Current status of GaN and related compounds as wide-gap semiconductors

The quaternary InGaAlN system is a wide gap semiconductor with direct bandgap tunable from 2 to 6.2 eV and a wurtzite structure. This material can be used to fabricate double-heterostructures which are indespensable for semiconductor laser diodes. This paper reports work on the MOVPE growth of the InGaAlN system, doping studies and its potential as a light emitting device. GaN crystalline quality has been drastically improved by the introduction of an AlN and a GaN buffer layer. InN single crystal has been obtained, whose nitrogen equilibrium vapor pressure is much larger than the phosphorus pressure of InP at growth temperatures. The keys to InN growth have been shown to be a low growth temperature and a high V/III ratio. The growth of AlN single crystals has also been reported. Single-crystal InGaN was successfully grown over the whole InN to GaN composition range. A near-band-edge emission in photoluminescence has been observed and its peak shifted toward a longer wavelength as the indium mole fraction was increased. It is possible to control the composition of single-crystal GaAlN and its bandgap bowing parameter has also been reported. InGaAlN has been grown, its optical transmission measured and its photoluminescence observed. Lattice-matching growth of InGaN was performed on a new substrate, zinc oxide single crystal. The lattice-matching has been confirmed to be effective in improving the crystalline quality. With respect to doping, n-type GaN with a carrier density of 10¹⁸ cm^-3 and a mobility of 300 cm²/V·s has been obtained by Si-doping. p-Type GaN, achieved by Mg-doping, has been reported. The hole density was 3 × 10¹⁷ cm^-3 with annealing and 3 × 10¹⁸ cm^-3 with electron-beam radiation after growth. The fabrication of p-n GaN light emitting diodes with electroluminescence at a wavelength of 430 nm has been reported. The optically pumped laser oscillation of GaN near the 373 nm wavelength has also been reported. The enhancement of mobility in the GaN/AlGaN heterostructure has been reported as an electrical characteristic of GaN.