Researchers from China's Jilin University, led by Prof. Yuantao Zhang, have developed wafer-scale high-efficiency blue III-nitride LEDs with a weak polarization electric field (weak-PEF), grown on the standard polar c-plane. The new LEDs offer high efficiency, excellent wavelength stability and strong lateral carrier confinement, which makes them much less sensitive to sidewall effects when shrunk down to microLED sizes.
III-nitride LEDs on the polar c-plane are the industry's mature platform for microLEDs, but the strong polarization electric field in these devices causes the quantum confined Stark effect (QCSE), which reduces the efficiency, makes the emission wavelength unstable and increases the lateral carrier diffusion length, a real problem for tiny microLED pixels. Weak-PEF quantum wells can solve this while remaining compatible with existing production processes, but until now such devices suffered from limited efficiency.
The researchers studied several pulse-growth modes for the InGaN/AlGaN digital alloy (DA) barriers in the MOCVD process, and found that the NH3 delivery during the AlGaN growth period is the key factor affecting the quality of the barriers. Modulating the NH3 flow rate improved the interface quality, suppressed defects and enhanced the optical performance.
The team then produced the weak-PEF LEDs on 4-inch wafers using an industrial MOCVD system. The wafer showed good wavelength uniformity, and the LEDs demonstrated a wavelength stability superior to that of conventional InGaN/GaN MQW LEDs. When made into microLEDs, the weak-PEF devices were about 50% less sensitive to sidewall effects compared to conventional designs.
