Epitaxy

Epitaxy (from Greek epi - "on ", "above", and taxis meaning " order " or " align " ) is a form of crystal growth, which can occur during the growth of crystals on crystalline substrates. One speaks of epitaxy, if at least one crystallographic orientation of the growing crystal ( the growing crystals) corresponds to an orientation of the crystalline substrate.

In natural processes epitaxy works is that several small crystals grow from each other in spatial distance on a large crystal. In industrial processes, the growing up crystals are usually not spatially separated, but form a continuous layer. Depending on whether the substrate and growing up crystals or layer consist of the same or different material, also the names homo- or hetero-epitaxy can be used.

Epitaxy in the nature

In nature, epitaxy occurs as oriented intergrowth of two minerals. But it can also be an adhesion of the same mineral (for example, as in the Rutilvarietät Sagenit ). Classic examples of epitaxy form of graphic granite ( intergrowth of quartz and feldspar, the crystals are reminiscent of writing), the intergrowths of rutile and hematite, and the star-shaped intergrowth of tetragonal- pyramidablem Cumengeit and würfeligem Boleite.

Epitaxy in the art

In art, the epitaxy is used primarily in the microelectronics and semiconductor technology use. An example of homoepitaxial layers are monocrystalline silicon layers on a silicon substrate. In this way, special doping profiles for transistors can be manufactured, for example, an abrupt transition in the dopant concentration, which is not possible with conventional processes such as diffusion and ion implantation. Furthermore, the epitaxial layers are much cleaner than conventional Czochralski silicon substrates. Examples of hetero-epitaxy, ie the growth of a layer whose material is different from the substrate, are silicon on sapphire substrates GaAs 1 - x P x or layers on GaAs, for example, conductive layers on SOI substrates. The resulting layers are monocrystalline, but have a crystal lattice that is different from the substrate.

Epitaxy

There are different technical process for the preparation of epitaxial layers or body:

• Liquid phase (liquid phase epitaxy LPE )
• Special methods of chemical vapor deposition (chemical vapor deposition, CVD) Chemical vapor deposition ( vapor phase epitaxy, VPE) Hydride ( hydride vapor phase epitaxy HVPE )
• Organometallic vapor phase epitaxy ( metal organic vapor phase epitaxy MOVPE)

• MBE ( molecular beam epitaxy, MBE)
• Ion beam assisted deposition ( ion beam assisted deposition, IBAD )

• Chemical beam epitaxy ( CBE)
• Metal-organic molecular beam epitaxy ( MOMBE )
• Gas source molecular beam epitaxy ( GSMBE )

For example, chemical vapor phase epitaxy of silicon layers

The production of single-crystal silicon layers on silicon substrates can be carried out using the chemical vapor phase epitaxy. The substrate is heated in a vacuum chamber to a temperature in the range from 600 ° C to 1200 ° C. For the deposition of gaseous silicon compounds are ( such as silane, dichlorosilane, trichlorosilane or ) is introduced in conjunction with hydrogen, which thermally decompose near the substrate. The " liberated " silicon atoms are deposited randomly distributed on the substrate surface and form nuclei. These germs then the further layer growth takes place. For energy reasons, the growth takes place in the lateral direction until the level is completely filled, only after the growth starts at the next level. By the addition of a gaseous boron compound ( diborane ), p- conducting layers, or by a phosphorus compound ( phosphine) or an arsenic compound ( arsine ) n-type silicon layers are formed.

The growth rates in an epitaxy reactor are limited by two factors. Based on the Arrheniusdarstellung ( the logarithmic growth rate is shown on 1 / ( absolute temperature) ) can be characterized two areas:

• The reaction-rate limited region in which enough atoms, although ready for the reaction at the surface of the substrate, but the adsorption process is running too slow, because the desorption of hydrogen from the silicon surface is the limiting process. The reaction can be accelerated by increasing the temperature, the increase in the Arrhenius plot is linear and steeper than in the transport limited range.
• The transport limited region ( at higher temperatures). Herein may not diffuse rapidly enough new gas atoms to the reaction site, the gas diffusion is the limiting process. The Arrhenius plot is linear and relatively flat, that is, the growth rate is only weakly dependent on temperature. Thus, the film growth process is relatively robust with respect to variations in the substrate surface temperature.