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5.2 Effects of the polarization of the light emitted from UV LEDs

5.3.3 Packaging of LEDs

Extraction of light from LEDs 73 coupled to the SP mode. In this case when an electron and hole recombine, a SP is created instead of a photon being emitted into free space. At the SP resonant energy, the SP density of states is very high. If the emission wavelength of the LED matches the SP resonant energy, the spontaneous emission will be greatly enhanced due to the Purcell effect. To extract the light from the SP mode it is necessary to scatter the SPs which can be effectively achieved if the metal layer is rough and has imperfections [143]. Okamoto et al. [143] demonstrated a 6.8 fold enhancement in theηint of 470 nm optically pumped LEDs with an Ag coating and a 3 fold enhancement for LEDs with an Al coating. However for the coupling of the QW with the SP it is necessary that the distance between the QW and the metal layer is within the SP fringing field penetration depth which is only 47 nm for Ag and 77 nm for Al on GaN.

An exponential increase in the luminescence intensity was found as the distance between the QW and the metal layer decreased. This becomes an issue for the fabrication of electrically pumped III-nitride LEDs where a thickness of at least 100 nm is needed to obtain a high quality p-current spreading layer.

Even if coupling between the QW and the SPs is not possible, they can still be used to enhance the LEE by SP-TM wave coupling. Gao et al. [144] reported a 217 % increase in the peak photoluminescence intensity at 294 nm for AlGaN-based UV LEDs with an 5 nm thick Al layer placed at a distance of 90 nm from the active region. No increase in the IQE was observed indicating that the QWs do not couple to the SPs. They also showed that the enhancement increased as the wavelength decreased which might be attributed to the increased TM emission from the LEDs at shorter wavelengths and stronger coupling as the photon energy approaches the SP resonant energy.

74 Extraction of light from LEDs problem a flip-chip bonding scheme is used for LEDs [145] in which the LED die is inverted and mounted on the submount with the epitaxial side down (Fig. 5.17).

Figure 5.17: (a) Schematic of a UV LED flip-chip mounted on a submount. The light is collected from the transparent substrate [146]. (b) Photograph of a 320 nm LED flip-chip mounted on an AlN submount

Although flip-chip mounting is a challenging technological process, flip-chip LEDs (FCLEDs) have a number of advantages over the traditional top emitter LEDs which include:

• reduced thermal resistance of the device due to efficient transfer of heat through the metal bonding pads. In the case of top-emitter LEDs grown on sapphire substrates, the thermal resistance of the device is high due to the low thermal conductivity and thickness of the sapphire substrate. The low thermal resistance of FCLEDs results in good thermal performance of the devices and subsequently increased lifetimes.

• good current spreading because of the presence of thick p-ohmic contacts as the con-tacts no longer have to be semi-transparent. Hence FCLEDs can be manufactured with large emission areas.

• compatibility with wafer scale packaging.

• no distortion of the radiation pattern due to the absence of bonding wires.

• enhanced LEE as the light is extracted from the transparent sapphire substrate avoiding absorption at the ohmic p-contact, bonding pads and the bonding wires. Furthermore if the contacts are replaced by highly reflective mirrors, light propagating downwards can be redirected up and extracted through the substrate increasing the LEE.

Extraction of light from LEDs 75 In Fig. 5.18, the output power of 305 nm AlInGaN LED measured on wafer is compared to the output power of the same LED after dicing and flip-chip mounting on an AlN sub-mount. The FCLED was measured in an integrating sphere under dc conditions. Due to the enhancement of the heat and light extraction, a maximum output power of 3 mW at 200 mA was achieved.

Figure 5.18: Emission characteristics of a LED emitting at 305 nm with micro-LED array contact geometry (l = 33μm, p = 183.5μm) measured on wafer (black line) and after flip-chip bonding (red/ gray line) [76].

Encapsulation of UV LEDs

The chip-encapsulating material is another factor of the packaging technology that can greatly influence the LEE of the LEDs. In order to increase the LEE an optically trans-parent encapsulant should be used, which decreases the index contrast at the semiconduc-tor/ substrate–air interface thus opening the escape cone [45]. Given that the geometry of the encapsulant is chosen such (e.g. hemispherical dome structure) that the light is always incident normal to the encapsulant–air interface, the LEE can be increased by a factor of two or three. For UV LEDs the chosen encapsulant should be optically transparent in the UV region, stable under UV exposure and high temperatures, chemically inert, hermetic, mold-able and have a refractive index similar or close to that of the semiconductor material or the substrate. However, the transparency of epoxy resins or silicones, presently used as encap-sulants for visible and IR LEDs, decreases drastically at wavelengths shorter than 350 nm.

Prolonged exposure to UV light and heat also degrades the encapsulant which results in a further decrease in the transparency [147]. As most commercial silicones are either strongly absorbing or degrade rapidly in the deep UV region, they are not used in the packaging of UV LEDs below 350 nm. Currently, for the packaging of deep UV LEDs, UV transparent quartz windows or lenses are used. However, the fabrication of quartz lenses and their inte-gration with LEDs is tedious and very expensive. Recently Yamada et al presented results on 265 nm and 285 nm UVC LEDs encapsulated with polymerized perfluoro(4-vinyloxy-1-butene) [148]. The stable end (s-type) version of the encapsulant was reported to have a transparency level above 90% down to 200 nm and no visible ageing or degradation of the encapsulant was found after more than 3000 hours of operation.

76 Extraction of light from LEDs

Figure 5.19: Absorption coefficient of PDMS and ethylene vinyl acetate (EVA) as a function of wavelength and time spent (a) under a Xe arc lamp at room temperature in air (b) at 85C and 85%

relative humidity [149].

Extraction of light from LEDs 77

Figure 5.20:(a) Comparison of the spectrum of a 390 nm InAlGaN LED chip without an encapsulant to an LED chip with PDMS as an encapsulant. Inset: Photograph of the encapsulated LED. (b) Inte-grating sphere measurements of the output power of 390 nm InAlGaN LED chips with and without a PDMS encapsulant.

In this work the use of polydimethylsiloxane (PDMS), with refractive index of 1.54 -1.55 [150], as an encapsulant for UV LEDs was investigated. PDMS is an elastomeric material and belongs to the group of polymeric organosilicon compounds or silicones. It is extensively used as a master mould for soft lithography due to its ease of use (easily mold-able), low cost and high transparency in the UV region [151]. However, it has never before been used as an encapsulant for UV LEDs. McIntosh et al. [149] demonstrated that PDMS is stable under 1948 hours of exposure to a Xenon arc lamp (5.6 kWh/m2) at room temperature (Fig. 5.19 a). Additionally, they showed after exposure to damp heat (85% relative humidity and 85C) for 1200 hours, the absorption in a 1.6 mm thick PDMS layer increased from less than 1% to 10% at 300 nm (Fig. 5.19 b). The stability of PDMS to UV exposure and its reasonable stability to heat and high humidity make it ideal for UV LED encapsulation.

To study the influence of PDMS, used as an encapsulant, on the LEE of UV LEDs, the change in the output power of a 380 nm LED after encapsulation with PDMS was measured.

The encapsulant was moulded into a hemispherical dome using an aluminium master mould.

No change in the emission spectrum of the LED was observed indicating no absorption of light in the PDMS layer or secondary emission of light from the layer (Fig. 5.20 a). A two-fold increase in the output power of the LED was obtained indicating a two-two-fold increase in the LEE of the LED with the use of a hemispherical PDMS encapsulant (Fig. 5.20 b). Hence PDMS, with its high UV transparency, UV and thermal stability and easy handling, can be used as an effective and low cost option as an UV and visible LED encapsulant and for the fabrication of integrated optical components to increase the LEE of LEDs.

78 Extraction of light from LEDs