Superior Optoelectronic Performance of N-Polar GaN LED to Ga-Polar Counterpart in the “Green Gap” Range

The low luminous efficiency of indium gallium nitride (InGaN) light-emitting diodes (LED) in the “green gap” range has been a long unsettled issue confounding the researchers. One of the main obstacles comes from the intrinsic polarization field in the incumbent Ga-polar LEDs (Ga-LEDs), where the polarization field will bend the energy band thus reducing the radiative recombination efficiency. The scenario will become different when we reverse the polarization field with the adoption of N-polar GaN, which should be a promising candidate to obtain LEDs with high luminous efficiency in the “green gap” range. In this study, the optical and electronic performances of InGaN LEDs in the “green gap” range with Ga- and N-polar have been numerically investigated. The results demonstrate that the light-output power of N-polar LED (N-LED) is ~1.69-fold higher than that of Ga-LED at a current density of 1250 A/cm2, thus leading to a significantly improved internal quantum efficiency. Meanwhile, the turn-on voltage of N-LED is lowered by ~17.3% compared to that of Ga-LED. As revealed by the energy band diagram, the superior optoelectronic performance of N-LED is mainly attributed to the stronger carrier confinement in the active region and the lower carrier injection barriers. This study suggests the prospective realization of high luminous efficiency InGaN LEDs in the “green gap” range by the implementation of N-LEDs.

wells (MQWs) bear a strong polarization field with the mag-23 nitude of MV/cm [2]. Such a field will spatially separate the 24 The associate editor coordinating the review of this manuscript and approving it for publication was Md. Selim Habib . electrons and holes in the QWs and bend the energy band 25 diagram, thus reducing the radiative recombination efficiency 26 and exacerbating the carrier leakage from QWs, which is 27 known as the quantum-confined Stark effect (QCSE) [3], [4], 28 [5]. A large efficiency droop occurs in Ga-polar InGaN LEDs 29 (Ga-LEDs) under high current injection and a large blue shift 30 of emission wavelength appears, which are the dilemmas 31 confronting the incumbent GaN-based light-emitting devices 32 as most of the devices are fabricated on the Ga-polar GaN 33 matrix. The efficiency droop will become even worse when 34 the emission wavelength shifts to a longer side, e.g. >550 nm. 35 This is because the high indium contents will cause a stronger 36 VOLUME 10,2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ piezoelectric polarization field and deteriorate the crystalline 37 quality of InGaN, which is known as the ''green gap'' phe-38 nomenon [6]. Many efforts have been made to bridge the 39 ''green gap'' [6], [7], [8], [9]. However, up to date, the 40 external quantum efficiency (EQE) of red InGaN LEDs is 41 still lower than 3% [10]. Recently, N-polar LED (N-LED) has 42 been reported to be an effective candidate to break through the 43 bottleneck faced by Ga-LED [10], [11], [12], [13]. The polar-44 ization field in N-LED is in a reverse direction compared to 45 that of Ga-LED, thus having some intrinsic merits to suppress 46 the efficiency droop effect and carrier leakage problem [14].
In addition, a higher indium and magnesium incorporation  tive recombination efficiency and carrier overflow [12], [14].

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Therefore, a theoretical investigation for the N-LED in the 74 ''green gap'' range should be necessary.

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In this study, we numerically investigated the optical

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The structure of Ga-LED and N-LED is depicted in Fig. 1(a).

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According to the default set of the APSYS software, only the 113 total built-in electric field (i.e. the vector sum of p-n junction 114 built-in field and the polarization field) could be extracted 115 from the simulation. In order to evaluate the polarization field 116 in the MQWs region, we simulated the built-in electric fields 117 with and without considering the polarization effect both for 118 Ga-LED and N-LED at the equilibrium state (viz. zero bias).

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The results are delineated in Fig. 1(b). The fields tend to be 120 a constant in the three middle QWs, therefore the polariza-  The emission spectra as well as the peak wavelength and and 2(b), respectively. Both of the two structures emit a light 144 with a peak wavelength from ∼560 nm to ∼530 nm as the cur-145 rent increases, which locates in the ''green gap'' range. When 146 the injection current increased to 1250 A/cm 2 , the Ga-LED 147 and N-LED all demonstrate a blue-shift of peak wavelength 148 and an expansion of FWHM, indicating a screening effect of 149 QCSE and a band filling effect [5]. However, N-LED exhibits 150 a smaller blue-shift value (∼26 nm) than that of Ga-LED 151 of ∼39 nm, indicating a more stable wavelength of N-LED 152 under different currents. And the FWHM of N-LED is also a 153 few nanometers narrower than that of Ga-LED, indicating an 154 outperformed optical performance of N-LED.

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The light-output power (LOP) and the IQE of Ga-LED 156 and N-LED are shown in Fig. 3(a). The LOP of N-LED 157 remains an almost constant impetus when the injection cur-158 rent increases to a large current density of 1250 A/cm 2 , 159 resulting in a nondrooping IQE (∼98%). The high IQE and 160 almost no efficiency droop in N-LED is in agreement with 161 the results reported by other researchers [12], [33], indi-162 cating the suppression of carrier overflow, which will be 163 discussed in detail in the following section. While the LOP 164 of Ga-LED tends to a slower increment when the injection 165 current increases to a high level, leading to a droop effect 166 of IQE. The LOP of N-LED at 1250 A/cm 2 is ∼1.69-fold 167 higher than that of Ga-LED, indicating the superior optical 168  To reveal the mechanism behind the superior optoelec-177 tronic performance of N-LED, the energy band diagram is 178 investigated for the two structures, as delineated in Fig. 4.