The synthesis of large single crystals of GaN (gallium nitride) is a matter of great importance in optoelectronic devices for blue-light-emitting diodes and lasers. Although high-quality bulk single crystals of GaN suitable for substrates are desired, the standard method of cooling its stoichiometric melt has been unsuccessful for GaN because it decomposes into Ga and N2 at high temperatures before its melting point. Here we report that applying high pressure completely prevents the decomposition and allows the stoichiometric melting of GaN. At pressures above 6.0 GPa, congruent melting of GaN occurred at about 2,220 °C, and decreasing the temperature allowed the GaN melt to crystallize to the original structure, which was confirmed by in situ X-ray diffraction. Single crystals of GaN were formed by cooling the melt slowly under high pressures and were recovered at ambient conditions.

Congruent melting of gallium nitride at 6 GPa and its application to single-crystal growth.

GaN single crystal growth using high-purity Na as a flux

We report a straightforward, nonaqueous, solvothermal method for the production of nanoscale gallium nitride structures. Nanoparticles with spherical and rodlike morphologies are produced via in situ gallium azide precursor synthesis and decomposition in superheated toluene or THF. The solution reaction between gallium chloride and sodium azide produces an insoluble azide precursor that solvothermally decomposes to GaN at temperatures below 260 °C. The resulting products are poorly crystalline but thermally stable and crystallize to hexagonal GaN upon annealing at 750 °C. Product morphologies include nanoparticles (ca. 50 nm) and nanorods (ca. 300 nm lengths). Upon annealing, the nanoparticles coalesce into larger organized crystalline structures with hexagonal facets.

Solvothermal azide decomposition route to GaN nanoparticles, nanorods, and faceted crystallites

Light and light based technologies have played an important role in transforming our lives via scientific contributions spanned over thousands of years. In this book we present a vast collection of articles on various aspects of light and its applications in the contemporary world at a popular or semi-popular level. These articles are written by the world authorities in their respective fields. This is therefore a rare volume where the world experts have come together to present the developments in this most important field of science in an almost pedagogical manner. This volume covers five aspects related to light. The first presents two articles, one on the history of the nature of light, and the other on the scientific achievements of Ibn-Haitham (Alhazen), who is broadly considered the father of modern optics. These are then followed by an article on ultrafast phenomena and the invisible world. The third part includes papers on specific sources of light, the discoveries of which have revolutionized optical technologies in our lifetime. They discuss the nature and the characteristics of lasers, Solid-state lighting based on the Light Emitting Diode (LED) technology, and finally modern electron optics and its relationship to the Muslim golden age in science. The book’s fourth part discusses various applications of optics and light in today's world, including biophotonics, art, optical communication, nanotechnology, the eye as an optical instrument, remote sensing, and optics in medicine. In turn, the last part focuses on quantum optics, a modern field that grew out of the interaction of light and matter. Topics addressed include atom optics, slow, stored and stationary light, optical tests of the foundation of physics, quantum mechanical properties of light fields carrying orbital angular momentum, quantum communication, and Wave-Particle dualism in action.

Optics in Our Time

Conditions for seeded growth of GaN crystals by the Na flux method

The present paper reviews presentation of the present status of high pressure solution growth of GaN single crystals and their characterization. The main aspects of the growth include: the thermodynamic properties of the system, the kinetic description of the growth which include the N, dissolution, the volume transport and the surface kineties. The important properties of GaN crystals derived from optical, X-ray, electron transport and positron annihilation measurements are presented. Also etching properties and their relation to the growth morphology and doping are discussed.

S. Krukowski

Papers

Growth of GaN Single Crystals under High Nitrogen Pressures and their Characterization