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Epitaxy is an important technique in crystallography where natural or artificial crystals are grown on a crystalline substrate; the underlying substrate acts as a seed crystal and determines the orientation of the crystals which grow upon it.
What is Epitaxy?
Derived from the Greek epi, meaning above, and taxis, an ordered manner, the process results in the formation of one or several crystalline thin films that may be of the same or different chemical compositions and structure as the substrate. The deposited film locks into one or more crystallographic orientations with respect to the substrate crystal, and the resulting epitaxial film or layer has a particular registry or location relative to the underlying layer.
The process is used in nanotechnology and in semiconductor fabrication where it is of commercial importance; in fact, epitaxy is the only affordable method of high-quality crystal growth for many semiconductor materials. For most thin film applications – hard or soft coatings, or optical coatings – it is of little importance but it is critical in semiconductor thin film technology, where the growth of semiconductor materials form layers and quantum wells in electronic and photonic devices such as computer video displays and telecommunication applications. For most technological applications, the desire is for the deposited material to form a crystalline film that has one well-defined orientation with respect to the substrate crystal structure.
Types of Epitaxy
There are various types of epitaxy:
- Homoepitaxy – This is performed with one material, so the substrate and thin film are the same, often silicon on silicon. This is often used to grow films that are purer than the substrate, and which can be doped independently of it.
- Heteroepitaxy – This is performed with different materials and often used to grow films of materials for which crystals can’t otherwise be obtained e.g. silicon on sapphire, or graphene on hexagonal boron nitride. This method allows for optoelectronic structures and bandgap engineered devices.
- Heterotopotaxy – This method is similar to heteroepitaxy except growth is not limited to two-dimensional growth; the substrate is similar only in structure to the thin-film material.
- Pendeo-epitaxy – In this process, a heteroepitaxial film grows vertically and laterally simultaneously. It is used in silicon-based manufacturing processes and is particularly important for compound semiconductors such as gallium arsenide.
Heteroepitaxy is often used for metal-semiconductor growth; many metal-semiconductor structures are used for contact applications and epitaxial growth allows for increased electron movement through a junction. However, trying to grow a layer of crystals atop a substrate that is different to it can present problems; matching lattices are important to minimize defects and increase electron mobility, but the process can lead to unmatched lattices. This mismatch can cause strained or relaxed growth, thus triggering interfacial defects, and straying from what is considered ‘normal’ can lead to changes in the electronic, optic, thermal and mechanical properties of the film.
Epitaxial Growth of Thin Film Materials and Its Applications
Epitaxial growth of thin film materials has numerous applications in electronics, optoelectronic and magneto-optics. Growth can occur in several ways, the most common being vapor phase epitaxy (a modification of chemical vapor deposition), where atoms for deposition on the substrate come from vapor and growth occurs at the gaseous/solid interface. Solid phase epitaxy deposits a thin non-crystalline film on the substrate which is then heated to form a crystalline layer, while liquid phase epitaxy sees layers grown from a liquid source.
The latter is by far the cheapest and easiest route for producing device quality layers, but metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are growing in use. Initial costs are expensive but MOCVD and MBE are more versatile and can readily produce multilayer structures with atomic-layer control, which is fundamental to the nanoengineering now required to produce device structures in as-grown multilayers.
References and Further Reading