Elsevier

Optics Communications

Volume 400, 1 October 2017, Pages 89-95
Optics Communications

Improved performance of InGaN/GaN MQW LEDs with trapezoidal wells and gradually thinned barrier layers towards anode

https://doi.org/10.1016/j.optcom.2017.04.079Get rights and content

Highlights

  • Novel InGaN/GaN trapezoidal quantum well (TQW) blue LEDs are designed and studied.

  • TQW LEDs exhibit efficiency droop of only 2% compared to 49% in rectangular QW LEDs.

  • Light output in TQW LEDs improves by 236.7% relative to rectangular QW LEDs.

  • TQW LEDs outperform due to better hole transportation and distribution in the wells.

Abstract

We design and evaluate the performance of three InGaN/GaN multiple quantum well blue LEDs – A. rectangular quantum wells with a fixed barrier width, B. trapezoidal quantum wells with a fixed barrier width, and C. trapezoidal quantum wells with a decreasing barrier width towards the anode end – in terms of efficiency droop and power output. We obtain band diagram, electric field, emission spectra and carrier concentration using well calibrated APSYS simulation. Use of trapezoidal quantum wells increases better overlapping between electron and hole wavefunctions thereby increasing radiative recombination events. Furthermore decreasing barrier width from n- to p- regions shortens hole transport path which results in better hole transport and distribution in the wells and hence larger radiative recombination rate. Our proposed structure C exhibits efficiency droop reduction of 2.1% and enhancement of optical power of 236.7% compared to conventional rectangular quantum well structure at injection current of 120 mA.

Introduction

In recent years InGaN based light emitting diodes (LEDs) have received a great deal of research interest around the globe for their wide range of commercial applications such as full-color displays, television liquid crystal display (LCD) back lighting, mobile platforms, and general lighting  [1], [2], [3], [4], [5]. Particularly outdoor displays and solid-state lighting demand for high power LEDs together with enhanced efficiency. Growth of good quality InGaN/GaN multiple quantum well (MQW) structures, their subsequent processing combined with physical, electrical and optical characterizations pose a critical role for designing and realizing high performance LEDs. There have been a few remarkable works on the growth and characterization of InGaN/GaN MQW structures [6], [7]. Experimental findings reveal that fabrication of InGaN/GaN MQWs with high In contents of more than 30% is a formidable challenge due to formation of threading dislocations at the well/barrier interface and of uncontrolled gradient of In composition leading to quantum dot (QD)–like regions [7]. However, scholarly work of Cheong et al. [7] suggests that the growth of InGaN/GaN MQWs with In content of 30% exhibits a very good quality of well/barrier interface as confirmed by HRXRD spectra, XTEM and HRTEM images and also supported by PL spectra analysis. Despite extensive efforts have been put forward during more than one decade, the blue LEDs suffer from efficiency droop which refers to the reduction of emission efficiency with increasing injection current [8], [9], [10]. To date, a number of mechanisms have been proposed to explain this phenomenon including carrier leakage from the active region  [11], Auger recombination [12], [13], lower hole injection efficiency [14], and inhomogeneous distribution of electrons and holes in the active region. Although the exact origin of efficiency droop remains debatable, the asymmetric transport properties of carriers and their inhomogeneous distribution in the QWs influence greatly this phenomenon. To improve optical performance and reduce efficiency droop numerous schemes are proposed and analyzed in the literature [15], [16], [17], [18], [19]. It is also established that the existence of piezoelectric polarization fields in InGaN/GaN quantum well LEDs plays a critical role in degrading their performance. As such, a few research groups have attempted to reduce the polarization field in the multiple quantum wells (MQWs) by adopting various techniques as described below. K.M. Song et al. [20] grew several non polar a-plane InGaN/GaN multiple quantum well LEDs with different well thicknesses and observed that the optimized a-plane InGaN well thickness is much thicker than that in c-plane InGaN because of the reduced polarization fields in a-plane QWs. Light output from such LEDs was reported to increase with increasing well thickness up to 4.3 nm, however showing a reverse trend for larger well thicknesses particularly at high injection currents due to the poor crystalline quality and associated defects. Schubert et al. [21] demonstrated GaN-based LEDs with InGaN MQWs together with polarization matched AlGaInN barrier in order to reduce the effect of polarization field and electron leakage from the active region resulting in an eventual reduction in efficiency droop at high current densities. In a more recent publication, Lin et al. [22] reported an elegant epitaxial growth technique of the InGaN/GaN structures in which low temperature GaN barriers are utilized to promote strain relaxation in MQWs thereby resulting in reduced polarization field. This in turn weakens the quantum confined Stark effect (QCSE) [23], [24] and thus improves the luminous properties of LEDs. Some research groups have adopted different kinds of electron blocking layers such as graded electron blocking layer (EBL) [25], AlInGaN/AlGaN super lattice electron blocking layer (EBL) [26] and In graded last quantum barrier [8]. Although the electron blocking layer reduces electron leakage, it provides additional hole blocking potential that acts as a barrier for hole injection thereby preventing holes from entering the active region. To improve hole injection some elegant techniques are reported including multiple step stage InGaN/GaN quantum wells with Si doped barrier [27], triangular shape last quantum barrier [28] and embedded AlGaN EBL inside active QWs [29]. However, the performance of InGaN blue LEDs is not yet satisfactory particularly at high injection current.

To mitigate the aforesaid problems, we propose a blue LED featuring InGaN/GaN multiple quantum wells (MQWs) with trapezoidal shape clubbed with decreasing barrier width from cathode towards the electron blocking layer (EBL). The trapezoidal QWs exhibit better electron and hole wave functions overlap than rectangular QWs leading to higher radiative recombination rate while reduced the barrier thicknesses from n-side to p-side decreases the hole transport path thereby improving hole injection in the active region. In this study we consider three different LED structures having five quantum wells each – A. InGaN rectangular QWs with GaN barriers of a given width, B. trapezoidal InGaN QWs with fixed width of GaN barriers, and C. trapezoidal InGaN QWs with decreasing width of GaN barriers from cathode to the EBL. The electrical and optical analyses of the three structures are performed using APSYS simulation program [30].

Section snippets

Device structure and numerical framework

The c-plane sapphire substrate is exploited to form the light-emitting diode (LED) structure which consists of InGaN/GaN multiple quantum wells (MQWs) as the active region sandwiched between n-type GaN cathode and p-type AlGaN electron blocking layer capped by p-type GaN and anode contact. A 4.5 μm thick n-type GaN cathode layer with doping concentration of 5 × 1018 cm3 is grown on a 50 nm thick undoped GaN buffer layer which sits on the top of the substrate as illustrated in Fig. 1. The

Results and discussion

Fig. 3 (a) compares the variation of output power with the input current, also called the LI characteristic curve, for the LED A obtained using APSYS software with the experimental LI curve reported in [43]. Furthermore, Fig. 3 (b) shows a comparison between our simulated curve for IQE vs. current and the reported data curve in [43] for the same device. A very good agreement between our simulated curves and the reported characteristics (Fig. 3 (a) and (b)) ensures the validity of our

Conclusion

We have analyzed the performance of three InGaN/GaN multiple quantum well blue LEDs- (i) rectangular quantum wells with a fixed barrier width, (ii) trapezoidal quantum wells with a fixed barrier width and (iii) trapezoidal quantum wells with a decreasing barrier width towards the anode end – in terms of efficiency droop and light output on the basis of band diagram, electric field, carrier concentrations and emission spectra. Our proposed InGaN/GaN trapezoidal structure with a decreasing

Acknowledgments

The first and second authors acknowledge SERB for providing financial support through Project No. SB/EMEQ-087/2013 dtd.10.07.2013. Also the first author acknowledges UGC for providing his fellowship vide No. is F1-17.1/2013-14/RGNF-2013-14-SC-WES-52737/ (SA-III/Website).

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