It is wide known that gallium nitride is a very attractive material for optoelectronic devices such as blue and ultraviolet light sources, UV detectors, for high temperature/power electronics and microwave devices. For these purpose one need suitable substrates for device structures epitaxy. In the nature there are no native crystals of the gallium nitride, synthesis is very difficult and it is hard to obtain single crystal with good properties. Crystallization of freestanding GaN substrates could be made by either high-pressure synthesis, by sublimation method or ammonothermal method. However, the size of GaN crystals obtained in these ways is still too small for practical use. The current largest freestanding GaN substrate was obtained by growth of a thick GaN layer on a sapphire substrate using Hydride Vapour Phase Epitaxy (HVPE) and separation of the grown layer from the sapphire substrate.

We have investigated thick GaN layers deposited in HVPE system on composite substrates which were made on sapphire in Metalorganic Vapour Phase Epitaxy (MOVPE) system. The following substrates: (00.1) sapphire substrates with AlN, AlN/GaN and GaN thin layers were used.

Composite GaN on sapphire Al₂O₃ substrates with different nucleation layers (GaN or AlN) were grown using an AIX 200/4 RF-S low-pressure reactor. The source gases were trimethylgallium (TMGa), trimethylaluminium (TMAl) and ammonia (NH₃). The reactor pressure was 200 mbar, and the layers were grown at 1048°C.

Thick GaN layers were deposited in conventional, open HVPE system: three-temperature zone furnace and horizontal quartz reactor. Nitrogen (6N) was used as the carrier gas. GaCl was formed by the reaction of gaseous HCl (6N) and liquid Ga (6N) at 920°C. HCl was diluted by nitrogen. NH₃ (7N) was used as the source gas. Total gas flow was about 4500 ml/min. The temperature in main grown zone was kept at 1065°C.

In this HVPE system thick GaN layers (in the range of 50 up to 80 micrometers) were deposited on the top of GaN/sapphire, AlN/sapphire and GaN/AlN/sapphire structures.

We have determined the optical parameters, crystallographic structure and quality of epitaxial thick GaN layers and made comparison of these three types thick layers. We have observed significant differences.

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