The Microstructure of Metalorganic-Chemical-Vapor-Deposition GaN on Sapphire

The group-III-N materials (GaN and its related alloys, InGaN and AlGaN) are a rapidly emerging family of compound semiconductors that have great potential for a wide range of optoelectronics and electronics applications. GaN-based blue light-emitting diodes (LEDs) are commercially available and can already be found in full-color, billboard-size displays in major cities in Japan. Achievement of continuous-wave (cw) laser operation at blue wavelengths will lead to the commercialization of these coherent sources probably within the next few years. Like other semiconductor lasers, these devices can be manufactured relatively cheaply (eventually for a few dollars), and they are expected to replace current ($1,000 and up) ultraviolet and blue-laser sources in medical spectroscopy, high-density information storage, and in projection displays. This article reviews the key role of the crystal structure of GaN grown on sapphire, which is currently the most widely used substrate. We trace the origin of this crystal structure from nucleation and show how it evolves as growth proceeds. We show that the final GaN film is an ordered polycrystal (rather than a single crystal), in which the individual grains are of high crystalline quality but are “tilted” and “twisted” with respect to one another. We conclude by speculating on how this microstructure may explain the unusual optical and electrical properties of GaN.