Although phosphor-coated white light LEDs have become the mainstream for bright illumination the colour-conversion phosphors used in the mix have largely varying photoluminescence lifetimes. As an example, inspecting the time-resolved yellow emission photoluminescence spectra of a white LED at 450 and 560nm, the researchers revealed a decay time for blue light emission of 1.4ns, much faster than the 54.4ns for the yellow light emission.
This means that when a white-light is modulated to transmit a digital signal, the blue light effectively turns on and off at a faster rate than the yellow phosphor emission, and a broadband Si photodetector ends up getting the slowest common denominator of the signal, slow rise and fall times.
To focus on the fast blue emission, the researchers devised a narrowband, green InGaN LED as the receiver. Their paper “Textured V‑Pit Green Light Emitting Diode as a Wavelength-Selective Photodetector for Fast Phosphor-Based White Light Modulation” published in the ACS Photonics journal details the fabrication of an InGaN LED with a sharp wavelength selectivity at 380nm, using V-pit texturing to improve the peak responsivity of the thin active layer in the InGaN LED.
As 3D finite-difference time-domain (FDTD) simulations of the device show, the textured V-pit surface improves the light absorption through increased angular scattering and optical paths.
The LED structure was grown by metal−organic chemical vapor deposition (MOCVD) on a sapphire substrate. It consists of an undoped GaN layer, a 1μm thick Si-doped n-GaN layer, 10 pairs of InGaN (4nm)/GaN (16nm) QWs, a 40nm thick Mgdoped AlGaN, and a 600nm thick Mg-doped GaN.
The textured top surface of the LED structure consists of randomly distributed hexagonal Vpits (at a density of around 1×109/cm2) with large modulation amplitudes of 300 to 500nm. It was obtained during the p-AlGaN layer growth, by controlling the trimethylgallium (TMGa) gas flow.
The researchers have calculated that the average absorption in the 420 to 480nm range (blue light) is about 2.23 times higher than that of a flat LED with the same emission wavelength and active layer thickness used for comparison. The peak responsivity for the textured LED detector was 0.23A/W, 63% higher compared to 0.139 A/W for a flat LED detector.
The idea behind this research is that using such blue-wavelength-selective photodetector in place of broadband Si photodectors would suffice to speed-up white-light Li-Fi communications four fold without having to replace any of the installed white-light LED bulbs and luminaires.