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Amelioration of internal quantum efficiency of green GaN-based light-emitting diodes by employing variable active region
Physica E 117 (2020) 113826
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Amelioration of internal quantum efficiency of green GaN-based light-emitting diodes by employing variable active region Muhammad Usman a, *, Abdur-Rehman Anwar a, Munaza Munsif a, Dong-Pyo Han b, Kiran Saba c a
Faculty of Engineering Sciences, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, 23460, Khyber Pakhtunkhwa, Pakistan Faculty of Science and Technology, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, 468-8502, Japan c Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142, Warsaw, Poland b
A B S T R A C T
We have designed and investigated green InGaN-based light-emitting diodes numerically. We have proposed an efficient device structure with improved carrier transport as well as distribution of holes in the active region. In our proposed structure, we employ a variable active region to improve the device performance. The variable active region comprises of quantum wells with increasing thickness and quantum barriers with decreasing thickness. Both quantum wells and quantum barriers are varied from n-side to p-side of the device. The transport and distribution of holes have significantly improved in our proposed structure exhibiting excellent performance in comparison with other structures.
1. Introduction Light-emitting diodes (LEDs) comprising of Gallium Nitride (GaN) materials have found extensive applications in solid-state lighting, backlight source of liquid crystal displays, color displays, automotive lightning, in addition to domestic applications [1–5]. In recent decades, Indium Gallium Nitride (InGaN) has been commercially employed as active region in GaN-based LEDs because of its tunable bandgap [3,6–9]. Nevertheless, the shift of wavelength towards green regime by employing the InGaN as active layer having high indium composition ~30% is still very challenging [10,11]. The high indium composition leads to severe problems, such as lattice misalignment in active region between quantum well and barrier which generates strong polarization effect [12]. Because of strong polarization, the rectangular shape of quantum well is distorted as a result, the electron-hole wavefunctions shift in opposite directions. As a consequence, the electron-hole over lapping probability decreases [13,14]. By Fermi’s golden rule, the rate of electronic transition from conduction band to valence band depends on square of the matrix element, which is directly proportional to overlap of electron-hole wavefunctions. Thus, the significant decrease in internal quantum efficiency (IQE) occurs at high indium content [15–17]. The internal field and quantum confined stark effect (QCSE) have been reported to have a dominant role in the separation of electron-hole wavefunctions decreasing the radiative recombination rate [18,19]. The origin of efficiency droop is still an open question, however a number of phenomena behind the droop have been reported,
such as Auger recombination [20], leakage of carriers from active region [21] and also non-homogenous distribution of carriers in active region and the asymmetric transport characteristics of carriers influences on efficiency [22]. For the improvement of optoelectronics characteristics, different device structures have been proposed such as lattice-matched AlGaInN barriers [23], graded electron blocking layer (EBL) by vary ing Al composition in AlGaN [24]. However, employing the EBL not only reduces the electron leakage, it also increases the effective barrier height impeding the injection of holes in the active region. Hole transport is a major issue in active region as compared to electron transport because of their greater effective mass [25]. For better hole injection, numerous approaches have been reported such as staggered-shaped InGaN quan tum well (QW) with silicon doped barriers [26], triangular-shaped quantum well near electron blocking layer [27], all-quaternary de vices [28,29], W-, zig-zag- and wedge-shaped quantum wells [30–32]. Nevertheless, the optoelectronics characteristics of GaN-based LEDs suffer at high current density. In this study, we proposed a green InGaN-based multiquantum well (MQW) LED having better hole trans port as compared to the regular structure. Due to the improvement of hole concentration in active region, the optoelectronic characteristics of our proposed LED are enhanced. 2. Device structure and parameters In this study, all the structures, under investigation, have green emission wavelengths and have same geometry which is 300 μm � 300
* Corresponding author. E-mail address: [email protected] (M. Usman). https://doi.org/10.1016/j.physe.2019.113826 Received 30 September 2019; Received in revised form 11 November 2019; Accepted 15 November 2019 Available online 18 November 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
M. Usman et al.
Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826
Fig. 1. (a) Illustration of the device structure. Illustration of the active region (b) fixed thickness of QW & QB (LED-A) and (c) by varying QW thickness only (LED-B) (d) by varying QB thickness only (LED-C) (e) by varying both QW & QB thickness (LED-D).
μm A three-pair In0.30Ga0.70N QW is used in the active region. The active
is 41.8 nm.
region is embedded between n- and p-GaN layers. n- and p-GaN layers thicknesses are 3 μm and 0.15 μm respectively, whereas the corre sponding doping concentrations n- and p-GaN layers are 5 � 1018 cm 3 and 1.2 � 1018 cm 3 respectively. Al0.15Ga0.85N EBL is also inserted between active region and p-GaN, having thickness is 0.020 μm with pdoping concentration of 3 � 1017 cm 3. Commercially available APSYS simulator has been employed in this study [33]. The entire structure is similar for all the LEDs excluding the active region. In LED-A, the thickness of QWs and QBs are fixed which is 2.6 nm and 8.5 nm respectively. For LED-B, the thickness of QBs are fixed (8.5 nm) and the thicknesses of QW1 to QW3 are 2.4 nm, 2.6 nm and 2.8 nm respectively. In LED-C, the QW thickness is fixed (2.6 nm) and the thickness of QB1 to QB 4 are 10 nm, 9 nm, 8 nm and 7 nm respectively. In the proposed structure i.e. LED-D, the thicknesses of QW1 to QW3 is 2.4 nm, 2.6 nm and 2.8 nm respectively while the thicknesses of QBs from QB1 to QB4 are 10 nm, 9 nm, 8 nm and 7 nm respectively. The thickness trend of QWs & QBs for all the LEDs is clear from Fig. 1. All the LEDs are operated at 300 K. The SRH lifetime and Auger recombination coefficients are 200 ns and 5 � 10 43 m6/s respectively and the internal loss is 2000 m 1. The active region thickness of each of the four devices
3. Result & discussion Fig. 2 shows the energyband diagrams of the LEDs under discussion. On the x-axis, position from 0.01 μm to 0.0518 μm shows the active region from n-to p-side. Fig. 2(a) shows the energyband diagram of LEDA with fixed thickness of all the QWs i.e. 2.6 nm and QBs i.e. 8.5 nm in the active region. Fig. 2(b) shows energyband diagram of LED-B with the thickness of QWs varying from 2.4 nm to 2.8 nm with a step size of 0.2 nm whereas the thickness of QBs is fixed i.e. 8.5 nm. Fig. 2(c) shows the energyband diagram of LED-C with the thickness of QBs varying from 10 nm to 7 nm with a step size of 1 nm by keeping the thickness of QWs fixed i.e. 2.6 nm. Fig. 2(d) shows the energyband diagram of LED-D with the increasing thickness of QWs from n-to p-side i.e. from 2.4 nm to 2.8 nm. Meanwhile, the thickness of QBs decreases from n-to p-side i.e. from 10 nm to 7 nm. It may be noted that the active region’s total thickness of all the LEDs is same. The QB near the p-side with reduced thickness is more favorable for better injection of holes [34]. In addition to the improved hole injection, the symmetric distribution of holes in the active layer is also necessary for the efficient LEDs. The hole transport 2
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Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826
Fig. 2. Energy band diagram of the LEDs under discussion at 40 A cm
can be improved by increasing the thickness of QW near the p-side [35]. Electrons have lower effective mass and high mobility, therefore the transport of electrons across the active region is generally not severely affected. To minimize the escape of electrons from the active region, EBL Al0.15Ga0.85N is inserted which has high bandgap as compared to GaN. By employing the EBL, the leakage of electrons is significantly reduced. But it also offers additional effective barrier height in the path of hole injection which is a well-reported downside of conventional AlGaN EBL [24,27]. Fig. 3(a) represents the electron distribution in active region of all the LEDs. In the proposed LED-D, the electron concentration is low in the first and second QWs but high in last QW as compared to LED-A and LED-B. Fig. 3(b) shows the comparison of hole distribution in the active region of the devices under investigation. In conventional structure, maximum concentration of holes is confined in the QW which is nearest to the p-side because holes have heavy mass and low potency to move, in comparison to electrons [34]. Therefore, conventionally, last QW has significant contribution in the radiative recombination. It may be recalled that the transport and uniform distribution of holes in active region is a serious issue [25]. The significant improvement in the hole transport as well as distribution in the active region of LED-B and LED-C, relative to LED-A, is consistent with the reported literature [35–37]. In proposed structure i.e. LED-D, the hole transport as well as their uniform distribution in the active region are remarkably improved. The overall hole concentration of LED-D is improved by ~80%, ~27% and ~21% in comparison to LED-A, LED-B and LED-C respectively. Fig. 3(c) compares the radiative recombination of the LEDs under discussion. The QW which is near the p-side, in case of all LEDs, shows maximum radiative recombination because the carrier confinement is maximum in this well.
2
.
The radiative recombination rate of LED-D is enhanced by ~28% with reference to both LED-A and LED-B. However, the radiative recombi nation is comparable with LED-C. The overlapping probability of electron-hole wavefunctions is enhanced, as a result, the radiative recombination rate is amplified [38]. The spontaneous emission rate spectra with respect to wavelength for all the LEDs are shown in Fig. 4. The peak emission spectra of all LEDs lie in the green regime, the emission wavelength of all the LEDs at maximum current is ~520 nm. Fig. 4 shows that by varying the QB thickness (i.e. LED-C), the peak of emission spectra increases but by varying the QW thickness (i.e. LED-B) the full width half maxima (FWHM) of emission spectra increases. These spectra results are consistent with published literature [35,39]. It may be pointed out that in case of high indium content i.e. green LEDs, band-filing effect leads to increase of FWHM with the increase of injection current [35]. Addi tionally, the integrated electroluminescence (EL) emission of LED-A, LED-B, LED-C and LED-D is ~4236.19 s 1 eV 1 m 1, ~4625.23 s 1 eV 1 m 1, ~5171.75 s 1 eV 1 m 1, and ~5458.76 s 1 eV 1 m 1 respectively. Moreover, it may also be recalled that the spontaneous emission rate is the distribution of radiative recombination [40,41]. IQE and light output power (LOP) with respect to current of all the LEDs are shown in Fig. 5. The IQE as well as LOP of the proposed LED (i. e. LED-D) is superior to LED-A, B and C. The efficiency droop ratio of LED-A, LED-B, LED-C and LED-D is ~55%, ~50%, ~45% and ~40% respectively at 100 A cm 2. In the proposed structure LED-D, the effi ciency droop ratio is decreased by ~28%. The LOP of LED-A, LED-B, LED-C and LED-D is ~65, ~72, ~77 and ~84 mW respectively at 100 A cm 2. LED-D shows best LOP among all the structures, which is improved by ~23% as compared to the reference structure. 3
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Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826
Fig. 3. (a–b) Comparison of electrons and holes distribution (c) radiative recombination in active region of the LEDs under discussion at 40 A cm
2
.
Fig. 4. Comparison of spontaneous emission rate spectra of the LEDs under discussion.
Fig. 5. Comparison of internal quantum efficiency and light output power of the LEDs under discussion.
4. Conclusion
Acknowledgments
We analyzed that the optoelectronic characteristics of proposed green LED-D are superior to LED-A, LED-B and LED-C. The enhancement of characteristics is basically attributed to good transport as well as uniform distribution of holes in active region. It is concluded that the increment in the thickness of QWs as well as decrement in the thickness of QBs from n-to p-side is the best solution to improve the optoelectronic characteristics.
The authors are obliged to Higher Education Commission of Pakistan and Semiconductor Photonics Laboratory, Hanyang University, South Korea for lending technical support for this work. We would also like to thank Dr. Muhammad Ajmal Khan from RIKEN Center for Advanced Photonics (RAP), Japan for the useful discussion. References [1] K. Ng, Y. Kuan, T. Chew, LED Backlight, Google Patents, 2006.
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Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826 [23] M.F. Schubert, et al., Polarization-matched Ga in N∕ Al Ga in N multi-quantumwell light-emitting diodes with reduced efficiency droop, Appl. Phys. Lett. 93 (4) (2008), 041102. [24] Y.-K. Kuo, J.-Y. Chang, M.-C. Tsai, Enhancement in hole-injection efficiency of blue InGaN light-emitting diodes from reduced polarization by some specific designs for the electron blocking layer, Opt. Lett. 35 (19) (2010) 3285–3287. [25] C. Wang, et al., Hole transport improvement in InGaN/GaN light-emitting diodes by graded-composition multiple quantum barriers, Appl. Phys. Lett. 99 (17) (2011) 171106. [26] Z. Zheng, et al., Improved carrier injection and efficiency droop in InGaN/GaN light-emitting diodes with step-stage multiple-quantum-well structure and holeblocking barriers, Appl. Phys. Lett. 102 (24) (2013) 241108. [27] L. Cheng, H. Chen, S. Wu, Improving hole injection and carrier distribution in InGaN light-emitting diodes by removing the electron blocking layer and including a unique last quantum barrier, J. Appl. Phys. 118 (8) (2015), 083104. [28] M. Usman, K. Saba, D.-P. Han, N. Muhammad, Efficiency improvement of green light-emitting diodes by employing all-quaternary active region and electronblocking layer, Superlattice Microstruct. 113 (2018) 585–591, 2018/01/01/. [29] M. Usman, et al., Improved optoelectronic performance of green light-emitting diodes by employing GaAlInN quantum wells without electron blocking layer, Phys. E Low-dimens. Syst. Nanostruct. 106 (2019) 68–72, 2019/02/01/. [30] M. Usman, U. Mushtaq, D.-G. Zheng, D.-P. Han, M. Rafiq, N. Muhammad, Enhanced internal quantum efficiency of bandgap-engineered green W-shaped quantum well light-emitting diode, Appl. Sci. 9 (1) (2019) 77. [31] M. Usman, et al., Zigzag-shaped quantum well engineering of green light-emitting diode, Superlattice Microstruct. 132 (2019) 106164, 2019/08/01/. [32] M. Islam, et al., Numerical analysis of the indium compositional variation on the efficiency droop of the GaN-based light-emitting diodes, in: SPIE Optical Engineering þ Applications, vol. 10755, SPIE, 2018, p. 5. [33] CrossLight®, APSYS (advanced physical models of semiconductor devices), Available: http://www.crosslight.com/products/apsys/. [34] S.-H. Yen, M.-L. Tsai, M.-C. Tsai, S.-J. Chang, Y.-K. Kuo, Investigation of optical performance of InGaN MQW LED with thin last barrier, IEEE Photonics Technol. Lett. 22 (24) (2010) 1787–1789. [35] C. Wang, et al., Efficiency droop alleviation in InGaN/GaN light-emitting diodes by graded-thickness multiple quantum wells, Appl. Phys. Lett. 97 (18) (2010) 181101. [36] Z. Ju, et al., Improved hole distribution in InGaN/GaN light-emitting diodes with graded thickness quantum barriers, Appl. Phys. Lett. 102 (24) (2013) 243504. [37] L. Cheng, D. Gong, W. Hu, Study on GaN-based light-emitting diodes with gradedthickness quantum barriers, in: 2015 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), IEEE, 2015, pp. 53–54. [38] T. Kuroda, A. Tackeuchi, Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells, J. Appl. Phys. 92 (6) (2002) 3071–3074. [39] M.-C. Tsai, S.-H. Yen, Y.-C. Lu, Y.-K. Kuo, Numerical study of blue InGaN lightemitting diodes with varied barrier thicknesses, IEEE Photonics Technol. Lett. 23 (2) (2010) 76–78. [40] N. Chen, W. Lien, Y. Yang, C. Shen, Y. Wang, J. Chen, Spectral shape and broadening of emission from AlGaInP light-emitting diodes, J. Appl. Phys. 106 (7) (2009), 074514. [41] A. Vaskuri, H. Baumgartner, P. K€ arh€ a, G. Andor, E. Ikonen, Modeling the spectral shape of InGaAlP-based red light-emitting diodes, J. Appl. Phys. 118 (20) (2015) 203103.
[2] G. Chen, et al., Performance of high-power III-nitride light emitting diodes, Phys. Status Solidi 205 (5) (2008) 1086–1092. [3] M.H. Crawford, LEDs for solid-state lighting: performance challenges and recent advances, IEEE J. Sel. Top. Quantum Electron. 15 (4) (2009) 1028–1040. [4] A. Laubsch, M. Sabathil, J. Baur, M. Peter, B. Hahn, High-power and highefficiency InGaN-based light emitters, IEEE Trans. Electron Devices 57 (1) (2009) 79–87. [5] L. Tai-Ping, et al., Blue InGaN light-emitting diodes with dip-shaped quantum wells, Chin. Phys. B 20 (10) (2011) 108504. [6] S. Nakamura, M. Senoh, N. Iwasa, S.-i. Nagahama, T. Yamada, T. Mukai, Superbright green InGaN single-quantum-well-structure light-emitting diodes, Jpn. J. Appl. Phys. 34 (10B) (1995) L1332. [7] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode: the Complete Story, Springer Science & Business Media, 2000. [8] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima, E. Kurimoto, Optical bandgap energy of wurtzite InN, Appl. Phys. Lett. 81 (7) (2002) 1246–1248. [9] J. Cho, J.H. Park, J.K. Kim, E.F. Schubert, White light-emitting diodes: history, progress, and future, Laser Photonics Rev. 11 (2) (2017) 1600147. [10] M.R. Krames, et al., Status and future of high-power light-emitting diodes for solidstate lighting, J. Disp. Technol. 3 (2) (2007) 160–175. [11] H. Li, et al., Quantum efficiency enhancement of 530 nm InGaN green lightemitting diodes with shallow quantum well, APEX 6 (5) (2013), 052102. [12] H.K. Cho, J.Y. Lee, C.S. Kim, G.M. Yang, N. Sharma, C. Humphreys, Microstructural characterization of InGaN/GaN multiple quantum wells with high indium composition, J. Cryst. Growth 231 (4) (2001) 466–473. [13] Z. Wu, et al., Effect of internal electrostatic fields in InGaN quantum wells on the properties of green light emitting diodes, Appl. Phys. Lett. 91 (4) (2007), 041915. [14] J.-H. Ryou, et al., Control of quantum-confined Stark effect in InGaN-based quantum wells, IEEE J. Sel. Top. Quantum Electron. 15 (4) (2009) 1080–1091. [15] H. Zhao, et al., Design and characteristics of staggered InGaN quantum-well lightemitting diodes in the green spectral regime, IET Optoelectron. 3 (6) (2009) 283–295. [16] H. Zhao, G. Liu, N. Tansu, Analysis of InGaN-delta-InN quantum wells for lightemitting diodes, Appl. Phys. Lett. 97 (13) (2010) 131114. [17] M. Usman, U. Mushtaq, D.-G. Zheng, D.-P. Han, N. Muhammad, Investigation of optoelectronic characteristics of indium composition in InGaN-based light-emitting diodes, Mater. Res. Express 6 (4) (2019), 045909. [18] M. Usman, H. Kim, J.-I. Shim, D.-S. Shin, Measurement of piezoelectric field in single-and double-quantum-well green LEDs using electroreflectance spectroscopy, Jpn. J. Appl. Phys. 53 (9) (2014), 098002. [19] U. Muhammad, N. Nawaz, K. Saba, K. Karimov, N. Muhammad, Experimental and numerical analysis of the indium-content on the internal electromechanical field in GaN-based light-emitting diodes, Optik 172 (2018) 1193–1198, 2018/11/01/. [20] Y. Shen, G. Mueller, S. Watanabe, N. Gardner, A. Munkholm, M. Krames, Auger recombination in InGaN measured by photoluminescence, Appl. Phys. Lett. 91 (14) (2007) 141101. [21] K.J. Vampola, M. Iza, S. Keller, S.P. DenBaars, S. Nakamura, Measurement of electron overflow in 450 nm InGaN light-emitting diode structures, Appl. Phys. Lett. 94 (6) (2009), 061116. [22] H.-Y. Ryu, J.-I. Shim, Effect of current spreading on the efficiency droop of InGaN light-emitting diodes, Opt. Express 19 (4) (2011) 2886–2894.
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Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Amelioration of internal quantum efficiency of green GaN-based light-emitting diodes by employing variable active region Muhammad Usman a, *, Abdur-Rehman Anwar a, Munaza Munsif a, Dong-Pyo Han b, Kiran Saba c a
Faculty of Engineering Sciences, Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, 23460, Khyber Pakhtunkhwa, Pakistan Faculty of Science and Technology, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, 468-8502, Japan c Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142, Warsaw, Poland b
A B S T R A C T
We have designed and investigated green InGaN-based light-emitting diodes numerically. We have proposed an efficient device structure with improved carrier transport as well as distribution of holes in the active region. In our proposed structure, we employ a variable active region to improve the device performance. The variable active region comprises of quantum wells with increasing thickness and quantum barriers with decreasing thickness. Both quantum wells and quantum barriers are varied from n-side to p-side of the device. The transport and distribution of holes have significantly improved in our proposed structure exhibiting excellent performance in comparison with other structures.
1. Introduction Light-emitting diodes (LEDs) comprising of Gallium Nitride (GaN) materials have found extensive applications in solid-state lighting, backlight source of liquid crystal displays, color displays, automotive lightning, in addition to domestic applications [1–5]. In recent decades, Indium Gallium Nitride (InGaN) has been commercially employed as active region in GaN-based LEDs because of its tunable bandgap [3,6–9]. Nevertheless, the shift of wavelength towards green regime by employing the InGaN as active layer having high indium composition ~30% is still very challenging [10,11]. The high indium composition leads to severe problems, such as lattice misalignment in active region between quantum well and barrier which generates strong polarization effect [12]. Because of strong polarization, the rectangular shape of quantum well is distorted as a result, the electron-hole wavefunctions shift in opposite directions. As a consequence, the electron-hole over lapping probability decreases [13,14]. By Fermi’s golden rule, the rate of electronic transition from conduction band to valence band depends on square of the matrix element, which is directly proportional to overlap of electron-hole wavefunctions. Thus, the significant decrease in internal quantum efficiency (IQE) occurs at high indium content [15–17]. The internal field and quantum confined stark effect (QCSE) have been reported to have a dominant role in the separation of electron-hole wavefunctions decreasing the radiative recombination rate [18,19]. The origin of efficiency droop is still an open question, however a number of phenomena behind the droop have been reported,
such as Auger recombination [20], leakage of carriers from active region [21] and also non-homogenous distribution of carriers in active region and the asymmetric transport characteristics of carriers influences on efficiency [22]. For the improvement of optoelectronics characteristics, different device structures have been proposed such as lattice-matched AlGaInN barriers [23], graded electron blocking layer (EBL) by vary ing Al composition in AlGaN [24]. However, employing the EBL not only reduces the electron leakage, it also increases the effective barrier height impeding the injection of holes in the active region. Hole transport is a major issue in active region as compared to electron transport because of their greater effective mass [25]. For better hole injection, numerous approaches have been reported such as staggered-shaped InGaN quan tum well (QW) with silicon doped barriers [26], triangular-shaped quantum well near electron blocking layer [27], all-quaternary de vices [28,29], W-, zig-zag- and wedge-shaped quantum wells [30–32]. Nevertheless, the optoelectronics characteristics of GaN-based LEDs suffer at high current density. In this study, we proposed a green InGaN-based multiquantum well (MQW) LED having better hole trans port as compared to the regular structure. Due to the improvement of hole concentration in active region, the optoelectronic characteristics of our proposed LED are enhanced. 2. Device structure and parameters In this study, all the structures, under investigation, have green emission wavelengths and have same geometry which is 300 μm � 300
* Corresponding author. E-mail address: [email protected] (M. Usman). https://doi.org/10.1016/j.physe.2019.113826 Received 30 September 2019; Received in revised form 11 November 2019; Accepted 15 November 2019 Available online 18 November 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
M. Usman et al.
Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826
Fig. 1. (a) Illustration of the device structure. Illustration of the active region (b) fixed thickness of QW & QB (LED-A) and (c) by varying QW thickness only (LED-B) (d) by varying QB thickness only (LED-C) (e) by varying both QW & QB thickness (LED-D).
μm A three-pair In0.30Ga0.70N QW is used in the active region. The active
is 41.8 nm.
region is embedded between n- and p-GaN layers. n- and p-GaN layers thicknesses are 3 μm and 0.15 μm respectively, whereas the corre sponding doping concentrations n- and p-GaN layers are 5 � 1018 cm 3 and 1.2 � 1018 cm 3 respectively. Al0.15Ga0.85N EBL is also inserted between active region and p-GaN, having thickness is 0.020 μm with pdoping concentration of 3 � 1017 cm 3. Commercially available APSYS simulator has been employed in this study [33]. The entire structure is similar for all the LEDs excluding the active region. In LED-A, the thickness of QWs and QBs are fixed which is 2.6 nm and 8.5 nm respectively. For LED-B, the thickness of QBs are fixed (8.5 nm) and the thicknesses of QW1 to QW3 are 2.4 nm, 2.6 nm and 2.8 nm respectively. In LED-C, the QW thickness is fixed (2.6 nm) and the thickness of QB1 to QB 4 are 10 nm, 9 nm, 8 nm and 7 nm respectively. In the proposed structure i.e. LED-D, the thicknesses of QW1 to QW3 is 2.4 nm, 2.6 nm and 2.8 nm respectively while the thicknesses of QBs from QB1 to QB4 are 10 nm, 9 nm, 8 nm and 7 nm respectively. The thickness trend of QWs & QBs for all the LEDs is clear from Fig. 1. All the LEDs are operated at 300 K. The SRH lifetime and Auger recombination coefficients are 200 ns and 5 � 10 43 m6/s respectively and the internal loss is 2000 m 1. The active region thickness of each of the four devices
3. Result & discussion Fig. 2 shows the energyband diagrams of the LEDs under discussion. On the x-axis, position from 0.01 μm to 0.0518 μm shows the active region from n-to p-side. Fig. 2(a) shows the energyband diagram of LEDA with fixed thickness of all the QWs i.e. 2.6 nm and QBs i.e. 8.5 nm in the active region. Fig. 2(b) shows energyband diagram of LED-B with the thickness of QWs varying from 2.4 nm to 2.8 nm with a step size of 0.2 nm whereas the thickness of QBs is fixed i.e. 8.5 nm. Fig. 2(c) shows the energyband diagram of LED-C with the thickness of QBs varying from 10 nm to 7 nm with a step size of 1 nm by keeping the thickness of QWs fixed i.e. 2.6 nm. Fig. 2(d) shows the energyband diagram of LED-D with the increasing thickness of QWs from n-to p-side i.e. from 2.4 nm to 2.8 nm. Meanwhile, the thickness of QBs decreases from n-to p-side i.e. from 10 nm to 7 nm. It may be noted that the active region’s total thickness of all the LEDs is same. The QB near the p-side with reduced thickness is more favorable for better injection of holes [34]. In addition to the improved hole injection, the symmetric distribution of holes in the active layer is also necessary for the efficient LEDs. The hole transport 2
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Fig. 2. Energy band diagram of the LEDs under discussion at 40 A cm
can be improved by increasing the thickness of QW near the p-side [35]. Electrons have lower effective mass and high mobility, therefore the transport of electrons across the active region is generally not severely affected. To minimize the escape of electrons from the active region, EBL Al0.15Ga0.85N is inserted which has high bandgap as compared to GaN. By employing the EBL, the leakage of electrons is significantly reduced. But it also offers additional effective barrier height in the path of hole injection which is a well-reported downside of conventional AlGaN EBL [24,27]. Fig. 3(a) represents the electron distribution in active region of all the LEDs. In the proposed LED-D, the electron concentration is low in the first and second QWs but high in last QW as compared to LED-A and LED-B. Fig. 3(b) shows the comparison of hole distribution in the active region of the devices under investigation. In conventional structure, maximum concentration of holes is confined in the QW which is nearest to the p-side because holes have heavy mass and low potency to move, in comparison to electrons [34]. Therefore, conventionally, last QW has significant contribution in the radiative recombination. It may be recalled that the transport and uniform distribution of holes in active region is a serious issue [25]. The significant improvement in the hole transport as well as distribution in the active region of LED-B and LED-C, relative to LED-A, is consistent with the reported literature [35–37]. In proposed structure i.e. LED-D, the hole transport as well as their uniform distribution in the active region are remarkably improved. The overall hole concentration of LED-D is improved by ~80%, ~27% and ~21% in comparison to LED-A, LED-B and LED-C respectively. Fig. 3(c) compares the radiative recombination of the LEDs under discussion. The QW which is near the p-side, in case of all LEDs, shows maximum radiative recombination because the carrier confinement is maximum in this well.
2
.
The radiative recombination rate of LED-D is enhanced by ~28% with reference to both LED-A and LED-B. However, the radiative recombi nation is comparable with LED-C. The overlapping probability of electron-hole wavefunctions is enhanced, as a result, the radiative recombination rate is amplified [38]. The spontaneous emission rate spectra with respect to wavelength for all the LEDs are shown in Fig. 4. The peak emission spectra of all LEDs lie in the green regime, the emission wavelength of all the LEDs at maximum current is ~520 nm. Fig. 4 shows that by varying the QB thickness (i.e. LED-C), the peak of emission spectra increases but by varying the QW thickness (i.e. LED-B) the full width half maxima (FWHM) of emission spectra increases. These spectra results are consistent with published literature [35,39]. It may be pointed out that in case of high indium content i.e. green LEDs, band-filing effect leads to increase of FWHM with the increase of injection current [35]. Addi tionally, the integrated electroluminescence (EL) emission of LED-A, LED-B, LED-C and LED-D is ~4236.19 s 1 eV 1 m 1, ~4625.23 s 1 eV 1 m 1, ~5171.75 s 1 eV 1 m 1, and ~5458.76 s 1 eV 1 m 1 respectively. Moreover, it may also be recalled that the spontaneous emission rate is the distribution of radiative recombination [40,41]. IQE and light output power (LOP) with respect to current of all the LEDs are shown in Fig. 5. The IQE as well as LOP of the proposed LED (i. e. LED-D) is superior to LED-A, B and C. The efficiency droop ratio of LED-A, LED-B, LED-C and LED-D is ~55%, ~50%, ~45% and ~40% respectively at 100 A cm 2. In the proposed structure LED-D, the effi ciency droop ratio is decreased by ~28%. The LOP of LED-A, LED-B, LED-C and LED-D is ~65, ~72, ~77 and ~84 mW respectively at 100 A cm 2. LED-D shows best LOP among all the structures, which is improved by ~23% as compared to the reference structure. 3
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Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826
Fig. 3. (a–b) Comparison of electrons and holes distribution (c) radiative recombination in active region of the LEDs under discussion at 40 A cm
2
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Fig. 4. Comparison of spontaneous emission rate spectra of the LEDs under discussion.
Fig. 5. Comparison of internal quantum efficiency and light output power of the LEDs under discussion.
4. Conclusion
Acknowledgments
We analyzed that the optoelectronic characteristics of proposed green LED-D are superior to LED-A, LED-B and LED-C. The enhancement of characteristics is basically attributed to good transport as well as uniform distribution of holes in active region. It is concluded that the increment in the thickness of QWs as well as decrement in the thickness of QBs from n-to p-side is the best solution to improve the optoelectronic characteristics.
The authors are obliged to Higher Education Commission of Pakistan and Semiconductor Photonics Laboratory, Hanyang University, South Korea for lending technical support for this work. We would also like to thank Dr. Muhammad Ajmal Khan from RIKEN Center for Advanced Photonics (RAP), Japan for the useful discussion. References [1] K. Ng, Y. Kuan, T. Chew, LED Backlight, Google Patents, 2006.
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Physica E: Low-dimensional Systems and Nanostructures 117 (2020) 113826 [23] M.F. Schubert, et al., Polarization-matched Ga in N∕ Al Ga in N multi-quantumwell light-emitting diodes with reduced efficiency droop, Appl. Phys. Lett. 93 (4) (2008), 041102. [24] Y.-K. Kuo, J.-Y. Chang, M.-C. Tsai, Enhancement in hole-injection efficiency of blue InGaN light-emitting diodes from reduced polarization by some specific designs for the electron blocking layer, Opt. Lett. 35 (19) (2010) 3285–3287. [25] C. Wang, et al., Hole transport improvement in InGaN/GaN light-emitting diodes by graded-composition multiple quantum barriers, Appl. Phys. Lett. 99 (17) (2011) 171106. [26] Z. Zheng, et al., Improved carrier injection and efficiency droop in InGaN/GaN light-emitting diodes with step-stage multiple-quantum-well structure and holeblocking barriers, Appl. Phys. Lett. 102 (24) (2013) 241108. [27] L. Cheng, H. Chen, S. Wu, Improving hole injection and carrier distribution in InGaN light-emitting diodes by removing the electron blocking layer and including a unique last quantum barrier, J. Appl. Phys. 118 (8) (2015), 083104. [28] M. Usman, K. Saba, D.-P. Han, N. Muhammad, Efficiency improvement of green light-emitting diodes by employing all-quaternary active region and electronblocking layer, Superlattice Microstruct. 113 (2018) 585–591, 2018/01/01/. [29] M. Usman, et al., Improved optoelectronic performance of green light-emitting diodes by employing GaAlInN quantum wells without electron blocking layer, Phys. E Low-dimens. Syst. Nanostruct. 106 (2019) 68–72, 2019/02/01/. [30] M. Usman, U. Mushtaq, D.-G. Zheng, D.-P. Han, M. Rafiq, N. Muhammad, Enhanced internal quantum efficiency of bandgap-engineered green W-shaped quantum well light-emitting diode, Appl. Sci. 9 (1) (2019) 77. [31] M. Usman, et al., Zigzag-shaped quantum well engineering of green light-emitting diode, Superlattice Microstruct. 132 (2019) 106164, 2019/08/01/. [32] M. Islam, et al., Numerical analysis of the indium compositional variation on the efficiency droop of the GaN-based light-emitting diodes, in: SPIE Optical Engineering þ Applications, vol. 10755, SPIE, 2018, p. 5. [33] CrossLight®, APSYS (advanced physical models of semiconductor devices), Available: http://www.crosslight.com/products/apsys/. [34] S.-H. Yen, M.-L. Tsai, M.-C. Tsai, S.-J. Chang, Y.-K. Kuo, Investigation of optical performance of InGaN MQW LED with thin last barrier, IEEE Photonics Technol. Lett. 22 (24) (2010) 1787–1789. [35] C. Wang, et al., Efficiency droop alleviation in InGaN/GaN light-emitting diodes by graded-thickness multiple quantum wells, Appl. Phys. Lett. 97 (18) (2010) 181101. [36] Z. Ju, et al., Improved hole distribution in InGaN/GaN light-emitting diodes with graded thickness quantum barriers, Appl. Phys. Lett. 102 (24) (2013) 243504. [37] L. Cheng, D. Gong, W. Hu, Study on GaN-based light-emitting diodes with gradedthickness quantum barriers, in: 2015 International Conference on Numerical Simulation of Optoelectronic Devices (NUSOD), IEEE, 2015, pp. 53–54. [38] T. Kuroda, A. Tackeuchi, Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells, J. Appl. Phys. 92 (6) (2002) 3071–3074. [39] M.-C. Tsai, S.-H. Yen, Y.-C. Lu, Y.-K. Kuo, Numerical study of blue InGaN lightemitting diodes with varied barrier thicknesses, IEEE Photonics Technol. Lett. 23 (2) (2010) 76–78. [40] N. Chen, W. Lien, Y. Yang, C. Shen, Y. Wang, J. Chen, Spectral shape and broadening of emission from AlGaInP light-emitting diodes, J. Appl. Phys. 106 (7) (2009), 074514. [41] A. Vaskuri, H. Baumgartner, P. K€ arh€ a, G. Andor, E. Ikonen, Modeling the spectral shape of InGaAlP-based red light-emitting diodes, J. Appl. Phys. 118 (20) (2015) 203103.
[2] G. Chen, et al., Performance of high-power III-nitride light emitting diodes, Phys. Status Solidi 205 (5) (2008) 1086–1092. [3] M.H. Crawford, LEDs for solid-state lighting: performance challenges and recent advances, IEEE J. Sel. Top. Quantum Electron. 15 (4) (2009) 1028–1040. [4] A. Laubsch, M. Sabathil, J. Baur, M. Peter, B. Hahn, High-power and highefficiency InGaN-based light emitters, IEEE Trans. Electron Devices 57 (1) (2009) 79–87. [5] L. Tai-Ping, et al., Blue InGaN light-emitting diodes with dip-shaped quantum wells, Chin. Phys. B 20 (10) (2011) 108504. [6] S. Nakamura, M. Senoh, N. Iwasa, S.-i. Nagahama, T. Yamada, T. Mukai, Superbright green InGaN single-quantum-well-structure light-emitting diodes, Jpn. J. Appl. Phys. 34 (10B) (1995) L1332. [7] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode: the Complete Story, Springer Science & Business Media, 2000. [8] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima, E. Kurimoto, Optical bandgap energy of wurtzite InN, Appl. Phys. Lett. 81 (7) (2002) 1246–1248. [9] J. Cho, J.H. Park, J.K. Kim, E.F. Schubert, White light-emitting diodes: history, progress, and future, Laser Photonics Rev. 11 (2) (2017) 1600147. [10] M.R. Krames, et al., Status and future of high-power light-emitting diodes for solidstate lighting, J. Disp. Technol. 3 (2) (2007) 160–175. [11] H. Li, et al., Quantum efficiency enhancement of 530 nm InGaN green lightemitting diodes with shallow quantum well, APEX 6 (5) (2013), 052102. [12] H.K. Cho, J.Y. Lee, C.S. Kim, G.M. Yang, N. Sharma, C. Humphreys, Microstructural characterization of InGaN/GaN multiple quantum wells with high indium composition, J. Cryst. Growth 231 (4) (2001) 466–473. [13] Z. Wu, et al., Effect of internal electrostatic fields in InGaN quantum wells on the properties of green light emitting diodes, Appl. Phys. Lett. 91 (4) (2007), 041915. [14] J.-H. Ryou, et al., Control of quantum-confined Stark effect in InGaN-based quantum wells, IEEE J. Sel. Top. Quantum Electron. 15 (4) (2009) 1080–1091. [15] H. Zhao, et al., Design and characteristics of staggered InGaN quantum-well lightemitting diodes in the green spectral regime, IET Optoelectron. 3 (6) (2009) 283–295. [16] H. Zhao, G. Liu, N. Tansu, Analysis of InGaN-delta-InN quantum wells for lightemitting diodes, Appl. Phys. Lett. 97 (13) (2010) 131114. [17] M. Usman, U. Mushtaq, D.-G. Zheng, D.-P. Han, N. Muhammad, Investigation of optoelectronic characteristics of indium composition in InGaN-based light-emitting diodes, Mater. Res. Express 6 (4) (2019), 045909. [18] M. Usman, H. Kim, J.-I. Shim, D.-S. Shin, Measurement of piezoelectric field in single-and double-quantum-well green LEDs using electroreflectance spectroscopy, Jpn. J. Appl. Phys. 53 (9) (2014), 098002. [19] U. Muhammad, N. Nawaz, K. Saba, K. Karimov, N. Muhammad, Experimental and numerical analysis of the indium-content on the internal electromechanical field in GaN-based light-emitting diodes, Optik 172 (2018) 1193–1198, 2018/11/01/. [20] Y. Shen, G. Mueller, S. Watanabe, N. Gardner, A. Munkholm, M. Krames, Auger recombination in InGaN measured by photoluminescence, Appl. Phys. Lett. 91 (14) (2007) 141101. [21] K.J. Vampola, M. Iza, S. Keller, S.P. DenBaars, S. Nakamura, Measurement of electron overflow in 450 nm InGaN light-emitting diode structures, Appl. Phys. Lett. 94 (6) (2009), 061116. [22] H.-Y. Ryu, J.-I. Shim, Effect of current spreading on the efficiency droop of InGaN light-emitting diodes, Opt. Express 19 (4) (2011) 2886–2894.
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