6. Increasing the electron injection efficiency (EIE) by introducing an MQB

EIE into the QW is reduced due to the electron leakage caused by the low hole concentrations in the p-type AlGaN layers. The EIE reduction is particularly severe for LEDs with wavelength shorter than 260 nm, because an electron barrier height of an EBL becomes smaller [17]. We introduced a MQB [51, 52] to serve as an EBL, and consequently achieved a marked increase in EIE [18].

than two orders of magnitude smaller than the main peaks. The output power of the 227 nm LED was 0.15 mW at an injection current of 30 mA, and the maximum EQE was 0.2% under pulsed operation at RT. Figure 14 shows (a) the EL spectra for various injection currents and (b) the current-output power (I-L) and current-EQE (ηext) (I-EQE) characteristics for a 222 nm AlGaN-MQW LED measured under pulsed operation at RT [20]. Single-peak operation at

Figure 14. (a) EL spectra for various injection currents and (b) the output power and EQE (ηext) vs current characteristics

for a 222 nm AlGaN-MQW LED measured under pulsed operation at RT.

Figure 13. EL spectra on a log scale of a 227 nm AlGaN DUV LED for various injection currents.

138 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

Figure 15 shows schematic illustrations of the electron flow for an AlGaN DUV LED with (a) a MQB EBL and (b) a conventional single barrier EBL. In usual case, we are using single barrier EBL for 250–280 nm UVC LEDs. However, the electron barrier height of the single barrier EBL is determined by the bandgap of the barrier material, and it is not sufficiently high for UVC LED with wavelength shorter than 260 nm. On the other hand, we can increase the 'effective' barrier height of the EBL by introducing MQB. Even electrons having higher energy above the MQB band-edge can be reflected by the multi-reflection effects of the MQB, and injected into the QWs, resulting in higher EIE.

Figure 16 shows the electron transmittance through an AlGaN MQB and a conventional single barrier EBL for a 250 nm AlGaN LED calculated by a transfer-matrix method. It was shown, using barriers with thickness modulation, that the 'effective' barrier height of an AlGaN/ AlGaN MQB is up to twice that of a conventional single-barrier EBL.

Figure 17 shows a schematic diagram of the structure of a 250 nm AlGaN QW DUV LED with an MQB EBL and a cross-sectional TEM image of a fabricated device. We carried out experiments to find an appropriate MQB structure, and found that the insertion of an initial thickbarrier is important for reflecting low energy electrons. We also found that thin barriers contribute to the reflection of higher-energy electrons. The optimum MQB comprised five layers of Al0.95Ga0.05N/Al0.77Ga0.23N with thicknesses of 7/4/5.5/4/4/2.5/4/2.5/4 nm, in which

the barriers are in bold type and the valleys are normal type. The coherence length for obtaining the multi-reflection effect of the MQB means the total thickness of the MQB should

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 141

Figure 17. Schematic structure and cross-sectional TEM image of a 250 nm AlGaN QW DUV LED with an MQB.

Figure 18 shows (a) the I-L and (b) I-EQE characteristics for 250 nm AlGaN MQW LEDs with an MQB and with a single-barrier EBL, both measured under cw operation at RT. These show significant increases in output power and EQE when the single-EBL is replaced by the MQB. The maximum output powers of LEDs with the MQB and with the single-barrier EBL are 15 mW and 2.2 mW, respectively, and the introduction of the MQB has increased the EQE by a

Figure 18. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for 250 nm AlGaN-MQW LEDs with

be less than 40 nm.

an MQB and with a single-EBL.

Figure 15. Schematic images of the electron flow in AlGaN DUV LEDs with (a) a MQB EBL and (b) a conventional single barrier EBL.

Figure 16. Electron transmittance through AlGaN/AlGaN MQB (red-line) and conventional single barrier EBL (blackline) calculated for a 250 nm-band AlGaN-QW LED.

Figure 17. Schematic structure and cross-sectional TEM image of a 250 nm AlGaN QW DUV LED with an MQB.

the barriers are in bold type and the valleys are normal type. The coherence length for obtaining the multi-reflection effect of the MQB means the total thickness of the MQB should be less than 40 nm.

Figure 18 shows (a) the I-L and (b) I-EQE characteristics for 250 nm AlGaN MQW LEDs with an MQB and with a single-barrier EBL, both measured under cw operation at RT. These show significant increases in output power and EQE when the single-EBL is replaced by the MQB. The maximum output powers of LEDs with the MQB and with the single-barrier EBL are 15 mW and 2.2 mW, respectively, and the introduction of the MQB has increased the EQE by a

Figure 15. Schematic images of the electron flow in AlGaN DUV LEDs with (a) a MQB EBL and (b) a conventional single

140 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

Figure 16. Electron transmittance through AlGaN/AlGaN MQB (red-line) and conventional single barrier EBL (black-

barrier EBL.

line) calculated for a 250 nm-band AlGaN-QW LED.

Figure 18. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for 250 nm AlGaN-MQW LEDs with an MQB and with a single-EBL.

factor of approximately 4. From Figure 18, we estimate that the EIE would have been improved from approximately 25% to more than 80% by introducing the MQB.

Figure 19 shows the I-L characteristics for a 237 nm AlGaN MQW LED with an MQB and a 234 nm LED with a single-barrier EBL, both measured under cw operation at RT. The increase in EIE when using the MQW was found to be extremely high. The output power has been increased by a factor of 12 by replacing the single-barrier EBL with a MQB.

Figure 20 shows the wavelength dependence of the EQE for AlGaN DUV LEDs with MQBs and single-barrier EBLs. Introducing the MQB has increased the EQE by 10, 4 and 3 times for 235, 250 and 270 nm AlGaN LEDs, respectively. We obtained a cw output power of 33 mW from a 270 nm AlGaN-MQW LED with an MQB on a bare chip, but expect to get higher output power by using flip-chip geometry and heat dissipation. The value of EQE was 3.8% in the absence of any means to increase LEE [21].

RIKEN and Panasonic have developed commercially available UVC LED modules for use in sterilization in 2014 [25, 26]. To develop commercially available devices with constant high EQEs and long device lifetimes, the reproducibility and uniformity of the AlN template and the AlGaN LED layer structure need to be maintained. Reproducibility is particularly difficult because the growth conditions are very sensitive to the vapor-reaction between NH3 and TMAl induced by high growth temperatures (1250–1400C). We achieved highly uniform ML-AlN templates on sapphire in a 3 2 inch MOCVD reactor using pulsed NH3 flow. The

> fluctuation in FWHM for these was within 4%. We consistently obtained FWHMs of the XRC (10–12) of 340 arcsec for these templates. These highly uniform and low TDD templates are suitable for producing commercial DUV LEDs. Figure 21 shows (a) a photograph of a 270 nm 10 mW DUV LED module containing 6 chips and (b) the operating properties of this module for applications to sterilization. Lifetimes longer than 10,000 h have already been achieved for

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 143

Figure 21. (a) Photograph of a commercially available 270 nm 10 mW DUV LED module developed by RIKEN and

Panasonic for applications to sterilization and (b) the operating properties of this module.

Figure 20. Wavelength dependence of the EQE of AlGaN DUV-LEDs with MQBs and single-EBLs.

devices with EQEs of 2–3% [25, 26].

Figure 19. Current-output power (I-L) characteristics for a 237 nm AlGaN-MQW LED with an MQB and a 234 nm AlGaN-MQW LED with a single-EBL.

Figure 20. Wavelength dependence of the EQE of AlGaN DUV-LEDs with MQBs and single-EBLs.

factor of approximately 4. From Figure 18, we estimate that the EIE would have been

Figure 19 shows the I-L characteristics for a 237 nm AlGaN MQW LED with an MQB and a 234 nm LED with a single-barrier EBL, both measured under cw operation at RT. The increase in EIE when using the MQW was found to be extremely high. The output power has been

Figure 20 shows the wavelength dependence of the EQE for AlGaN DUV LEDs with MQBs and single-barrier EBLs. Introducing the MQB has increased the EQE by 10, 4 and 3 times for 235, 250 and 270 nm AlGaN LEDs, respectively. We obtained a cw output power of 33 mW from a 270 nm AlGaN-MQW LED with an MQB on a bare chip, but expect to get higher output power by using flip-chip geometry and heat dissipation. The value of EQE was 3.8% in the

RIKEN and Panasonic have developed commercially available UVC LED modules for use in sterilization in 2014 [25, 26]. To develop commercially available devices with constant high EQEs and long device lifetimes, the reproducibility and uniformity of the AlN template and the AlGaN LED layer structure need to be maintained. Reproducibility is particularly difficult because the growth conditions are very sensitive to the vapor-reaction between NH3 and TMAl induced by high growth temperatures (1250–1400C). We achieved highly uniform ML-AlN templates on sapphire in a 3 2 inch MOCVD reactor using pulsed NH3 flow. The

Figure 19. Current-output power (I-L) characteristics for a 237 nm AlGaN-MQW LED with an MQB and a 234 nm

improved from approximately 25% to more than 80% by introducing the MQB.

142 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

increased by a factor of 12 by replacing the single-barrier EBL with a MQB.

absence of any means to increase LEE [21].

AlGaN-MQW LED with a single-EBL.

fluctuation in FWHM for these was within 4%. We consistently obtained FWHMs of the XRC (10–12) of 340 arcsec for these templates. These highly uniform and low TDD templates are suitable for producing commercial DUV LEDs. Figure 21 shows (a) a photograph of a 270 nm 10 mW DUV LED module containing 6 chips and (b) the operating properties of this module for applications to sterilization. Lifetimes longer than 10,000 h have already been achieved for devices with EQEs of 2–3% [25, 26].

Figure 21. (a) Photograph of a commercially available 270 nm 10 mW DUV LED module developed by RIKEN and Panasonic for applications to sterilization and (b) the operating properties of this module.
