The principle of MOVPE is relatively simple.
In order to produce a GaAs film, for example, gaseous compounds of gallium or arsenic, so-called
precursors, are needed as starting materials. For arsenic, the arsenic hydride (AsH3) is chiefly
used, for gallium a metal-organic compound such as trimethyl gallium (TMGa). They are fed into
the reactor with the aid of a carrier gas (hydrogen or nitrogen).
The reactor contains the
so-called substrate, a very thin single-crystal wafer of GaAs which is heated. The temperatures
range from about 500 °C to 1500 °C depending on the material system
to be produced. For pure GaAs
to be deposited, the compounds must be broken up. This takes already place in part in the gas
phase due to the heat emitted from the substrate or by collisions with the molecules of the
carrier gas. The fragments move to the substrate surface, together with undamaged arsine and
TMGa, where they settle and migrate over the wafer. Due to the high temperature and the reactions
accelerated by the substrate, additional bonds are split up so that ultimately pure gallium or
arsenic can be incorporated. In this way, a new GaAs layer grows on the wafer monolayer by
monolayer. The remainder of the starting molecules, the methyl groups of TMGa and the hydrogen of
arsine, partially combine forming the well-known methane, a constituent of natural gas. Together
with molecules which have not reacted they detach from the surface and are flushed out of the
reactor by the carrier gas stream into a gas purification system, the scrubber.
The metal-organic sources are also toxic, but their spontaneous flammability in air is even more dangerous. For this reason, alternatives have been sought both for the group-III and the group-V sources, which are less dangerous and nevertheless largely fulfil the above requirements. back
However, hydrogen is highly explosive in contact with oxygen. Even a fraction of 4 % in air is sufficient to form an ignitable mixture. A large portion of the production costs of layers produced with MOVPE is therefore spent on operational safety. Hydrogen sensors must be installed which, in conjunction with a sophisticated safety technology, immediately interrupt the process in case of an alarm and disconnect all ignition sources such as light switches, telephones etc. In order to increase operational safety and simultaneously lower the costs, efforts have been made for quite a number of years to replace hydrogen by inert nitrogen.
But nitrogen is not only favoured from the safety aspect. The molecule is about 14 times heavier than the hydrogen molecule, which is positively reflected in the decomposition of the source materials. The bonds break more easily upon collision with heavy nitrogen. Other physical properties lead e.g. to a more homogeneous heat distribution in the region of the substrate or a more homogeneous distribution of the starting materials, which significantly improves the quality of the epitaxed layers.back
The substrate has various functions. It serves as a support onto which the desired layer is deposited. At the same time, it participates as a catalyst in the reactions taking place in the reactor. Last, but not least the crystal pattern of the substrate serves as a "building plan" for new atoms so that the epitaxial layer is also monocrystalline.
The production of GaAs or InP wafers is similar to silicon wafers for computer chips: giant single-crystal cylinders are drawn from the melt of the two elements. These are then sawn into wafers about half a millimetre in thickness. Subsequently, the wafer thus produced is polished on one or both sides.
The epitaxy of nitride compounds is more difficult because it is not possible at present to draw large single crystals from these materials. Sapphire (Al2O3) or silicon carbide (SiC) are therefore used as substrates because their crystal properties are similar to those of the nitrides.
For an epitaxy, the wafer is placed on a graphite plate in the reactor. In many cases, this so-called susceptor is pivoted to achieve a more homogeneous distribution of temperature and precursors in the gas phase over the substrate. The susceptor and the substrate are heated by infrared emitters or radiofrequency heating. back
MOVPE can be used to produce layers which consist not only of two elements - so-called binary systems, e.g. GaAs, InP, AlN - but of three or four elements, the so-called ternary compounds such as GaInP, AlGaAs, AlGaN or the quaternary systems (example: AlGaInP, InGaAsP). In addition, foreign atoms are often selectively incorporated into the layers, so-called dopings, to obtain particular physical properties. Moreover, modern components require a particular sequence of different layers, so-called heterostructures, so that a large variety of different sources are often used in an epitaxial process. In III-V semiconductors these are compounds with the elements aluminium, gallium, indium, phosphorus, arsenic and nitrogen. The doping materials used are magnesium, zinc, carbon and silicon in the form of metal-organic molecules or hydrogen compounds (e.g. silanes, i.e. silicon-hydrogen compounds). More details on the research page. back
Not all the precursor molecules are converted so that toxic and flammable substances are contained in the gas mixture leaving the reactor. Added to this is the highly explosive carrier gas in hydrogen processes. The mixture must therefore be filtered, neutralized or diluted before the purified gas can be passed into the exhaust air. This takes place in a gas scrubber. Depending on the consistency of the purification material, a differentiation is made between dry and wet scrubbers. The type of scrubber used depends on the substances used for epitaxy.back