Gallium nitride (GaN) has become an important semiconductor for the optoelectronics and power electronics fields in the pursuit of high efficiency devices. However, the lack of a natural native GaN substrate has forced growth of GaN devices on foreign substrates such as sapphire, silicon carbide, and silicon. To further enhance efficiency and develop devices with longer lifetimes, the number of defects present in devices must be reduced. The development of a native GaN substrate of high crystalline quality would directly enable defect reduction.
The ammonothermal method of GaN growth has shown significant promise as a technique for the production of high quality GaN crystals of industrially significant size (crystals on the order of centimeters in the largest dimension). The ammonothermal method is a solvothermal method that uses a mineralizer (here ammonium chloride) with supercritical ammonia to transport GaN from a source material from one temperature zone to grow a seed crystal in another temperature zone. High pressures, high temperatures, and the presence of a highly corrosive chemistry make development of an economical growth reactor challenging. This body of work outlines the development of a growth reactor capable of high temperature ammonothermal growth of GaN using ammonium chloride mineralizer.
Initial development of the ammonothermal reactor required identification of suitable reactor materials. A materials stability study was conducted by exposing samples of materials to the ammonothermal environment and measuring mass loss as well as any chemical or mechanical changes that occurred. An Inconel 625 alloy reactor was employed, although the reactor itself was somewhat susceptible to corrosion from the ammonothermal environment. The study yielded a subset of materials that may be suitable for use as gaskets and other single use items which include niobium, molybdenum, titanium, vanadium, tungsten, gold, and platinum. Alloys of molybdenum and cobalt may also be useful. High strength titanium-zirconium-molybdenum (TZM) was also identified as a corrosion resistant material and was selected for reactor design.
A TZM reactor was then designed and fabricated. Subsequent high pressure, high temperature tests indicated that TZM was essentially inert and growth of GaN crystals followed. All GaN growth was accomplished at or above 650°C using seed crystals grown by hydride vapor phase epitaxy. Seeds were characterized by micrometer measurements for growth thickness, x-ray diffraction (XRD) for crystalline quality, and secondary ion mass spectrometry (SIMS) for impurity concentrations. The growth quality appeared to match the seed quality as measured by XRD. Growth coloration ranged from slightly gray to green or yellow with growth rates up to 191 µm/day. Most seeds exhibited significant faceting at the edges of the sample, forming semipolar planes. SIMS was performed on a couple of samples which indicated oxygen concentrations of ~1018 cm¬-3.
