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, suitable for advanced microdisplays used in augmented reality (AR) and virtual reality (VR) installations, in which case, The higher cost of self-supporting Gans can be compensated for by improved efficiency.

The researchers are affiliated with the University of Science and Technology of China, Suzhou Institute of Nanotechnology and Nanobionics, Jiangsu 3rd Generation Semiconductor Research Institute, Nanjing University, Soozhou University and Suzhou Nawei Technology Co., LTD. The research team believes that this micro-LED is expected to be used in displays with ultra-high pixel density (PPI) submicron or nanometer LED configurations.

The researchers compared the performance of micro-leds manufactured 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 were made into leds with a diameter of 10μm and with indium tin oxide (ITO) transparent contact and silicon dioxide (SiO2) sidewall passivation.

The chips manufactured on the heteroepitaxial GaN/ sapphire template show a large performance difference. In particular, the intensity and peak wavelength vary greatly depending on the location within the chip. At a current density of 10A/cm2, a chip on the sapphire showed a wavelength shift of 6.8nm between the center and the edge. Of the two chips from the sapphire wafer, one is only 76 percent as strong as the other.

For chips made on self-supporting GaN, the wavelength variation is reduced to 2.6nm, and the strength performance of the two different chips is more similar. The researchers attribute the wavelength uniformity variation to different stress states in the homogeneous and heterogeneous structures: Raman spectroscopy shows residual stresses of 0.023GPa and 0.535GPa, respectively.

The cathode luminescence shows that the dislocation density of heteroepitaxial plates is about 108/cm2, while that of homoepitaxial plates is about 105/cm2. "The lower dislocation density can minimize the leakage path and improve the luminous efficiency," commented the research team.

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

Based on the simulation work, the researchers designed and implemented an optimized epitaxial structure on a self-supporting GaN that improves the external quantum efficiency and voltage performance of the microdisplay at lower injection current densities (Figure 2). In particular, homoepitaxy achieves a thinner barrier and sharp interface, whereas the same structures achieved in heteroepitaxy show a more blurred profile under TEM examination.

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.

The thinner barrier partly simulates the V-shaped pits that can easily 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 luminous region, in part due to a thinning barrier in the multi-quantum well structure around the V-shaped pits.

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

ZMSH Sulotion for GaN wafer

The growing demand for high-speed, high-temperature and high power-handling capabilities has madethe semiconductor industry rethink the choice of materials used as semiconductors. For instance,

as various faster and smaller computing devices arise, the use of silicon is making it difficult to sustain Moore’s Law. But also in power electronics, So GaN semiconductor wafer is grown out for the need.

Due to its unique characteristics (high maximum current, high breakdown voltage, and high switching frequency), Gallium Nitride GaN is the unique material of choice to solve energy problems of the future. GaN based systems have higher power efficiency, thus reducing power losses, switch at higher frequency, thus reducing size and weight.