Gallium Nitride (GaN) is a new generation semiconductor of wide, direct energy gap of 3.44 eV at room temperature. Among many advantages of GaN, a special attention is attracted  to its high thermal, mechanical and chemical resistance, as well as high breakdown voltage and operating current. Wide spectrum of GaN applications includes both production of optoelectronic devices (blue and green light emitting diodes and laser diodes, UV detectors) and high power-high frequency devices used in modern satellite and surface communication systems (including mobile) and navigation.

At present commercially available GaN-based electronic devices are manufactured mainly by heteroepitaxy of quantum structures on non-native substrate (sapphire, SiC), leading to generation of large threading dislocation density, limiting power, efficiency and lifetime of the devices. High dislocation density (at the level of 10⁶-10⁹ cm^-2) is the reason of narrowing the range of operation parameters. The ideal solution of this problem would be use of bulk GaN substrates for homoepitaxy. Recently, large interest has been devoted to ammonothermal method, which is at present regarded as one of the key technologies of bulk GaN substrates manufacturing. It uses supercritical ammonia to dissolution of feedstock material and crystallization of GaN on native seeds due to convection-driven transport and supersaturation of the solution.

The ammonothermal method enables growth of large diameter crystals (now surpassing two inches) of high crystalline quality. It is well controlled, reproducible process performed at relatively low temperature and is perfectly scalable method (with autoclave size), enabling simultaneous growth of many crystals in one run under minimal material costs. The produced GaN crystals demonstrate exceptionally high crystalline properties curve, large lattice curvature radius  and the lowest dislocation density (of the order of 10⁴ cm^-2). Different orientations GaN substrates (polar, nonpolar, semipolar) and wide spectrum of electronic properties can be achieved. Substantial progress has been done to obtain high purity and transparency both n-type and semi‑insulating substrates. Special technological procedures were applied to reduce concentration of oxygen and other contaminants, leading to a reduction of free carrier and impurity absorption bands, decreasing by several times the absorption coefficient to very few cm^-1 at 450 nm.

The usefulness of ammonothermal GaN substrates in production of high quality devices was confirmed by latest achievement of Watt-class violet laser diode made on such substrates. New examples of  devices, such as Schottky diodes, HEMT transistors (of low leakage current), blue and green lasers, grown on ammonothermal GaN wafers of various orientations, will be shown. Low dislocation density ammonothermal GaN substrates may then cause next breakthrough in production of high power optoelectronic and electronic devices.

This work was partially supported by the National Centre for Research and Development, under Applied Research Programme Contract Number PBS1/A3/9/2012 and PBS1/B5/8/2012 and under Innotech Programme, Contract Number INNOTECH -K1/IN1/44/158851/NCBR/12

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1. Preparation of free-standing GaN substrates from thick GaN layers crystallized by Hydride Vapor Phase Epitaxy on ammonothermally grown GaN seeds
2. Semipolar (2021) UV LEDs and LDs grown by PAMBE
3. Podłoża AMMONO-GaN przyszłością elektroniki półprzewodnikowej
4. Flat lattice of truly bulk ammonothermal GaN