Micro-Raman Spectroscopy: Self-Heating Effects In Deep UV Light Emitting Diodes
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Micro-Raman Spectroscopy: Self-Heating Effects In Deep UV Light Emitting Diodes A. Sarua1, M. Kuball1, M. J. Uren2, A. Chitnis3, J. P. Zhang3, V. Adivarahan3, M. Shatalov3, and M. Asif Khan3 1 H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom 2 QinetiQ Ltd, Malvern WR14 3PS, United Kingdom 3 Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina 29208 ABSTRACT Ultraviolet light emitting diodes (LED) based on GaN and its ternary alloy AlGaN are key devices for applications such as solid state white lighting and chemical sensing. Ultraviolet LEDs are prone to self-heating effects, i.e., temperature rises during operation, contributing significantly to the commonly observed saturation of light output power at relatively low input currents. Rather little, however, is known about the actual device temperature of an operating ultraviolet LED. Using micro-Raman spectroscopy temperature measurements were performed as a function of input current on 325nm-Al0.18Ga0.82N/Al0.12Ga0.88N multiple quantum wells LEDs grown on sapphire substrates, flip-chip mounted on SiC for heat-sinking. Temperature maps were recorded over the active device area. Temperature rises of about 65 °C were measured at input currents as low as 50mA (at 8V) for 200 µm x 200 µm size LEDs despite flipchip mounting the devices. Temperature rises at the device edges were found to be higher than in the device center, due to combined heat sinking and current crowding effects. Finite difference heat dissipation simulations were performed and compared to the experimental results.
INTRODUCTION Wide bandgap materials, such as GaN, AlN, InN and their alloys offer a range of key properties such as the possibility to cover a bandgap range from 0.9eV up to 6.2eV, high breakdown electric field, high mobility, and high temperature and chemical stability. Development of nitride based LEDs has recently been targeted towards low power consumption white light illumination as well as light sources for chemical and biological sensor applications. Starting from high-efficiency blue-LEDs based on InGaN/GaN demonstrated in the early 90’s1, there has recently been a major breakthrough into the deep UV (below 350nm) range with the introduction of quantum well structures based on ternary and quaternary nitride alloys (Al,In)GaN2-9. Recently, LEDs with main peak emission wavelength as low as 278 nm with milliwatt operation have also been realized10. Nevertheless, there are several issues to be resolved before these devices can be introduced into the commercial market. In particular, selfheating effect is known to be responsible for saturation of light output power in nitride based LEDs11. The device thermal management is, therefore, of great importance for LED designers. In this work we report on the use of the micro-Raman technique to study self-heating in a deep UV LED based on AlGaN quantum well active structures operated using DC pump current. Finite difference heat dissipation simulation was used
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