Room and Cryogenic Temperature Operation of 280 nm Deep Ultraviolet Light Emitting Diodes
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Room and Cryogenic Temperature Operation of 280 nm Deep Ultraviolet Light Emitting Diodes Maxim Shatalov, Vinod Adivarahan, Jian Ping Zhang, Ashay Chitnis, Shuai Wu, Radhika Pachipulusu, Vasavi Mandavilli, and M. Asif Khan Department of Electrical Engineering, University of South Carolina, Columbia, SC 29208, U.S.A. ABSTRACT We present a study of the electrical and optical characteristics of 280 nm emission deep ultraviolet light emitting diodes (LED) at room and cryogenic temperatures. At low bias the defect assisted carrier tunneling primarily determines the current conduction. The roomtemperature spectral performance and optical power are limited mostly by pronounced deep level defect assisted radiative and non-radiative recombination as well as poor electron confinement in the active region. At temperatures below 100 K the electroluminescence peak intensity increases by more than one order of magnitude due to suppression of non-radiative recombination channels indicating that with a proper device design and improved material quality, milliwatt power 280 nm LED are viable.
INTRODUCTION Due to their enormous potential applications several research groups are actively developing deep ultraviolet (UV) light-emitting diodes (LEDs). Nishida et al. have reported on milliwatt power UV LEDs with emission at 352 nm over hydride vapor phase epitaxial (HVPE) GaN substrates [1]. Recently using sapphire substrates, we have also reported on deep UV LEDs with peak emission at 325 nm and 280 nm [2,3]. Our reported devices utilized an innovative AlN/Al0.5Ga0.5N superlattice (SL) approach for deposition of 2 µm thick AlxGa1-xN (x>0.35) buffer layers with significantly lower defect levels [4,5]. In addition we also employed a p+GaN/p-AlGaN hole accumulation layer for improving the p-doping and thereby hole injection into the active region, which, for the initial design of 285 nm LED, comprised of an Al0.46Ga0.54N/Al0.42Ga0.58N single quantum well [3,6]. The emission spectra of these 280 nm LEDs contained a sharp quantum well band edge peak at 280 nm and a deep level assisted emission band at 330 nm. Our recent studies show that at very low pump currents the 330 nm emission is stronger than that at 280 nm [7,8]. At higher currents it rapidly saturates with a simultaneous increase in the 280 nm peak, which then dominates the spectra at pump currents in excess of 200 mA. For the 280 nm emission, we previously reported room temperature powers as high as 0.25 mW for a pulsed pump current of 650 mA [9]. The number of nonradiative defects is a key factor that controls the quantum efficiency of LED devices [3]. The number of nonradiative defects is itself a strong function of the buffer and the active layers material quality. Additionally, our studies indicated that the emission band at 330 nm results from a recombination of the electrons via deep neutral acceptor levels in the p-AlGaN layer of our device structure [8, 9]. This data also suggests that the weak carrier confinement not only resulted in the long wavelength emission (
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