Hydride gas phase epitaxy

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Hydrid gas phase epitaxy ( English hydride vapor phase epitaxy , HVPE) is an epitaxial coating process . It is used in microelectronics, among other things, for the production of layers from III-V compound semiconductor materials; wherein the starting materials on the one hand in metallic form (the source material of the component of the III. exist main group ) and on the other hydrogen compounds of elements of main group V .

Example process: gallium nitride

Procedure

One example process is the overgrowth of carrier material with gallium nitride (GaN) (Ga = III element , N = V element) in order to produce substrates for blue emitting LEDs or blue lasers with a long service life. Ultimately, the LEDs and lasers themselves cannot be manufactured using this process, since the layer structures are very complicated and have to be deposited with precise atomic precision. The HVPE is a process with very high growth rates that do not allow such control. However, since the components require base crystals that are as perfect as possible in crystalline form, this process is ideal for this.

In this case, hydrogen chloride (HCl, gas of hydrochloric acid) and gallium at high temperature, about 600-800 ° C to gallium implemented, this continues to flow and makes the further course together with gaseous ammonia to the carrier material, which is also the substrate is called. At controlled pressure and high temperatures, this mixture reacts to form gallium nitride. It is deposited on the carrier and grows into a GaN layer. Typical growth rates that can be achieved with good material quality are between 50 and 150 µm / h.

Problems

The main problem with the carriers overgrown for GaN is that, unlike other semiconductor materials produced in this way, e.g. B. gallium arsenide (GaAs) or indium phosphide (InP), is the same material as the layer, but mostly sapphire or silicon carbide . As a result, the crystal lattice of the substrate and the layer do not match.

With skillful process management, one can nevertheless achieve that a monocrystalline layer is produced, which, however , is afflicted with a large number of crystal defects . These defects can then be reduced by further thick growth of this layer. Qualities as are usually achieved with other III-V semiconductor crystals, however, have not yet been achieved. The best defect densities in the so-called bulk gallium nitride are still a factor of 100,000 above the best values ​​e.g. B. in gallium arsenide. A particularly low defect density of the substrate is necessary for the service life of laser components . With HVPE gallium nitride pseudo-substrates , however, considerable service life values have already been achieved.

It is not possible to draw low-defect gallium nitride bulk material, as is usual with gallium arsenide or indium phosphide, from a melt of the group III element that is saturated with the group V element. The reason for this is that the nitrogen in the material has an immensely high vapor pressure at the required growth temperatures ; this would have to be set in the crystal growing apparatus. When drawing indium phosphide, the phosphorus already has a pressure of more than 30 bar, and the crystal growing apparatus looks monstrous because of the extreme demands placed on the atmosphere to be set. In the case of gallium nitride, the vapor pressure of the nitrogen is several orders of magnitude higher, which makes it difficult to work economically. Attempts are currently being made to produce larger, thick single crystal rods ( ingots ) from the gas phase using HVPE . There are already some publications on this.

Gallium nitride LEDs are currently extremely efficient in converting electrical energy into light. Their efficiency is currently only surpassed by fluorescent tubes, the applications of which are quite limited by the required designs. Since LEDs get by with low voltages and the currents are also moderate, their areas of application are extremely diverse. There are worldwide efforts to bring light generation into households with semiconductor elements made of gallium nitride. These efforts are particularly strong in the People's Republic of China .

history

HVPE was used for gallium nitride in its early days, but had only limited success back then, mainly because too little was known about the possibilities of gallium nitride. Only with the development of the blue LED , which is based on gallium nitride, but manufactured with organometallic gas phase epitaxy (MOVPE), did the material gain in importance. In the time between these dates, the HVPE was used on an industrial scale to manufacture other compound semiconductors , in part for red, orange and yellow LEDs. One reason for this was that the HVPE can be used to produce thick layers of good crystallinity very quickly. On the other hand, the more modern forms of these components with higher light yields consist of sequences of very thin layers , the thickness of which must be controlled with extreme precision, which is why HVPE hardly plays a role in this area. An exception is the deposition of gallium phosphide pseudo-substrates in certain types of red to yellow ultra-high-brightness LEDs. They are applied as a transparent carrier layer as the top layer before the absorbing gallium arsenide substrate is removed.