Superlattices and Microstructures 132 (2019) 106167 Available online 25 June 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved. Double-side step-graded AlGaN electron blocking layer for nearly droop-free GaN-based blue LEDs Suraj Prasad a, b , Ramit Kumar Mondal a, b , Vijay Chatterjee a, b, * , Suchandan Pal a, b a Optoelectronics and MOEMS Group, CSIR- CEERI, Pilani, Rajasthan, 333031, India b Academy of Scientifc and Innovative Research (AcSIR), CSIR-CEERI Campus, Pilani, Rajasthan, 333031, India A R T I C L E INFO Keywords: Effciency droop Double side step grading Electron blocking layer (EBL) Internal quantum effciency (IQE) ABSTRACT Double-side step-graded AlGaN electron blocking layer (EBL) for InGaN/GaN based LEDs with emission wavelength at 415 nm is proposed to develop nearly droop-free performance. The performance comparison of LEDs with double-side step-graded EBL and with conventional EBL reveals that the proposed structure yields 31.50% improved IQE, three-fold higher output power with a very low effciency droop ~ (5%) at the current density of 200 A/cm 2 . Such noticeable improvement is justifed by critically analyzing and comparing both of the structures in terms of band structure, carrier concentration, electrostatic feld and radiative recombination rate. Anal- ysis confrms that double-side step-graded EBL results in superior electron confnement, increased hole injection and higher radiative recombination rate for LEDs. 1. Introduction Over the last few decades, III- Nitrides have appeared as a potential alloy system due to its tunable direct bandgap property. III- Nitride semiconductors have fetched revolutionary advancement in the feld of energy effcient solid-state lighting, producing a great societal impact along with countless applications [1–4]. The state-of-the-art InGaN/GaN based blue light emitting diodes (LEDs) are capable of producing external quantum effciency (EQE) as high as 80% [5]. However, at relatively higher current density, LEDs suffer from large effciency droop, limiting their credibility in high power applications, which still remains a major challenge to the scientifc community [6–9]. The possible origins behind effciency droop is still required proper investigation. Various mechanism such as electron outfow from active region [7,10], Auger recombination [11,12], poor hole injection to the active region [9], higher threading dislocation density [13,14], polarization feld [15], non-uniform carrier distribution [16] have been identifed by several research groups. Among them, the main mechanism behind the effciency droop is expected due to severe electron leakage and poor hole injection in the active region [7,9]. Higher mobility of electron due to lighter effective mass leads to inadequate confnement in the active region. On the other hand, owing to lower mobility and heavy effective mass of the holes, lower injection and non-uniform distribution of charge carriers in multiple quantum well is a common phenomenon. So, to overcome the above issues at higher cur- rent density, a suppression in electron outfow and enhancement in hole injection are desirable for highly effcient and powerful InGaN based blue LEDs. In order to prevail, different design and structural modifcations are employed to improve the effciency droop. LEDs with staggered InGaN quantum well [17], n-AlGaN EBL [18,19], graded composition AlGaN/GaN [20], AlGaN/InGaN superlattice EBL [21], AlGaN/GaN/AlGaN EBL [22], lattice matched InAlN EBL [23], P-N type quantum barrier [24] have already been reported. * Corresponding author. Optoelectronics and MOEMS Group, CSIR- CEERI, Pilani, Rajasthan, 333031, India. E-mail address: vc@ceeri.res.in (V. Chatterjee). Contents lists available at ScienceDirect Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices https://doi.org/10.1016/j.spmi.2019.106167 Received 16 April 2019; Received in revised form 7 June 2019; Accepted 23 June 2019