Theoretical and Applied Physics

Since the first synthesization of gallium nitride nanorods (GaN) in 1932, high quality GaN could not be obtained for a long term because the equilibrium vapor pressure of nitrogen is a several orders of magnitude higher than the equilibrium pressure of As in gallium arsenide (GaAs) at the growth temperature. For the lack of a GaN substrate, a GaN thin film was epitaxially grown on a sapphire substrate in 1969. A MIS-type GaN LED was fabricated and the first blue emission was observed in 1971. Accidentally, the high quality GaN film with a smooth surface was successively grown through a low-temperature grown AlN buffer layer by the metalorganic vapor phase epitaxy (MOVPE) for the first time in 1986. To construct a double-heterostructure (DH structure) indispensable for a highly efficient LED, InGaAlN was proposed in 1989. This year, InGaN as a material for the blue emission was grown under the sophisticated conditions, and p-GaN was also obtained with Mg doping. The blue LED was fabricated, and became commercially available in 1993. A white LED was developed in 1996, and has been widely used in the solid state lighting. Taking the material characteristics of nitride semiconductors, the first field-effect-transistors of nitride semiconductors was fabricated in 1993. The high electron mobility transistor has been already used in the base stations of cellar phones. The development of high power and high frequency transistors has been increasingly promoted for realizing the sustainable society. Recently, the polarity characteristic in GaN has been attracted for devices with higher performance.