LED epitaxial wafers form the core of LED devices, directly determining key optoelectronic properties such as emission wavelength, brightness, and forward voltage. Among all manufacturing techniques, Metal-Organic Chemical Vapor Deposition (MOCVD) plays a dominant role in the epitaxial growth of III-V and II-VI compound semiconductors. Below are several technological advancements and trends shaping the future of LED epitaxy.

1. Optimization of Two-Step Growth Technique

The commercial standard involves a two-step epitaxial growth process. However, current MOCVD reactors can accommodate only a limited number of substrates per cycle—commonly 6 wafers—while 20-wafer configurations are still under optimization. This limitation affects uniformity across wafers. Future directions include:

• Scaling up: Developing reactors that support higher wafer loads to reduce per-unit cost.

• Automation: Emphasis on single-wafer tools with high reproducibility and process automation.

2. Hydride Vapor Phase Epitaxy (HVPE)

HVPE enables rapid growth of thick GaN layers with low threading dislocation density. These films can serve as substrates for homoepitaxial growth via other methods. Additionally, freestanding GaN films separated from the original substrates could serve as alternatives to bulk GaN. Nonetheless, HVPE suffers from poor thickness control and corrosive by-products, which limit material purity.

3. Selective or Lateral Epitaxial Overgrowth

This method significantly improves crystal quality by reducing defect density in GaN layers. A GaN layer is first deposited on a substrate (typically sapphire or SiC), followed by a polycrystalline SiO₂ mask layer. Photolithography and etching expose windows in the GaN layer. GaN then grows vertically in these windows before expanding laterally across the mask.

4. Pendeo-Epitaxy for Defect Reduction

Pendeo-epitaxy offers a way to mitigate lattice and thermal mismatch-induced defects. GaN is grown on substrates like 6H-SiC or Si using a two-step process. Patterned etching creates alternating GaN pillar and trench structures, upon which lateral growth forms suspended GaN layers. This method eliminates the need for a mask layer and avoids material contamination.

5. UV LED Material Development

Efforts are underway to develop short-wavelength UV LED materials, providing a solid foundation for UV-excited white LEDs using trichromatic phosphors. These phosphors, more efficient than conventional YAG:Ce-based systems, have the potential to significantly improve luminous efficacy.

6. Multi-Quantum Well (MQW) Chip Technology

MQW structures introduce layers with varying dopants and compositions during growth, creating quantum wells that emit photons of various wavelengths. This technique allows direct white light emission and reduces complexity in circuit and package design, though it presents considerable fabrication challenges.

7. Photon Recycling Technology

Sumitomo Electric developed a white LED using ZnSe and CdZnSe in 1999. Blue light emitted from the CdZnSe layer excites the ZnSe substrate, producing complementary yellow light, resulting in white emission. Similarly, Boston University achieved white light by layering AlInGaP over GaN-based blue LEDs.

Process Flow of LED Epitaxial Wafers

Epitaxial Growth:
Substrate → Structural Design → Buffer Layer → N-type GaN Layer → MQW Emission Layer → P-type GaN Layer → Annealing → Optical/X-ray Inspection → Wafer Completion

Chip Fabrication:
Wafer → Mask Design & Lithography → Ion Etching → N-electrode Deposition/Annealing → P-electrode Deposition/Annealing → Dicing → Sorting & Binning