Silica‑Nanoparticle‑Enhanced PDMS Encapsulation Significantly Improves Light‑Output and Thermal Performance of AlGaN Deep‑UV LEDs

Background

AlGaN‑based DUV‑LEDs (200–300 nm) are increasingly used in sterilization, water treatment, security, and sensing, offering a mercury‑free, highly reliable alternative to conventional UV lamps【1–10】. Their limited output power stems from quantum‑well defects, absorption losses, and total internal reflection (TIR) at the sapphire–air interface【15–17】. While silicon encapsulants have improved LEE in visible LEDs【18–30】, DUV devices still suffer. Here, we propose a fluid encapsulation using PDMS—nontoxic, chemically stable, with a refractive index of 1.43 and >85 % transmittance at 275 nm【31–32】—and enhance it further by incorporating SiO₂ NPs to scatter light and raise the average refractive index.

Methods and Materials

Figure 1 outlines the fabrication sequence: (a) ceramic substrate with alumina electrodes; (b) 275 nm peak DUV‑LED chip bonded to the substrate via hot‑pressure; (c) aluminum reflector cavity positioned around the chip; (d) PDMS fluid (optionally doped with SiO₂ NPs) filled into the cavity; (e) a 3 mm × 1.3 mm hemispherical quartz lens placed on the outer ring; (f) dicing along scribe lines; and (g) the final DUV‑LED with the NP‑doped PDMS encapsulation. Figure 2a shows a conventional DUV‑LED, while Figure 2b illustrates our design with a SiO₂‑NP‑doped PDMS layer and quartz cover. The PDMS transmittance curve (Figure 2c) confirms 85 % throughput at 275 nm. Emission spectra measured with an SLM‑20 integrating sphere (Isuzu Optics) confirm a dominant 275 nm peak with a 12 nm full‑width at half‑maximum. Table 1 lists all component specifications.

Fabrication steps: a ceramic substrate, b DUV‑LED chip bonded to substrate, c aluminum plate bonded to chip, d doped binder dispensed into cavity, e quartz lens cover, f cut‑out finished LEDs, g final device with SiO₂‑NP‑doped PDMS.

Encapsulation details: a conventional flip‑chip, b our NP‑doped PDMS design, c PDMS transmittance (200–600 nm), d photo and emission spectrum at 200 mA, e TEM image of 14 nm SiO₂ NPs【26】.

SiO₂ NP preparation: 150 °C dehydration, 48 h N₂ drying, 14 nm average size to avoid agglomeration.

Results and Discussion

Four encapsulation variants were evaluated (Figure 3). DUV‑LED (I) uses a 60° aluminum reflector with an air cavity; (II) replaces the air with PDMS fluid; (III) fills the cavity with slightly less PDMS and adds a quartz hemisphere; (IV) fully fills the cavity with PDMS and includes the quartz cover. Integrating‑sphere measurements show that at 200 mA, (I) delivers 42.07 mW, (II) 36.11 mW (−14.16 % due to TIR in PDMS), (III) 48.13 mW (+14.39 %), and (IV) 70.05 mW (+66.49 %). The highest output occurs with the complete PDMS cavity, eliminating the air gap and maximizing light transmission. Figure 3f explores SiO₂ NP loading (0, 0.1, 0.2, 0.3 wt%). At 200 mA, the 0.2 wt% doped device achieves 80.58 mW, a 15 % LEE improvement over the undoped fluid and 81.45 % over the baseline (I). Light scattering from the NPs reduces TIR and boosts extraction. Thermal imaging (Figure 4, Table 2) confirms that the doped PDMS structure lowers the interface temperature by 4 °C at 200 mA, demonstrating superior heat dissipation.

Encapsulation comparison: (a) DUV‑LED (I), (b) DUV‑LED (II), (c) DUV‑LED (III), (d) DUV‑LED (IV), (e) output power under each configuration, (f) output versus SiO₂ NP concentration.

Average surface temperature comparison between DUV‑LED (I) and (IV) across currents.

Conclusions

We demonstrate that doping the PDMS encapsulant with 0.2 wt% SiO₂ NPs markedly enhances LEE, achieving an 81.45 % increase in light output and a 4 °C reduction in interface temperature at 200 mA. The reduced TIR and additional scattering provided by the NPs underpin these gains. This compact, thermally efficient architecture offers a practical route to high‑performance AlGaN DUV‑LEDs.

Availability of Data and Materials

Not applicable

Abbreviations

DUV‑LEDs:

Deep‑ultraviolet light‑emitting diodes

NPs:

Nanoparticles

PDMS:

Polydimethylsiloxane