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Gallium Nitride (GaN) based light emitting diodes (LEDs) which utilize a quantum well structure have been used in the ultra-violet (UV) range for the past decade and have gathered a significant amount of attention for various applications. Despite their usage, they do have several drawbacks that need to be addressed. A team of Researchers from Taiwan have incorporated graphene quantum dots (GQDs) into conventional GaN UV-LEDs in an effort to alleviate some of the issues whilst simultaneously improving the performance these LEDs.
GaN LEDs are used in a wide variety of applications, with common use in sterilization, water/air purification and medical device technologies. These LEDS are generally composed of multiple derivatives of GaN, with InGaN, InAlGaN and p-GaN, and has prompted this specific class to be known as InGaN/InAlGaN UV LEDs.
These LEDs currently work in the UV-range but suffer from several issues, including low material quality in the AlGaN layer, incomplete current spreading, high optical absorption in the p-GaN contact layers and high current-induced degradation.
Whilst many novel strategies, from changing the geometrical design to introducing photonic-crystal structures, have been proposed to improve the overall efficiency through charge carrier confinement mechanisms, many have failed to sufficiently improve the optical output.
In GaN LED structures, photons become emitted outside of the escape cone due to its large refractive index. These emissions lead to a loss of emitted light and a subsequent reduction of the light extraction.
Photon recycling is a way of recapturing the emitted photons which propagate outside of the escape cone, and re-emit them back into the active layer of the escape cone. This causes an overall increase in the extraction efficiency of LEDs. However, in quantum well structures, such as those in GaN, these effects are very seldom seen and is the reason as to why the Researchers chose to incorporate GQDs into the LEDs.
The Researchers grew the UV LEDs on a sapphire substrate using a modified chemical vapor deposition (CVD) method to form multiple quantum well layers. The GQDs were produced by laser ablation methods using an optical parametric oscillator (OPO) laser and were subsequently deposited on top of the InGaN/InAlGaN layered structure, with a concentration of 0.9 mg/ml and an average diameter of 3.5 nm.
To characterize the new LEDs, the Researchers utilized a range of techniques, including transmission electron microscopy (TEM), time-resolved photoluminescence spectroscopy (PL), atomic force microscopy (AFM, PSIA XE-100) and UV-Vis spectroscopy. The Researchers also measured current-voltage (I-V) characteristics using source meter (Keithley-2400), light output-current (L-I) characteristics using a calibrated integrating sphere, and electroluminescence (EL) methods with a 0.75m spectrometer and a photomultiplier tube (PMT).
GQDs have a high optical absorption and improved both the optical and electrical properties of the UV-LEDs. Upon incorporation of the GQDs on to the LED surface, the device experienced an enhancement in to electroluminescence (by up to 71%) and decrease in the series resistance (up to 15.5%).
As the Researchers increased the concentration of GQDs deposited on top of the device, the emitted light, i.e. the series resistance, within the LED increased. The same proportional relationship was seen upon a decrease in the concentration. The light output power achieved a maximum increase of 71%, which was achieved at 0.9 mg/ml.
The significant improvement of the LED device was attributed to photon recycling processes through light extraction from the waveguide mode; and due to the transfer of emitted charge carriers captured by the GQDs and returned to the active layer of the escape cone within the GaN layers.
The research has provided a solution to an old problem and now allows for photon recycling processes to be utilized in GaN UV-LEDs, allowing for more efficient devices to be constructed with fewer drawbacks compared to current GaN UV-LEDs.
Sources and Further Reading
- “Enhanced Performance of GaNbased Ultraviolet Light Emitting Diodes by Photon Recycling Using Graphene Quantum Dots”– Lin T-N., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-07483-3