Deep-ultraviolet lasing was achieved at 243.5 nm from an Al x Ga 1Àx N-based multi-quantum-well structure using a pulsed excimer laser for optical pumping. The threshold pump power density at room-temperature was 427 kW/cm 2 with transverse electric (TE)-polarization-dominant emission. The structure was epitaxially grown by metalorganic chemical vapor deposition on an Al-polar free-standing AlN (0001) substrate. Stimulated emission is achieved by design of the active region, optimizing the growth, and the reduction in defect density afforded by homoepitaxial growth of AlN buffer layers on AlN substrates, demonstrating the feasibility of deep-ultraviolet diode lasers on free-standing AlN. V

We demonstrate transverse-magnetic (TM) dominant deep-ultraviolet (DUV) stimulated emission from photo-pumped AlGaN multiple-quantum-well lasers grown pseudomorphically on an AlN/sapphire template by means of photoluminescence at room temperature. The TM-dominant stimulated emission was observed at wavelengths of 239, 242, and 243 nm with low thresholds of 280, 250, and 290 kW/cm2, respectively. In particular, the lasing wavelength of 239 nm is shorter compared to other reports for AlGaN lasers grown on foreign substrates including sapphire and SiC. The peak wavelength difference between the transverse-electric (TE)-polarized emission and TM-polarized emission was approximately zero for the lasers in this study, indicating the crossover of crystal-field split-off hole and heavy-hole valence bands. The rapid variation of polarization between TE- and TM-dominance versus the change in lasing wavelength from 243 to 249 nm can be attributed to a dramatic change in the TE-to-TM gain coefficient ratio for the sapphire-based DUV lasers in the vicinity of TE-TM switch.

No abstract

Optically pumped deep-ultraviolet (DUV) lasing with low threshold was demonstrated from AlGaN-based multiple-quantum-well (MQW) heterostructures grown on sapphire substrates. The epitaxial layers were grown pseudomorphically by metalorganic chemical vapor deposition on (0001) sapphire substrates. Stimulated emission was observed at wavelengths of 256 nm and 249 nm with thresholds of 61 kW/cm2 and 95 kW/cm2 at room temperature, respectively. The thresholds are comparable to the reported state-of-the-art AlGaN-based MQW DUV lasers grown on bulk AlN substrates emitting at 266 nm. These low thresholds are attributed to the optimization of active region and waveguide layer as well as the use of high-quality AlN/sapphire templates. The stimulated emission above threshold was dominated by transverse-electric polarization. This work demonstrates the potential candidacy of sapphire substrates for DUV diode lasers.

Data and analysis are presented for the study of efficiency droop in visible III-nitride light-emitting diodes (LEDs) considering the effects of both electron spill-over out of active region and hole injection into the active region. Performance characteristics of blue LEDs with lattice-matched In0.18Al0.82N electron-blocking layers (EBLs) with different thicknesses were measured in order to exclude the effects of strain and doping efficiency of the EBL, and the quantum efficiencies were analyzed taking account of the electron spill-over current and the relative hole concentration. The results suggest that the highest efficiency in LEDs with a 15-nm In0.18Al0.82N EBL is due to relatively lower hole-blocking effect, hence higher hole injection than in LEDs with a 20-nm EBL, while providing a higher potential barrier for reduced electron spill-over than in LEDs with thinner EBLs. This study suggests that the EBL hole-blocking and electron-confinement effects should be considered in order to achieve higher light output power and reduced efficiency droop at high currents.