## 1 Introduction  
Recently, organic Electro Luminescence (EL) displays are being increasingly adopted in smartphones. This material functions as a self-luminous display, which requires no backlight, and boasts excellent contrast in addition to a wide viewing angle. However, owing to low color reproducibility and concerns regarding durability in high-temperature environments, research and development of micro light emitting diode (\(\upmu\)LED) displays is still ongoing [1, 2]. In a \(\upmu\)LED display, the sub-pixels for colors R (red), G (green), and B (blue) independently emit light, power consumption is low, and the inorganic material is durable over a long lifetime. In particular, \(\upmu\)LED displays are deemed useful for applications in wearable devices, in which flexibility and power consumption are important. However, the realization of \(\upmu\)LED displays faces many challenges. \(\upmu\)LED chips are 10-20 \(\upmu\)m\({}^{2}\) in size, and a smartphone display requires 2 million pieces of micro LEDs. Moreover, the \(\upmu\)LED needs to emit all three colors uniformly with linear control of intensity. Productivity and cost are also important. Display systems with three different LEDs, shown in Figure 1a [3], or a blue LED system in which red and green (RG) phosphors are excited by the blue LED shown in Figure 1b [4] are mainstream. Our group is developing an ultraviolet (UV)-excited RGB system that excites red, green, and blue phosphors using underlying UV-LEDs, as shown in Figure 1c [5].  
Figure 1: Types of Micro light emitting diode (\(\upmu\)LED) displays. (**a**) Three-LED system (**b**) Blue LED + RG phosphor system (**c**) UV-LED + RGB phosphor system.  
In the three-LED system, Al\({}_{\text{x}}\)In\({}_{\text{y}}\)Ga\({}_{\text{1-x-y}}\)N is grown on a sapphire substrate to form blue and green LEDs, whereas Al\({}_{\text{x}}\)In\({}_{\text{y}}\)Ga\({}_{\text{1-x-y}}\)P is generally grown on a GaAs substrate to form the red LED; however, microchips cannot be fabricated in this manner because the materials are fragile. This challenge led to the development of GaN-based red LEDs [6; 7]. Although InGaN-based blue LEDs have been reported to achieve external quantum efficiency (EQE) of 80% or more [8], the EQE of InGaN-based green LEDs is still about 30% [9]. Because of droop [10], the green LED exhibits much lower luminous efficiency than the blue LED [11]. Additionally, even if these problems are addressed, synchronous control may be complicated because the band gaps, driving voltages, and reaction speeds are different for the different colors. Furthermore, though the problem of mounting can be avoided by unifying the structure with a blue LED if we adopt an InGaN blue LED + RG phosphor system, this raises difficulties in achieving color balance and synchronization because the blue LED is self-luminous and the red and green LEDs are excited by a blue light; in this case, the light intensities are significantly different and some differences in reaction speeds are also expected. In the UV-LED + RGB phosphor system where each RGB color is realized by exciting phosphors using a UV-LED with a wavelength of 385 nm, the inside of the RGB element is a flip chip with the same AlInGaN material and the same lateral chip-electrode structure, and these features decrease the difficulty of mounting. Furthermore, as the luminescent efficiency of the phosphor material is improved on the side having shorter wavelength, the range of available RGB phosphor materials broadens. UV light from a UV-LED contains some purple but does not greatly affect the excitation light color, and the high color reproducibility due to high excitation efficiency simplifies the achievement of color balance, synchronization, and controllability [12; 13]. This article reports tests of the internal quantum efficiency (IQE) to reveal how the material, composition, and structure of UV \(\upmu\)LEDs affects performance characteristics in the near-UV region.