Epitaxy

Typical form of an epitaxial intergrowth of cumengeit (tetragonal pyramids) on Boleit cubes

Epitaxial growth of nepheline on hematite

Star-shaped epitaxy of rutile on hematite

Epitaxial intergrowth of two generations of galena crystals form a "herringbone pattern"

Epitaxy (from ancient Greek ἐπί epí "on, over" and τάξις taxis , "order, alignment") is a form of crystal growth that can occur when crystals grow on crystalline substrates. One speaks of epitaxy when at least one crystallographic orientation of the growing crystal (s) corresponds to an orientation of the crystalline substrate.

In natural processes, epitaxy works in such a way that several small crystals grow on a large crystal at a spatial distance from one another. In technical processes, the growing crystals are usually not spatially separated from one another, but rather form an uninterrupted layer. Depending on whether the substrate and growing crystals or layer consist of the same or different material, the terms homo- or heteroepitaxy are also used.

Epitaxy in nature

In nature, epitaxy occurs as an oriented fusion of two minerals . But it can also be an intergrowth of one and the same mineral (e.g. as in the rutile variety Sagenite). Classic examples of epitaxy are written granite (intergrowth of quartz and feldspar , where the quartz is reminiscent of writing), the intergrowth of rutile and hematite, and the star-shaped intergrowth of tetragonal-pyramidal cumengeit and cubic boleit .

Epitaxy in Technology

In technology, epitaxy is mainly used in microelectronics and semiconductor technology. An example of homoepitaxial layers are monocrystalline silicon layers on a silicon substrate, used e.g. B. the epitaxial transistor (1960). In this way, special doping profiles for transistors can be produced, for example an abrupt transition in the dopant concentration, which is not possible with conventional methods such as diffusion and ion implantation . Furthermore, the epitaxial layers are far cleaner than the usual Czochralski silicon substrates. Examples of heteroepitaxy, ie the growth of a layer whose material differs from the substrate, are silicon on sapphire substrates or GaAs _{1 − x} P _x layers on GaAs, for example conductive layers on SOI substrates . The resulting layers are monocrystalline, but have a crystal lattice that differs from the substrate.

Epitaxial process

There are different technical processes for the production of epitaxial layers or bodies:

• Liquid phase ( liquid phase epitaxy , LPE)
• Specific methods of chemical vapor deposition ( chemical vapor deposition , CVD)
  • Chemical vapor phase epitaxy ( vapor phase epitaxy , VPE)
    • Hydride ( hydride vapor phase epitaxy , HVPE)
    • Metalorganic vapor phase epitaxy ( metal organic vapor phase epitaxy , MOVPE)
  • Atomic layer epitaxy ( atomic layer epitaxy , ALE)
• Specific methods of physical vapor deposition ( physical vapor deposition , PVD)
  • Laser beam evaporation (pulsed laser deposition, PLD)
  • Molecular beam epitaxy ( molecular beam epitaxy , MBE)
  • Ion beam-assisted deposition ( ion beam assisted deposition , IBAD)
• Mixed forms
  • chemical beam epitaxy (CBE)
  • metal-organic molecular beam epitaxy (MOMBE)
  • gas source molecular beam epitaxy (GSMBE)

Example: chemical gas phase epitaxy of silicon layers

The production of monocrystalline silicon layers on silicon substrates can be carried out with the aid of chemical gas phase epitaxy. The substrate is heated to temperatures in the range of 600 ° C to 1200 ° C in a vacuum chamber. For the deposition, gaseous silicon compounds (such as silane , dichlorosilane or trichlorosilane ) are introduced in combination with hydrogen, which thermally decompose near the substrate. The "released" silicon atoms are randomly distributed on the substrate surface and form crystallization nuclei. The further layer growth then takes place on these nuclei. For energetic reasons, the growth takes place in the lateral direction until the level is completely filled, only then does the growth in the next level begin. By adding a gaseous boron compound ( diborane ), p-conducting layers or n-conducting silicon layers can be produced using a phosphorus compound ( phosphine ) or an arsenic compound ( arsine ).

The growth rates in an epitaxial reactor are limited by two factors. With the help of the Arrhenius representation (the logarithmic growth rate is represented as 1 / ( absolute temperature )) two areas can be identified:

• the reaction rate-limited area in which there are enough atoms available for the reaction on the surface of the substrate, but the adsorption process runs too slowly because the desorption of hydrogen from the silicon surface is the limiting process. The reaction can be accelerated by increasing the temperature; the rise in the Arrhenius curve is linear and steeper than in the transport-limited area.
• the transport-limited area (at higher temperatures) . Here new gas atoms cannot diffuse to the reaction site quickly enough, gas diffusion is the limiting process. The Arrhenius curve is linear and relatively flat, which means that the growth rate is only slightly dependent on temperature. As a result, the layer growth process is relatively robust with regard to fluctuations in the substrate surface temperature.

See also

• Allotaxic
• Layer growth

Web links

Commons : Epitaxy  - collection of images, videos and audio files

Individual evidence

1. ^ Anthony C. Jones, Paul O'Brien: CVD of Compound Semiconductors: Precursor Synthesis, Development and Applications . John Wiley & Sons, 2008, ISBN 978-3-527-61462-2 , Section 1.8.3 Basic Principles of MOVPE, CBE, ALE.
2. ↑ Ulrich Hilleringmann: silicon semiconductor technology . Vieweg + Teubner Verlag, 2004, ISBN 978-3-519-30149-3 , pp. 120 .