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Abstract
The performance of AlGaN-based mid and deep ultraviolet light emitting diodes (LEDs) is severely limited by electron overflow and by the poor hole injection into the device active region. We have studied the effect of various electron blocking layers on the performance of AlGaN LEDs operating at ~280 nm. It is observed that, compared to conventional p-type electron blocking layer, the incorporation of an n-type AlN/AlGaN superlattice electron blocking layer before the active region can significantly improve the device performance by reducing electron overflow without compromising hole injection. Direct on-wafer measurement showed an external quantum efficiency ~4.4% and wall-plug efficiency ~2.8% by optimizing the design of n-type AlN/AlGaN superlattice electron blocking layer, which is nearly a factor of five to ten times better than identical devices but with the incorporation of a conventional p-type electron blocking layer.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
AlGaN-based mid and deep ultraviolet (UV) light emitting diodes (LEDs) have the potential to replace existing UV light sources, such as mercury lamps, for a broad range of applications including water purification, disinfection, analytic sensing, and biomedical photonics [1–4]. Such solid-state UV lamps can offer significantly reduced power consumption, smaller footprint, lower cost and tunable emission across the entire UV-A and UV-B and a large part of the UV-C spectra [5–8]. To date, however, AlGaN LEDs operating in the UV-B and UV-C bands still exhibit very low efficiency, primarily due to the presence of large densities of defects and dislocations in the device active region, poor current conduction, and inefficient light extraction [9–14]. For example, it has remained difficult to achieve efficient p-type conduction in high Al content AlGaN, due to several issues associated with Mg-dopant, including extremely high activation energy (up to ~600 meV), poor solubility, and the tendency towards self-compensation through the formation of point defects [15,16]. The ineffective Mg-doping leads to very low concentrations of free holes in Al-rich AlGaN [17–19]. On the other hand, electron concentrations in the range of ~1018-1019 cm−3 can be readily achieved in n-type Al-rich AlGaN using Si as a dopant [20–22]. This large disparity in the electron and hole concentrations, together with the difference in the carrier mobility (~25-50 cm2/V∙s [20,23] and ~1-5 cm2/V∙s [19] for electrons and holes in Al-rich AlGaN, respectively) leads to highly asymmetric charge carrier transport properties of n and p-type AlGaN cladding layers employed in mid and deep UV LED structures. The resistivity of p-type (Mg-doped) AlGaN epilayers increases rapidly with increasing Al content, with values generally in the range of 10~104 Ω∙cm for AlGaN layers with over 70% Al composition [11,24–27]. The corresponding resistivity values reported for n-type (Si-doped) Al-rich AlGaN are nearly three to six orders of magnitude lower, for similar Al composition, in the range of 0.01-1 Ω∙cm [23]. Consequently, there is a highly imbalanced electron and hole injection, leading to significant electron overflow in the device active region and poor carrier injection efficiency under moderate current injection conditions [28]. Such an issue becomes more severe for AlGaN LEDs operating at shorter wavelengths, due to the further reduced free hole concentration [29].
To prevent electron overflow, a high Al composition p-type (Mg-doped) AlGaN electron blocking layer (EBL) has been commonly incorporated between the device active region and p-AlGaN layer [30–32]. Such a technique has shown to be highly effective to reduce electron overflow and to improve the efficiency of GaN-based visible LEDs [33–36]. To date, however, there have been few studies on the effectiveness of this technique on the performance of mid and deep UV AlGaN LEDs [37–41]. In wide bandgap AlGaN LEDs, the incorporation of a high Al composition AlGaN EBL can severely compromise hole injection into the device active region, due to the large valence band offset at the hetero-interface and the significantly reduced hole concentration with increasing Al composition. The resulting increase in device resistivity also leads to undesired heating effect and reduced wall-plug efficiency.
In an effort to minimize electron overflow without compromising hole injection into the device active region, we have studied the effect of various n-type EBLs, incorporated between the active region and the n-AlGaN cladding layer, on the performance of high Al-content AlGaN UV LEDs. The LED heterostructures were grown on sapphire wafer by plasma-assisted molecular beam epitaxy (MBE) and were designed to operate at ~280 nm. The n-type EBL consists of Si-doped AlN/AlGaN short period superlattices, which allow for effective cooling of “hot” electrons before their injection into the device active region and therefore can minimize electron overflow. The polarization enhanced doping through the use of superlattice can simultaneously improve lateral conductivity and therefore leads to more uniform injection of electrons into the device active region [42,43]. Efficient injection of holes into the device active region, on the other hand, can be achieved due to the absence of any potential barrier. Direct on wafer measurements showed a maximum EQE ~4.4%, which is significantly higher compared to that (~0.5-1%) of conventional mid-UV AlGaN LEDs grown by MBE [42–46]. The reduced barrier to hole injection also contributes to a lower turn-on voltage for the n-type EBL samples. The peak wall-plug efficiency is improved from ~0.5% to ~2.8%, when we switch from a p-type EBL to an n-type EBL.
2. Epitaxy of AlGaN LED heterostructures
AlGaN mid-UV LED heterostructures were grown using a Veeco Gen 930 MBE system on AlN-on-sapphire templates from DOWA Holdings Co. Ltd. The growth parameters include a substrate temperature of 750 °C and a growth rate of ~150 nm/hr. The samples were grown under slightly metal (Ga) rich conditions to enhance Mg dopant incorporation [11,44,47–49]. Figure 1(a) illustrates the schematic for the LED structure using a conventional p-type EBL (Mg-doped AlN/Al0.7Ga0.3N superlattice). The LED heterostructure incorporating an n-type AlN/Al0.7Ga0.3N superlattice EBL is shown in Fig. 1(b). All the growths were initiated with a ~50 nm undoped AlN buffer, followed by a 300 nm thick n+-Al0.7Ga0.3N contact layer. The active region consists of six periods of Al0.45Ga0.55N (~2 nm)/Al0.7Ga0.3N (~5 nm) quantum wells. The p-AlGaN cladding layer consists of Mg-doped AlGaN with Al content graded linearly from 70% to 50% in ~20 nm, followed by the growth of ~20 nm p-type Al0.5Ga0.5N and 2 nm p-GaN contact layer. Further details about the growth and composition control of the AlGaN epilayers can be found elsewhere [50]. The measured internal quantum efficiency of the quantum wells is ~30-50%, as has been detailed in a previous work [51]. SIMS measurements performed on the samples confirmed that differently doped layers were well formed with extremely abrupt interfaces. Hall measurements using an Ecopia HMS-3000 Hall measurement setup showed resistivity values of ~0.001 Ω∙cm and ~0.7 Ω∙cm for the n-type and p-type Al0.7Ga0.3N layers, respectively.
Fig. 1 Schematic illustration of AlGaN mid-UV LEDs with the incorporation of (a) a p-type AlN/AlGaN superlattice electron blocking layer (EBL) and (b) an n-type AlN/AlGaN superlattice EBL. Equilibrium energy band diagram for the LED heterostructures with (c) a p-type AlN/AlGaN superlattice EBL and (d) an n-type AlN/AlGaN superlattice EBL.
Listed in Table 1, we have studied AlGaN mid-UV LEDs incorporating EBLs with different thicknesses and dopants. Three representative designs, referred to as A, B, and C, are described below. LED A has a p-type EBL which consists of ten periods of Mg-doped AlN (~1.5 nm)/Al0.7Ga0.3N (~1 nm) superlattice placed between the active region and the p-AlGaN cladding layer. 1-D Poisson-Schrödinger equations, considering the effect of the strong spontaneous polarization present in these alloys, were used to generate the equilibrium band diagrams for the different structures [52]. From the equilibrium band diagram shown in Fig. 1(c), a significant barrier to hole injection is observed, which has a deleterious effect on the device performance, especially at low voltages. The designs of LEDs B and C are identical to that of LED A, except that the p-type EBL is replaced by an n-type EBL incorporated between the n-AlGaN cladding layer and the active region. The n-type EBLs in LEDs B and C consist of ten and twenty periods of Si-doped AlN (~1.5 nm)/Al0.7Ga0.3N (~1 nm) superlattice, respectively. The equilibrium energy band diagram for LED B is schematically shown in Fig. 1(d). It is observed that there is a reduced barrier for hole injection to the active region, while presenting a barrier to electron injection to minimize electron overflow. Similar effect is also seen from the energy band diagram of LED C with twenty periods of n-type AlN/Al0.7Ga0.3N superlattice (not shown).
Table 1. List of AlGaN mid-UV LED samples with different electron blocking layer (EBL) designs.
3. Fabrication and characterization of mid-UV LEDs
In the LED fabrication, Al (200 nm)/Au (100 nm) was first deposited as the p-metal contact, followed by inductively coupled plasma reactive ion etching (ICP-RIE) to define mesas and to expose the n+-AlGaN contact layer. A Ti (40 nm)/Al (120 nm)/Ni (40 nm)/Au (50 nm) metal stack was deposited to form n-metal contact. The device areal sizes varied from 40 μm × 40 μm to 100 μm × 100 μm. The measured current-voltage characteristics for LEDs A, B, and C are shown in Fig. 2. It is seen that the best current-voltage characteristics are obtained for LED B with the incorporation of ten periods of n-AlN/Al0.7Ga0.3N superlattices, which has a turn-on voltage ~7 V. Significantly, current density over 1 kA/cm2 was measured at a moderate voltage ~13.5 V [45,46]. The incorporation of 20 periods of n-AlN/Al0.7Ga0.3N superlattices (LED C), however, increases the turn-on voltage, due to the large resistivity of the EBL. In both samples B and C with n-type EBLs, the current-voltage characteristics are better than that of LED A, which has a p-type EBL. The reduced hole injection into the active region with the use of a p-type EBL may contribute to the increased turn-on voltage for LED A, which operates at ~12.5 V for a current density of 20 A/cm2. It is worthwhile mentioning that the current-voltage characteristics of LED A are similar to some LEDs operating at ~280 nm reported previously [53,54]. We also performed studies on AlGaN LEDs with the incorporation of thirty period n-AlN/Al0.7Ga0.3N superlattice EBL as well as a 25 nm thick n-AlN EBL, which showed worse current-voltage characteristics compared to LEDs B and C. In this study Al/Au was used as the p-metal contact layer to enhance the light reflection and emission from the backside of the wafer (sapphire) [53]. Better turn-on voltage is expected with the use of Ni/Au p-metal contact for these devices, which would enhance current injection from the p-contact.
Fig. 2 I-V characteristics for AlGaN mid-UV LEDs measured at room temperature. Blue curve: LED A with 10 × p-AlN/Al0.7Ga0.3N superlattice EBL; Black curve: LED B with 10 × n-AlN/Al0.7Ga0.3N superlattice EBL; Red curve: LED C with 20 × n-AlN/Al0.7Ga0.3N superlattice EBL. The inset shows a top-emitting 100 μm × 100 μm device from sample B at a current density of ~50 A/cm2.
To measure the electroluminescence spectra of the fabricated LEDs, a Keithley 2400 SMU was used to apply a CW bias, and the emission was collected using an optical fiber coupled to a high-resolution spectrometer and detected by a charge coupled device detector. The normalized electroluminescence spectra for LEDs A and B at a current density of 100 A/cm2 are shown in Fig. 3(a). Variations of the spectral linewidth and peak emission wavelength with injection current are further shown in Figs. 3(b) and (c), respectively. The devices with the n-EBLs show a relatively narrow linewidth (~12 nm) and highly stable operation. Both the spectral linewidths and emission wavelengths exhibit a negligible dependence on the injection current. For comparison, the sample with p-type EBL (LED A) shows a broader electroluminescence spectrum (~22 nm) than that of the sample with n-type EBL (LED B), despite identical device active regions. The presence of a shoulder at ~292 nm, apart from the main peak at ~282 nm for the LED sample with p-type EBL is explained by the presence of significant electron overflow and the resulting parasitic emission from the p-AlGaN layer.
Fig. 3 (a) Normalized electroluminescence spectra measured at 100 A/cm2 at room temperature for LED A (10 × p-AlN/Al0.7Ga0.3N superlattice EBL) and LED B (10 × n-AlN/Al0.7Ga0.3N superlattice EBL). (b) Spectral linewidths versus current density for LEDs A and B. (c) Peak wavelength versus current density for LEDs A and B. The measurement error bars are also shown in (b) and (c).
On-wafer EQE of unpackaged LED devices was measured at room temperature using a Newport 818-ST2-UV silicon photodiode detector connected to a Newport Model 1919-R power meter [55,56]. To minimize heating effect, voltage pulses with a duty cycle of 1% and a period of 100 μs were supplied to the devices using an AV-1010B pulse generator. The measured EQE is expected to increase significantly if the devices were packaged to optimize light extraction. The EQE plotted against the current density is depicted in Fig. 4(a), for each of the LED samples. It is evident that the EQE is significantly higher for the samples grown with an n-type EBL instead of a p-type EBL. The maximum measured EQE is ~4.4% for LED B with ten periods of AlN/Al0.7Ga0.3N superlattice n-EBL, while the EQE peaks at only ~1.3% for LED A with p-type EBL. This further indicates the detrimental impact of p-type EBL on hole injection. As the thickness of n-type EBL superlattice is increased to twenty periods in LED C, the peak EQE also decreases compared to LED B, as a result of less efficient electron injection to the active region. Similar measurements performed on LED samples grown with thirty periods of AlN/Al0.7Ga0.3N superlattice and with a 25 nm AlN n-EBL (not shown) showed a further decrease in EQE, confirming the important role of n-EBL on the device performance. For comparison, the EQE of previously reported LEDs operating in this wavelength range grown by MBE is generally limited to ~0.5-1% [42–45]. With the use of a polarization-engineered tunnel junction to enhance hole injection, an EQE ~2.8% was recently reported for an AlGaN LED operating at ~287 nm grown by MBE [46].
Fig. 4 (a) External quantum efficiency (EQE), (b) power density, and (c) wall-plug efficiency (WPE) versus current density measured at room temperature for LED A (10 × p-AlN/Al0.7Ga0.3N superlattice EBL), LED B (10 × n-AlN/Al0.7Ga0.3N superlattice EBL), and LED C (20 × n-AlN/Al0.7Ga0.3N superlattice EBL).
Variations of the output power vs. injection current were further studied. Shown in Fig. 4(b), LED B exhibits a power density ~7.6 W/cm2 at 60 A/cm2, which is almost a factor of five times higher than that measured for LED A, which has a p-EBL. We also observe a significant droop in the maximum EQE of the fabricated devices when operating at higher biases. This could be related to the presence of electron flow under high biasing voltage, heating effect and/or Auger recombination [57]. The wall-plug efficiency of these devices was also measured and plotted in Fig. 4(c). The maximum wall-plug efficiency of the device with ten periods of the n-AlN/AlGaN superlattice EBL (LED B) is ~2.8%, which is significantly higher than that (~0.5%) for the LED with p-EBL (LED A). Improved performance was reported previously for InGaN based visible LEDs with the use of n-EBL, instead of p-EBL [35,36,58,59]. However, the performance improvement is much more dramatic for AlGaN UV LEDs. In mid and deep UV AlGaN LEDs wherein poor p-type conduction is a primary limiting factor for the device performance, the replacement of the conventional p-type EBL by an n-type EBL can suppress electron overflow without negatively impacting hole injection. The more balanced charge carrier transport allows for more efficient recombination in the device active region, thereby improving device performance. Such a unique design is expected to further improve the device performance for LEDs operating at 200-265 nm, wherein p-type doping is further hindered due to the even higher Al content required for these shorter wavelengths.
4. Summary
In summary, we have investigated the effect of EBL on the performance of AlGaN mid-UV LEDs. We have demonstrated the unique advantages of using an n-type EBL, compared to the conventional p-type EBL in significantly enhancing the performance of AlGaN UV LEDs. The use of an optimally designed n-type AlN/AlGaN superlattice EBL can reduce electron overflow without compromising hole transport and injection into the device active region, thereby better balancing the hole and electron injection to the device active region, while simultaneously increasing the lateral conductivity of electrons and allowing for better current spreading. A significant improvement in both the EQE and WPE was measured for AlGaN mid-UV LEDs, compared to previous reports by MBE. Further improved device performance is expected by optimizing the p-type doping and device fabrication process and by improving the light extraction efficiency.
Funding
College of Engineering, University of Michigan.
References
1. A. Khan, K. Balakrishnan, and T. Katona, “Ultraviolet light-emitting diodes based on group three nitrides,” Nat. Photonics 2(2), 77–84 (2008). [CrossRef]
2. P. J. Parbrook and T. Wang, “Light emitting and laser diodes in the ultraviolet,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1402–1411 (2011). [CrossRef]
3. L. Kemény, Z. Csoma, E. Bagdi, A. H. Banham, L. Krenács, and A. Koreck, “Targeted phototherapy of plaque-type psoriasis using ultraviolet B-light-emitting diodes,” Br. J. Dermatol. 163(1), 167–173 (2010). [CrossRef] [PubMed]
4. K. Davitt, Y.-K. Song, W. Patterson, A. Nurmikko, M. Gherasimova, J. Han, Y.-L. Pan, and R. Chang, “290 and 340 nm UV LED arrays for fluorescence detection from single airborne particles,” Opt. Express 13(23), 9548–9555 (2005). [CrossRef] [PubMed]
5. H. Hirayama, N. Maeda, S. Fujikawa, S. Toyoda, and N. Kamata, “Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes,” Jpn. J. Appl. Phys. 53(10), 100209 (2014). [CrossRef]
6. H. Sun and T. D. Moustakas, “UV emitters based on an AlGaN p–n junction in the form of graded-index separate confinement heterostructure,” Appl. Phys. Express 7(1), 012104 (2013). [CrossRef]
7. Y. Kashima, N. Maeda, E. Matsuura, M. Jo, T. Iwai, T. Morita, M. Kokubo, T. Tashiro, R. Kamimura, Y. Osada, H. Takagi, and H. Hirayama, “High external quantum efficiency (10%) AlGaN-based deep-ultraviolet light-emitting diodes achieved by using highly reflective photonic crystal on p-AlGaN contact layer,” Appl. Phys. Express 11(1), 012101 (2018). [CrossRef]
8. M. Jo, N. Maeda, and H. Hirayama, “Enhanced light extraction in 260 nm light-emitting diode with a highly transparent p-AlGaN layer,” Appl. Phys. Express 9(1), 012102 (2015). [CrossRef]
9. M.-H. Chang, D. Das, P. V. Varde, and M. Pecht, “Light emitting diodes reliability review,” Microelectron. Reliab. 52(5), 762–782 (2012). [CrossRef]
10. J. S. Speck and S. J. Rosner, “The role of threading dislocations in the physical properties of GaN and its alloys,” Physica B 273–274, 24–32 (1999). [CrossRef]
11. X. Liu, A. Pandey, D. A. Laleyan, K. Mashooq, E. T. Reid, W. J. Shin, and Z. Mi, “Charge carrier transport properties of Mg-doped Al 0.6 Ga 0.4 N grown by molecular beam epitaxy,” Semicond. Sci. Technol. 33(8), 085005 (2018). [CrossRef]
12. S. Inoue, N. Tamari, and M. Taniguchi, “150 mW deep-ultraviolet light-emitting diodes with large-area AlN nanophotonic light-extraction structure emitting at 265 nm,” Appl. Phys. Lett. 110(14), 141106 (2017). [CrossRef]
13. T. D. Moustakas, “Ultraviolet optoelectronic devices based on AlGaN alloys grown by molecular beam epitaxy,” MRS Commun. 6(3), 247–269 (2016). [CrossRef]
14. M. H. Crawford, “Chapter One - Materials Challenges of AlGaN-Based UV Optoelectronic Devices,” in Semiconductors and Semimetals, Z. Mi and C. Jagadish, eds., III-Nitride Semiconductor Optoelectronics (Elsevier, 2017), 96, pp. 3–44.
15. C. Stampfl and C. G. Van de Walle, “Doping of AlxGa1−xN,” Appl. Phys. Lett. 72(4), 459–461 (1998). [CrossRef]
16. C. Stampfl and C. G. Van de Walle, “Theoretical investigation of native defects, impurities, and complexes in aluminum nitride,” Phys. Rev. B Condens. Matter Mater. Phys. 65(15), 155212 (2002). [CrossRef]
17. Y. Chen, H. Wu, E. Han, G. Yue, Z. Chen, Z. Wu, G. Wang, and H. Jiang, “High hole concentration in p-type AlGaN by indium-surfactant-assisted Mg-delta doping,” Appl. Phys. Lett. 106(16), 162102 (2015). [CrossRef]
18. K. Ebata, J. Nishinaka, Y. Taniyasu, and K. Kumakura, “High hole concentration in Mg-doped AlN/AlGaN superlattices with high Al content,” Jpn. J. Appl. Phys. 57(4S), 04FH09 (2018). [CrossRef]
19. Y.-H. Liang and E. Towe, “Progress in efficient doping of high aluminum-containing group III-nitrides,” Appl. Phys. Rev. 5(1), 011107 (2018). [CrossRef]
20. T. M. Al tahtamouni, A. Sedhain, J. Y. Lin, and H. X. Jiang, “Si-doped high Al-content AlGaN epilayers with improved quality and conductivity using indium as a surfactant,” Appl. Phys. Lett. 92(9), 092105 (2008). [CrossRef]
21. K. Takeda, M. Iwaya, T. Takeuchi, S. Kamiyama, and I. Akasaki, “Electrical properties of n-type AlGaN with high Si concentration,” Jpn. J. Appl. Phys. 55(5S), 05FE02 (2016). [CrossRef]
22. Y. Taniyasu, M. Kasu, and N. Kobayashi, “Intentional control of n-type conduction for Si-doped AlN and AlXGa1−XN (0.42⩽x<1),” Appl. Phys. Lett. 81(7), 1255–1257 (2002). [CrossRef]
23. R. Collazo, S. Mita, J. Xie, A. Rice, J. Tweedie, R. Dalmau, and Z. Sitar, “Progress on n-type doping of AlGaN alloys on AlN single crystal substrates for UV optoelectronic applications,” Phys. Status Solidi., C Curr. Top. Solid State Phys. 8(7–8), 2031–2033 (2011). [CrossRef]
24. K. B. Nam, M. L. Nakarmi, J. Li, J. Y. Lin, and H. X. Jiang, “Mg acceptor level in AlN probed by deep ultraviolet photoluminescence,” Appl. Phys. Lett. 83(5), 878–880 (2003). [CrossRef]
25. Z. Li, J. Li, H. Jiang, Y. Han, Y. Xia, Y. Huang, J. Yin, and S. Hu, “High conductivity of Mg-Doped Al0.3Ga0.7N with Al0.4Ga0.6N/AlN superlattice structure,” Adv. Condens. Matter Phys. 2014, 784918 (2014). [CrossRef]
26. T. C. Zheng, W. Lin, R. Liu, D. J. Cai, J. C. Li, S. P. Li, and J. Y. Kang, “Improved p-type conductivity in Al-rich AlGaN using multidimensional Mg-doped superlattices,” Sci. Rep. 6(1), 21897 (2016). [CrossRef] [PubMed]
27. A. Kakanakova‐Georgieva, D. Nilsson, M. Stattin, U. Forsberg, Å. Haglund, A. Larsson, and E. Janzén, “Mg-doped Al0.85Ga0.15N layers grown by hot-wall MOCVD with low resistivity at room temperature,” Physica Status Solidi (RRL) –. Rapid Research Letters 4(11), 311–313 (2010).
28. Q. Dai, Q. Shan, J. Wang, S. Chhajed, J. Cho, E. F. Schubert, M. H. Crawford, D. D. Koleske, M.-H. Kim, and Y. Park, “Carrier recombination mechanisms and efficiency droop in GaInN/GaN light-emitting diodes,” Appl. Phys. Lett. 97(13), 133507 (2010). [CrossRef]
29. M. L. Nakarmi, N. Nepal, J. Y. Lin, and H. X. Jiang, “Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys,” Appl. Phys. Lett. 94(9), 091903 (2009). [CrossRef]
30. E. F. Schubert, Light-Emitting Diodes, 3rd ed. (Cambridge University, 2018).
31. Z. Liu, J. Ma, X. Yi, E. Guo, L. Wang, J. Wang, N. Lu, J. Li, I. Ferguson, and A. Melton, “p-InGaN/AlGaN electron blocking layer for InGaN/GaN blue light-emitting diodes,” Appl. Phys. Lett. 101(26), 261106 (2012). [CrossRef]
32. C. S. Xia, Z. M. S. Li, W. Lu, Z. H. Zhang, Y. Sheng, W. D. Hu, and L. W. Cheng, “Efficiency enhancement of blue InGaN/GaN light-emitting diodes with an AlGaN-GaN-AlGaN electron blocking layer,” J. Appl. Phys. 111(9), 094503 (2012). [CrossRef]
33. G. Alahyarizadeh, M. Amirhoseiny, and Z. Hassan, “Effect of different EBL structures on deep violet InGaN laser diodes performance,” Opt. Laser Technol. 76, 106–112 (2016). [CrossRef]
34. S.-H. Han, D.-Y. Lee, S.-J. Lee, C.-Y. Cho, M.-K. Kwon, S. P. Lee, D. Y. Noh, D.-J. Kim, Y. C. Kim, and S.-J. Park, “Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes,” Appl. Phys. Lett. 94(23), 231123 (2009). [CrossRef]
35. Y. Ji, Z.-H. Zhang, Z. Kyaw, S. Tiam Tan, Z. Gang Ju, X. Liang Zhang, W. Liu, X. Wei Sun, and H. Volkan Demir, “Influence of n-type versus p-type AlGaN electron-blocking layer on InGaN/GaN multiple quantum wells light-emitting diodes,” Appl. Phys. Lett. 103(5), 053512 (2013). [CrossRef]
36. Z.-H. Zhang, Y. Ji, W. Liu, S. Tiam Tan, Z. Kyaw, Z. Ju, X. Zhang, N. Hasanov, S. Lu, Y. Zhang, B. Zhu, X. Wei Sun, and H. Volkan Demir, “On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer,” Appl. Phys. Lett. 104(7), 073511 (2014). [CrossRef]
37. B. So, J. Kim, E. Shin, T. Kwak, T. Kim, and O. Nam, “Efficiency improvement of deep-ultraviolet light emitting diodes with gradient electron blocking layers,” Phys. Status Solidi 215(10), 1700677 (2018). [CrossRef]
38. L. Li, Y. Zhang, S. Xu, W. Bi, Z.-H. Zhang, and H.-C. Kuo, “On the hole injection for III-nitride based deep ultraviolet light-emitting diodes,” Materials (Basel) 10(10), 1221 (2017). [CrossRef] [PubMed]
39. Z.-H. Zhang, S.-W. Huang Chen, Y. Zhang, L. Li, S.-W. Wang, K. Tian, C. Chu, M. Fang, H.-C. Kuo, and W. Bi, “Hole transport manipulation to improve the hole injection for deep ultraviolet light-emitting diodes,” ACS Photonics 4(7), 1846–1850 (2017). [CrossRef]
40. H. Hirayama, Y. Tsukada, T. Maeda, and N. Kamata, “Marked enhancement in the efficiency of deep-ultraviolet AlGaN light-emitting diodes by using a multiquantum-barrier electron blocking layer,” Appl. Phys. Express 3(3), 031002 (2010). [CrossRef]
41. T. Kolbe, A. Knauer, J. Rass, H. K. Cho, S. Hagedorn, S. Einfeldt, M. Kneissl, and M. Weyers, “Effect of electron blocking layer doping and composition on the performance of 310 nm light emitting diodes,” Materials (Basel) 10(12), 1396 (2017). [CrossRef] [PubMed]
42. Y. Liao, C. Thomidis, C. Kao, and T. D. Moustakas, “AlGaN based deep ultraviolet light emitting diodes with high internal quantum efficiency grown by molecular beam epitaxy,” Appl. Phys. Lett. 98(8), 081110 (2011). [CrossRef]
43. T. D. Moustakas, Y. Liao, C. Kao, C. Thomidis, A. Bhattacharyya, D. Bhattarai, and A. Moldawer, “Deep UV-LEDs with high IQE based on AlGaN alloys with strong band structure potential fluctuations,” in Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XVI (International Society for Optics and Photonics, 2012), 8278, p. 82780L (2012).
44. Y. H. Liang and E. Towe, “Heavy Mg-doping of (Al,Ga)N films for potential applications in deep ultraviolet light-emitting structures,” J. Appl. Phys. 123(9), 095303 (2018). [CrossRef]
45. X. Liu, B. H. Le, S. Y. Woo, S. Zhao, A. Pofelski, G. A. Botton, and Z. Mi, “Selective area epitaxy of AlGaN nanowire arrays across nearly the entire compositional range for deep ultraviolet photonics,” Opt. Express 25(24), 30494–30502 (2017). [CrossRef] [PubMed]
46. Y. Zhang, Z. Jamal-Eddine, F. Akyol, S. Bajaj, J. M. Johnson, G. Calderon, A. A. Allerman, M. W. Moseley, A. M. Armstrong, J. Hwang, and S. Rajan, “Tunnel-injected sub 290 nm ultra-violet light emitting diodes with 2.8% external quantum efficiency,” Appl. Phys. Lett. 112(7), 071107 (2018). [CrossRef]
47. T. D. Moustakas and A. Bhattacharyya, “The role of liquid phase epitaxy during growth of AlGaN by MBE,” Phys. Status Solidi., C Curr. Top. Solid State Phys. 9(3–4), 580–583 (2012). [CrossRef]
48. A. M. Fischer, S. Wang, F. A. Ponce, B. P. Gunning, C. M. Fabien, and W. A. Doolittle, “Origin of high hole concentrations in Mg-doped GaN films,” Phys. Status Solidi 254(8), 1600668 (2017). [CrossRef]
49. S. D. Burnham, G. Namkoong, D. C. Look, B. Clafin, and W. A. Doolittle, “Reproducible increased Mg incorporation and large hole concentration in GaN using metal modulated epitaxy,” J. Appl. Phys. 104(2), 024902 (2008). [CrossRef]
50. D. A. Laleyan, X. Liu, A. Pandey, W. J. Shin, E. T. Reid, K. Mashooq, M. Soltani, and Z. Mi, “Molecular beam epitaxy and characterization of Al0.6Ga0.4N epilayers,” J. Cryst. Growth 507, 87–92 (2019). [CrossRef]
51. A. Aiello, A. Pandey, A. Bhattacharya, J. Gim, X. Liu, D. A. Laleyan, R. Hovden, Z. Mi, and P. Bhattacharya, “Optical and interface characteristics of Al0.56Ga0.44N/Al0.62Ga0.38N multiquantum wells with ∼280 nm emission grown by plasma-assisted molecular beam epitaxy,” J. Cryst. Growth 508, 66–71 (2019). [CrossRef]
52. See http://my.ece.ucsb.edu/mgrundmann/bandeng.htm for one-dimensional Poisson-Schrödinger solver.
53. N. Maeda, M. Jo, and H. Hirayama, “Improving the efficiency of AlGaN deep-UV LEDs by using highly reflective Ni/Al p-type electrodes,” Phys. Status Solidi 215(8), 1700435 (2018). [CrossRef]
54. T. Kinoshita, T. Obata, T. Nagashima, H. Yanagi, B. Moody, S. Mita, S. Inoue, Y. Kumagai, A. Koukitu, and Z. Sitar, “Performance and reliability of deep-ultraviolet light-emitting diodes fabricated on AlN substrates prepared by hydride vapor phase epitaxy,” Appl. Phys. Express 6(9), 092103 (2013). [CrossRef]
55. B. T. Tran and H. Hirayama, “Growth and fabrication of high external quantum efficiency AlGaN-based deep ultraviolet light-emitting diode grown on pattern Si substrate,” Sci. Rep. 7(1), 12176 (2017). [CrossRef] [PubMed]
56. M. A. Khan, N. Maeda, M. Jo, Y. Akamatsu, R. Tanabe, Y. Yamada, and H. Hirayama, “13 mW operation of a 295–310 nm AlGaN UV-B LED with a p-AlGaN transparent contact layer for real world applications,” J. Mater. Chem. C Mater. Opt. Electron. Devices 7(1), 143–152 (2019). [CrossRef]
57. J. Cho, E. F. Schubert, and J. K. Kim, “Efficiency droop in light-emitting diodes: Challenges and countermeasures,” Laser Photonics Rev. 7(3), 408–421 (2013). [CrossRef]
58. S. Yen, M. Tsai, M. Tsai, Y. Shen, T. Hsu, and Y. Kuo, “Effect of n-type AlGaN layer on carrier transportation and efficiency droop of blue InGaN light-emitting diodes,” IEEE Photonics Technol. Lett. 21(14), 975–977 (2009). [CrossRef]
59. Y. Li, Y. Gao, M. He, J. Zhou, Y. Lei, L. Zhang, K. Zhu, and Y. Chen, “Effect of polarization-matched n-type AlGaInN electron-blocking layer on the optoelectronic properties of blue InGaN light-emitting diodes,” J. Disp. Technol. 9(4), 244–248 (2013). [CrossRef]
