# Micro-LEDs, a Manufacturing Perspective
## 4 Full-Color Schemes  
As for most micro-display applications, full-colors (composed of the three primary colors red, green, and blue) are needed for each pixel. The widespread display technology LCD is based on light generated by an LED backlight and passing through a matrix of liquid crystal \"light switches\" and color filters constituting the individual subpixel. While micro-LED display is based on a matrix of tiny light emitters made of micro-LEDs for each subpixel in which brightness can be individually controlled. However, III-nitride LEDs, similar to other heterojunction-based semiconductor structures, are nearly monochromatic emission sources, emitting light at a single color, with its wavelength determined mainly by the indium content in the InGaN/GaN multiple quantum wells (MQWs). Although several approaches exist to produce full-color micro-displays from single color micro-LEDs, it remains to be a challenge for developers. The most straightforward approach for full color is to assemble discrete commercial LEDs with the three different colors to form an array, with each pixel containing red, green, and blue LED dies. In theory, InGaN/GaN MQWs LEDs can generate all these three colors by tuning the indium content in the MQWs. Unfortunately, InGaN-based green and red LEDs still possess a relatively low luminescence efficiency, which partially results from large lattice mismatch between the InGaN active layer and GaN buffer for green and red wavelengths. This limits the progress in developing full-color displays using GaN family alone. GaP/GaAs based LEDs can be used to emit red light, but may suffer from even more significant drop in EQE as the size shrinks [30]. To enhance the color quality, Soitec, a French company, has developed a substrate technology called InGaNOX [54; 55]. The initially strained 200 nm thick InGaN layer with indium content from 1.5% to 8% is grown on a GaN/sapphire substrate. Then the thin InGaN layer is transferred using Soitec's Smart Cut(tm) technology onto a compliant layer deposited on a substrate. After patterning, the strain-relaxed, Ga-face-exposed InGaN layer with an in-plane lattice parameter ranging from 3.190 to 3.205 A is transferred to a final handle substrate with sizes up to 300 mm, which can be sapphire, glass, or silicon. By using this technology, InGaN micro-LEDs with full-color emission (without using phosphor conversion) grown on the same wafer have been fabricated. Red LEDs with emission wavelength up to 630 nm and a chip size of 100 \(\times\) 100 \(\upmu\)m\({}^{2}\) have been demonstrated by Soitec. Another approach utilizes single color micro-LED array but with color converters. The color of the pump LEDs in the LED arrays can be blue or ultraviolet (UV), and phosphors or quantum dots (QDs) can be employed as color converters. When blue micro-LED pump is used, only red and green color-converters are needed with blue partially getting through; while the employment of UV micro-LEDs requires blue, red, and green color-converters. One major challenge in obtaining full-color with this method is the precise placement of the color-converters on to the sub-pixels with a very small size. A variety of technologies can be used to deposit color converters for LED color-conversion, including spin coating, mist coating, pulse-spray coating, stamp printing, ink jet printing, and aerosol jet printing [56; 57; 58; 59]. Among all of these technologies, aerosol jet printing has the advantages of being precise, non-contact, masks-less, and capable of depositing highly viscous inks and depositing more materials in one printing step. Therefore, this method is deemed as a suitable approach for application in color-conversion in high-resolution micro-LED displays.