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• New developments about the electron-blocking mechanism in the UVB LED. • Al-graded AlGaN electron-blocking and p-AlGaN hole-source layers are implemented. • Efficiency droop at 500 mA is remarkably suppressed. • The operating voltages at 150 mA are significantly reduced from 64 to 24 V. • High IQE and high hole injection efficiency are confirmed in the UVB LED. Aluminum gallium nitride (AlGaN) based UVB LEDs are delivering an increase in efficiency under low current drive, however, the devices are confronted with high efficiency droop and high operating voltages during the measurement on bare-wafer under high current drive. Such issues in UVB LEDs are attributed to using conventional p-type multi-quantum-barrier electron-blocking layer (p-MQB EBL) and fixed Al-contents in the p-AlGaN hole-source layer (HSL). When the conventional p-MQB EBL and bulk p-AlGaN HSL was replaced by Al-graded ud-AlGaN EBL and Al-graded p-AlGaN HSL, respectively, the efficiency droop has been remarkably suppressed and almost uniform external-quantum efficiency (EQE) under high injection current was confirmed. Also, the operating voltages under 150 mA drive were significantly reduced from 64 V (conventional p-MQB EBL) to 24 V (new polarized ud-AlGaN EBL) in UVB LEDs. These improvements are attributed to the smooth valance band edge without any potential barrier in both Al-graded ud-AlGaN EBL and Al-graded p-AlGaN HSL for efficient injection efficiency. At the same time the new structure was considered useful for both blocking of high energy electron overshooting toward the p-side as well as Mg-diffusion toward the multi-quantum wells (MQWs) under high current drive. These experimental results were also confirmed by simulations. [ABSTRACT FROM AUTHOR]

Highlights • The crystalline quality improved with increasing the annealing temperature. • The crystalline quality was improved by further growth of AlN. • Decreased anisotropy of XRC implies a reduction in the density of stacking faults. Abstract AlGaN light-emitting diodes (LEDs) have been studied for their use as highly efficient light sources in the deep ultraviolet (DUV) region. The spontaneous and piezoelectric polarization in c -plane AlGaN, produces a strong electric field, which degrades the performance of LEDs. To overcome this polarization problem, growth on less widely used substrates, such as those with semipolar/nonpolar orientations has been carried out. However, the expected improvements have not been forthcoming for AlGaN alloys in the DUV region because of the poor crystallinity obtained. Here, we report on the improved crystallinity of semipolar AlN grown on m -plane sapphire using a high-temperature annealing process. For (10-1-3) and (11-22) AlN layers grown on m -plane sapphire, the crystal quality improved with increasing annealing temperature up to 1700 °C. The crystallinity of (11-22) AlN layer was further improved by further growth of AlN. A best FWHM of 280arcsec was obtained for a 3-μm thick (11-22) AlN layer grown after high-temperature annealing. [ABSTRACT FROM AUTHOR]