The term epitaxy comes from the Greek roots epi (ἐπί), meaning "above", and taxis (τάξις), meaning "an ordered manner".
One of the main commercial applications of epitaxial growth is in the semiconductor industry, where semiconductor films are grown epitaxially on semiconductor substrate wafers. For the case of epitaxial growth of a planar film atop a substrate wafer, the epitaxial film's lattice will have a specific orientation relative to the substrate wafer's crystalline lattice such as the [001] Miller index of the film aligning with the [001] index of the substrate. In the simplest case, the epitaxial layer can be a continuation of the same exact semiconductor compound as the substrate; this is referred to as homoepitaxy. Otherwise, the epitaxial layer will be composed of a different compound; this is referred to as heteroepitaxy.
Types
Homoepitaxy
is a kind of epitaxy performed with only one material, in which a
crystalline film is grown on a substrate or film of the same material.
This technology is used to grow a film which is more pure than the
substrate and to fabricate layers having different doping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".
Homotopotaxy is a process similar to homoepitaxy except that the thin-film growth is not limited to two-dimensional growth. Here the substrate is the thin-film material.
Heteroepitaxy is a kind of epitaxy performed with
materials that are different from each other. In heteroepitaxy, a
crystalline film grows on a crystalline substrate or film of a different
material. This technology is often used to grow crystalline films of
materials for which crystals cannot otherwise be obtained and to
fabricate integrated crystalline layers of different materials. Examples
include silicon on sapphire, gallium nitride (GaN) on sapphire, aluminium gallium indium phosphide (AlGaInP) on gallium arsenide (GaAs) or diamond or iridium, and graphene on hexagonal boron nitride (hBN).
Heterotopotaxy is a process similar to heteroepitaxy
except that thin-film growth is not limited to two-dimensional growth;
the substrate is similar only in structure to the thin-film material.
Pendeo-epitaxy is a process in which the heteroepitaxial film is growing vertically and laterally at the same time.
In 2D crystal heterostructure, graphene nanoribbons embedded in hexagonal boron nitride give an example of pendeo-epitaxy.
Epitaxy is used in silicon-based manufacturing processes for bipolar junction transistors (BJTs) and modern complementary metal–oxide–semiconductors (CMOS), but it is particularly important for compound semiconductors such as gallium arsenide.
Manufacturing issues include control of the amount and uniformity of
the deposition's resistivity and thickness, the cleanliness and purity
of the surface and the chamber atmosphere, the prevention of the
typically much more highly doped substrate wafer's diffusion of dopant
to the new layers, imperfections of the growth process, and protecting
the surfaces during manufacture and handling.
Applications
Epitaxy is used in nanotechnology and in semiconductor fabrication. Indeed, epitaxy is the only affordable method of high quality crystal growth for many semiconductor materials. In surface science, epitaxy is used to create and study monolayer and multilayer films of adsorbed organic molecules on single crystalline
surfaces. Adsorbed molecules form ordered structures on atomically flat
terraces of single crystalline surfaces and can directly be observed
via scanning tunnelling microscopy. In contrast, surface defects and their geometry have significant influence on the adsorption of organic molecules.
Methods
Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor deposition. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductors. Solid-phase epitaxy is used primarily for crystal-damage healing.
Vapor-phase
Silicon is most commonly deposited by doping with silicon tetrachloride and hydrogen at approximately 1200 to 1250 °C:
- SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g)
This reaction is reversible, and the growth rate depends strongly
upon the proportion of the two source gases. Growth rates above 2
micrometres per minute produce polycrystalline silicon, and negative
growth rates (etching) may occur if too much hydrogen chloride
byproduct is present. (In fact, hydrogen chloride may be added
intentionally to etch the wafer.) An additional etching reaction
competes with the deposition reaction:
- SiCl4(g) + Si(s) ↔ 2SiCl2(g)
Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:
- SiH4 → Si + 2H2
This reaction does not inadvertently etch the wafer, and takes place
at lower temperatures than deposition from silicon tetrachloride.
However, it will form a polycrystalline film unless tightly controlled,
and it allows oxidizing species that leak into the reactor to
contaminate the epitaxial layer with unwanted compounds such as silicon dioxide.
VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE.
Liquid-phase
Liquid-phase
epitaxy (LPE) is a method to grow semiconductor crystal layers from the
melt on solid substrates. This happens at temperatures well below the
melting point of the deposited semiconductor. The semiconductor is
dissolved in the melt of another material. At conditions that are close
to the equilibrium between dissolution and deposition, the deposition of
the semiconductor crystal on the substrate is relatively fast and
uniform. The most used substrate is indium phosphide (InP). Other
substrates like glass or ceramic can be applied for special
applications. To facilitate nucleation, and to avoid tension in the
grown layer the thermal expansion coefficient of substrate and grown
layer should be similar.
Centrifugal liquid-phase epitaxy is used commercially to make thin layers of silicon, germanium, and gallium arsenide. Centrifugally formed film growth is a process used to form thin layers of materials by using a centrifuge. The process has been used to create silicon for thin-film solar cells and far-infrared photodetectors. Temperature and centrifuge spin rate are used to control layer growth. Centrifugal LPE has the capability to create dopant concentration gradients while the solution is held at constant temperature.
Solid-phase
Solid-phase
epitaxy (SPE) is a transition between the amorphous and crystalline
phases of a material. It is usually done by first depositing a film of
amorphous material on a crystalline substrate. The substrate is then
heated to crystallize the film. The single crystal substrate serves as a
template for crystal growth. The annealing step used to recrystallize
or heal silicon layers amorphized during ion implantation is also
considered one type of Solid Phase Epitaxy. The Impurity segregation and
redistribution at the growing crystal-amorphous layer interface during
this process is used to incorporate low-solubility dopants in metals and
Silicon.
Molecular-beam epitaxy
In molecular beam epitaxy (MBE), a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum (10−8 Pa; practically free space) to the substrate, where they condense. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing periodic groups III, IV, and V semiconductor crystals.
Doping
An epitaxial layer can be doped during deposition by adding impurities to the source gas, such as arsine, phosphine, or diborane.
The concentration of impurity in the gas phase determines its
concentration in the deposited film. As in chemical vapor deposition
(CVD), impurities change the deposition rate.
Additionally, the high temperatures at which CVD is performed may allow
dopants to diffuse
into the growing layer from other layers in the wafer
("out-diffusion"). Also, dopants in the source gas, liberated by
evaporation or wet etching of the surface, may diffuse into the
epitaxial layer ("autodoping"). The dopant profiles of underlying layers
change as well, however not as significantly.
Minerals
Rutile epitaxial on hematite nearly 6 cm long. Bahia, Brazil
In mineralogy, epitaxy is the overgrowth of one mineral on another in an orderly way, such that certain crystal directions of the two minerals are aligned. This occurs when some planes in the lattices of the overgrowth and the substrate have similar spacings between atoms.
If the crystals of both minerals are well formed so that the directions of the crystallographic axes are clear then the epitaxic relationship can be deduced just by a visual inspection.
Sometimes many separate crystals form the overgrowth on a single
substrate, and then if there is epitaxy all the overgrowth crystals will
have a similar orientation. The reverse, however, is not necessarily
true. If the overgrowth crystals have a similar orientation there is
probably an epitaxic relationship, but it is not certain.
Some authors
consider that overgrowths of a second generation of the same mineral
species should also be considered as epitaxy, and this is common
terminology for semiconductor scientists who induce epitaxic growth of a film with a different doping level on a semiconductor substrate of the same material. For naturally produced minerals, however, the International Mineralogical Association (IMA) definition requires that the two minerals be of different species.
Another man-made application of epitaxy is the making of artificial snow using silver iodide, which is possible because hexagonal silver iodide and ice have similar cell dimensions.
Isomorphic minerals
Minerals that have the same structure (isomorphic minerals) may have epitaxic relations. An example is albite NaAlSi
3O
8 on microcline KAlSi
3O
8. Both these minerals are triclinic, with space group 1, and with similar unit cell parameters, a = 8.16 Å, b = 12.87 Å, c = 7.11 Å, α = 93.45°, β = 116.4°, γ = 90.28° for albite and a = 8.5784 Å, b = 12.96 Å, c = 7.2112 Å, α = 90.3°, β = 116.05°, γ = 89° for microcline.
3O
8 on microcline KAlSi
3O
8. Both these minerals are triclinic, with space group 1, and with similar unit cell parameters, a = 8.16 Å, b = 12.87 Å, c = 7.11 Å, α = 93.45°, β = 116.4°, γ = 90.28° for albite and a = 8.5784 Å, b = 12.96 Å, c = 7.2112 Å, α = 90.3°, β = 116.05°, γ = 89° for microcline.
Polymorphic minerals
Rutile on hematite, from Novo Horizonte, Bahia, Northeast Region, Brazil
Hematite pseudomorph after magnetite, with terraced epitaxial faces. La Rioja, Argentina
Minerals that have the same composition but different structures (polymorphic minerals) may also have epitaxic relations. Examples are pyrite and marcasite, both FeS2, and sphalerite and wurtzite, both ZnS.
Rutile on hematite
Some pairs of minerals that are not related structurally or compositionally may also exhibit epitaxy. A common example is rutile TiO2 on hematite Template:Fe. Rutile is tetragonal and hematite is trigonal, but there are directions of similar spacing between the atoms in the (100) plane of rutile (perpendicular to the a axis) and the (001)
plane of hematite (perpendicular to the c axis). In epitaxy these
directions tend to line up with each other, resulting in the axis of the
rutile overgrowth being parallel to the c axis of hematite, and the c
axis of rutile being parallel to one of the axes of hematite.
Hematite on magnetite
Another example is hematite Fe3+
2O
3 on magnetite Fe2+Fe3+
2O
4. The magnetite structure is based on close-packed oxygen anions stacked in an ABC-ABC sequence. In this packing the close-packed layers are parallel to (111) (a plane that symmetrically "cuts off" a corner of a cube). The hematite structure is based on close-packed oxygen anions stacked in an AB-AB sequence, which results in a crystal with hexagonal symmetry.
2O
3 on magnetite Fe2+Fe3+
2O
4. The magnetite structure is based on close-packed oxygen anions stacked in an ABC-ABC sequence. In this packing the close-packed layers are parallel to (111) (a plane that symmetrically "cuts off" a corner of a cube). The hematite structure is based on close-packed oxygen anions stacked in an AB-AB sequence, which results in a crystal with hexagonal symmetry.
If the cations
were small enough to fit into a truly close-packed structure of oxygen
anions then the spacing between the nearest neighbour oxygen sites would
be the same for both species. The radius of the oxygen ion, however, is
only 1.36 Å and the Fe cations are big enough to cause some variations. The Fe radii vary from 0.49 Å to 0.92 Å, depending on the charge (2+ or 3+) and the coordination number (4 or 8). Nevertheless, the O spacings are similar for the two minerals hence hematite can readily grow on the (111) faces of magnetite, with hematite (001) parallel to magnetite (111).
