Following a specialized Masters in Nanosciences and Nanotechnologies, Lou Denaix completed a doctorate at CEA-Leti, which is affiliated with the Grenoble-Alpes University. Alongside her studies, she excels in mountain bike orienteering. A regular participant in world championships and international competitions, she has earned several podium finishes. Her research focuses on improving the production of ultraviolet (UV) and visible emitting LEDs made from III-N semiconductor materials: Aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and alloys of these materials. This technology has numerous applications, including in medical and food sterilization processes, and in UV water purification.
A major constraint of using III-N semiconductor materials stems from the presence of major internal electric fields, which reduce the quantum efficiency of LEDs. Because this constraint can lessen the yield, it’s important to have methods that can manage the electric fields and a technique to characterize them. The proposed method involves using delta doping to reduce bias field effects, or even to reverse the electric field locally. To characterize the doping effect on electrostatic fields, Lou uses an unusual characterization technique called electron holography, produced by Transmission Electron Microscopy (TEM).
For Lou, the greatest challenge was to control TEM manipulation and to reproduce delta doping findings that would fine-tune the electric field in III-nitride semiconductors.
“Presenting my research in Fukuoka, Japan, was a fulfilling experience for several reasons. But one of the highlights of the conference was without a doubt my chance encounter with a samurai.”
Frédéric Barbier holds a diploma in physical measurements. To bridge the gap between art and physics, he studied Art History for a year, which gave him the opportunity to analyze part of the Lascaux caves using synchrotron (ESRF) in Grenoble.
After a few years in the industry, he worked at CEA-Liten at INES, the French National Institute for Solar Energy, before joining CEA-Leti to work on producing red emitting microLEDs. Although it was initially a challenge in the field of micro-displays, it is now included in most embedded augmented reality devices.
The building blocks of these micro-displays are microLEDs produced using MOCVD epitaxy incorporating gallium nitride (GaN). By stacking InGaN/GaN or InGaN/InGaN materials, which make up the LED active region, light emission is possible using electric stimulation.
The indium concentration in the active region must be adapted to generate pixels in different colors (RGB). The efficiency of blue and green LEDs is now sufficient, thanks to their low indium concentration, and they can be used for the native emission of these devices.
However, producing pure red light is more challenging. Generating a light emission of approximately 630 nm requires a very high concentration of indium, leading to major material degradation, and thereby reducing the efficiency of this type of LED. Different GaN and InGaN lattice parameters cause numerous defects during epitaxy, along with a limited indium concentration in the LED's active region.
To address this problem, Frédéric and his team created a new relaxed InGaN substrate by micro-structuring a planar (2D) InGaN layer mounted by epitaxial deposit on GaN, which was itself on a sapphire substrate. This InGaN block structure relaxes the material, making for more compatibility when indium is incorporated into the active region.
One of the challenges of micro-structuration is its characterization at room temperature, as well as anticipating its behavior at high temperatures during the epitaxy of the LED active region.
Characterizing the blocks at temperatures of up to 700 °C revealed a second relaxation phenomenon, which occurs during the annealing process. Our next challenge will therefore be to better understand this phenomenon and its distribution among the blocks. The same study is also underway for micro-LEDs formed on a silicon substrate.
Participating in a conference that specializes in nitrides and presenting the findings of my team were already quite an experience. Because our materials are innovative, having the chance to talk about our findings among nitride colleagues was very rewarding!”
Conclusions and next steps
While Lou will continue contributing to our understanding of the electrostatic properties of III-N semiconductors and the use of delta doping to improve its reproducibility, while Frédéric would like to improve the efficacy of red LEDs.