Molecular beam epitaxy

Molecular beam epitaxy (English molecular beam epitaxy, MBE) process is a physical vapor deposition (PVD ) to form crystalline thin films (or layer system ).

The method is mainly used in the semiconductor art, including single crystal structures to semiconductor compounds such as gallium arsenide (GaAs), indium phosphide (InP ), GaInNAs, gallium antimonide ( GaSb ) to generate on a substrate.

It was developed in the late 1960s at Bell Laboratories by Alfred Y. Cho and John R. Arthur.

Basics

Epitaxy means that the crystal structure of the growing layer of the substrate, the adapting, as long as the physical properties (particularly the lattice parameters ) of the two substances is not too different from each other. This is called homoepitaxy when substrate and layer consist of the same compound, otherwise by heteroepitaxy. In heteroepitaxy occurs because of the generally different lattice parameters to tensions in the grown layer. Above a critical layer thickness dislocations form (defects) and the stress decays exponentially.

MBE requires an ultra high vacuum to avoid contamination by the residual gas atoms. But while the growth process, the pressure increases due to the effusion into the high vacuum range. The materials from which the film is to be made ​​, heated in Evaporationstiegeln ( effusion ) and enter as a directed molecular beam (without collisions with the background gas ) to the substrate. This is also heated and thus allows an orderly growth of the layer.

By controlling the crucible temperature and a controlled opening and blocking of the molecular beam sources individual complicated multi-layer structures with varying compositions, and dopings may be prepared. The layer thicknesses few atomic layers (ie less than a nanometer ) amounted to micrometer.

The MBE process can be monitored and controlled by appropriate in - situ method ( RHEED, ellipsometry ), which do not affect the growth process.

Applications

Molecular beam epitaxy is used mainly in the production of optoelectronic devices using, inter alia, in the laser diode, quantum cascade laser or dielectric mirrors.

The precise thickness control, also can be realized structures with very small physical dimensions, as it is typical for nanotechnology. They have novel properties, which are based on quantum phenomena. This natural roughness or self-organization are often exploited within the boundary layers at heteroepitaxy. In particular, the strain epitaxially grown heterostructures leads to zero-, one - and higher-dimensional structures such as the already mentioned

• Quantum dots,
• Quantum wires, as well as the
• Quantum wells.

In the basic research MBE is therefore used for the growth of strained Si / SiGe. With this technology, it should be possible in the future, so-called HEMTs in Si / SiGe technology ( MODFETs called ) to implement and reduce costs through the use of materials such as GaAs.

Other variants

Organic molecular beam epitaxy

Evaporation also well-ordered films of organic molecules can be produced on atomically flat inorganic surfaces. This process is also referred to as organic molecular (English organic molecular beam epitaxy, ombe ).

In inorganic chemistry, the low-temperature Molekularstrahlexpitaxie is used for the synthesis of thermodynamically unstable, but kinetically inhibited substances.

Allotaxie

A special MBE method is the Allotaxie that help can be produced in monocrystalline silicon, for example, buried Cobaltdisilicid layers.