Fig. 1. (Color online) Schematic diagram of the sample structures for two different growth methods: ELOG method (a) and the method grown on AlN strain modulation layer (b).

Fig. 2. (Color online) Schematic diagram of in-situ reflectance curve in initial stage of AlGaN growth^[25].

Fig. 3. (Color online) The in-situ reflectance curves of AlGaN samples with different growth rate (GR). The GR controlled by TMGa flow rate, it is 34.7 sccm for the sample with small GR, 69.4 sccm for the sample with large GR.

Fig. 4. (Color online) Dependence of the low momentum parameter S (a) and high momentum parameter W (b) on positron incident energy in these five samples. (c) Relationship between S and W of the five samples. (d) S parameter and peak responsivity of AlGaN detector versus AlN mole fraction of AlGaN.

Fig. 5. (Color online) Previously reported resistivity value as a function of the AlN mole fraction of n-AlGaN layers. Our results are shown as red solid star symbols^[33].

Fig. 6. (Color online) PL peak energy each as a function of temperature for samples A (3 nm) and B (6 nm).

Fig. 7. (Color online) The schematic diagram of LD structure.

Fig. 8. (Color online) Spatial distribution of electron concentration around WG layers and active region under different injection current values from 20 to 300 mA for samples LD1 with 50 nm-thick WG layer (a), LD2 with 150 nm-thick WG layer (b). (c) Shows lasing wavelength of 357.9 nm in the electroluminescence spectrum of another UV LD under RT pulsed operation condition at an injection current of 1550 mA. (d) Shows the RT output power of the 357.9 nm LD as a function of injected current. The inset shows a photo of the laser with the blue far field pattern of the laser beam image formed on the white paper screen.

Fig. 9. (Color online) Pulsed light output power versus injection current of LD A (a) and LD B (b) as a function of temperature. Temperature dependences of CW P−I−V characteristics of LD A (c) and LD B (d). The insets of (a) and (b) show ln(I_th) as a function of temperature for LD A (a) and LD B (b).

Fig. 10. (Color online) P−I and I−V curves of LD A (empty symbols) and LD B (full symbols) under CW operation for LD A and LD B.

Fig. 11. (Color online) I−V curves (square points) and P−I curves (circle points) of LD B’ (red) and LD B’’(black) under CW operation.

Fig. 12. (Color online) Optical output power curve (red) and the corresponding voltage curve (blue) as a function of the aging time for the unsealed LD under a CW operation with an injection current of 800 mA.

Fig. 13. (Color online) Optical micrographs of the top view of LD front cavity facet for virgin (a) and aged (b) LD, respectively, where the waveguide ridge region can be seen by two green parallel lines. The deposits at front facet are marked by a red ellipsoidal circle in (b). SEM images (c) of the front cavity facet for aged LD. A biforked deposit is clearly observed in the side view.

Fig. 14. (Color online) (a) SEM images of the front cavity facet for aged flip chip packaged LD with lasing wavelength of 384 nm, where the QW region is located between two parallel dashed blue lines; (c) is the enlarged image of (a) under ridge region; (b) and (d) are the EDS images for the regions in (a) marked with red ellipsoidal circle and thick red crosshair, respectively.