Molecular beam epitaxy
Molecular beam epitaxy ( English molecular beam epitaxy , MBE) is a method of physical vapor deposition (PVD) to crystalline thin films to produce (or layer systems).
The method is especially in the semiconductor technology used, inter alia, single crystal structures from semiconductor compounds such as gallium arsenide (GaAs), indium phosphide (InP), Ga In N As , gallium antimonide (GaSb) on a substrate to produce.
It was developed by Alfred Y. Cho and John R. Arthur at Bell Laboratories in the late 1960s .
Basics
Epitaxy means that the crystal structure of the growing layer adapts to that of the substrate as long as the physical properties (in particular the lattice parameters ) of the two substances do not differ too greatly from one another. One speaks of homoepitaxy if the substrate and layer consist of the same compound, otherwise of heteroepitaxy . In heteroepitaxy, the generally different lattice parameters result in tension in the grown layer. Above a critical layer thickness, dislocations (defects) form and the tension decays exponentially.
MBE requires an ultra-high vacuum to avoid contamination by residual gas atoms. During the growth process, however, the pressure rises due to the effusion in the high vacuum area. The substances that make up the layer are heated in evaporation crucibles ( effusion cells ) and reach the substrate as a directed molecular beam (without collisions with the background gas). This is also heated and thus allows the layer to grow in an orderly manner.
By controlling the crucible temperatures and the controlled opening and blocking of the molecular beam of individual sources, complex multilayer structures with changing compositions and dopings can be produced. The layer thicknesses can range from a few atomic layers (i.e. less than a nanometer ) to micrometers .
The MBE process can be controlled and controlled by suitable in-situ methods ( RHEED , ellipsometry ), which do not influence the growth process.
Applications
Molecular beam epitaxy is primarily used in the manufacture of optoelectronic components, including laser diodes , dielectric mirrors or quantum cascade lasers .
Thanks to the precise layer thickness control, structures with very small spatial dimensions can also be realized, as are typical for nanotechnology . These have novel properties based on quantum phenomena . Often natural roughness or self-organization within the boundary layers in heteroepitaxy are used. In particular, the tensioning of epitaxially grown heterostructures leads to zero-, one- and higher-dimensional structures like those already mentioned
- Quantum dots ,
- Quantum wires , as well as the
- Quantum wells .
In basic research, MBE is therefore also used for the growth of strained Si / SiGe. With this technology it should be possible in the future to implement so-called HEMTs in Si / SiGe technology ( called MODFETs ) and to save costs by using materials such as GaAs.
Other variants
Organic molecular beam epitaxy
Well-ordered layers of organic molecules can also be produced on atomically level inorganic surfaces by evaporation. This method is also called organic molecular beam epitaxy (engl. Organic molecular beam epitaxy , OMBE), respectively.
In inorganic chemistry , low-temperature molecular beam epitaxy is used for the synthesis of thermodynamically unstable, but kinetically inhibited substances.
Allotaxic
A special MBE process is allotaxy , with the help of which, for example, buried cobalt disilicide layers can be produced in monocrystalline silicon .
See also
literature
- Siegfried Mantl, Helge L. Bay: New method for epitaxial heterostructure layer growth. In: Applied Physics Letters . 61, No. 3, 1992, pp. 267-269, doi : 10.1063 / 1.107965 .
- G. Biasiol, L. Sorba: Molecular beam epitaxy: principles and applications . In: Crystal Growth of Materials for Energy Production and Energy-Saving Applications . 2001, p. 66-83 .
Web links
- epitaxy.net : a central, non-commercial forum about epitaxy, with an extensive list of links and a world map with locations of epitaxy systems.
References and footnotes
- ↑ Such structures are e.g. B. quantum wells , quantum wires and so-called quantum dots .