Development of Device Technology for Micro-Light-Emitting Diodes
Due to rapid developments of wearable and portable devices in recent years, displays with better efficiency and higher resolution performances are greatly desired. Inorganic micro-light-emitting diodes (μLEDs) have been considered as one of the promising candidates for next-generation display applications, and demonstrations of μLED displays have shown outstanding performances as compared to liquid crystal displays (LCDs) and organic-light-emitting-diode (OLED) displays. However, it has been shown that the maximum efficiency drops as the device dimensions shrink. This dissertation describes post-etch fabrication techniques that suppress or mitigate the size effect in InGaN and AlGaInP μLEDs.
In this dissertation, the concepts of current display technologies, namely LCDs and OLED displays, and μLEDs will be first introduced. Moreover, the mechanism of the reduction in efficiency will also be addressed. Followed by the first chapter, a series of fabrication methods and their improvements to the device performances will be discussed in the chapters 2-4. In chapter 2, the optoelectrical improvements in the InGaN μLEDs with dielectric sidewall passivation using atomic layer deposition (ALD) will be illustrated. This is the first demonstration that the InGaN μLED efficiency can be partially recovered using post-etch technique. Due to the greater surface recombination velocity and minority carrier diffusion length, the effects of ALD sidewall passivation on AlGaInP μLEDs are investigated in chapter 3. This chapter also reveals the excellent optical properties of AlGaInP red emitters, and the potential elimination of the size effect in AlGaInP devices is also shown. In chapter 4, a detailed report on the performances of InGaN devices with the combination of chemical treatment and ALD sidewall passivation is presented, where this is the first demonstration of size-independent efficiency from 100×100 to 10×10 μm2 devices. Since the size effect is mitigated in InGaN and AlGaInP devices, chapter 5 shows a possible mass transfer method that employs fluidic assembly with external forces. Lastly, other critical challenges in μLED displays, brief μLED device process follower, and outlook are discussed concisely in chapter 6.