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Abstract
We propose the use of optical films to enhance the light extraction efficiency (LEE) and wide-angle emission of traditional packaged deep-ultraviolet light-emitting diodes (DUV-LEDs). Total internal reflection occurs easily in DUV-LEDs because they contain sapphire, which has a high refractive index. DUV-LEDs also contain an aluminum nitride (AlN) ceramic substrate, which has high light absorption in the ultraviolet band. Photons are absorbed by the sapphire and AlN ceramic substrate, which reduces the LEE of DUV-LEDs. By adding a brightness enhancement film (BEF) on the sapphire surface and a high-reflection film (HRF) on the surface of the AlN ceramic substrate, the LEE of DUV-LEDs can be increased. Moreover, we designed a single-layer metal reflective film (SMRF) on the upper surface of the quartz glass in order to achieve wide-angle emission. Experimental results indicated that compared with traditional packaged DUV-LEDs, the light output power and external quantum efficiency of DUV-LEDs with a plated BEF, HRF, and SMRF increased by 18.3% and 18.2%, respectively. Moreover, an emission angle of 160° was achieved. In a reliability test, DUV-LEDs maintained more than 95% of the initial forward voltage and light output power after 1000 h of operation at 25°C, which indicated that the addition of an optical film can improve the light efficiency and long-term reliability of DUV-LEDs.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Compared with mercury lamps, deep-ultraviolet light-emitting diodes (DUV-LEDs) composed of aluminum gallium nitride (AlGaN) materials offer the advantages of small size, high efficiency, low energy consumption, environmental friendliness, and adjustable wavelength [1,2]. DUV-LEDs are widely used in sterilization, deodorization, air and water purification, ultraviolet (UV) curing, photomedicine, photocatalysis, and health lighting [3–5] and are expected to replace mercury lamps completely in the future. In the field of sterilization and disinfection, DUV-LEDs provide high-energy UV light for irradiating microorganisms and destroying their deoxyribonucleic acid or ribonucleic acid [6,7]. DUV-LED disinfection is an effective method for eliminating severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has caused coronavirus disease 2019 (COVID-19) [8,9]. To prevent and control the COVID-19 pandemic, DUV-LED disinfection technology is being used in public places, transportation, personal protection, and other fields. However, the light efficiency, light output power, and reliability of AlGaN DUV-LEDs are still considerably lower than those of InGaN blue LEDs and thus are insufficient for most practical applications [10–12].
The external quantum efficiency (EQE) of InGaN near-UV LEDs with wavelengths above 365 nm is 46%–76%, whereas that of AlGaN DUV-LEDs is generally less than 20%. In particular, the EQE of DUV-LEDs with a wavelength of 265–280 nm (used for sterilization) is less than 5%, which seriously affects the light efficiency of DUV-LEDs [4]. With the shortening of the emission wavelength of DUV-LEDs, technical challenges are being encountered in the epitaxy and effective doping of high-quality AlGaN materials. Moreover, higher requirements are being placed on DUV-LED device packaging technology. Traditional LED packaging structures mainly use organic encapsulation materials (e.g., epoxy and silicone) for packaging. However, because of the short wavelength and high energy of deep-UV light, organic encapsulation materials undergo UV degradation under long-term deep-UV light radiation and exhibit low transmittance in the DUV region. This phenomenon seriously affects the light efficiency and reliability of DUV-LEDs [13–15]. Considering the aforementioned shortcomings of organic materials, in the mainstream design of DUV-LED packaging, a quartz glass lens is used as a cover layer to achieve hermetic packaging, such as three-dimensional (3D) glass packaging and 3D ceramic substrate packaging, to improve reliability [16–18]. However, because of the large differences in refractive indices, Fresnel reflection losses occur at the glass–air and chip–air interfaces. These reflection losses can cause reflected glare and reduce light transmittance, which results in a decrease in the light output power extracted from glass-encapsulated DUV-LEDs [19]. Although roughened surfaces, patterned substrates, photonic crystals, sapphire substrate light guide layers, and chips with optimized shapes can be used in chip manufacturing [20–24] to reduce total internal reflection (TIR), the packaging method can also aid in suppressing TIR loss. Various packaging methods have been proposed to reduce the TIR loss. Chen et al. [25] incorporated aluminum nitride (AlN) nanoparticles into a fluoropolymer to take advantage of the high refractive index and wide bandgap of AlN particles to increase the refractive index of the fluoropolymer, which reduced the difference between the refractive indices of the fluoropolymer and chip. AlN nanoparticles also provide excellent light scattering ability, which increases the light output power of DUV-LEDs with AlN-doped fluoropolymer encapsulation by 16.4%. Moreover, a microlens array fluoropolymer encapsulation layer was proposed in [26] to suppress the TIR on the surface of DUV-LED chips. A porous template was fabricated using the waterdrop condensation method, which was used to form microlens arrays with different micromolding curvature radii on the fluoropolymer. The critical angle and light output power of the DUV-LEDs was increased by the scattering effect of the microlens array. Liang et al. [27] used nanophotolithography and wet etching to fabricate nanolens arrays (NLAs) on sapphire lens for light extraction in DUV-LEDs. By optimizing the NLAs with a radius of 350 nm, the light output power and emission angle of the DUV-LEDs was increased by 24.7% and 14°, respectively. Chen et al. [19] prepared dual-side nanostructures on a flat quartz glass surface. To obtain subwavelength (deep-UV band) nanostructures, they fabricated gold nanoparticles through the rapid thermal annealing of gold thin films and dry etching. Moth-eye-shaped nanostructures with an average height and period of 140 and 90 nm, respectively, were prepared through etching on the glass surface. The deep-UV light transmittance of the dual-side nanostructured glass was as high as 97.2%, and the light efficiency of DUV-LEDs increased by 8.6%.
For the hermetic packaging of DUV-LEDs, 3D ceramic substrates improve the heat dissipation performance. However, the low reflectivity of the inner wall of the ceramic substrate dam (e.g., ceramic or copper material) of DUV-LEDs in the deep-UV band makes it difficult to extract light effectively from the side wall of the chip (absorbed by the packaging material), which affects the light efficiency of the DUV-LEDs [28]. In view of the high reflectivity (>90%) of metal aluminum (Al) to deep-UV light, Ye et al. [29] used plate-level Al reflector cups to achieve the batch packaging of DUV-LEDs. In this step, not only the packaging efficiency but also the light extraction from the sidewall of the chip was enhanced using high-mirror Al reflectors. The addition of plate-level Al reflector cups to the DUV-LEDs increased the light output power of the DUV-LEDs by 18.38%. Ye et al. [30] stated that a nanoparticle-doped polydimethylsiloxane (PDMS) fluid and quartz hemispherical glass cover can be used for sidewall-packaged DUV-LEDs with Al reflectors to increase the light extraction. When they placed PDMS fluid doped with SiO2 nanoparticles in a cavity instead of air, the light output power increased by 81.45%. This enhancement was attributed to the reduction of the TIR and additional light scattering in PDMS fluids by SiO2 nanoparticles.
Although the aforementioned methods can promote light extraction by reducing the light trapping and absorption losses inside the packaging structure, the preparation and packaging processes are relatively complex. Moreover, most methods enhance the light extraction from the top of the DUV-LED chip. Studies that have suggested methods to improve the light extraction efficiency (LEE) of DUV-LEDs have not conducted research on the design of optical thin films on sapphire substrates, AlN ceramic substrates, and quartz glass surfaces. The present study proposes the addition of a brightness enhancement film (BEF) on a sapphire surface and a high-reflection film (HRF) on an AlN ceramic substrate to increase the LEE. To increase the emission angle for achieving a stronger wide-angle effect, a single-layer metal reflective film (SMRF) was designed on the outer surface of the semi-inorganic encapsulated quartz glass. The advantage of this method over methods, such as substrate surface microstructure and chip surface roughening, is that it does not require complicated lithography and etching processes.
2. Experiment
The 280 nm DUV-LED epi-structure wafer was grown on a c -plane sapphire substrate using a metal organic chemical vapor deposition (MOCVD) system. The epi structure includes a 2-µm -thick AlN buffer layer, 2.5 µm-thick n-type (Si-doped) Al0.55Ga0.45N layer, an active region consists of 5 pairs of 2.5-nm Al0.37Ga0.63N/12.5-nm Al0.51Ga0.49N multi-quantum wells, 30 nm-thick p-type (Mg-doped) Al0.75Ga0.25N electron blocking layer, and 20 nm-thick p-type Al0.45Ga0.55N contact layer. The epi-structure wafer was then mesa-etched by inductively coupled plasma etching (ICP-RIE) to expose the n-AlGaN contact layer, where Ti/Al/Ni/Au (20 nm/120 nm/50 nm/50 nm) as an n-contact was electron beam evaporated. Last, the p-type electrode made of Ni/Au (20 nm/200 nm) was e-beam deposited. The size of the DUV-LED chip used in this study was 60 mil × 60 mil, and the peak wavelength was 280 nm. A sapphire substrate (refractive index: 1.7163) was used as the light guiding layer. By using the traditional direct plated copper ceramic substrate process, an AlN ceramic substrate with a Ni/Au metal covering layer on the surface of the Cu pad was prepared. The chip was manufactured through eutectic flip-chip bonding on an AlN ceramic substrate. Then, a low-viscosity paste was printed on the dam surface of the AlN ceramic substrate as an adhesive layer between the glass cap and the ceramic substrate. Finally, a quartz glass was placed and aligned with the dam of the AlN ceramic substrate, which was the traditional DUV-LED packaging [ Fig. 1(a)]. This paper proposes the use of optical films to optimize DUV-LED packaging for preparing DUV-LEDs with high light extraction and wide-angle emission. A BEF, an HRF, and an SMRF were added on the sapphire substrate, AlN ceramic substrate, and on a quartz glass surface, respectively [Fig. 1(b)]. Figure 1 displays the optical paths of the two DUV-LED packaging structures. In the traditional DUV-LED packaging structure, only the light emitted or scattered upward can be extracted. Moreover, a part of the light undergoes TIR in the sapphire substrate. However, a considerable light emitted directly from the sidewalls was absorbed by the AlN ceramic substrate and cannot be extracted. However, after the addition of a BEF on the sapphire substrate and an HRF on the AlN ceramic substrate, the diffuse reflections at the sapphire substrate and HRF layer decrease, which increases the light extraction. This phenomenon explains the considerable increase in the light output power of the high-light-extraction, wide-emission-angle DUV-LED structure in comparison with the traditional DUV-LED structure. All the optical films were deposited using an evaporation deposition system. The detailed experimental steps are described in the following text. All measured data for these two types of DUV-LEDs were averaged from 30 different samples.
Fig. 1. Optical paths of the (a) traditional DUV-LED packaging and (b) high-light-extraction, wide-emission-angle DUV-LED packaging.
2.1 BEF on an UVC-LED chip
To reduce the TIR of the DUV-LED chip, a BEF composed of a SiO2/HfO2 stack was e-beam deposited on the sapphire substrate (total thickness of 85.67 nm) for improving the light extraction. The deposition rates were 0.2 nm/s and 1 nm/s for HfO2 and SiO2, respectively, and the oxygen partial pressure was 1×10−2 Pa for hafnia evaporation and 4×10−3 Pa for silica evaporation. Figure 2 depicts the reflectance spectra of pure sapphire and the BEF in the wavelength range of 250–300 nm measured by a UV/VIS/NIR spectrophotometer (UH-4150, Hitachi, Tokyo, Japan). The average reflectivity at 260–280 nm reduced from 7% for pure sapphire to 0.21% for the sapphire substrate with the BEF.
2.2 HRF on an AlN ceramic substrate
In the traditional DUV-LED packaging [Fig. 1(a)], the light originating from the chip is reflected toward the AlN ceramic substrate and absorbed, which results in a decrease in the LEE. To increase the reflectivity of the AlN ceramic substrate, an HRF was designed using a metal and dielectric film. The incident surface of the light path was from air to the dielectric film and then to the metal film. Figure 3 illustrates the ANSYS SPEOS simulation results for the light output power (Po) of the AlN ceramic substrate with different HRF substrate reflectance (SR) values, and the relevant data are presented in Table 1. The simulation architecture and optical-mechanical diagram were shown in Fig. 1(b). The relationship between light output power when the equivalent reflectivity is set to 10%−100%. The orange frame indicates the reflectance of the AlN ceramic substrate without HRF. A total of 80% of the reflected light was absorbed, and Po was only 193.4 mW. The red frame indicates that the reflectivity of the plated HRF was 95%, and Po was 209.3 mW. Thus, the HRF increased the LEE.
Fig. 3. Relationship between the SR and light output power (Po) of the simulated HRF on an AlN ceramic substrate.
Table 1. SR Values, Absorption, and Light Output Power of Different AlN Ceramic Substrates
In the HRF film, HfO2 and SiO2 were used as the high-refractive-index material (H) and low-refractive-index material (L), respectively. Use optical simulation software Essential Macleod to simulate HRF film. The HRF film consists of six groups of HfO2 and SiO2 pairs and an Al layer, where air was the incident medium, the refractive index of SiO2 was 1.49603, the refractive index of HfO2 was 2.0887, the AlN ceramic substrate refractive index was 2.24312, the Al layer refractive index was 3.06, and the reference wavelength was 275 nm. The light normal incidence from the air passes through HfO2/SiO2 pairs, incidence to the Al layer and reflected out. We optimized the HRF film by using the transfer matrix method, in order to maximize the reflections in the range of the wavelength, the individual layer thicknesses were varied iteratively by a random method, changing the layer thicknesses of HfO2 and SiO2 to adjust the reflectivity in the wavelength range. The simulation results show a total film thickness of 920.78 nm, as shown schematically in Fig. 4(a). The HRF film containing HfO2 and SiO2 were deposited using either e-beam evaporation with a rate of 0.2 and 1 nm/s for HfO2 and SiO2, respectively, and the oxygen partial pressure was 1×10−2 Pa for hafnia evaporation and 4×10−3 Pa for silica evaporation. Figure 4(b) depicts the reflectance spectra of the AlN ceramic substrate without HRF and with the HRF at angles of incidence of 0° and 45°. For the AlN ceramic substrate with the HRF, the average reflectance of light with a wavelength of 260–280 nm incident at 0° and 45° was as high as 95.75% and 85%, respectively. Figure 5(a) depicts a prototype with BEF plating on the DUV-LED chip and HRF plating on the AlN ceramic substrate. Figure 5(b) depicts the encapsulation of quartz glass in DUV-LED.
Fig. 4. (a) HRF film structure diagram of the simulation and (b) reflectance spectra of the AlN ceramic substrates with and without the HRF.
Fig. 5. (a) Prototype with BEF plating on the DUV-LED chip and HRF plating on the AlN ceramic substrate and (b) Prototype of DUV-LEDs with quartz glass packaging.
2.3 SMRF on a quartz glass surface
An SMRF (a single-layer, 50.930-nm Al film) was deposited onto a quartz glass surface by e-beam evaporator. Air entered the quartz glass and was then reflected by the SMRF, which allowed the existence of wide-angle light emission. Figure 6 shows the reflectance spectrum of the SMRF on the quartz glass surface at 250–300 nm measured by a UV/VIS/NIR spectrophotometer. The average reflectance of the SMRF was 92.1%.
2.4 Simulation results of optical ray tracing
The models of the traditional DUV-LED packaging and high-light-extraction, wide-emission-angle DUV-LED packaging were established and simulated using the ANSYS SPEOS optical ray tracing simulation software. The simulated optical path diagrams are shown in Figs. 7(a) and 7(b), with only 1% of the actual simulated rays being depicted in the diagram. In the traditional packaging model displayed in Fig. 7(a), most of the light incident terminated on the AlN ceramic substrate. The forward light was absorbed by the sapphire substrate, and TIR loss occurred. However, in the high-light-extraction, wide-emission-angle DUV-LED packaging model [Fig. 7(b)], almost all the light incident on the HRF was reflected, and the BEF on the sapphire increased the forward light extraction. In addition, the light reflected by the SMRF escaped from the side wall of the quartz glass, which resulted in a larger emission angle. The light distributions of the two packaging models are displayed in Figs. 7(c) and 7(d). The extraction efficiency of the traditional DUV-LED packaging was low, and the light was concentrated in the central portion of the packaging. The extraction efficiency of the high-light-extraction, wide-emission-angle DUV-LEDs was considerably higher than that of the traditional LEDs. The light intensity distribution range increased, and the center intensity weakened. These phenomena were consistent with the experimental results. In addition, compared with the traditional DUV-LED packaging (60.5%), the high-light-extraction, wide-emission-angle DUV-LED packaging exhibited a higher uniformity (approximately 87.2%).
Fig. 7. Simulation results of optical ray tracing. (a) and (b) Optical path diagram and (c) and (d) light distribution of the traditional DUV-LED packaging and high-light-extraction, wide-emission-angle DUV-LED packaging.
3. Results and discussion
Figure 8(a) presents the measured light output power of the traditional DUV-LED packaging model and high-light-extraction, wide-emission-angle DUV-LED packaging model at an injection current of 10–400 mA. For the injection current of 20–400 mA, the light output power of the high-light-extraction, wide-emission-angle packaged DUV-LED structure was always greater than that of the traditional DUV-LED packaged structure. At 350 mA, the light output power of the high-light-extraction, wide-emission-angle DUV-LED structure was 201.1 mW, which was 18.3% higher than that of the traditional DUV-LED structure (170 mW). This light extraction enhancement was attributed to the increase in bottom emission caused by the reflection effect of the HRF on the AlN ceramic substrate, and the addition of the BEF on the sapphire substrate to reduce the TIR loss. Moreover, the inset of Fig. 8(a) shows the current–voltage (I–V) characteristic of DUV-LED. Because both use eutectic bonding and the same packaging conditions, they did not affect the electrical properties. The voltages were approximately 5.7 and 8.6 V for current at 20 and 350 mA, respectively. The WPEs (at 350 mA) of the packaged traditional and high-light-extraction, wide-emission-angle DUV-LEDs were 5.65% and 6.68%, respectively. Based on this result, the WPEs of the packaged high-light-extraction, wide-emission-angle DUV-LED was improved 18.3% over the packaged traditional DUV-LED. The EQEs (at 350 mA) of the packaged traditional and high-light-extraction, wide-emission-angle DUV-LEDs were 10.97% and 12.97%, respectively; thus, the high-light-extraction, wide-emission-angle DUV-LEDs had a 18.2% higher EQE than did the traditional DUV-LEDs [Fig. 8(b)]. This result indicates that the efficiency of DUV-LEDs can be improved using a BEF and HRF because they considerably improve the optical performance. The inset of Fig. 8(b) presents the electroluminescence (EL) spectra of the high-light-extraction, wide-emission-angle DUV-LED packaged structure. For this structure, the peak emission wavelength was 280 nm, and the full width at half-maximum was approximately 21.7 nm.
Fig. 8. Plots of the (a) light output power and (b) EQE versus the injection current for the traditional DUV-LED packaging and high-light-extraction, wide-emission-angle DUV-LED packaging. Inset: (a) I–V characteristic and (b) EL spectra of the DUV-LEDs.
Figure 9(a) illustrates the far-field emission pattern of the high-light-extraction, wide-emission-angle DUV-LED packaging with and without the SMRF on quartz glass at the injection current of 350 mA. Compared with the DUV-LED without quartz glass (148°), the DUV-LED with quartz glass had a higher emission angle (160°). The wider emission angle of the DUV-LED with quartz glass can be attributed to the primary optical design, where an SMRF was coated on the quartz glass surface. When light enters the quartz glass, the light is reflected by the SMRF and escapes from the sidewalls. Figures 9(b) and 9(c) displays the 3D light distribution patterns of the high-light-extraction, wide-emission-angle DUV-LED packaging with and without the SMRF on quartz glass, respectively. Compared with the DUV-LED without the SMRF, the DUV-LED with the SMRF had a wider emission angle (peak moved to 40°). The light intensity of the DUV-LED with the SMRF was distributed on the sidewall. However, the central intensity of the DUV-LED with the SMRF drops to 53.03%, which indicates the conversion of a point light source into a surface light source. Figures 9(d) and 9(e) depicts the emission images of the high-light-extraction, wide-emission-angle DUV-LED packaging with and without the SMRF on quartz glass, respectively. The light emission angle increased but the center intensity decreased after SMRF coating.
Fig. 9. (a) Far-field radiation patterns, (b) and (c) 3D light distribution patterns, and (d) and (e) emission images of the high-light-extraction, wide-emission-angle DUV-LED packaging with and without the SMRF on quartz glass at an injection current of 350 mA.
The forward voltage, light output power, and reflectance of the HRF were measured at 25°C for 1000 h in the aging tests of high-light-extraction, wide-emission-angle DUV-LEDs at 350 mA (Fig. 10). High-light-extraction, wide-emission-angle DUV-LED burn-in test was better than traditional DUV-LED. For deep UV wavelengths, HfO2 and SiO2 materials were selected as the source of the optical film, and the emission wavelength does not change during the aging test. After 1000 h of operation, the normalized forward voltage, light output power, and reflectance of the HRF reduced by 3.5%, 5%, and 3.7%, respectively. The results indicate that through the use of an optimized optical film in high-light-extraction, wide-emission-angle DUV-LEDs, high UV durability can be achieved, which confirms that DUV-LEDs have high reliability and high optical stability under UV radiation.
Fig. 10. (a) Relative forward voltage, (b) relative light output power, and (c) relative reflectance of the HRF of high-light-extraction, wide-emission-angle DUV-LEDs in the aging tests conducted at 25°C for 1000 h.
4. Conclusions
In this study, a BEF, an HRF, and an SMRF were deposited onto a sapphire substrate, an AlN ceramic substrate, and a quartz glass surface, respectively, to increase the LEE, EQE, and emission angle of DUV-LEDs. After the addition of the BEF, HRF, and SMRF, the light output power increased to 201.1 mW (from 170 mW), the LEE increased by 18.3%, the EQE increased by 18.2%, and the emission angle increased to 160°. In a reliability test, more than 95% of the forward voltage, light output power, and reflection of the HRF were maintained after 1000 h of operation, which indicates the high reliability of the high-light-extraction, wide-emission-angle DUV-LED packaging design. In future applications, for a given area of the sterilization light source module, the pitch between the LEDs can be increased to reduce the number of LEDs used for optimizing the uniformity of UV light on the irradiated surface.
Funding
Ministry of Science and Technology, Taiwan (109-2218-E-027-003-MY2, MOST 110-2622-E-194-007).
Acknowledgments
The authors would like to thank department of R&D division of Harvatek corporation for the measurement support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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