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]