Techniques for processing materials in supercritical fluids include processing in a capsule disposed within a high-pressure apparatus enclosure. The invention is useful for growing crystals of: GaN; AN; InN; and their alloys, namely: InGaN; AlGaN; and AlInGaN; for manufacture of bulk or patterned substrates, which in turn can be used to make optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors.