Liquid phase epitaxy

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Liquid phase epitaxy ( English liquid phase epitaxy , LPE) is a chemical epitaxy , for the preparation of single crystal thin films on a single crystal substrate from a solution .

functionality

For liquid phase epitaxy there are several different versions of devices (e.g. the tilting process) in which a melt is guided over the substrate in the saturation range. The material to be deposited (usually a semiconductor such as gallium arsenide or silicon ) is dissolved in the melt (the solvent) at temperatures well below its melting point . If required, desired doping materials can also be added. The melt is brought into contact with the substrate on which the epitaxial layer is to grow under a hydrogen atmosphere . To do this, the substrate is cooled, which reduces the solubility limit of the material in the melt. In the area of ​​the saturation concentration, the solubility limit is exceeded and the desired layer grows.

A frequently used method is the tilting process, in which the saturated melt is tilted over the substrate. Once the desired layer thickness has been reached, the remaining melt is simply dumped to end the process. With this method, it is also possible to produce layer sequences such as those required for double heterostructure lasers.

Advantages and disadvantages

In liquid phase epitaxy, the epitaxial deposition takes place near the equilibrium state. This is associated with a high structural quality of the layers, which is expressed in almost atomically smooth surfaces and interfaces. In addition, a very precise layer stoichiometry can be achieved, which is sometimes problematic in gas phase epitaxy processes. The deposition rates are also relatively high due to the comparatively high concentrations, which means that in many cases the process is a very economical epitaxial process, especially in mass production .

The technical implementation of the theoretically determined process parameters is sometimes difficult to realize because the process is sensitive to the growth conditions and hardly tolerates any deviations from the optimized process parameters. Larger steps or wave structures in the surface can be the result. Such defects can be avoided by careful selection and cleaning of the substrate, the purity of the gas atmosphere and an exact setting of the deposition conditions.

application

The method is mainly used in the epitaxial deposition of compound semiconductors , where the deposition using gas-phase epitaxial methods is difficult when the partial pressures of the individual components differ greatly. The method is for example for the large scale production of GaAs and GaP - light emitting diodes used. The metallic component, i.e. gallium in the case of gallium arsenide, often serves as the solvent, but other low-melting metals such as tin or lead are also possible. Here, however, attention must also be paid to possible parasitic doping by the solvent, for example tin in GaAs leads to n-doped layers, since it is built into the arsenic lattice sites.

For element semiconductors such as silicon, liquid phase epitaxy is of practically no importance. Gas-phase epitaxial processes are primarily used here. Possible solvents for silicon liquid phase epitaxy would be tin, aluminum and gallium.

Another field of application is the production of glass ceramics in which crystallization begins on an admixed nucleus in the melt phase. In this application, however, not only a thin layer is produced, but a compact material which is only partially crystallized.

literature

  • Liquid phase epitaxy . In: Dieter Sautter, Hans Weinerth (Hrsg.): Lexicon Electronics and Microelectronics . Springer, 1997, ISBN 3-540-62131-8 , pp. 338 .
  • Peter Capper, Michael Mauk: Liquid Phase Epitaxy of Electronic, Optical and Optoelectronic Materials . Wiley-Interscience, 2007, ISBN 978-0-470-85290-3 .