Optik 123 (2012) 1287–1292 Contents lists available at SciVerse ScienceDirect Optik jou rnal homepage: www.elsevier.de/ijleo Optimization towards reduction of efficiency droop in blue GaN/InGaN based light emitting diodes Sumitra Singh ∗ , Navin K. Rohila, Suchandan Pal, C. Dhanavantri Optoelectronic Devices Group, Central Electronics Engineering Research Institute (Council of Scientific and Industrial Research), Pilani 333031, India a r t i c l e i n f o Article history: Received 8 March 2011 Accepted 26 July 2011 Keywords: Light-emitting diodes (LEDs) Multi-quantum-well (MQW) Efficiency droop GaN/InGaN a b s t r a c t Light emitting diodes (LEDs) based on GaN/InGaN material suffer from efficiency droop at high current injection levels. We propose multiple quantum well (MQW) GaN/InGaN LEDs by optimizing the bar- rier thickness and high–low–high indium composition to reduce the efficiency droop. The simulation results reflect a significant improvement in the efficiency droop by using barrier width of 10 nm and high–low–high indium composition in MQW LED. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction In recent time, GaN/InGaN based LEDs have been commercial- ized for indoor and outdoor lighting and displays, however, they suffer from reduction in efficiency at relatively high injection cur- rent levels, which has been named as the “efficiency droop” [1]. The quantum efficiency reaches its peak at low current density and thereafter monotonically decreases for further increasing the drive current [2–4]. The radiative recombination rate and output power of the InGaN based LEDs are thus degraded accordingly [5]. Kim et al. [2] reported the effect of polarization field in the active region due to the lattice mismatch among InGaN well layers and GaN bar- rier layers of the MQWs, which causes inadequate confinement of electrons in the active region, and subsequently causes electron overflow to the p-type region leading to an efficiency droop. In addition, it has also been reported that at relatively high current density, the carriers escape from indium localized states and recombine non-radiatively in high-density defect sites [6,7]. The dependence of the dislocation density and the well thickness on efficiency droop has also been reported as an explanation for electron overflow as a major cause of efficiency droop [8,9]. Recently, the effect of the Auger recombination at high injection current has been reported, which leads to the efficiency droop [10]. The reason to the essential Auger recombination presented may come from the unusual Auger coefficient of the GaN-based mate- rial, which varies from 1 × 10 -34 to 5.37 × 10 -28 cm 6 /s obtained ∗ Corresponding author. E-mail address: sumitra@ceeri.ernet.in (S. Singh). from experimental measurements and theoretical estimations [11]. A double-heterostructure has therefore been proposed as an active layer to solve the problem [2,6]. However, the origin of efficiency droop is still under debate. One of the approaches to improve the overall efficiency of III- nitride Blue LED is to reduce the electron overflowing problem [12] by optimizing the blue LED structure. The III-nitride compound semiconductors require relatively large injection currents (for their operation due to lower hole concentration), higher series resistance and lower material gain, ultimately making the electron overflow- ing problem more serious than other compound semiconductors. At present, incorporation of electron blocking layer (EBL) [13] is known to be one of the most effective approaches in reducing this problem. Moreover, it plays an important role in filling the pits, which are initially caused by the lattice mismatch between GaN and the sapphire substrate. The subsequent strained InGaN–GaN MQW that is grown at relatively lower temperature (750 ◦ C) would further intensify density and/or size of the emerging pits [14]. To improve the efficiency of MQW LEDs, EBL plays an important role in confining electrons effectively in the MQW region [12]. The p- type AlGaN EBL is usually used in blue LEDs to reduce the electron leakage current. However, the p-type AlGaN layer also retards the injection of holes, which leads to the degradation of efficiency at higher current level. Yen et al. [15] suggested to use n-type AlGaN layer below the active region and Kuo at al [16] suggested to use InGaN barrier instead of traditional GaN barrier for reduction of efficiency droop in InGaN/InGaN MQW LED. In this paper, we have optimized an InGaN–GaN MQW blue LED structure for the reduction of efficiency droop at relatively higher current without using n-type AlGaN layer with the same 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.07.061