Electrical Properties of Nichia AlGaN/InGaN/GaN Blue LEDs in a Wide Current/Temperature Range
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ABSTRACT Studies of electrical characteristics of nitride-based light emitting diodes (LEDs) are of interest as they can shed light on carrier transport across the p-n heterojunction. In addition, they provide a convenient way of investigating degradation processes associated with high electrical stress. We present electrical characteristics of Nichia NLPB500 blue LEDs based on AlGaN/InGaN/GaN material system in the temperature range of 9-340 K. Two components of current are identified. High-density current stress leads to diode degradation by shunt formation caused by metal electromigration. At 8 K, blue emission was observed at currents as small as 20 nA. INTRODUCTION Wide-bandgap semiconductors GaN and InGaN are promising materials for application as laser active media in the emission range from visible to UV. Carrier injection across a heterojunction is of great interest as the most convenient technique for pumping the active material. So far, electrical characteristics of GaN-based diodes have not been extensively investigated, and carrier transport mechanism has not been well identified. In this paper, we discuss electrical properties of high-brightness Nichia NLPB500 AlGaN/InGaN/GaN double-heterostructure blue LEDs grown on sapphire substrate [1], with emphasis on carrier transport through the diode. Carriers injected into the active region can recombine radiatively, resulting in an efficient blue emission related to impurity centers at intermediate currents, and UV bandedge-related emission at high currents. Previously, we have identified the mechanism of photopumped
visible emission in these devices at room temperature (RT) and at 20 K as radiative transitions involving three acceptor levels related to Zn impurity, combined with series of local (configuration-coordinate) phonon-replicas [2]. EXPERIMENT The LED structure [1] is shown schematically in Fig. 1. Both electrical contacts are situated at upper side of the diode chip, with insulating sapphire substrate at the bottom side. The junction area is typically 4.6x1I0 4 cm 2. The active In1Gal.,N layer (x = 0.06) is codoped with Zn and Si and is -50 nrm thick. Mgdoped nitrides form the p-side of the diode with 0.15-gtm-thick Alo1. 5Gao.85N cladding and 0.5-R.m-thick GaN contact layers. Two groups of devices were studied - "old generation" LEDs acquired in early 1994 (lot S403024) and "new generation" LEDs acquired a year later (lot 4B0001). Several devices were tested at different conditions of electrical load, including dc measurements of I-V characteristics in a wide temperature range and stress-testing under high-bias electrical pulses. Electrical characteristics were studied in the range of 9 - 340 K, with no total freeze-out of the diode conductance even at the lowest temperatures. Direct-current I-V measurements were performed using an HP 4140B picoamperometer, which enables the measurements down to the level of 1015 A. For the temperature measurements above 80 K, the diode was fixed inside a micro refrigeration device (MMR Technologies, model K200
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