The Challenges of Micro LED Display Technology

In the era of artificial intelligence and big data, display devices are no longer simple information displays, but interactive, high-fidelity, and immersive information interaction terminals, which put forward requirements for display devices such as spatial three-dimensional images, interactive, energy-saving, thin, flexible, foldable and curly, and large size. Micro-LED display technology was born. With the industry players continuing to increase the layout, it’s crucial to develop new technology.

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What is Micro LED?

Micro-LED display technology is a self-luminous display technology, through the array of micro-level LED light-emitting devices (μLED) integrated into the active addressable driver substrate, in order to achieve individual control and lighting, so as to output the display image. The micro-LED display has many advantages such as self-illumination, high resolution, low response time, high integration, high reliability, etc., and small size, high flexibility, easy to disassemble and merge, can be used in any existing display applications from small to large size. In many application scenarios, compared with liquid crystal displays (LCD) and organic light-emitting diode displays (OLED), Micro LED displays can play a more excellent display effect.

Micro LED epitaxial technology challenges

Although Micro-LED display technology is rapidly developing, the shift from lighting applications to display applications makes it more demanding and challenging for LED epitaxy.

  • Substrate Material Selection

The choice of substrate material and epitaxial technology has a critical impact on the performance of Micro-LED devices. Since the micro-LED chip is smaller than the traditional chip less than 50μm, its extremely high yield and uniformity requirements put forward higher requirements and challenges for the substrate selection and epitaxy technology. When applied to high-resolution displays, the injection current density of Micro-LED is very low, and the non-radiative compound caused by defects is particularly prominent, which greatly reduces the light output efficiency of Micro-LED, and therefore Micro-LED requires epitaxial wafers with lower defect density.
The substrates that can be used commercially on a large scale include sapphire, SiC, and Si substrates, but these substrates are used as GaN epitaxy for heterogeneous epitaxy, which has a high dislocation density due to the lattice mismatch and thermal mismatch between the heterogeneous substrate and GaN epitaxial layer. Compared with heterogeneous substrates such as sapphire, SiC, and Si, the choice of GaN materials as substrates can greatly improve the crystal quality of epitaxial wafers, reduce the dislocation density, and improve the device operating life, luminous efficiency, and device operating current density. However, the preparation of GaN single crystal substrate is very difficult, GaN substrate is very expensive, and the maximum size of only 4 inches (10.16 cm), so it is difficult to meet the needs of commercialization.

  • Wavelength uniformity control

Micro-LED display technology is a self-emitting display technology. In high-resolution display applications, the difference in color rendering caused by the uneven emission wavelength of Micro-LEDs can greatly affect the display effect. To ensure the display effect, the standard deviation of wavelength variation of Micro-LED epitaxial wafers needs to be controlled at 0.8 nm or less. Therefore, the control of airflow and temperature uniformity is particularly important in the epitaxial growth of InGaN/GaN quantum wells by metal-organic chemical vapor deposition (MOCVD).

Optimizing the airflow uniformity in the MOCVD epitaxial growth process plays a crucial role in the improvement of LED wavelength uniformity. At present, Prismo UniMax, the latest domestic micro MOCVD equipment, adopts zonal temperature control technology to ensure the temperature field balance throughout the epitaxial growth and uses a series of strain control technologies such as MO source and airflow uniformity to enhance the wavelength uniformity of LED epitaxial wafers to meet the demand of micro-LED display. For micro-LED applications with high requirements for wavelength uniformity, the graphite tray design can be optimized to make it have a certain curvature to better match the epitaxial wafer warpage during the epitaxial growth process to achieve further improvement of temperature uniformity control.

  • Defect control

Dislocations as non-radiative composite centers and leakage channels can significantly affect the chip Micro-LED performance. Due to the small size of Micro-LED and low injection current density, its optoelectronic performance is very sensitive to the dislocation density. Currently, patterned substrate technology and buffer layer technology are more often applied to the heterogeneous epitaxial growth of GaN on sapphire or silicon substrates to reduce the dislocation density and improve the crystal quality. The homogeneous epitaxy technology on high-quality GaN substrate can effectively reduce the dislocation density of LED epitaxial wafers.

From general LED lighting to Mini-LED display and then to Micro-LED display, the chip size is shrinking and the difficulties are increasing. micro-LED not only needs breakthroughs in epitaxial technology but also the renewal of MOCVD equipment is equally important. At present, Micro-LED is subject to cost and yield, looking ahead, the large-scale commercialization of Micro-LED is still has a long way to go.