Multicolour luminescence from InGaN quantum wells grown over GaN nanowire arrays by molecular-beam epitaxy.

The luminescence of InGaN single quantum wells grown by molecular-beam epitaxy under fixed conditions over a series of c-axis GaN nanowire arrays with different geometrical parameters was studied. For arrays with variable GaN average wire diameters and fixed wire densities, the InGaN luminescence peak shifted to higher energy with decreasing wire diameter. It is shown that this trend cannot be attributed to lateral quantum confinement or diameter-dependent InGaN strain. For arrays with variable wire densities and fixed average diameters, the InGaN emission appeared as two distinct bands of different colours, the relative intensities of which depended on the wire density. By optimizing both the GaN wire density and InGaN growth conditions, the colours of the two different bands were combined to realize phosphor-free white light-emitting diodes. The mechanisms for the dependence of the InGaN luminescence on the geometrical parameters of the GaN nanowire array are discussed.