8. Highly reflective (HR) PhC for increasing LEE

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

Figure 27. (a) Current-output power (I-L) and (b) current-EQE (ηext) characteristics for the conventional and LEE

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

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

p-AlGaN contact layer, an Rh mirror electrode, a PSS, and the lens-like encapsulating.

enhanced UVC-LED. The inset in (a) shows the EL spectra of the LEDs at a direct current of 20 mA.

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

1.5, respectively.

AlGaN:Mg/Rh layers and the PSS, respectively.

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 resistance, resulting in a higher operating voltage.

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 high LEE but also high WPE in DUV LEDs.

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

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

We performed a finite-difference time-domain (FDTD) analysis for obtaining an appropriate HR-PhC design using the following Bragg equation [57]:

$$m\lambda/n\_{\text{eff}} = 2a \tag{1}$$

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

Figure 28. Schematic diagram of the structure of DUV LEDs with and without a reflective PhC on the p-AlGaN contact layer.

Figure 29 shows the schematic cross-sectional structures and electronic-field (E-field) mappings calculated by using FDTD method for 280 nm UVC LEDs (a) with and (b) without reflective HR-PhC, which is fabricated in the p-AlGaN/p-GaN contact layer. To obtain high reflectivity of UV radiation from the QW emitting region, we set the distance between the bottom of PhC air-rod and the QW to be 60 nm [57]. As can be seen in Figure 29(b), the UV light from the QWs propagates equally in all directions for a usual Led case. On the other hand, if we introduce the HR-PhC, radiation from the QWs does not penetrate into the PhC, as shown in Figure 29(a), resulting in realizing a highly-reflection of radiated light. From the FDTD analysis, we found that the LEE is increased by factors of approximately 2.8 and 1.8 at maximum by introducing the HR-PhC into the p-GaN and p-AlGaN contact layers, respectively. We also found that the LEE enhancement factor significantly depends on the value of R/ a and that the appropriate R/a value is around 0.4 [57].

Based on these design, we fabricated DUV LEDs with HR-PhCs on the p-AlGaN contact layer. We used nano-imprinting and inductively-coupled plasma (ICP) dry-etching to fabricate a low-damage PhC. Figure 30 shows (a) a cross-sectional scanning electron microscopy (SEM) image and (b) a high-resolution (HR) TEM image of the hexagonally configured PhC, as well as (c) surface and (d) cross-sectional SEM images of the Ni-electrode.

transparent p-AlGaN contact layer. The LEDs were measured under continuous wave operation on the bare wafers at room temperature. The maximum EQEs of the LEDs with and without the HR-PhC were 10 and 7.9%, respectively. The introduction of the PhC increased the EQE by a factor of 1.23, which is almost the same as obtained by FDTD simulation [57]. We also performed the same experiments using low-reflective Ni p-electrode, and obtained the maximum EQEs with and without the HR-PhC of 6 and 4.8%, respectively. The relatively low EQE of 4.8% is attributed to the low reflectivity of Ni. According to a simple estimate of the relationships between the EQEs of 4.8% (Ni; 30%), and 7.9% (Ni/Mg; 80%) for the LEDs without the PhC, the reflectance for the HR-PhC p-AlGaN with the Ni/Mg electrode is

Figure 29. Cross-sectional structures and electronic-field mappings calculated by using FDTD method for 280 nm DUV

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 151

LEDs (a) with and (b) without reflective PhC.

The period, diameter and depth of the air-holes were 252, 100 and 64 nm, respectively, confirmed by HR-TEM. Also, three-layer MQW and the 2-layer MQB-EBL located just below the air-holes of the PhC were observed in the HR-TEM. Finally, Ni and Ni/Mg electrodes were deposited via a tilted-evaporation method. We confirmed that the air-holes remained clear, with partial evaporation of Ni at the edges.

Figure 31 shows (a) the I-L and (b) the I-EQE characteristics of 283 nm AlGaN DUV LEDs with high-reflectivity Ni/Mg electrodes (reflectivity of >80%) with and without a PhC on the

Figure 29 shows the schematic cross-sectional structures and electronic-field (E-field) mappings calculated by using FDTD method for 280 nm UVC LEDs (a) with and (b) without reflective HR-PhC, which is fabricated in the p-AlGaN/p-GaN contact layer. To obtain high reflectivity of UV radiation from the QW emitting region, we set the distance between the bottom of PhC air-rod and the QW to be 60 nm [57]. As can be seen in Figure 29(b), the UV light from the QWs propagates equally in all directions for a usual Led case. On the other hand, if we introduce the HR-PhC, radiation from the QWs does not penetrate into the PhC, as shown in Figure 29(a), resulting in realizing a highly-reflection of radiated light. From the FDTD analysis, we found that the LEE is increased by factors of approximately 2.8 and 1.8 at maximum by introducing the HR-PhC into the p-GaN and p-AlGaN contact layers, respectively. We also found that the LEE enhancement factor significantly depends on the value of R/

Figure 28. Schematic diagram of the structure of DUV LEDs with and without a reflective PhC on the p-AlGaN contact

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

Based on these design, we fabricated DUV LEDs with HR-PhCs on the p-AlGaN contact layer. We used nano-imprinting and inductively-coupled plasma (ICP) dry-etching to fabricate a low-damage PhC. Figure 30 shows (a) a cross-sectional scanning electron microscopy (SEM) image and (b) a high-resolution (HR) TEM image of the hexagonally configured PhC, as well

The period, diameter and depth of the air-holes were 252, 100 and 64 nm, respectively, confirmed by HR-TEM. Also, three-layer MQW and the 2-layer MQB-EBL located just below the air-holes of the PhC were observed in the HR-TEM. Finally, Ni and Ni/Mg electrodes were deposited via a tilted-evaporation method. We confirmed that the air-holes remained clear,

Figure 31 shows (a) the I-L and (b) the I-EQE characteristics of 283 nm AlGaN DUV LEDs with high-reflectivity Ni/Mg electrodes (reflectivity of >80%) with and without a PhC on the

a and that the appropriate R/a value is around 0.4 [57].

layer.

with partial evaporation of Ni at the edges.

as (c) surface and (d) cross-sectional SEM images of the Ni-electrode.

Figure 29. Cross-sectional structures and electronic-field mappings calculated by using FDTD method for 280 nm DUV LEDs (a) with and (b) without reflective PhC.

transparent p-AlGaN contact layer. The LEDs were measured under continuous wave operation on the bare wafers at room temperature. The maximum EQEs of the LEDs with and without the HR-PhC were 10 and 7.9%, respectively. The introduction of the PhC increased the EQE by a factor of 1.23, which is almost the same as obtained by FDTD simulation [57]. We also performed the same experiments using low-reflective Ni p-electrode, and obtained the maximum EQEs with and without the HR-PhC of 6 and 4.8%, respectively. The relatively low EQE of 4.8% is attributed to the low reflectivity of Ni. According to a simple estimate of the relationships between the EQEs of 4.8% (Ni; 30%), and 7.9% (Ni/Mg; 80%) for the LEDs without the PhC, the reflectance for the HR-PhC p-AlGaN with the Ni/Mg electrode is

Figure 30. (a) Cross-sectional SEM and (b) HR-TEM images of the PhC fabricated on the p-AlGaN contact layer, along with (c) surface and (d) cross-sectional SEM images of a Ni electrode deposited on the PhC p-AlGaN layer by the tiltedevaporation method.

estimated to exceed 90%. These results indicate that the surface damage caused to the PhC during fabrication was negligible.

The value of R/a used in the experiments (R/a = 0.2) were not optimized value. If we had used a larger value of R/a, i.e., R/a = 0.4, we would have expected to obtain a significantly higher LEE. The LEE can be further increased by adopting FC technology and encapsulation. By introducing a PhC into the contact layer and reducing the operating voltage, it is expected that LEDs with higher WPE can be obtained.

DUV emission have been achieved for AlGaN-QWs by developing a low-TDD AlN layer grown on sapphire substrate. 222–351 nm DUV LEDs were made using this technology. The EIE of the LEDs was increased significantly by controlling the electron flow using an MQB. We also demonstrated improvements in LEE by using a transparent p-AlGaN contact layer, a highly reflective p-electrode, an AlN buffer layer on a PSS, and an encapsulating resin. The maximum EQE obtained was 20.3% for a 275 nm UVC LED, which is the highest EQE reported so far. We also demonstrated that an HR-PhC fabricated on the p-contact layer significantly

Figure 31. Comparison between the (a) I-L and (b) I-EQE characteristics of 283 nm AlGaN DUV LEDs with highreflectivity Ni/Mg electrodes (reflectance of >80%) with and without a PhC on the transparent p-AlGaN contact layer.

Recent Progress in AlGaN Deep-UV LEDs http://dx.doi.org/10.5772/intechopen.79936 153

increases the efficiency.

#### 9. Summary

We have demonstrated on the technologies to develop high-efficiency AlGaN-based DUV LEDs from the view point of increasing IQE, EIE and LEE. Significant increases in IQE of

Figure 31. Comparison between the (a) I-L and (b) I-EQE characteristics of 283 nm AlGaN DUV LEDs with highreflectivity Ni/Mg electrodes (reflectance of >80%) with and without a PhC on the transparent p-AlGaN contact layer.

estimated to exceed 90%. These results indicate that the surface damage caused to the PhC

Figure 30. (a) Cross-sectional SEM and (b) HR-TEM images of the PhC fabricated on the p-AlGaN contact layer, along with (c) surface and (d) cross-sectional SEM images of a Ni electrode deposited on the PhC p-AlGaN layer by the tilted-

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

The value of R/a used in the experiments (R/a = 0.2) were not optimized value. If we had used a larger value of R/a, i.e., R/a = 0.4, we would have expected to obtain a significantly higher LEE. The LEE can be further increased by adopting FC technology and encapsulation. By introducing a PhC into the contact layer and reducing the operating voltage, it is expected that LEDs

We have demonstrated on the technologies to develop high-efficiency AlGaN-based DUV LEDs from the view point of increasing IQE, EIE and LEE. Significant increases in IQE of

during fabrication was negligible.

with higher WPE can be obtained.

9. Summary

evaporation method.

DUV emission have been achieved for AlGaN-QWs by developing a low-TDD AlN layer grown on sapphire substrate. 222–351 nm DUV LEDs were made using this technology. The EIE of the LEDs was increased significantly by controlling the electron flow using an MQB. We also demonstrated improvements in LEE by using a transparent p-AlGaN contact layer, a highly reflective p-electrode, an AlN buffer layer on a PSS, and an encapsulating resin. The maximum EQE obtained was 20.3% for a 275 nm UVC LED, which is the highest EQE reported so far. We also demonstrated that an HR-PhC fabricated on the p-contact layer significantly increases the efficiency.
