Highly Efficient RED Superluminescent Light-Emitting Diodes at 625-650 nm
T Superluminescent light emitting diodes (SLEDs) are an interesting alternative to laser diodes (LDs) for applications where short coherence lengths or low speckle noise are required.
SLEDs combine the directionality typical of LDs with the spectral width of LEDs. Their beam-like output enables efficient coupling to external elements while keeping a broadband emission spectrum. These characteristics stem from amplifying the spontaneous emission by stimulated emission within a single or double pass in the waveguide 1.
Therefore, visible SLEDs targeting the red, blue, and green primary colors represent an ideal light source candidate for compact and efficient projection systems with low power consumption and for sequential lighting of DLP-based micro-displays.
As a consequence of the eye sensitivity, the luminous efficiency in the lower red spectral range at 625 nm is almost three times higher than at 650 nm. For typical end user applications, luminous flux values of about 10 lm are desired. Hence, for this spectral region light sources are required that deliver output powers between 40 mW and 120 mW. As a consequence, red SLEDs should operate at wavelengths below 640 nm, where the luminous efficacy of the human eye is much higher.
The best efficacy vs. output power compromise: 635 nm
Based on full 3D simulations of the electro-optical performance, EXALOS has proposed and verified different ways for shifting the emission wavelength down below 640 nm 1.
Simulations of device performance based on the same architecture, but changing the active region composition to achieve emission at shorter wavelength, show that for the current epitaxial design the optimum wavelength for achieving highest luminous flux values is around 635 nm due to the compromise between increasing luminous efficacy and decreasing the internal efficiency of the light emitting active regions.
Using measurement results obtained from fabricated SLEDs operating at 650 nm with the current epitaxial layer structure in the simulation, it was found that the highest luminous flux values can be obtained for wavelengths around 635 nm. Moreover, 635 nm would be the best compromise, the “Sweet Spot”, between the increasing luminous efficacy and the decreasing of the internal efficiency of the light emitting active regions.
To face the challenges of the new design, the shift from 650 nm can be achieved by properly adjusting the material composition of the Quantum Wells – not their thickness – and by using a reflective SLED (R-SLED) geometry. Based on this proposal two grown SLED structures operating at 650 nm and at 635 nm were compared. On this basis, efficient SLEDs in the 635 nm spectral region were produced for the first time.
The measurement results of the experimental designs agreed quite well with the simulations.
The L-I curves obtained from the 635-nm SLED chip are a bit inferior compared to the L-I curves for the 650-nm chip. However, the corresponding luminous flux values at 70 mA were measured with 1.2 lm (650 nm) and 2.1 lm (635 nm) and, therefore, 75% larger for the structure operating at the lower wavelength.2
(b) ex-facet output power obtained at 25°C and an injection current of 100 mA. The luminous
efficacy plotted on the right-hand scale shows the opposite trend with wavelength compared to the output power.
Marco Rossetti, Antonino Castiglia, Marco Malinverni, Christian Mounir, Nicolai Matuschek, Marcus Duelk, Christian Vélez, RGB Superluminescent Diodes for AR Micro-Displays, SID 2018 Digest.