Epitaxy#Liquid-phase

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

Operation

For the liquid phase, there are several different designs of devices (such as the tilting method ) in which a melt in the saturation region is performed on the substrate. The material to be deposited (generally a semiconductor, such as gallium arsenide or silicon ) is in this case ( the solvent) in the melt at temperatures well below its melting point. As needed and desired dopants can be added. The melt is placed under a hydrogen atmosphere in contact with the substrate on which the epitaxial layer is grown. , The substrate is cooled, whereby the solubility of the material is reduced in the melt. In the range of the saturation concentration, the solubility limit is exceeded and the desired layer is grown.

A frequently used method is the tilting method, besides the saturated melt is tilted over the substrate. Once the desired thickness is reached, the residual melt the end of the process is simply dumped. With this method, it is also possible to produce the layer sequences, as required for double-heterostructure lasers.

Pros and Cons

In the liquid phase epitaxial deposition of the near equilibrium state takes place. This high structural quality of the layers is connected, which is expressed in nearly atomically flat surfaces and interfaces. In addition, a very accurate film stoichiometry can be achieved, which is particularly problematic in vapor phase epitaxy. The deposition rate, due to the comparatively high concentration is also relatively high, which makes the method in many cases is quite economical epitaxy, especially in the 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 tolerates little variation from the optimized process parameters, larger steps or wave structures in the surface may result. Such defects can be avoided by careful selection and cleaning of the substrate, the purity of the gas atmosphere and a precise adjustment of the deposition conditions.

Application

The method is particularly for the epitaxial deposition of compound semiconductor, for widely different partial pressures of the individual components of the deposition over gasphasenepitaktische method is difficult. The method is used for example for large scale production of GaAs and GaP light-emitting diodes. Is often used as a solvent, the metal component, that is, in the case of gallium, gallium arsenide, but also other low-melting metals, such as tin or lead, possible. However, it must also be paid to possible parasitic doping by the solvent, so tin leads to GaAs n- doped layers, since it is incorporated at lattice sites of arsenic.

For elemental semiconductors such as silicon, the liquid phase has virtually no meaning. Here are primarily gasphasenepitaktische method used. Possible solvents for a silicon liquid phase would be tin, aluminum and gallium.

Another field of application is the production of glass-ceramics, in which a crystallization on a seed used admixed in the melt phase. In this application, however, not only a thin layer is formed, but a compact material which is crystallized to a certain extent only.