Gallium nitride

Yellow, odorless solid

Fixed

6.1 g · cm -3

800 ° C.

Insoluble in water

Attention

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Gallium nitride (GaN ) is a III-V semiconductor having a wide bandgap (wide bandgap ) is used in the opto-electronics in particular, for blue and green light emitting diodes ( LED) and for high-performance, high-temperature and high-frequency field-effect transistors used. In addition, it is suitable for sensor applications.

History

The material was synthesized for the first time in 1930 and 1969 grew by Maruska and Tietjen first time by hydride vapor phase epitaxy epitaxially as a layer. 1971 succeeded Manasevit, Erdmann and Simpson for the first time on MOCVD growth of GaN, which can be regarded as an important step in the further development.

Properties

GaN crystallized preferably in the ( hexagonal ) wurtzite structure, the cubic zincblende modification is not stable.

The compound is dissolved by hot concentrated sulfuric acid and hot concentrated sodium hydroxide solution slowly, but not of concentrated hydrochloric acid, nitric acid, and aqua regia. It is air resistance and sublimes without decomposition at 800 ° C.

Production

The main problem in the production of GaN-based devices was and is on the difficulty of producing large single crystals of GaN to it to produce high-quality GaN wafer. It must therefore be still evaded on foreign substrates, mainly sapphire and SiC are used. The quality of the ( heteroepitaxial ) layers on foreign substrates was very driven by the work of the group of Akasaki and Nakamura in the late 1980s. Another challenge is the p-doping of the semiconductor material, which is necessary for almost all of the optoelectronic components. You first, the group managed to Akasaki in 1988, then 1992, Shuji Nakamura with a modified approach.

GaN single crystals are now mainly by hydride vapor phase epitaxy (English hydride vapor phase epitaxy ) produced, which is driven by a handful of global technology companies. Initially reacts gaseous hydrogen chloride with liquid about 880 ° C hot gallium to gallium. In a reaction zone, the gallium is brought at temperatures between 1000 ° and 1100 ° C in the vicinity of a GaN seed crystal. Here, the gallium reacts with ammonia to liberate the flowing of hydrogen chloride into crystalline gallium nitride. Under optimum conditions, can be made of a few millimeters with the HVPE method now crystals up to 50 mm in diameter and thickness.

Gallium nitride is in the laboratory by the reaction of gallium with ammonia at 1100 ° C.

Or by ammonolysis of Ammoniumhexafluorogallat at 900 ° C.

Areas of application

This led to the first commercial blue LED which is marketed by Nichia in 1993, and later to the first blue semiconductor laser (1997, Nichia ). Until then, based blue LEDs on the material silicon carbide, which is an indirect semiconductor for efficient light emission poorly suited. Having a higher indium content in the active zone of the GaInN quantum films also green and yellow light emission is possible. The efficiency of such LEDs decreases but increasingly with higher In content based on several physical and chemical facts.

New fame GaN by the WiMax technology. For the high frequency ( 3.5 GHz) to current technologies are not suitable. In addition to the foreign substrate is sapphire GaN can now also on silicon carbide ( SiC) and silicon (Si ) can be prepared. From a purely technical GaN on SiC, due to the high thermal conductivity of SiC, advantageous for power applications. Compared to silicon, however, the cost of silicon carbide substrate are significantly higher ( about $ 1000 per 4-inch wafer). Currently, GaN transistors are already commercially available from Asian and American companies. In Germany N / GaN transistors at various institutes and companies with high pressure in the development of (Ga Al ) worked. First samples in RF power have been available since 2010 from commercial suppliers from Germany. The main focus of the development work is directed towards the device reliability. So researchers at the Fraunhofer Institute for Solar Energy Systems ISE have novel gallium nitride transistors successfully used in power electronic circuits. In power electronics, only silicon transistors are used in the voltage range up to 600 V so far. The properties of gallium nitride are interesting not only because of their higher efficiency, but also allow much higher switching frequencies.

Through the use of GaN-based LEDs today's lighting techniques will be revolutionized. A first step is taking place for several years, conversion of traffic lights on reliable and fuel-efficient LED technology which only based by those available since the early 1990s, green GaN LEDs (instead of inefficient green GaP -based LEDs) has been made possible and the introduction LED daytime running lights based in cars. In the latter, a white light impression through the use of blue LEDs in combination with phosphors, so-called luminescence converters generated. These phosphors convert part of the blue light to yellow, and the human eye perceives this as white light. Improved phosphors, which emit broad band, or the combination of several phosphors with different wavelengths, a high-quality white light may be produced which also satisfies high requirements, i.e., has a high color rendering index. More detailed information particular to the art of LEDs can be found in articles on LEDs.

It was predicted that. , By doping with transition metals or rare earths, a goal could be achieved at room temperature magnetic semiconductors for spintronics as it is of interest The actual detection of room temperature ferromagnetism in GaN is very difficult, since the coming of the doping elements in question usually have a very low solubility in GaN and are therefore often embedded in the form of ferromagnetic nanocrystals in the GaN crystal, which in due to their small size conventional X-ray diffraction experiments are easy to overlook, and their presence only at high X-ray intensity as it can for instance be achieved with synchrotron radiation reveal. Many previous reports of ferromagnetic at room temperature, doped with transition metals GaN can be attributed to such nanocrystals. For the often -mentioned case of GaMnN this argument is valid, although limited, in contrast to gallium arsenide, however, manganese is in GaN no acceptor, but a deep impurity, ie at a level near the band center, so is not itself for the mediation of the magnetic interaction required in diluted magnetic semiconductors defect electrons available. In addition Coming seem GaMnN the interaction between the electron holes and the manganese ions to be so strong that the former are bound to the latter, which makes it difficult to achieve the ferromagnetic order. This means that the concentration required for ferromagnetism at room temperature in GaMnN to defect electrons is much higher than previously thought. A possible way to meet this requirement is to dope addition to the magnetic ions with acceptors such as Mg, which, however, new challenges, such as the large activation energy of Mg in GaN, brings with it.