Fig. 1. (Color online) State of the art of UV-LEDs in the wavelength of 210−320 nm. The wall-plug efficiency values are plotted as reported or calculated from reported voltage, current, and light output power. Data obtained from Refs. [9, 16−47].

Fig. 2. (Color online) (a) Schematic band diagram and carrier injection in AlGaN-based UV-LEDs. (b) Equivalent circuit diagram for AlGaN-based UV-LEDs with the flip-chip configuration. The voltage drops are estimated under the current densities of 50−100 A/cm^{2 [12]}.

Fig. 3. (Color online) State of the art of (a) conductivity and (b) electron concentration in n-doped Al-rich AlGaN. Data obtained from Refs. [50, 52−65, 70, 74−76].

Fig. 4. (Color online) Theoretical formation energy change of (a) V_Ⅲ and (b) C_N as a function of Ⅴ/Ⅲ ratio and growth rate (proportional to the metalorganic flow)^[52]. Experimental trade-off of the (c) growth temperature and (d) Ⅴ/Ⅲ ratio to suppress the formation of V_Ⅲ-nSi and C_N^[53].

Fig. 5. (Color online) (a) Conductivity and (b) electron concentration as a function of Si concentration in n-Al_0.7Ga_0.3N^[59].

Fig. 6. (Color online) (a) The activation energy of Si in Al-rich AlGaN with Al composition higher than 80%^[66]. (b) Configuration coordinate diagram of the DX center in Si-doped Al(Ga)N^[74].

Fig. 7. (Color online) (a) Activation energy of Mg in dependence of Al composition in AlGaN. Data obtained from Refs. [9, 79−87]. (b) Formation enthalpies of Mg_Ga/Mg_Al as a function of Al composition in bulk AlGaN under N-rich growth condition^[88].

Fig. 8. (Color online) (a) Schematic illustration of valence band for p-type superlattice (SL) doping^[92]. (b) The vertical miniband transport of holes in Al_0.63Ga_0.37N/Al_0.46Ga_0.54N SLs with a constant barrier thickness of 0.75 nm^[96].

Fig. 9. (Color online) (a) Schematic illustration of polarization-induced p-type doping in graded polar heterostructures^[79]. (b) Directions of the spontaneous and piezoelectric polarization in strained N-polar AlGaN/GaN and Al-polar AlGaN/AlN heterostructures^[98].

Fig. 10. (Color online) (a) Formation enthalpies of Mg_Ga/Mg_Al as a function of Al composition on the surface of AlGaN under N-rich growth condition^[88]. (b) Schematic diagram of the Mg-rich profiles by periodic interruptions of Al and Ga source during growth^[88].