Fig. 1. Comparison of performance of mainstream display technologies

Fig. 2. Diagrams of Micro-LED lighting structures. (a) Diagram of light emission for formal Micro-LED; (b) diagram of light emission for flip-chip Micro-LED

Fig. 3. Techniques used to increase light extraction efficiency in LED chips^[61]. (a)‒(c) Light propagation from high-refractive-index medium to low-refractive-index medium; (d)‒(f) surface roughening; (g)‒(i) designed specific shapes of chips to redirect light rays at each bounce and increase subsequent$q_{\mathrm{L}\mathrm{E}\mathrm{E}}$

Fig. 4. Design diagrams for Micro-LED^[89]. (a) Reference sample; (b) after hydrogen passivation

Fig. 5. PhC structures on surfaces of GaN-based LED^[101]. (a) Schematic diagram of traces of light emitting from PhC-based LED; (b) side view SEM image of PhC on ITO surface of GaN-based LED; (c) side view SEM image of PhC on surfaces of p-GaN layer, side-wall and n-GaN layer

Fig. 6. Design schematics for LED with PhC. (a) Micro-LED with etching of p-GaN^[105]; (b) cross-sectional view of devices A and B as well as 3D diagram of 3D-PhC backside reflector^[106]; (c) diagram of structure of DUV LEDs with and without PhC on p-AlGaN contact layer^[107]; (d) PhC phosphors^[108]; (e) 3D illustration of nanowire LED with graphene electrode^[112]; (f) 2D cross-sectional diagram of nanowire LED with graphene electrode^[112]

Fig. 7. Design diagrams of LED with microlens. (a) High efficiency OLED device structure with microlens^[132]; (b)(c) diagrams of lens operation and structure of lens matrix^[133]; (d) cross-section schematic of alternative tandem device structure^[134]; (e) basic architecture of NE-LFD^[135]; (f) structure of Micro-LED array chip and structure of Micro-LED array with MLA^[136]; (g) schematic of collimated LED array encapsulated with Mushroom-Cap and cross-sectional view and typical optical path of LED array^[137]

Fig. 8. Design schematics for LED with metasurface. (a) Schematic of conventional lens or metasurface for achieving beam collimation^[157]; (b) traditional Micro-LED light output^[157]; (c) schematic of beam collimation of metasurface Micro-LED^[157]; (d) working principle of LED with metasurface^[150]; (e) emission diagrams of LED in various configurations^[150]; (f) schematic of resonant cavity light-emitting diode with TiO₂ metasurface^[151]; (g)(h) optoelectronic performance of GaN-based LEDs with disordered hypersurface structures^[154]

Table 1. Comparison of research schemes of PhD techniques of LED

Table 2. Summary of research on MLA of LEDs

Table 3. Summary of research on LED metasurface techniques

Table 4. Summary of research on light extraction efficiency and beam shaping of LEDs