Fig. 1. Schematic and applications of GaN-based LDs

Fig. 2. Schematics of three-dimensional (3D) hexagonal serpentine mask^[52]. (a) Bottom mask; (b) top mask; (c) 3D hexagonal serpentine mask; (d) sectional diagram of 3D hexagonal serpentine mask

Fig. 3. Surface morphologies of epitaxially grown GaN based on 3D hexagonal serpentine mask in different growth stages^[52]. (a) SEM image at beginning of growth; (b) SEM image at coalescence stage; (c) SEM image of fully coalesced GaN film

Fig. 4. CL images of fully coalesced GaN films. (a) GaN film based on striped serpentine mask^[51]; (b) GaN film based on 3D hexagonal serpentine mask^[52]

Fig. 5. Fitting results calculated using leakage current model. (a) L-I experimental curve and fitting result of green LD^[31]; (b) leakage current ratios of blue LDs with different Al mole fractions in EBL from simulation (circle) and fitting (line) results^[54]

Fig. 6. Schematics of p-type region of LD structure. (a) Schematic of energy band structure and drift leakage current^[54]; (b) diagram of main electron motion process^[16]

Fig. 7. Surface SEM images of ultra-thick MQW samples^[64]. (a) MQW sample with spacers grown at low temperatures; (b) MQW sample with spacers grown at high temperatures

Fig. 8. Structural diagrams and surface morphologies of three MQW samples^[64]. (a) HT spacer, no CR layer; (b) LT spacer, CR layer directly beneath well; (c) LT spacer, CR layer directly above well; (d)‒(f) corresponding surface atomic force microscope (AFM)images

Fig. 9. Room-temperature PL spectra of three MQW samples^[64]

Fig. 10. Schematics of growth structures of five single quantum well (SQW) samples^[64]. (a) Sample A; (b) sample B; (c) sample C; (d) sample D; (e) sample E

Fig. 11. Room-temperature PL spectra for samples with CR layer in different positions relative to quantum well region^[64]. (a) Samples B, C, and D with CR layer below quantum well; (b) sample E with CR layer above quantum well and reference sample A

Fig. 12. Improvement of surface morphology by CR layer^[64]. (a) AFM image before waveguide layer growth; (b) AFM image after waveguide layer growth; (c) AFM image after waveguide layer growth with CR layer; (d) schematic of sample structure before waveguide layer growth; (e) schematic of sample structure after waveguide layer growth; (f) schematic of sample structure after waveguide layer growth with CR layer