rate :

Nucleation rate describes the number of nuclei of critical size forming on a substrate per unit time. Nuclei can grow through direct impingement of gaseous phase atoms, but this does not happen commonly in the beginning stage of film formation when the nuclei are spaced far apart. Instead, the rate at which critical nuclei grow does depend on the rate at which adsorbed monomers ( adatoms) get attached to it.

Fig: Ohring Schematic of basic atomistic processes on substrate surface during vapour deposition.

In the above figure, the energetic vapour atoms impinging on the substrate usually remain on its surface for a length of time τs which is given by:

Here, ν is the vibrational frequency of the adatom on the surface of substrate (typically ~ 10¹² sec ^-1),

E_des is the energy required to desorb it back into the vapour. Changes in E_des are usually expected at substrate heterogeneities, such as cleavage steps or ledges where binding energy of adatoms is greater relative to planar surface.

Growth processes controlling microstructure evolution:

 

a)      Condensation of atom, surface diffusion

b)      Nucleation of isolated islands

c)      Growth of islands

d)     impingement and coalescence of islands

e)      Formation of polycrystalline islands and channels

f)       Development of continuous film

g)      Local epitaxy on grains and columns

h)      Competitive colunm growth and grain coarsening

i)        Renucleation

Thin film structures

Poly-crystalline thin films- with grain sizes ranging from a few nanometers to cm (Are the Zn-covered steel lamp posts, letter boxes, etc. with huge grains products of the thin film industry?) Single crystalline thin films, but full of defects like dislocations, precipitates, point defects. Nearly perfect single crystalline thin films – what we often would like to have, but not always get. If we just look at polycrystalline thin films, we may have just regular grains, or all kinds of textures. Again, we deal with it when we run across it.

Amorphous thin films, like amorphous Si (a-Si) or many other materials. You just can’t have amorphous bulk Si or most everything else that usually likes to form a crystal

Amorphous-crystalline mixes, like a-Si:H containing nanometer-sized embedded islands of crystalline silicon ( c- Si and then called µc-Si:H. This is the base of the so-called “microcrystalline Si thin-film solar cell”, one of the hottest contender for the solar cell market of the future.

Structural defects

Defects have a profound impact on the macroscopic properties of materials. In this lecture we will discuss the three basic classes of defects in crystals:

Point defects – atoms missing or in irregular places in the lattice (lattice vacancies, substitutional and interstitial impurities, self-interstitials).

Fig. 34: Shows the point defect in the crystal.

Linear defects – groups of atoms in irregular positions (e.g. screw and edge dislocations).

Fig. 34: Shows the edge dislocation defect in the crystal.

Planar defects – the interfaces between homogeneous regions of the material (grain boundaries, stacking faults, external surfaces).

Fig. 35: Shows the stacking fault defect in the crystal.

Nucleation and growth of thin films

By placing a suitable substrate in the flux of atoms sputtered from the target surface, one can grow a thin film. When an atom or molecule arrives at the substrate it must be absorbed onto the surface, which is a two-stage process. The molecule approaches the substrate and interacts with atoms at the surface. If its momentum is dissipated during the interaction it may no longer have enough energy to escape and will be physically trapped on the surface – this process is known as physisorption. The molecule is still able to diffuse across the surface and may either desorb if it gains sufficient energy, or it may interact with other surface atoms to form chemical bonds (chemisorption). The surface diffusion step is very important as it enables the adsorbed atoms to find low energy sites. Following initial nucleation there are three basic modes of film growth.

Fig. 29: Shows nucleation and growth of thin film.

Conclusion

For thin film preparation chemical and physical methods were discussed in detail. Electroplating is the application of a metal coating to a metallic or other conducting surface by an electrochemical process. How vapour phase growth methods work we have seen. Vacuum evaporation is normally done in the ballistic regime. Sputtering and sputtering yield was discussed in detail and their experimental setup have also been shown. Nucleation and growth of thin film have also been studied.

Conclusion

Growth and Structure of Thin Metallic Films was discussed. Nucleation theory has been discussed. Post-nucleation has also been discussed. Different kind of thin film structure have been discussed based on literature. Three types of defect were discussed such as point, linear and planner defects.

REFERENCES

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2. Frank, F. C.; van der Merwe, J. H. (1949). “One-Dimensional Dislocations. II. Misfitting Monolayers and Oriented Overgrowth”. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences. 198 (1053): 216–225.
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