Fig. 1. Cross-section SEM images of (a) stripe-shaped PSS, (b) AlN/PSS template, and (c) AlGaN epitaxial film grown on AlN/PSS template. Reproduced with permission^[16]. Copyright 2013, Elsevier.

Fig. 2. (a) Schematic diagram of the DUV LED grown on AlN/Gr/NPSS. (b) EL spectra of the DUV-LEDs with and without the Gr interlayer. Cross-sectional SEM images of AlN films on NPSS (c) without and (d) with the Gr interlayer. AlN has realized complete coalescence below a thickness of 1 µm in (d), which is less than half of the thickness (about 2.4 µm) on bare NPSS in (c). Reproduced with permission^[17]. Copyright 2019, American Institute of Physics Publishing.

Fig. 3. AFM images of the NLs deposited at (a) 950°C, (b) 1050°C, and (c) 1150°C after being recrystallized at 1250°C. AFM images of AlN epitaxial films on NLs deposited at (d) 850°C, (e) 950°C, (f) 1050°C, (g) 1150°C, and (h) 1250°C. Reproduced with permission^[65]. Copyright 2014, Elsevier.

Fig. 4. Cross-section TEM images of AlN/FSS without AlN SL in the (a) g = (0002) direction and (b) g = $(11\overline{2}0)$ direction. Cross-section TEM images of AlN/FSS without AlN SL in the (c) g = (0002) direction and (d) g = $(11\overline{2}0)$ direction. Reproduced with permission^[32]. Copyright 2021, American Institute of Physics Publishing.

Fig. 5. (a) Illustration of samples with four types of distinct AlN layer structures. (b) Dislocation density of samples A–D. (c) FWHM and individual rocking curves of samples A–D. (d) Schematic illustration of the growth mechanism and dislocation annihilation for sample C. Reproduced with permission^[82]. Copyright 2016, Elsevier.

Fig. 6. Cross-section TEM images of (a) AlN/FSS and (b) AlN/NPSS in the g = $(11\overline{2}0)$ direction. FWHM for AlN/FSS and AlN/NPSS at different stages: (c) (002) reflection peak and (f) (102) reflection peak. (d) Enlarged TEM image of the void in AlN/FSS. (e) Schematic diagrams of dislocation behavior in AlN epitaxial film. Reproduced with permission^[85]. Copyright 2020, Elsevier.

Fig. 7. (a) Schematic diagram of AlGaN-heterostructure MSM SBPD. (b) Responsivity of the MSM SBPD with and without photonic crystals. (c) Reflectivity and transmissivity of the AlGaN MSM SBPD with and without photonic crystals. (d) I-V characteristics of AlGaN MSM SBPD. Reproduced with permission^[97]. Copyright 2021, American Institute of Physics Publishing.

Fig. 8. (a) Device structure of the AlGaN MQWs SBPD. (b) I-V characteristic of Al_0.64Ga_0.36N/Al_0.34Ga_0.66N SBPD in the dark. (c) Responsivity spectrum of AlGaN MQWs SBPD at -0.5 V. Reproduced with permission^[89]. Copyright 2017, Institute of Physics.

Fig. 9. Schematic diagrams of (a) Al_0.15Ga_0.85N/Al_0.15Ga_0.85N SAM-APD SBPD and (b) dual-periodic DBR. (c) Reflectivity of single- and dual-periodic DBR. (d) Responsivity of the SAM-APD SBPDs with single-/dual-periodic and without DBR at 10 V reverse bias. The single-periodic DBR is composed of 25 pairs of AlN/Al_0.55Ga_0.45N (A/B). Reproduced with permission^[133]. Copyright 2017, Institute of Physics. (e) Cross-section TEM image of the DBR with 20-pair AlGaN/AlInN/AlInGaN layers. (f) Measured and simulated reflectivity of the DBR with 20-pair AlGaN/AlInN/AlInGaN layers. Reproduced with permission^[134]. Copyright 2016, Springer Nature Publishing Group.

Fig. 10. (a) Schematic and physical images of AlGaN-based p-i-n SBPD. (b) I-V curve and the corresponding current density of the device. (c) Spectral responsivity of AlGaN-based p-i-n SBPD under different bias. The inset shows the responsivity plot in semi-log scale. (d) The corresponding EQE in semi-log scale. The inset shows the variation trend of EQE with applied bias. Reproduced with permission^[149]. Copyright 2020, Elsevier.

Fig. 11. (a) Schematic structure of HDT-modified AlGaN SBPD. (b) Top-view photograph of HDT-modified AlGaN SBPD. Responsivity of AlGaN SBPD (c) without and (d) with HDT modification at various bias. (e) Dark current of AlGaN SBPD with and without HDT modification. (f) Electrical breakdown characteristics of the AlGaN SBPD with and without HDT modification. Reproduced with permission^[151]. Copyright 2021, Optical Society of America.

Fig. 12. (a) Schematic illustration of Pd-decorated Al_0.4Ga_0.6N MSM SBPD. (b) I-V characteristics of Al_0.4Ga_0.6N MSM SBPD with and without Pd NPs in the dark and in the 280/500 nm irradiation. The inset shows the top-view image of the Al_0.4Ga_0.6N MSM SBPD. (c) PDCR-V characteristics for the incident wavelength of 500 and 280 nm for the Al_0.4Ga_0.6N MSM SBPD with and without Pd NPs. (d) Responsivity spectra of Al_0.4Ga_0.6N solar-blind PD with and without Pd NPs at -10 V with incident wavelength from 220 to 300 nm. The inset shows the variation in a broad spectral range from 220 to 500 nm. (e) Responsivity spectra of Al_0.4Ga_0.6N solar-blind PD with and without Pd NPs in the 280 nm irradiation at different voltages. (f) The plot of responsivity with voltage at 500, 280, and 220 nm for Pd-decorated Al_0.4Ga_0.6N MSM SBPD. Reproduced with permission^[155]. Copyright 2022, Institute of Physics.

Fig. 13. (a) Schematic of TiO₂ PEC-UVPDs. (b) Energetics of operation of TiO₂ PEC-UVPDs. Reproduced with permission^[175]. Copyright 2012, Elsevier.

Fig. 14. Schematic illustrations of (a) Pt/AlGaN nanostructures on Si and (b) self-powered Pt/AlGaN PEC-SBPDs. (c)–(e) TEM images and STEM-EDS elemental mapping of Pt/AlGaN-50 nanostructures. Photocurrent densities of AlGaN nanostructures (f) at UV radiation of 254 and 365 nm and (g) at different incident 254 nm solar-blind light intensities. (h) Photocurrent densities of Pt/AlGaN-50 nanostructures at different incident light power intensities. (i) Photocurrent densities and ratios of Pt/AlGaN nanostructures with various Pt loading amounts. (j) Response and recovery time of Pt/AlGaN PEC-SBPDs with different Pt loading amounts. (k) Photocurrent densities and responsivities of Pt/AlGaN-50 nanostructures at different incident light power intensities. Reproduced with permission^[157]. Copyright 2020, American Chemical Society.

Fig. 15. Schematic diagrams of (a) MPC-DUV LED, (b) carrier recombination and generation in MPC-DUV LED, and (c) photon recycle, gain, and output process in the MPC-DUV LED. Reproduced with permission^[182]. Copyright 2019, Elsevier.

Table 1. Summary of the Reported AlN/Sapphire Template

Table 2. Summary of Performance Parameters on the Reported AlGaN SBPDs

Table 3. Summary of Performance Parameters on the Enhanced Techniques of Reported AlGaN SBPDs