On the origin of blue-green emission from heteroepitaxial non-polar a-plane InGaN quantum wells
Kappers, M J and Badcock, T J and Hao, R and Moram, M A and Hammersley, S and Dawson, P and Humphreys, C J
The optical and structural properties of nonpolar a-plane InGaN/GaN multiple quantum wells grown on r-plane sapphire substrates were investigated as a function of defect density. The photoluminescence spectrum is characterised by a peak at 380 nm (3.24 eV) and a blue-green emission band from 425-550 nm (2.25-3.10 eV). The near-UV peak is assigned to carrier recombination in the quantum wells lying on the a-plane. On the basis of microscopy, photoluminescence spectroscopy and cathodoluminescence imaging, the blue-green emission band is attributed to emission from sidewall quantum wells formed on the various semipolar facets of small (∼100 nm) surface pits which form during quantum well growth at the lower temperatures and high NH3 flow in a nitrogen atmosphere. The density of the surface pits equals that of partial dislocations and threading dislocations in the quantum wells structure. Thus, the 300 K photoluminescence spectrum is dominated by the blue-green emission band at a high defect density (>1010 cm-2) and its intensity is reduced with lower defect densities. For a spectrum with a single emission from the nonpolar a-plane InGaN/GaN quantum wells the defect density needs to be below ∼109 cm-2s.
Department of Materials
Imperial College London
Royal School of Mines
London, SW7 2AZ - UK