The best quality crystals of gallium nitride can be grown from the solution in supercritical ammonia [1]. This approach, which is known as ammonothermal method, is analogous to hydrothermal crystallization of quartz, however, ammonia is used in the place of water. The ammonothermal GaN crystals (Am-GaN) have many great attributes. They are extremely flat, with bowing radii of (0001) crystallographic planes reaching up to 100 m. Dislocations density is of the order of 10⁴ cm^-2 and free carrier concentration (for n-type crystals) can be varied from 5x10¹⁷ cm^-3 to 5x10¹⁹ cm^-3. One of the biggest drawbacks of the ammonothermal growth of GaN is that it is very slow: growth at best 0.1 mm per day. A technique that has a relatively high growth rate, even up to 500 µm per hour, is a Hydride Vapor Phase Epitaxy (HVPE). This is the most common approach for manufacturing GaN substrates today. The HVPE involves crystallization from the vapor phase at ambient pressure, with GaN deposited on a foreign substrate through the reaction of ammonia with gallium chloride at temperatures of about 1300 K. Etching or self-lift-off techniques are used to separate the nitride film from the foreign substrate (typically sapphire or GaAs) and yield a large-diameter, free-standing (F-S) GaN substrates. Unfortunately, the F-S HVPE-GaN crystals often suffer from lattice bowing. This follows from significant differences between the lattice constants and thermal expansion coefficients of the foreign substrate and the nitride film. It seems, therefore, that a synergy of the HVPE method (the highest growth rate) and the ammonothermal crystallization (the highest structural quality) can create new opportunities for an efficient production of the GaN bulk crystals (then substrates). This can also be helpful to answer a few general questions. First of all, is it possible to combine the HVPE and ammonothermal methods and crystallize perfect HVPE-GaN on a perfect ammonothermal GaN seed? Second of all, is a nature andstructural quality of the seed a main barrier for crystallization of the bulk HVPE-GaN? Finally, if it is possible to multiply the Am-GaN crystal by the HVPE method, obtaining a new F-S HVPE-GaN by slicing it from the seed. Some answers for these questions are being given in this paper. Thus, the results of the HVPE-GaN crystallization on the ammonothermal GaN crystals are described. The starting conditions for the HVPE growth on the Am-GaN seeds are determined, presented and discussed. Smooth GaN layers of excellent crystalline quality, from 0.6 mm to 1.1 mm thick, without cracks, and with dislocation density of the order of 5x10⁴ cm^-2 are shown. The result of slicing of a new HVPE grown material is demonstrated. Structural, optical and electrical properties of this new sliced F-S HVPE-GaN are examined and presented.

1.     R. Doradziński et al., in Technology of Gallium Nitride Crystal Growth, edited by D. Ehrentraut et al., (Springer-Verlag, Heidelberg, ISBN 978-3-642-04828-9), 137-158, (2010)

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