The goal of the project is to develop high-performance white light and green LEDs suitable for visible light communications (VLC)and POF communications, respectively. The current commercial white LEDs are mainly fabricated by the combination of blue LEDs and yellow phosphors. Although the blue LED's response time is at least less than 10^-9 seconds, the response time of the phosphors is typically 10^-3 to 10^2 seconds. Therefore, the bit rate in the VLC system would be limited when the phosphor-based white LEDs are directly served as data transmitters without the use of filter to cut yellow-green light from the phosphors. In this project, we propose to develop GaN-based LEDs with photon-recycling structure (PRS-LEDs) as the core, which multi-quantum well conversion layers are used to replace phosphors. In the PRS LED structures, short-wavelength photons generated by electrical pumping are absorbed in the conversion layers to reemit light in different wavelength to replace the conventional phosphor-based LED. They should exhibit better high-frequency response to meet the requirements of future VLC system. In this project, another target is to develop green LEDs with high performance suitable for POF communications. LED light sources, used in POF communications as data transmitters, are often designed to have a small device area to circumvent RC limited for high-speed operation, but this aggravates the negative effects due to high current densities(e.g., efficiency droop). In the project, we will develop GaN-based green LEDs featuring a blue or violet LED structure, that emits light in the form of electrical pumping (EP), stacked with green quantum wells(QWs), that emits light in the form of optical pumping (OP). In other words, the green QW emits green light and its carrier injection mechanism is no longer electrically excited. Comparing with conventional EP green LEDs, the advantage of this design is that the blue-violet light from the bottom of a PRS LED has a better chance to pump all green QW emitting layers in the PRS LED to reduce the current density of active layer. At the same time, because the electric field does not directly cross the active layer, the energy band bending should be alleviated to reduce the carrier overflow outside the QW layers and thereby exhibit minor efficiency droop. In addition to the improvement in the quantum efficiency, the PRS green LEDs will be designed with thin active layer sandwiched with reflectors, which function as a cavity, to boost its band width in high-speed operation.
|Effective start/end date||08-08-01 → 09-07-31|
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