High-speed nanowire LED in the blue/green spectral range

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High-speed optical data communication via free space or polymer optical fibre requires fast, safe, and low-cost light sources. At present red light sources (650 nm) are routinely used for polymer fibre data transport although a blue/green light source (< 520 nm) would provide a much lower intensity loss during fibre transmission. GaN/GaInN light emitting diodes emitting in the blue/green spectral regime are commercially available. However, the layer stacks of these diodes are grown in the c-direction of the wurtzite lattice causing a pronounced spatial separation of electrons and holes in the active InGaN quantum well due to spontaneous polarization. This separation results in long radiative lifetimes up to some 10 ns and fundamentally limits the achievable data rate using GaN/GaInN light emitting diodes.The key idea of this project is to completely eliminate the spontaneous polarization in order to increase the radiative recombination rate and to push the achievable modulation frequency up to above 2,000 MHz. Our approach is a core-shell nanowire arrangement consisting of a n-GaN core followed by a polarization-free m-plane shell composed of an active InGaN quantum well and a p-GaN contact layer. The device is realized by an array of core-shell GaN/InGaN nanowires grown on a conductive n-type silicon substrate. The free-space between the wires will be filled-up and a transparent top layer will serve as the anode of the device. A combination of self-catalytic and selective-area epitaxy shall be developed in order to provide the mandatory homogeneity of the nanowires within an array of nanowires and across the wafer.A distinct challenge is the elucidation of limiting recombination and relaxation mechanisms providing key information for the design and technology of a high-speed light source for optical data transmission. Time-resolved (< 100 ps) optical and optoelectronic measurements will be performed on the nanowire light sources with high spatial resolution (< 0.3 µm) in order to localize device limitations with respect to e.g. speed, intensity, and spectral line width of emission. The results will be directly incorporated into the design and technology of the devices. In parallel, the electrical high-frequency performance of the devices will be evaluated and optimized by scattering parameter measurements and equivalent circuit analysis. The fundamental goal of this project is the scientific development of a light source at frequencies above 2,000 MHz for free-space and polymer optical fibre data transmission.