Just in Time for the Holidays, Reds and Greens Glow Bright With New InGaN LED Research
InGaN may be a forerunning substrate for up-and-coming LED technology. Now, researchers are devising native red LEDs and bridging the "green gap."
GaN-based light emitters are said to hold great promise in the search for more efficient light sources and laser emitters. Significant research in indium gallium nitride (InGaN) devices is underway to realize a single-source technology for white light generation.
MicroLEDs are a key component of AR technologies. Image used courtesy of Porotech
Here is a round-up of research on InGaN-based LED technologies. While looking at three different announcements, we will examine some of the milestones and challenges associated with bringing InGaN mainstream.
"World’s First" Native Red InGaN MicroDisplay
Porotech, a start-up that has its roots in the material science department at the University of Cambridge, claims to have the world’s first indium gallium nitride micro-display with natively emitted red color.
The breakthrough, which produced a microdisplay of 0.55 inches diagonally at 960x540, is said to be a key milestone in the development of augmented reality technology.
First microdisplay in native red with the use of InGaN technology. Image used courtesy of Porotech
This finding is a significant material sciences achievement because of previous limitations with GaN-based light emitters—that is, GaN light emitters are typically characterized as having limited scalability and efficiency due to different chemistries among the light-emitting devices.
GaN-based LEDs originally were used natively in the blue/violet spectrum and later added green wavelengths. Up until this year, red wavelength emissions were produced using aluminum indium gallium phosphide (AllnGaP). However, GaN-on-silicon has been used in microdisplays for augmented reality in the past.
This mix of technology complicates manufacturing and suffers from quantum efficiency limitations at small device sizes.
MIT’s SMART Partnership Also Takes On Red InGaN LEDs
In a year where red means go (for InGaN LEDs), the Singapore-MIT Alliance for Research and Technology (SMART) has also created a novel method to produce yellow/red/orange wavelengths in indium gallium nitride devices.
By taking advantage of natural defects in the growth of the GaN materials, the researchers were able to form richer indium quantum wells that can emit longer red wavelengths. The researchers call these indium-heavy quantum wells v-pits.
Using growth defects to produce longer wavelength LEDs. Image used courtesy of MIT
This is important because the higher concentration of indium has been a pain point for manufacturing semiconductor structures in the past. Additionally, the researchers' novel approach uses a silicon substrate, which simplifies the growth process.
NC State Is Bridging the “Green Gap” for InGaN LEDs
Rounding out the advancements in InGaN LED research, NC State has developed a new process that is said to better address the “green gap” in III-nitride semiconductor LEDs and improve the density of holes in p-type materials. Their objective is to develop more efficient lasers and LEDs.
The green gap in two popular LED chemistries (2018). Image used courtesy of Reeves and Ze
The most common growth method for manufacturing these types of LEDs is metal-organic chemical vapor deposition (MOCVD). The MOCVD manufacturing process is described as a limiting factor in the production of hole-rich p-type III-nitrides.
NC State researchers applied a growth technique called semi-bulk growth to make templates of indium gallium nitride, composed of dozens of layers of InGaN on GaN.
In addition to using this template to grow the n-type materials, they discovered they could use the semi-bulk approach to grow the p-type materials as well.
NC State says this new process increases the holes available in a p-type III-nitride by an order of magnitude over existing methods, up to 5 x 1019 cm-3. The overall result is more light and less wasted heat.
Further work led to a shift in the spectrum output of the InGaN LEDs. By using their newly developed templates, instead of a GaN substrate, the team produced more efficient green/yellow LEDs.
Addressing the green gap with a new process to produce efficient InGaN yellow light. Image used courtesy of NC State
InGaN Challenges to Producing Single-Source White LEDs
Researchers have had trouble with using AlInGaP for LEDs because it isn't compatible with InGaN in a small-scale pixel design. It also has low thermal stability and poor mechanical properties.
Some of the remaining challenges with InGaN include efficiency droop with a higher current injection or high power. The source of the efficiency droop may be an electron leakage in p-doped layers or Auger recombination inside the InGaN quantum wells, for instance.
Porotech's native red microdisplay—along with its SMART partnership using crystal defects in GaN—appears to be addressing issues in AlInGaP. While tackling issues related to hole density and the generation of green/yellow light, NC State appears to be working on the overall quality of quantum well photon generation.
These advancements in InGaN technology may indicate that researchers are beginning to solve the challenges associated with GaN-based light sources.