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2010
Book Article
Titel
Gallium nitride and oxonitrides
Abstract
In this article a detailed description of the gallium-based nitride, oxide and Oxonitride phases, their synthesis methods and properties is provided. The stoichiometric gallium nitride (GaN) crystallizes in three known polymorphs. The stable phase at ambient conditions is the hexagonal wurtzite-type structure, where the gallium atoms are tetrahedral coordinated by their nearest neighbours. The problem in synthesizing single crystals for substrates and wafers due to decomposition of GaN below the melting point is presented as well as the approach of heteroepitaxial growth techniques for GaN thin films. Concerning gallium oxide (Ga2O3) the main focus lies on the description of the numerous polymorphs. The stable monoclinic betha-Ga2O3 exhibits like GaN a direct wide-band gap, which can be tuned within a large range by synthesis conditions and dopants. The other phases of Ga2O3 are less investigated and often show low crystallinity. The importance of the gallium nitride and gallium oxide for a large family of electronic and optical applications leads to an increasing interest in the ternary system Ga-O-N. Thus the article mainly deals with gallium oxide nitride, characterized by direct gallium-Nitrogen and gallium-oxygen bonds. The right nomenclature following IUPAC conventions for these materials is said to be gallium oxonitrides. Beside the amorphous oxonitrides known since the 1970s the approaches in theoretical and experimental investigations on crystalline gallium oxonitride are presented here. The defect spinel structure is according to first principal methods believed to be the lowest in energy, nevertheless still showing positive formation energy. Experimental results confirmed the formation of a defective spinel structure exhibiting vacancies on cation and anion sites. Because of the positive formation energy, high temperature high pressure synthesis techniques are necessary. One approach is the formation of the spinel phase by the reaction of GaN-Ga2O3 mixtures. Here pressures and temperatures in the range of 5 GPa and 1700 °C are needed because of the low energy state of betha-Ga2O3. The advantage of a single source molecular precursor approach is pointed out. The pre-coordination of gallium with nitrogen and oxygen leads to formation temperatures of about 1100 °C. Showing also a direct wide band gap these gallium oxonitrides are believed to have high potential for future electronic and optical applications.