Micro-LED based on self-supporting GaN

Micro-LED based on self-supporting GaN

Chinese researchers have been exploring the benefits of using free-standing (FS) gallium nitride (GaN) as a substrate for micro light-emitting diodes (LEDs) [Guobin Wang et al., Optics Express, v32, p31463, 2024]. Specifically, the team developed an optimized indium gallium nitride (InGaN) multiple quantum well (MQW) structure that performs better at lower injection current densities (around 10 A/cm²) and lower driving voltages, making it suitable for advanced microdisplays used in augmented reality (AR) and virtual reality (VR) devices. In these cases, the higher cost of free-standing GaN can be compensated by improved efficiency.

The researchers are affiliated with the University of Science and Technology of China, Suzhou Institute of Nano-Tech and Nano-Bionics, Jiangsu Research Institute of Third Generation Semiconductor, Nanjing University, Soochow University, and Suzhou NanoLight Technology Co., Ltd. The research team believes that this micro-LED technology holds promise for displays with ultra-high pixel density (PPI) in submicron or nanometer LED configurations.

The researchers compared the performance of micro-LEDs fabricated on free-standing GaN templates and GaN/sapphire templates.

The epitaxial structure of Metal Organic Chemical Vapor Deposition (MOCVD) includes a 100 nm n-type AlGaN carrier spreading layer (CSL), a 2 μm n-GaN contact layer, a 100 nm unintentionally doped (u-) GaN high electron mobility layer with low silane, a 20x (2.5 nm/2.5 nm) In0.05Ga0.95/GaN strain relief layer (SRL), 6x (2.5 nm/10 nm) blue InGaN/GaN multiple quantum wells, 8x (1.5 nm/1.5 nm) p-AlGaN/GaN electron blocking layer (EBL), an 80 nm p-GaN hole injection layer, and a 2 nm heavily doped p+-GaN contact layer.

These materials are made into 10 μm diameter LEDs with indium tin oxide (ITO) transparent contacts and silicon dioxide (SiO2) sidewall passivation.

The chips fabricated on heteroepitaxial GaN/sapphire templates exhibited significant performance variations. Notably, the intensity and peak wavelength varied greatly depending on the location within the chip. At a current density of 10 A/cm², one chip on sapphire showed a wavelength shift of 6.8 nm between the center and the edge. Among two chips from the sapphire wafer, one chip's intensity was only 76% of the other.

In contrast, the chips fabricated on free-standing GaN showed a reduced wavelength variation of 2.6 nm, and the intensity performance between different chips was much more consistent. Researchers attributed the wavelength uniformity change to the different stress states in homoepitaxial and heteroepitaxial structures: Raman spectroscopy showed residual stresses of 0.023 GPa and 0.535 GPa, respectively.

Cathodoluminescence revealed a dislocation density of about 10⁸/cm² for the heteroepitaxial wafer and about 10⁵/cm² for the homoepitaxial wafer. The research team commented, "The lower dislocation density can minimize leakage paths and improve light emission efficiency."

Although the reverse leakage current of homoepitaxial LEDs was reduced compared to heteroepitaxial chips, the current response under forward bias was also lower. Despite the lower current, the chips on free-standing GaN exhibited higher external quantum efficiency (EQE): in one case, it was 14%, compared to 10% for the chips on sapphire templates. By comparing the photoluminescence performance at 10K and 300K (room temperature), the internal quantum efficiency (IQE) of the two types of chips was estimated to be 73.2% and 60.8%, respectively.

Based on simulation work, the researchers designed and implemented an optimized epitaxial structure on free-standing GaN, improving the external quantum efficiency and voltage performance of micro-displays at lower injection current densities (Figure 2). Notably, homoepitaxy achieved thinner barriers and sharper interfaces, whereas the same structure achieved in heteroepitaxy showed a more blurred profile under transmission electron microscopy inspection.

Thinner barriers to some extent simulate the formation of V-shaped pits around dislocations. In heteroepitaxial LEDs, V-shaped pits have been found to have beneficial performance effects, such as improving hole injection into the emission region, partly due to the thinning of barriers in the multi-quantum well structures around the V-shaped pits.

At an injection current density of 10 A/cm², the external quantum efficiency of the homoepitaxial LED increased from 7.9% to 14.8%. The voltage required to drive a current of 10 μA decreased from 2.78 V to 2.55 V.

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