Fig. 1. Device performance and configuration of DUV-LED^[9]. (a) Development of WPE; (b) device structure and light propagation path

Fig. 2. Regulation of polarization degree controlled by energy band. (a) Variation of AlGaN energy band under different stress conditions^[17]; (b) polarization degrees of AlGaN quantum wells under different stress conditions^[19]; (c) variation of polarization degree with thickness of AlGaN quantum well ^[17]; (d) polarization degree of staggered quantum wells^[21]

Fig. 3. Extraction of TM polarization light by shrinking size of active region. (a) Device configuration and (b) far-field light distribution of DUV-LED with full-spatial omnidirectional reflector (FSODR)^[22]; (c) device configuration and (d) WPE of parallel-arrayed planar (PAP) µ-LED ^[29]

Fig. 4. Improvement of light extraction efficiency by roughing interfaces. (a) Cross-section scanning electron microscope (SEM) image of n-AlGaN on nanoporous template (NPT) ^[30]; (b) polarization degree and (c) external quantum efficiency and light output power of DUV-LED on NPT and as-grown template (AGT)^[30]; (d) schematic of nano-patterned sapphire substrate (NPSS) ^[31];extraction efficiency of (e) TE and (f) TM polarization light of DUV-LED on NPSS and traditional substrate^[31]

Fig. 5. Improvement of light extraction efficiency by removing substrates or native substrates. (a) Schematic of nanophotonic structure on DUV-LED with AlN single crystal as substrate^[44]; effect of (b) flat surface and (c) surface with nanophotonic structure on far-field distribution of light ^[44]; schematics of (d) device structure in substrate removal route by electrochemical etching and (e) current flow during electrochemical etching^[47]; device (f) structure and (g) performance in substrate removal route by laser lift-off technology^[50]

Fig. 6. Efficient doping of p-AlGaN with high transmittance. (a) Mechanism of doping in superlattices^[56]; (b) activation energy and (c) UV transmittance of p-AlGaN superlattice with Al mass fraction higher than 50%^[58]; (d) mechanism of polarization-induced doping and (e) variation of hole concentration with temperature^[60]

Fig. 7. Improvement of light extraction efficiency by electrodes or dielectric layer with high reflectivity. (a) Cross-section transmission electron microscopy (TEM) image and (b) UV transmittance of composited p-type region^[66]; (c) UV reflectivity of Al nanodots/Al complex electrodes^[66]; structure of DUV-LED with (d) conventional p-GaN layer and (e) p-type Si nanomembranes, and (f) comparison of device performance^[67]