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New possibility of MOVPE-growth in GaN and InN: polarization in GaN and nitrogen-incorporation in InN

In the application of nitride semiconductors for electronic and optical devices, spontaneous and piezoelectric polarizations have been discussed recently. On the contrary, in light emitting devices, polarization is expected to be absent. To suppress the polarization effect, GaN growth on A-plane and R-plane sapphire substrates has been attempted. A-plane sapphire has crystallographical symmetry different from GaN. R-plane sapphire has large lattice-mismatch from GaN. In this paper, GaN grown on an M-plane sapphire substrate which has been focused in 1990 is reviewed. M-plane sapphire has a lattice-mismatched to GaN by less than 3%. Single-phase GaN was grown on sapphire tilted 15 degrees from an M-plane and its inclination of c-axis to the nominal axis of a substrate was by 32 degree. This number is much attractive to suppress the polarization effect in light emitting devices. This paper also describes N-polar GaN grown by MOVPE. Differently from the reported data about N-polar GaN this N-polar GaN with a surface as smooth as Ga-polar one was obtained and the density of threading dislocations was in order of 1018/cm2. p-type doping was also possible. This N-polar is very suitable for the growth of InN, which has the high equilibrium-vapor-pressure of nitrogen, because N polarity has the advantage in the capture of nitrogen. The growth and the properties of N-polar InN on N-polar GaN templates are reviewed. Finally, the perspectives of InN in device applications are introduced.