Feb 19, 2013
(In,Ga)N/GaN nanowires shine green and red
Light-emitting diodes (LEDs) based on GaN nanowires are expected to benefit from dislocation-free interfaces and reduced piezoelectric fields. Demonstrators have been made by several groups, which indicate the possibility of white-light generation without phosphor coating. To advance the tailoring and modelling of light emission, researchers at the Paul-Drude-Institut in Berlin have developed specific characterization methods to determine the average Indium (In) content in nanowire ensembles with heterostructures.
GaN nanowires free of extended defects were grown in a self-induced way vertically on silicon by plasma-assisted molecular beam epitaxy. A characteristic of growth processes based on self-organization is a large statistical fluctuation of the diameters and lengths. The samples contained an active region of axial (In,Ga)N/GaN multi-quantum wells grown at the tip of the nanowires.
Statistical information about the thickness and the composition of the quantum wells was obtained thanks to the kinematic simulation of X-ray diffraction (XRD) data, and the systematic variation of the average (In,Ga)N alloy composition in a series of samples was confirmed by resonant Raman spectroscopy.
Complex light emission
Given the composition and thickness of the (In,Ga)N quantum wells, the energy of optical transitions can be computed by solving Schrödinger-Poisson equations in a self-consistent way. The one-dimensional calculation is an established tool to study the behaviour of planar quantum wells. In contrast, the axial nanowire structures in the current work showed a much lower shift in emission energy as a function of alloy composition compared with values predicted by the calculation.
Remarkably, the emission intensity increases with increasing In content, while for planar quantum wells the opposite trend is observed. The scientists consider several types of carrier localizations to play a role in this behaviour. Notably, the non-uniform strain inside the nanowire quantum wells may give rise to lateral carrier separation.
The quantitative experimental analysis of these structures forms the basis for future work on three-dimensional simulations.
Full details on the work can be found in the journal Nanotechnology.
About the author
Martin Wölz is a doctoral candidate at Humboldt-Universität, and a researcher at the Paul-Drude-Institut für Festkörperelektronik, Berlin, under the supervision of Dr Lutz Geelhaar. His research focuses on epitaxial crystal growth.