7. Increasing the light-extraction efficiency (LEE)

Improving the LEE is particularly important for the development of AlGaN DUV LEDs, because LEE is currently quite low in comparison with that of InGaN-based blue LEDs. However, increasing LEE is not so easy because of the scarcity of suitable transparent, conducting p-type contact layers and transparent p-type electrodes, and also the lack of highly reflective p-type electrodes applicable to UVB-UVC range.

Figure 22 shows schematic diagrams of several structures designed to improve LEE, and the approximate values of LEE calculated for them [24]. In a conventional DUV LED, the light going upward from the QWs is completely absorbed by the p-GaN contact layer. The light going downward is reflected at the sapphire/air interface by total internal reflection. As a result, the LEE is less than 8%. Although we have used photonic nanostructures on the surface of the sapphire substrate or an encapsulating technique, the improvement in LEE is not sufficiently high (a maximum of approximately 15% is expected). To improve LEE, we must introduce a transparent contact layer and a highly reflective p-type electrode. If we use a transparent p-AlGaN contact layer and an electrode with a reflectivity of 80%, LEEs more than 20% can be obtained. Further improvements can be made from light scattering effects obtained by having an AlN buffer layer grown on a patterned sapphire substrate (PSS). LEEs of approximately 35% are expected by combining a transparent contact layer with a reflective electrode and a PSS. Yet more improvements can be made by having a vertical LED with a back-surface photonic structure, which can be realized by removing the sapphire substrate. LEEs of >70% are expected for such LEDs, as analyzed in Ref. [53]. We also proposed using a highly reflective (HR) PhC for the p-contact layer, as we discuss later, which has almost perfect reflectivity for UV light. Using a structure with a transparent p-AlGaN contact layer, a HR-PhC on p-AlGaN, and vertical geometry with a backside photonic patterned structure for light extraction, EQEs of more than 40% are expected for UVC LEDs.

for Ni (20 nm)/Au (100 nm), normalized to the reflectivity of Al (92%). Although the reflectivity of Al metal is high (92%) in the DUV range, ohmic contact to p-AlGaN cannot be obtained. The insertion of a thin layer of Ni to improve this causes a significant reduction in reflectivity. Therefore, we did a thorough examination of the reflectivities of Ni/Al electrodes with very thin Ni layers. We confirmed normal operation of a UVC LED with a Ni (1 nm)/Al (200 nm) ptype electrode. The reflectivities of Ni (1 nm)/Al (200 nm) and Ni (20 nm)/Au (100 nm) elec-

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 145

Figure 23. Wavelength dependence of the reflectivity for various types of p-type electrode for AlGaN DUV LEDs.

Figure 24 compares the effect of the different p-type electrodes on (a) the I-L and (b) the I-EQE characteristics of 279 nm AlGaN DUV LEDs with transparent p-AlGaN contact layers [54]. Using the highly reflective Ni(1 nm)/Al(200 nm) in place of the conventional Ni(20 nm)/Au(100 nm) electrode increases the EQE from 5 to 9% owing to the increase in LEE [54]. We also confirmed that Ni(1 nm)/Mg and rhodium (Rh) p-electrodes are effective to increase the LEE of UVC LED [55]. Increases in LEE were demonstrated for a 275 nm UVC flip-chip (FC) LED by using a transparent p-type AlGaN contact layer, an Rh mirror electrode, an AlN buffer layer grown on PSS, and an encapsulating resin. The effects of each of these were systematically investigated [56]. Conventional and LEE enhanced type LED structures were fabricated to investigate the effects of the aforementioned features on LEE. Schematics of these are shown in Figure 25(a) and (b), respectively. The structures were grown by MOCVD for a conventional LED on a 4 μm thick AlN/sapphire template, and for a LEE enhanced type on an AlN/PSS. The detail layer and

Figure 26 shows a photograph of the FC LED sample. An Al-coated Si submount was used for the FC LED. The chip was encapsulated in hemispherical lens-like by silicon resin. We

trodes are approximately 76 and 31%, respectively, as shown in Figure 23.

devise structures were described in Ref. [56].

We demonstrated a DUV LED with a transparent p-AlGaN contact layer and a reflective ptype electrode [24]. We replaced the conventional Ni (20 nm)/Au (100 nm) p-type electrode with a highly reflective Ni (1 nm)/Al (200 nm) electrode [54]. Figure 23 shows the relative reflectivities of Ni/Al (200 nm) electrodes for various thicknesses of Ni (0.5–4 nm), and also that

Figure 22. Schematic illustrations of structures designed to improve the LEE of a DUV LED and rough estimates of the values of LEE for each structure.

7. Increasing the light-extraction efficiency (LEE)

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

reflective p-type electrodes applicable to UVB-UVC range.

extraction, EQEs of more than 40% are expected for UVC LEDs.

values of LEE for each structure.

Improving the LEE is particularly important for the development of AlGaN DUV LEDs, because LEE is currently quite low in comparison with that of InGaN-based blue LEDs. However, increasing LEE is not so easy because of the scarcity of suitable transparent, conducting p-type contact layers and transparent p-type electrodes, and also the lack of highly

Figure 22 shows schematic diagrams of several structures designed to improve LEE, and the approximate values of LEE calculated for them [24]. In a conventional DUV LED, the light going upward from the QWs is completely absorbed by the p-GaN contact layer. The light going downward is reflected at the sapphire/air interface by total internal reflection. As a result, the LEE is less than 8%. Although we have used photonic nanostructures on the surface of the sapphire substrate or an encapsulating technique, the improvement in LEE is not sufficiently high (a maximum of approximately 15% is expected). To improve LEE, we must introduce a transparent contact layer and a highly reflective p-type electrode. If we use a transparent p-AlGaN contact layer and an electrode with a reflectivity of 80%, LEEs more than 20% can be obtained. Further improvements can be made from light scattering effects obtained by having an AlN buffer layer grown on a patterned sapphire substrate (PSS). LEEs of approximately 35% are expected by combining a transparent contact layer with a reflective electrode and a PSS. Yet more improvements can be made by having a vertical LED with a back-surface photonic structure, which can be realized by removing the sapphire substrate. LEEs of >70% are expected for such LEDs, as analyzed in Ref. [53]. We also proposed using a highly reflective (HR) PhC for the p-contact layer, as we discuss later, which has almost perfect reflectivity for UV light. Using a structure with a transparent p-AlGaN contact layer, a HR-PhC on p-AlGaN, and vertical geometry with a backside photonic patterned structure for light

We demonstrated a DUV LED with a transparent p-AlGaN contact layer and a reflective ptype electrode [24]. We replaced the conventional Ni (20 nm)/Au (100 nm) p-type electrode with a highly reflective Ni (1 nm)/Al (200 nm) electrode [54]. Figure 23 shows the relative reflectivities of Ni/Al (200 nm) electrodes for various thicknesses of Ni (0.5–4 nm), and also that

Figure 22. Schematic illustrations of structures designed to improve the LEE of a DUV LED and rough estimates of the

Figure 23. Wavelength dependence of the reflectivity for various types of p-type electrode for AlGaN DUV LEDs.

for Ni (20 nm)/Au (100 nm), normalized to the reflectivity of Al (92%). Although the reflectivity of Al metal is high (92%) in the DUV range, ohmic contact to p-AlGaN cannot be obtained. The insertion of a thin layer of Ni to improve this causes a significant reduction in reflectivity. Therefore, we did a thorough examination of the reflectivities of Ni/Al electrodes with very thin Ni layers. We confirmed normal operation of a UVC LED with a Ni (1 nm)/Al (200 nm) ptype electrode. The reflectivities of Ni (1 nm)/Al (200 nm) and Ni (20 nm)/Au (100 nm) electrodes are approximately 76 and 31%, respectively, as shown in Figure 23.

Figure 24 compares the effect of the different p-type electrodes on (a) the I-L and (b) the I-EQE characteristics of 279 nm AlGaN DUV LEDs with transparent p-AlGaN contact layers [54]. Using the highly reflective Ni(1 nm)/Al(200 nm) in place of the conventional Ni(20 nm)/Au(100 nm) electrode increases the EQE from 5 to 9% owing to the increase in LEE [54]. We also confirmed that Ni(1 nm)/Mg and rhodium (Rh) p-electrodes are effective to increase the LEE of UVC LED [55].

Increases in LEE were demonstrated for a 275 nm UVC flip-chip (FC) LED by using a transparent p-type AlGaN contact layer, an Rh mirror electrode, an AlN buffer layer grown on PSS, and an encapsulating resin. The effects of each of these were systematically investigated [56]. Conventional and LEE enhanced type LED structures were fabricated to investigate the effects of the aforementioned features on LEE. Schematics of these are shown in Figure 25(a) and (b), respectively. The structures were grown by MOCVD for a conventional LED on a 4 μm thick AlN/sapphire template, and for a LEE enhanced type on an AlN/PSS. The detail layer and devise structures were described in Ref. [56].

Figure 26 shows a photograph of the FC LED sample. An Al-coated Si submount was used for the FC LED. The chip was encapsulated in hemispherical lens-like by silicon resin. We

Figure 24. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for 279 nm AlGaN-MQW DUV LEDs measured under cw operation at RT. comparison is made between LEDs with different p-type electrodes (conventional Ni/Au and highly reflective Ni/Al p-electrodes).

evaluated the transmittance of p-Al0.65Ga0.35N prior to introducing it as a p-type contact layer. We confirmed almost perfect transparency for the p-AlGaN contact layer, and even the Mg doping concentration was as high as 8 1019 cm<sup>3</sup> as measured by secondary ion mass spectrometry (SIMS).

Figure 27(a) and (b) show the I-L and I-EQE characteristics for the conventional and LEE enhanced type UVC LEDs under RT cw operation. The inset in Figure 27(a) shows the EL spectra at 20 mA. Each spectrum has the same peak at 275 nm. The output power for both

Figure 26. Photograph of the flip-chip (FC) LED mounted on a Si submount with an Al coating. The chip size is 0.5 0.5 mm2 and it is encapsulated in resin with a hemispherical shape. The Si submount is in contact with the Al

baseplate. The inset shows a side view of the encapsulating resin.

Figure 25. Schematics of (a) conventional and (b) LEE enhanced UV-LED structures. In the LEE enhanced UV-LED structure, we introduced a transparent p-type AlGaN:Mg contact layer, a Rh mirror electrode, a PSS, and an encapsulat-

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 147

ing resin.

Figure 25. Schematics of (a) conventional and (b) LEE enhanced UV-LED structures. In the LEE enhanced UV-LED structure, we introduced a transparent p-type AlGaN:Mg contact layer, a Rh mirror electrode, a PSS, and an encapsulating resin.

Figure 26. Photograph of the flip-chip (FC) LED mounted on a Si submount with an Al coating. The chip size is 0.5 0.5 mm2 and it is encapsulated in resin with a hemispherical shape. The Si submount is in contact with the Al baseplate. The inset shows a side view of the encapsulating resin.

evaluated the transmittance of p-Al0.65Ga0.35N prior to introducing it as a p-type contact layer. We confirmed almost perfect transparency for the p-AlGaN contact layer, and even the Mg doping concentration was as high as 8 1019 cm<sup>3</sup> as measured by secondary ion mass

Figure 24. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for 279 nm AlGaN-MQW DUV LEDs measured under cw operation at RT. comparison is made between LEDs with different p-type electrodes (conventional

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

spectrometry (SIMS).

Ni/Au and highly reflective Ni/Al p-electrodes).

Figure 27(a) and (b) show the I-L and I-EQE characteristics for the conventional and LEE enhanced type UVC LEDs under RT cw operation. The inset in Figure 27(a) shows the EL spectra at 20 mA. Each spectrum has the same peak at 275 nm. The output power for both

remained 5.7%. Improving the conductivity of the p-AlGaN contact layer is important issue in

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 149

In summary of this section, LEE of the 275 nm AlGaN UVC LED was increased by approximately five times by introducing a transparent p-AlGaN contact layer, an Rh reflective electrode, a PSS, and a lens-like encapsulating. A maximum EQE of 20.3% at 275 nm was obtained

To improve the LEE of UVB and UVC LEDs, the introduction of a transparent contact layer and a highly reflective electrode is important as indicated in the previous section. A p-AlGaN layer with high Al composition (50–70%) is used for the transparent p-contact layer for UVC LEDs, however, the low hole concentration of this layer leads an increase in the contact

In order to realize both high LEE and low voltage operation in DUV LEDs, we proposed using a highly-reflective photonic crystal (HR-PhC) [57–60]. It is possible to reflect UV light efficiently by using a 2-dimensional (2D) PhC on the surface of the p-GaN top contact layer. We can obtain low contact resistance because the top p-GaN layer has a high hole concentration. Therefore, a HR-PhC fabricated on p-GaN contact layer makes it possible to achieve not only

It is possible to increase LEE by lens bonding or lens-shaped encapsulation [34, 56], or by fabrication of a PhC on the backside of the device for suppressing the total reflection [40]. However, LEE

We fabricated DUV LEDs with a HR-PhC on the p-AlGaN contact layer. The fabrication of a uniform PhC with low damage made it possible to obtain high EQE [57]. Figure 28 shows a schematic structure of DUV LED with and without a HR-PhC region fabricated on the p-AlGaN contact layer. We used two-types of p-type electrodes, i.e., low-reflective (30%) Ni

We performed a finite-difference time-domain (FDTD) analysis for obtaining an appropriate

where m is an integer, λ is the wavelength of the light, neff is the effective refractive index of the PhC, and a is the lattice period of the PhC. Electromagnetic field analysis by the FDTD method is suitable for analyzing structures with sub-wavelength size geometry, and is usually used for the analysis of optical PhC devices. An air hole-type 2D PhC with hexagonal configuration was assumed. We observe that a larger photonic bandgap is obtained with larger R/a, i.e., with R/ a = 0.4, where R is the radius of the holes in the PhC. The values used in this work were

mλ=neff ¼ 2a (1)

still remains low if we are unable to eradicate the strong absorption in the p-GaN layer.

electrode and highly-reflective Ni(1 nm)/Mg (80%) [55] electrode.

HR-PhC design using the following Bragg equation [57]:

λ = 280 nm, neff = 2.3, m = 4 and a = 250 nm [57–60].

by combining all of the aforementioned light extraction features.

8. Highly reflective (HR) PhC for increasing LEE

resistance, resulting in a higher operating voltage.

high LEE but also high WPE in DUV LEDs.

future for obtaining high WPE.

Figure 27. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for the conventional and LEE enhanced UVC-LED. The inset in (a) shows the EL spectra of the LEDs at a direct current of 20 mA.

samples has good linearity, and the EQEs are almost constant up to 50 mA. The output power was increased from 3.9 to 18.3 mW at 20 mA and from 9.3 to 44.2 mW at 50 mA, both by a factor of five, by introducing LEE enhanced structure. These values correspond to EQEs of 4.3 and 20.3%, respectively. Thus, the EQE was substantially improved by including a transparent p-AlGaN contact layer, an Rh mirror electrode, a PSS, and the lens-like encapsulating.

To clarify the individual effects on the EQE, each structure for LEE enhancement was introduced step-by-step. Table 2 summarizes the device structures and the LED characteristics. From Table 2, we found that the enhancement factors for introducing a transparent p-AlGaN contact layer and Rh electrode, PSS, and lens-like encapsulation were approximately 3, 1.5, and 1.5, respectively.

The driving voltage of the LED was increased from 9 to 16 V at 20 mA by introducing p-AlGaN contact layer. The main reason for the increase in driving voltage is the increase of contact resistant by introducing p-AlGaN contact layer. The WPE of the LEE enhanced type LEE


Table 2. Summary of the device structures and their LED characteristics. Samples no. 1 and 4 correspond to the conventional and novel UV-LED structures, respectively. Samples no. 2 and 3 demonstrate the effects of including the AlGaN:Mg/Rh layers and the PSS, respectively.

remained 5.7%. Improving the conductivity of the p-AlGaN contact layer is important issue in future for obtaining high WPE.

In summary of this section, LEE of the 275 nm AlGaN UVC LED was increased by approximately five times by introducing a transparent p-AlGaN contact layer, an Rh reflective electrode, a PSS, and a lens-like encapsulating. A maximum EQE of 20.3% at 275 nm was obtained by combining all of the aforementioned light extraction features.
