Micro-LED based on self-supporting GaN

Micro-LED based on self-supporting GaN

Chinese researchers have been exploring the benefits of using self-supporting (FS) gallium nitride (GaN) as a substrate for miniature light-emitting diodes (LEDs) [Guobin Wang et al, Optics Express, v32, p31463, 2024]. In particular, the team has developed an optimized indium gallium nitride (InGaN) multi-quantum well (MQW) structure that performs better at lower injection current densities (about 10A/cm2) and lower drive voltages for advanced microdisplays used in augmented reality (AR) and virtual reality (VR) devices, where the higher cost of self-supporting GaN can be compensated by increased efficiency.

The researchers are affiliated with the University of Science and Technology of China, Suzhou Institute of Nanotechnology and Nanobionics, Jiangsu Third Generation Semiconductor Research Institute, Nanjing University, Soochow University, and Suzhou Navi Technology Co., Ltd. The research team believes that this micro-LED is expected to be used in displays with ultra-high pixel density (PPI) sub-micron or nano-LED configurations.

The researchers compared the performance of micro-LEDs fabricated on a self-supporting GaN template and a GaN/sapphire template (Figure 1).

Figure 1: a) micro-LED epitaxial scheme; b) micro-LED epitaxial film; c) micro-LED chip structure; d) Transmission electron microscope (TEM) cross-section images.

Metal-organic chemical vapor deposition (MOCVD) epitaxial structure includes 100nm N-type aluminum Gallium nitride (n-AlGaN) carrier diffusion/expansion layer (CSL), 2μm n-GaN contact layer, 100nm low silane unintentional doping (u-) GaN high electron mobility layer, 20x(2.5nm/2.5nm) In0.05Ga0.95/GaN strain release layer (SRL), 6x(2.5nm/10nm) blue InGaN/GaN multi-quantum well, 8x(1.5nm/1.5nm) p-AlGaN/GaN Electron Barrier layer (EBL), 80nm P-gan hole injection layer and 2nm heavily doped p+-GaN contact layer.

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

Chips fabricated on heteroepitaxial GaN/sapphire templates exhibit large performance differences. In particular, the intensity and peak wavelength vary greatly depending on the position within the chip. At a current density of 10 A/cm2, a chip on the sapphire shows a wavelength shift of 6.8 nm between the center and the edges. Of the two chips from the sapphire wafer, one chip is only 76% stronger than the other.

In the case of chips fabricated on self-supporting GaN, the wavelength variation is reduced to 2.6nm, and the intensity performance of the two different chips is much closer. The researchers attributed the wavelength uniformity change to different stress states in homogeneous and heterostructures: Raman spectroscopy showed that the residual stresses were 0.023 GPa and 0.535 GPa, respectively.

The cathodoluminescence showed that the dislocation density of heteroepitaxial wafers was about 108/cm2, while that of homogeneous epitaxial wafers was about 105/cm2. The research team commented, "The lower dislocation density minimizes the leakage path and improves the luminous efficiency. ”

Compared with heteroepitaxial chips, although the reverse leakage current of homogeneous epitaxial LEDs is reduced, the current response under forward bias is also reduced. Despite the lower current, chips on self-supporting GaN have a higher external quantum efficiency (EQE): 14% in one case compared to 10% on sapphire templates. By comparing the photoluminescence performance at 10K and 300K (room temperature), the internal quantum efficiency (IQE) of the two chips was estimated to be 73.2% and 60.8%, respectively.

Based on the simulation work, the researchers designed and implemented an optimized epitaxial structure on self-supporting GaN, which improved the external quantum efficiency and voltage performance of the microdisplay at lower injection current densities (Figure 2). In particular, the homogeneous epitaxy achieves a thinner potential barrier and a sharp interface, while the same structure achieved in the heteroepitaxy shows a more fuzzy outline under transmission electron microscopy.

Figure 2: Transmission electron microscope images of the multi-quantum well region: a) original and optimized homoepitaxy structures, and b) optimized structures realized in heterogeneous epitaxy. c) External quantum efficiency of homogeneous epitaxial micro-LED chip, d) current-voltage curve of homogeneous epitaxial micro-LED chip.

To some extent, the thinner barrier simulates the V-shaped pits that tend to form around the dislocation. In heteroepitaxial LEDs, V-shaped pits have been found to have beneficial performance effects, such as improved hole injection into the light-emitting region, in part due to the thinning of the barrier in the multi-quantum well structure around the V-pit.

At an injection current density of 10A/cm2, the external quantum efficiency of the homogeneous epitaxial LED increases from 7.9% to 14.8%. The voltage required to drive a 10μA current is reduced from 2.78V to 2.55V.