Over many decades, photodetectors have been optimised for optical fibre communications in the near-infrared spectrum ranging from the O-band (1260 nm - 1360 nm) to the U-band (1625-1675 nm). Many visible light communication (VLC) systems and demonstrations are based on these devices, resulting in sub-optimum system performance. In order to improve VLC systems, it is imperative that new detectors are developed whose responsivity peaks at around 400 nm. This paper reports such device based on Gallium Nitride (GaN) technology.

We know that light emitting diodes (LEDs) can be operated as photodectors if they are subjected to a negative bias [S. Schmid, G. Corbellini, S. Mangold and T. R. Gross, "An LED-to-LED Visible Light Communication system with software-based synchronization,"2012 IEEE Globecom Workshops, 2012, pp. 1264-1268, doi: 10.1109/GLOCOMW.2012.6477763]. This paper builds on this principle and reports a new GaN micro-LED that is operated as a photodector. Unlike many micro-LEDs, which are grown on a sapphire substrate, this new device uses a silicon substrate for cost and thermal management reasons. It is demonstrated that the new device indeed exhibits peak responsivity at around 400 nm and a maximum 20-dB bandwidth of around 800 MHz. A remarkable data rate of 10.14 Gbps at 1 m distance is achieved with a square shaped device of 50 mm side length using standard practice of orthogonal frequency division multiplexing (OFDM) in conjunction with bit and power loading. To achieve 10 Gbps, an incident power of 60 mW is required. Future work will have to focus on improving the sensitivity by three to four orders of magnitude to be able to use such devices in practical VLC systems.

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