## 1 Introduction  
III-Nitride-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of applications. For example, the emission wavelength of AlGaN-based LEDs can be tuned by alloying GaN with AlN to cover almost the entire ultraviolet (UV) spectral range (200-365 nm), making them perfectly suited to applications in biological, environmental, industrial, and medical fields [1, 2]. Furthermore, AlGaN-based ultraviolet-B LEDs (UVB-LEDs) have been attracting great attention since the implementation of the Minamata Convention on Mercury. There is much potential for such LEDs to be used in medical (e.g., cancer immunotherapy; treatment of psoriasis, vitiligo, and pemphphus vulgaris) and agricultural (e.g., plant growth under UVB lighting; production of phytochemicals in the green leaves of vegetables) applications [3, 4, 5, 6, 7]. For example, narrow-band (NB-UVB)emission therapy at 310 nm has been found to be more effective than broad-band (BB-UVB) therapy at 280-320 nm in the treatment of psoriasis and atopic dermatitis; furthermore, NB-UVB phototherapy with emission at 310 nm appears suitable for cancer therapy and skin cure therapy [8; 9; 10; 11]. Several reports have disclosed that high external quantum efficiencies (EQEs) are possible for UVB-LEDs when combining a low-dislocation-density AlN template substrate, a well-optimized n-AlGaN buffer layer, an electron injection layer, a transparent p-AlGaN layer, and reflective electrodes [12; 13; 14; 15]. At present, it is difficult to prepare highly conductive p-AlGaN contact layers with high Al contents (>50%); therefore, it has been necessary to use p-GaN as the contact layer to lower the series resistance of UVB-LEDs, even though its presence significantly decreases the light-extraction efficiency (LEE) as a result of strong absorption of UVB light. Moreover, several reports have suggested that silicone is unreliable as an encapsulating material against DUV light [16; 17; 18]. Therefore, to improve the reliability of UVB-LEDs, the conventional packaging design has involved flipping the LED chip bonded onto the AlN-based direct plating copper ceramic lead frames (AlN-DPC LFs) and using an attached quartz glass as the hermetic cover. This kind of packaging structure for UVB-LEDs results, however, in severely decreased light extraction and limited optical performance [19; 20; 21]. Furthermore, quartz glass is more expensive than silicone. Some DUV-transparent encapsulation materials have been proposed to ameliorate these drawbacks, including sol-gel methyl siloxane hybrid materials, fluorine resins, and silicone oil [22; 23; 24]. Therefore, in terms of simplifying the manufacture process, it would be profoundly useful to develop an inexpensive packaging structure using silicone as the transparent encapsulant that displays improved LEEs and acceptable reliability.  
In this study, we developed a simple, inexpensive, and highly durable packaging structure, using reliable silicone as the transparent encapsulation material, to improve the LEEs of UVB-LEDs. We have investigated the effects on the LEEs of various encapsulation methods applied in AlN-DPC LFs, and compared them with a conventional packaging structure. Our results suggest that such reliable silicone has great potential to induce long-term stability when removing the hermetic cover.