RESEARCH REVIEW Air gap boosts LED extraction efficiency
Output power increases with the insertion of an air gap between the chip and the remote phosphor
ENGINEERS from Yonsei University in Korea have shown that the insertion of an air gap can significantly increase the extraction efficiency of a white LED.
The team produced two types of device – a relatively conventional LED structure with an epoxy layer between the chip and the quantum dot phosphor, and a second, more radical design that replaced the epoxy with an air gap. This latter LED had a 33 percent higher optical extraction efficiency.
Both these remote phosphor designs have advantages over the more common ‘phosphor-in-a-cup’ architecture, which has phosphors dispersed in a liquid resin that is in direct contact with the chip.
This design, which is widely used in industry, impairs light extraction and colour quality, due to isotropic emission from the phosphors and the relatively narrow range of re-emitted spectra. The Korean team introduced
its air-gap structure to minimise light trapping in the phosphor layer of a remote-phosphor LED.
With their design, the light that is scattered or re-emitted from the quantum- dot-phosphor layer back towards the chip has a better chance of undergoing total internal reflection at the interface with air, due to the high difference in refractive index. This back-scattered light then has the potential to exit the device in the preferred direction. Calculations by the team suggest that when the quantum dot phosphors are embedded in a polymer with a refractive index of 1.53, about 45 percent of the re-emitted and scattered light heading in the backwards direction can be reflected at the air-polymer interface.
Further gains to the optical extraction efficiency are realised by adding a dome- shaped polymer lens on the LED surface. This increases the angle of the escape cone for the light emitted from the chip.
The team forms white-emitting LEDs by combining an InGaN-based chip emitting at 452 nm with two quantum dot polymer layers emitting at 597 nm and 530 nm. Driven at 2.8 V and 6 mA, the design with an airgap produced 1.66 mW, compared with 1.28 mW for the LED with an epoxy. The air-gap structure also had a slightly superior colour-rendering index of 81.8, compared with 80.7 for the control.
To determine the contribution made by the polymer lens on the LED, the team constructed a device without this optical element. Optical efficiency was then only 19.5 percent higher than that of the control, implying that the relative contributions from the air gap and the polymer lens were 66 percent and 34 percent, respectively.
M.-H. Shin et. al. Appl. Phys. Express 7 052101 (2014)
High-temperature CVD to cut SiC costs Researchers deposit high-quality 4H SiC at growth rates of more than 2 mm/hr
A JAPANESE PARTNERSHIP has significantly increased the growth rate for forming high-quality SiC by high- temperature CVD.
Growth rates of more than 2 mm/hour are now possible with this deposition technique, making it an attractive alternative to the standard method for forming SiC boules, physical vapour transport. The incumbent technology is only capable of growth rates of up to about 0.5 mm/hour, and the length of the SiC bulk crystal is limited, due to the use of sealed crucibles that make it tricky to replace the source material. These weaknesses partly account for the high cost of SiC substrates, which are an impediment to the growth of the SiC power electronics industry.
To cut SiC production costs, several research groups have investigated
alternative methods for boule growth. A solution approach using a silicon-metal solvent can realise growth rates in excess of 2 mm/hour.
However, another alternative, high- temperature CVD, is preferable – it allows a continuous source supply and reduces metal contamination by employing high- purity source gases.
However, prior to the work of the Japanese partnership between CRIEPI, FUPET and Denso, the downside of high- temperature CVD was the growth rate, which was limited to below 1 mm/hr.
Higher growth rates are possible in the Japanese team’s vertical CVD reactors that combine upward gas flow with a gas injector at the bottom and a hot zone surrounded by a graphite cylinder in the middle.
In the team’s smaller reactor, a growth rate of 1.4 mm/hr for the 4H polytype of SiC was possible at 2200 °C, using a mixture of H2
, SiH4 , C3 H8 and HCl,
with the latter at a flow rate of 2 m/s. Removing the HCl enabled an increase in the growth rate to 2.1 mm/hr at a growth temperature of 2350 °C, while maintaining the low threading screw dislocation density of the seed.
Armed with this knowledge, the researchers scaled up growth, moving to a larger reactor and a 3-inch seed. In this chamber, growth rates in the centre of the seed could reach 1.3 mm/hr when using HCl and a temperature of 2230 °C, and 2.4 mm/hr without HCl at 2300 °C.
N. Hoshino et. al. Appl. Phys. Express 7 052101 (2014)
June 2014
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