ABSTRACT
Since the first high-brightness GaN-based blue light-emitting diode (LED) was successfully achieved on flat c-plane sapphire substrate, LEDs have been widely used in diverse areas [1–3], such as displays, traffic signals, automobile headlamps, lightings, etc. The development direction of LED is high-power and high-efficiency general lighting devices based on sapphire substrates. However, further development of such devices is hampered by two main obstacles. First, the crystalline defect density in GaN is high, resulting from the relatively large lattice mismatch between GaN and sapphire [4–6], which would impact the internal quantum efficiency (IQE) of LEDs. Second, the light extraction efficiency (LEE) is low due to severe total reflection effect between GaN and sapphire [7,8]. To alleviate these problems, significant breakthroughs have been achieved. On one hand, two main approaches are proposed to improve IQE. The first is to employ special epitaxial technologies to reduce the crystalline defect density of epitaxial wafers [9–11]. The second is to adopt advanced epitaxial structures to improve carrier radiative recombination rate and avoid carrier leakage [12–14]. On the other hand, the approaches to improve LEE are under active research with various proposed methods, including patterned sapphire substrate (PSS) [15,16], Bragg reflection layers [17], photonic crystal [18], surfaces roughing [19], flip-chip packing [20], etc. However, some of these approaches have drawbacks, increasing the hardness for them in real applications. For example, Bragg reflection layers and photonic crystal bring foreign material layers into the LED epitaxial structures, which produces extra epitaxial difficulties and thus deteriorates the crystalline quality of GaN. Among these approaches, PSS shows great potential because it is capable of improving LEE and crystalline quality simultaneously. It has attracted intensive interests and become a standard procedure for GaN-based LEDs’ manufacturing so far.
