Design and Performance of Nitride-based UV LEDs
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Design and Performance of Nitride-based UV LEDs M. H. Crawford, J. Han, R. J. Shul, M. A. Banas, J. J. Figiel and L. Zhang Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT We overview several of the challenges in achieving high efficiency nitride-based UV (< 400 nm) LEDs. The issue of optical efficiency is presented through temperature-dependent photoluminescence studies of various UV active regions. These studies demonstrate enhanced optical efficiencies for active regions with In-containing alloys ( InGaN, AlInGaN). We compare the performance of two distinct UV LED structures. GaN/AlGaN quantum well LEDs with λ < 360 nm emission have demonstrated output powers > 0.1 mW, but present designs suffer from internal absorption effects. InGaN/AlInGaN quantum well LEDs with 370 nm< λ < 390 nm emission and > 1 mW output power are also presented.
INTRODUCTION While much of the research in the nitride field has focused on the development of blue and green LEDs and laser diodes, UV ( λ < 400 nm) LEDs based on GaN, AlGaN and/or AlGaInN active regions are also of great interest and are well suited to a number of applications. One of the most promising uses of a UV LED is as a high energy excitation source. In the biomedical and chemical sensing fields, UV LEDs can be used as compact and robust excitation sources of fluorescence. UV LEDs may also prove to be superior to blue LEDs in exciting phosphors for white lighting applications, due to improved color rendering. To date, there have been relatively few reports of UV LEDs based on the wide-bandgap nitride semiconductors. In particular, Akasaki et. al. [1] reported on a GaN/AlGaN double heterostructure (DH) LED with emission at 370 nm and up to 1.5% external quantum efficiency. Mukai, et. al. [2] have achieved an impressive 5 mW output at 371 nm from an InGaN/AlGaN DH LED with very low levels of indium (In) in the active region. That LED was reported to have up to 7.5% external quantum efficiency but the efficiency dropped by more than an order of magnitude as the emission wavelength shifted to 368 nm with the total elimination of In from the active region. Shorter wavelengths have been achieved by Han, et. al. [3] through a GaN/AlGaN multiquantum well (MQW) structure. These LEDs demonstrated a 354 nm emission peak with a narrow FWHM linewidth of 5.8 nm and relatively low output powers of 12 µW at 20 mA. Thus, while one group has demonstrated high ( > 5%) efficiency LED performance for λ > 370 nm, many of the challenges inherent to the shorter wavelength emission regime still remain. In this paper, we discuss challenges for achieving high performance UV LEDs and present the performance of two distinct UV LED structures. We first describe the materials growth and the general heterostructure designs for the UV LEDs that we have developed. In the next section, we review critical materials and design issues such as optical efficiency of UV active regions and internal absorption effects. We then report on the performance of GaN/AlGaN MQW LEDs with emission wavelengths
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