Abstract
A method of measuring the precise temperature distribution of GaN-based light-emitting diodes (LEDs) by quantitative infrared micro-thermography is reported. To reduce the calibration error, the same measuring conditions were used for both calibration and thermal imaging; calibration was conducted on a highly emissive black-painted area on a dummy sapphire wafer loaded near the LED wafer on a thermoelectric cooler mount. We used infrared thermal radiation images of the black-painted area on the dummy wafer and an unbiased LED wafer at two different temperatures to determine the factors that degrade the accuracy of temperature measurement, i.e., the non-uniform response of the instrument, superimposed offset radiation, reflected radiation, and emissivity map of the LED surface. By correcting these factors from the measured infrared thermal radiation images of biased LEDs, we determined a precise absolute temperature image. Consequently, we could observe from where the local self-heat emerges and how it distributes on the emitting area of the LEDs. The experimental results demonstrated that highly localized self-heating and a remarkable temperature gradient, which are detrimental to LED performance and reliability, arise near the p-contact edge of the LED surface at high injection levels owing to the current crowding effect.
Highlights
Available GaN-based visible light-emitting diodes (LEDs) have become a vital optical component in various applications, such as full-color displays, backlights in liquid crystal displays, automotive lights, and traffic signal lights [1]
By comparing the light and temperature distributions, we show how current crowding causes a local hot spot and temperature gradient on the LED surface at a high injection current level
To measure the thermal radiation distribution on the LED surface, we used an infrared microscope system consisting of an infrared thermal imaging camera (640 × 512 cooled InSb infrared focal plane array (IRFPA) with 15 μm pixel pitch) and a microscope objective lens (5× magnification with instantaneous field of view of μm) manufactured by FLIR Systems Inc
Summary
Available GaN-based visible light-emitting diodes (LEDs) have become a vital optical component in various applications, such as full-color displays, backlights in liquid crystal displays, automotive lights, and traffic signal lights [1]. Infrared thermography is the most popular method of thermal imaging and temperature mapping of an object’s surface It is currently used in various applications that require highly spatially resolved temperature distribution measurements [14,15,16,17,18,19] because it is a rapid non-contact method offering high spatial and thermal resolution. It has rarely been used in precise temperature mapping of LEDs because of its limited accuracy. By comparing the light and temperature distributions, we show how current crowding causes a local hot spot and temperature gradient on the LED surface at a high injection current level
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