Abstract
This tutorial paper focuses on the physical origin of thermal droop, i.e., the decrease in the luminescence of light-emitting diodes (LEDs) induced by increasing temperature. III-nitride-based LEDs are becoming a pervasive technology, covering several fields from lighting to displays, from automotive to portable electronics, and from horticulture to sensing. In all these environments, high efficiency is a fundamental requirement, for reducing power consumption and system cost. Over the last decade, a great deal of effort has been put in the analysis of the efficiency droop, the decrease in LED internal quantum efficiency (IQE) induced by high current density. On the other hand, an IQE decrease is observed also for increasing temperature, a phenomenon usually referred to as thermal droop. For commercial LEDs, the IQE decrease related to thermal droop can be comparable to that of efficiency droop: for this reason, understanding thermal droop is a fundamental step for making LEDs capable of operating at high temperature levels. In several fields (including street lighting, automotive, photochemical treatments, projection, entertainment lighting, etc.), compact and high-flux light sources are required: typically, to reduce the size, weight, and cost of the systems, LEDs are mounted in compact arrays, and heat sinks are reduced to a minimum. As a consequence, LEDs can easily reach junction temperatures above 85–100 °C and are rated for junction temperatures up to 150–175 °C (figures from commercially available LED datasheets: Cree XHP70, Osram LUW HWQP, Nichia NVSL219CT, Samsung LH351B, and LedEngin LZP-00CW0R) and this motivates a careful analysis of thermal droop. This paper discusses the possible physical causes of thermal droop. After an introduction on the loss mechanisms in junctions, we will individually focus on the following processes: (i) Shockley–Read–Hall (SRH) recombination and properties of the related defects; (ii) Auger recombination and its temperature dependence, including the discussion of trap-assisted Auger recombination; (iii) impact of carrier transport on the thermal droop, including a discussion on carrier delocalization, escape, and freeze out; (iv) non-SRH defect-related droop mechanisms. In addition, (v) we discuss the processes that contribute to light emission at extremely low current levels and (vi) the thermal droop in deep ultraviolet LEDs, also with reference to the main parasitic emission bands. The results presented within this paper give a tutorial perspective on thermal droop; in addition, they suggest a pathway for the mitigation of this process and for the development of LEDs with stable optical output over a broad temperature range.
Highlights
For commercial light-emitting diodes (LEDs), the internal quantum efficiency (IQE) decrease related to thermal droop can be comparable to that of efficiency droop: for this reason, understanding thermal droop is a fundamental step for making LEDs capable of operating at high temperature levels
The results presented within this paper give a tutorial perspective on thermal droop; in addition, they suggest a pathway for the mitigation of this process and for the development of LEDs with stable optical output over a broad temperature range
Dislocations may act as dark spots, as reported in previous publications,[4,5] but the overall performance of state-of-the-art GaN LEDs is only slightly affected by dislocation density,[6,7] thanks to the existence of localization effects,[8,9,10] that are discussed in detail in Sec
Summary
Since the fabrication of the first blue LED, a continuous research work has been carried out by the scientific community, with the final goal of increasing the quantum efficiency from less than 1% internal quantum efficiency (IQE) of the early days[2] to more than 90% demonstrated more recently.[3]. The efficiency of state-of-the-art LEDs may be limited by thermal droop, which is a decrease in the IQE induced by operation at high temperatures.[22,23,24,25] A schematic representation of thermal droop is given in Fig. 2 (right): at high temperatures, the increase in parasitic recombination and leakage processes leads to a drop in internal quantum efficiency In this figure, we only consider the effect of two mechanisms, namely, SRH recombination [involving defects located at ET − EFi = 0.1 eV, with τ0 = 10−6 s, see Eq (5)] and a dominant escape process with energy barrier equal to 0.125 eV [see Eq (13) and Ref. 26]. Schematic band diagram of a single-quantum-well LED, focusing on the carrier injection and escape processes
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