By Marta Kargol, Ph.D.Dec 7 2018.jpg)
Image Credits: gonin/shuttersotck.com
Surface modification by means of thin film deposition is an important industrial process used to protect basic materials against wear, fatigue, corrosion, and many other surface related damage phenomena. Thin film technology is used in many applications such as microelectronics, optics, hard and corrosion resistant coatings, and micromechanics. The properties of a thin film are closely related to its microstructure, especially to its morphology and surface roughness and affect the optical reflectivity, conductivity, and porosity characteristics. That’s why the knowledge of atomistic details of the thin film growth process is useful for the development of new complex devices and the control of thin films and new materials.
In general, the formation of a thin film takes place via nucleation and growth processes1. Atoms that deposited on the substrate diffuse around until they meet other atoms, nucleate and form islands. The islands grow with further deposition and will eventually grow to come in contact with other islands.
- Steps of thin films growth are as follow
- Absorption (physisorption)
- Surface diffusion
- Chemical bond formation (chemisorption): molecule-molecule or substrate-molecule
- Nucleation
- Microstructure formation: crystal structure or defects
- Bulk changes: diffusion or grain growth
As has been shown, thin films go through several distinct stages during growth, each affecting the resulting film microstructure and hence it’s physical properties. The growth mode of the thin film is decided by the relationship between the surface energy of the substrate (γs), and the film, (γf), and the energy of the interface between the film and the substrate, (γi)2. All systems tend to minimize their free energy by maximizing the area of lowest energy surfaces whenever possible while minimizing the interface energy. The three involved interface energies, plus possibly some strain are the decisive inputs for the resulting growth mode. Nucleation and growth of thin films depend on the thermodynamic parameters of the deposit and the substrate surface interaction between the atoms and the substrate material3,4.
Thin film formation on clean crystal substrates can be classified into three basic growth mode, including:
- Frank–Van der Merwe model
- Volmer–Weber model
- Stranski–Krastanov model
In the Frank and Vander Merwe model, the interaction between film atoms is smaller than their bonding to the substrate. First, a monolayer is formed and then the deposition of the second layer begins, e.g. rare gases on graphite. Thus, layer-by-layer growth is two dimensional, indicating that complete films form prior to the growth of subsequent layers. This growth mode usually happens when the substrate and film are homogeneous materials or some particular dissimilar materials such as the epitaxial growth of semiconductor and oxide materials. In the Volmer–Weber model, the bonding between atoms is larger than that to the substrate, causing the film atoms bonding strongly to each other and growing into many three-dimensional nucleus islands, e.g. lead on graphite. Polycrystalline thin films with a rough surface are usually obtained as the continual growth of the islands. This island growth mode often occurs when the substrate and film are heterogeneous. The Stransky and Krastanov (S-K) model combine the features of layer-by-layer growth and island growth. In the S-K mode, thin films firstly grow two-dimensionally in layer-by-layer mode and then grow three-dimensionally in island mode. It happens in the case of the generated stress impact after two-dimensional growth. Transition from the layer-by-layer to island-based growth occurs at a critical layer thickness which is highly dependent on the chemical and physical properties, such as surface energies and lattice parameters, of the substrate and film.
The pattern (i.e., the microstructure and morphology) of metal films deposited on oxides depends strongly on growth conditions, especially for thin layers. Which mode occurs can be determined by energetics, by kinetics or by a mixture of these two. Thin films can be deposited by different methods commonly known as physical vapor deposition (PVD) or chemical vapor deposition (CVD). The properties of a film and thereby its area of application is, of course, mainly determined by the choice of material. Moreover, the structure of the film on a nanometer scale or micrometer scale, known as the microstructure, can also affect the film properties substantially. The great example of the film growth way is epitaxial growth, which refers to the formation of an extended single-crystal overlayer on a crystalline substrate, achieved through layer-by-layer growth mode. Such advanced structures have led to significant improvement in the performance of many devices including lasers, light emitting diodes or detectors.
The formation of thin films due to the deposition of particles on a substrate allows the development of materials with important technological applications. The requirement to control the properties of the deposited films, and thus the development of materials with important technological applications, indicate the great importance to control the growth surface process.
References
- J. A. Venables, G. D. T. Spiller, M. Hanbucken, Nucleation and growth of thin films, Rep. Prog. Phys. 47, 399 (1984).
- D. L. Smith, Thin-Film deposition Principles & Practice, McGrawHill, Boston (1995).
- J. E Greene, Nucleation, Film Growth and Microstructural Evolution, Deposition Processes for Films and Coating, 2nd edition, Chapter 13, Noyes Publications (1994).
- M. Ohring, Materials Science of Thin Films. Academic Press, San Diego, CA, 2nd edition (2002).
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