# A Review of AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Sapphire
## 11 EQEs, WPEs and IQEs of AlGaN-Based Bare Dies with Uneven QWs  
Due to the thickness of the p-GaN layer, approximately half of the generated EL propagating to the p-side is immediately absorbed in the p-GaN. To avoid the waste of EL propagating to the n-cladding layer, 430-\(\upmu\)m-thick sapphire was used without thinning. Four samples of DUV-LEDs were fabricated: 266 and 271 nm LEDs were grown on a 265 nm template group (x = 0.67) and 283 and 297 nm LEDs were fabricated on a 285 nm template group (x = 0.61). Dies were placed on an AlN submount (3 \(\times\) 3 mm\({}^{2}\)) with a thickness of 0.015 inch (0.381 mm) and then mounted on TO-5 headers. The specific heat dissipation for TO-5 is limited to 120 mW. Figure 12 shows the EL intensity as a function of the injected current (I-L characteristics) with the I-V characteristics shown in the inset. Figure 13 shows corresponding WPEs. The peak EQEs were obtained at 40 mA. EQEs of 3.5% at 266 nm, 3.8% at 271 nm, 6.1% at 283 nm, and 6.0% at 298 nm were achieved. The load of 40 mA is slightly larger than the specific heat dissipation of TO-5 headers because V\({}_{\text{f}}\) is between 5 and 6 V. The peak WPEs were 3.0% at 266 nm, 3.4% at 271 nm, 5.4% at 283 nm, and 5.5% at 298 nm at DC currents of 40, 20, 10, and 5 mA, respectively. At 100 mA, the WPEs were reduced to 2.7, 3.1, 4.4, and 4.3% at 266, 271, 283, and 298 nm, respectively. The IQEs of these samples could not be precisely determined. However, rough estimates of IQEs are possible using I-L curves [129]. The LEE of the structure with p-GaN has been estimated by several groups to be 8-9% using ray tracing [24,41]. We also estimated on LEE of 8% for our DUV-LEDs. Assuming a fixed LEE and eliminating the effects of the shunt and parallel resistances of the LEDs, the relationship between the EL intensity and injected current density in the junction area enables the current injection efficiency (CIE) and IQE to be calculated. However, this method is based on a fixed LEE and the effect from such as a chipping of sapphire sidewalls was neglected. Therefore, the IQEs were conservatively estimated assuming a LEE of 10% and an improbable CIE of 100%. Thus, conservative IQEs of 35, 39, 61, and 60% were estimated for the 266, 271, 283, and 298 nm LEDs, respectively. The high IQEs of these samples are recognized. Because the TDD was on the order of 10\({}^{8}\)/cm\({}^{2}\), these IQEs led us to consider the carrier localization effect. From our many CIE estimations based on I-L characteristics, CIE at around 265 nm is considered to be about 20-30% smaller compared with that at around 285 nm. The CIE of 70-80% at around 265 nm is considered to be moderately conservative, and the expected IQE ranges are shown in Table 2. The samples correspond to the solid red circles in Figure 1.  
Figure 11: Laser microscopic image of p-GaN surface. The thickness of the p-GaN layer is 0.5 \(\upmu\)m. The roughness (Ra) is 5.86. The p-GaN surface was obtained by erasing the hexagonal pyramids via three-dimensional growth. (Permitted to be reproduced from Jpn. J. Appl. Phys. [13]. © Japan Society Applied Physics). Figure 12: EQEs and I-V characteristics of AlGaN-based DUV-LEDs. The four samples correspond to the solid red circles in Figure 1. (Permitted to be reused from Jpn. J. Appl. Phys. [13]. © Japan Society Applied Physics). \begin{table}
\begin{tabular}{c c c c c} \hline
**CIE/LEE (\%)** & **266 nm** & **271 nm** & **283 nm** & **298 nm** \\ \hline
100/10 & (35) & (38) & 61 & 60 \\
90/10 & (39) & 42 & 68 & 67 \\
80/10 & 43 & 48 & 76 & 75 \\
70/10 & 50 & 54 & & \\ \hline \end{tabular}
\end{table} 
Table 2: IQEs estimated from the fixed LEE and CIE. To show the expected range of the IQE at 40 mA, the values in the lower rows are considered to be the more probable IQEs. The values with CIE/LEE of 100/10 are conservative estimates for 283 and 298 nm LEDs. The values with CIE/LEE of 80/10 are the possible conservative estimates for the 266 nm LEDs, assuming that the CIE at 265 nm is 20% less than those of 283–298 nm LEDs.