Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures
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Comparison of the Electroluminescence of Blue and Deep-UV Light-Emitting Diodes at Elevated Temperatures X. A. Cao, T. E. Stecher and S F. LeBoeuf GE Global Research Center, Niskayuna, New York 12309 ABSTRACT The performance of InGaN and AlGaN-based blue (465nm) and deep ultraviolet (UV) (280 nm) light-emitting diodes (LEDs) at elevated temperatures (25-175 oC) were investigated. As compared to the InGaN LED, the deep-UV LED showed dominant band-edge emission, much smaller alloy broadening and weaker localization effects. Strong carrier localization was retained in the blue LED up to 175 oC, leading to temperature-independent emission intensity at lowenergy tails. The light output of the UV LED decreased much more rapidly with increasing temperature than that of the blue LED. The characteristic temperature was 37 K, compared to 270 K for the blue LED. These findings implicate the lack of localization effects in AlGaN alloys as one of the causal factors in the poor thermal performance of the deep UV LED and suggest that increasing carrier confining potentials will provide a critical means to improve its thermal stability. INTRODUCTION Ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN alloys have received a lot of attention over the past several years due to their promise for numerous applications including biological agent detection, water purification, and high-density data storage. With recent progress in the epitaxy of high-Al AlGaN materials and heterostructures, efficient UV LEDs with milliwatt power output and peak wavelengths as short as 250 nm have been demonstrated on sapphire substrates [1-9]. Self-heating is greater for AlGaN LEDs compared to InGaN LEDs for the same light output. This is due to lower radiative efficiency and higher electrical power requirements. Poor electrical conductivity of AlGaN materials was found to exacerbate the current crowding and localized heating problems [9]. An efficient and robust packaging scheme is needed to prevent premature saturation of light output and ensure long-term stability. On the other hand, it is desirable that the UV LEDs have high temperature tolerance, which would enable high current operation, simplify the thermal designs, and allow LED use in a wide range of thermal environments. Little work has been done to understand the optical characteristics of AlGaN based LEDs at elevated temperatures. In this work, we compare the temperature-dependent performance of an AlGaN-based deep-UV LED and an InGaN-based blue LED. The deep-UV light emission is from band-to-band transition while the blue light results from dominant localized state emission. The light intensity of the UV LED is more sensitive to the junction temperature due to smaller carrier confining and localization potentials. EXPERIMENTAL DETAILS The deep-UV LED was grown on sapphire using migration-enhanced metalorganic chemical vapor deposition (MEMOCVD) by Sensor Electronic Technology [3,4]. The LED
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structure consisted of a Si-doped AlGaN n-type cladding laye
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