Open Access
Add to BibSonomy
Abstract
In this paper, AlInN nanowire ultraviolet light-emitting diodes (LEDs) with emission at ∼299 nm have been successfully demonstrated. We have further studied the light extraction properties of these nanowire LEDs using photonic crystal structures with square and hexagonal lattices of nanowires. The light extraction efficiency (LEE) of the periodic nanowire LED arrays was found to be significantly increased as compared to random nanowire LEDs. The LEEs reach ∼ 56%, and ∼ 63% for the square and hexagonal photonic crystal-based nanowire structures, respectively. Moreover, highly transverse-magnetic polarized emission was observed with dominant vertical light emission for the AlInN nanowire ultraviolet LEDs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Efficient ultraviolet (UV) light-emitting diodes (LEDs) are essentially needed for a broad range of applications such as water and air purification, resin curing for 3D printing, sterilization, bioagent detection, chemical, and biochemical sensing [1–4]. Currently, AlGaN semiconductor for UV LED structure has been studied extensively due to its direct energy bandgap in the wavelength range of ∼200 nm to 365 nm. However, the performance of AlGaN based UV LEDs is severely limited by poor external quantum efficiency (EQE). Some probable reasons for the low EQE in AlGaN based UV LEDs are efficiency droop, mostly due to electron leakage into the p-region [5], poor light extraction efficiency (LEE) [6] which is mainly due to the total internal reflection and the absorption of UV light in the p-GaN contact layer [7]. Furthermore, unique optical polarization properties of high Al composition AlGaN quantum wells results in dominant transverse-magnetic (TM) [E // c-axis] polarized output in the UV regime [8,9]. As the TM polarized light propagates horizontally, LEE of light emitting from the top surface is severely limited. The EQE of AlGaN based UV LEDs has been reported to be less than 10% for λ < 300 nm [10,11]. Various techniques have been explored to improve the LEE of deep UV LEDs including patterned sapphire substrates, surface roughening, rolled-up nanotubes, photonic crystal patterns, tunnel junction, and flip-chip design, but with limited success [12–18]. By engineering the bandgap energy as well as the structure of the electron blocking layer, quantum well, and quantum barrier of the device active region, the enhanced efficiency and output power of the UV LEDs have been recorded [19–21].
Recently, nanowire based UV LEDs have been demonstrated and show significant improvement in the performance compared with thin-film based UV LEDs [22–24] due to the effective strain relaxation in the lateral dimension, leading to the achievement of significantly improved light extraction efficiency and drastically reduced dislocations and polarization fields. Studies have been performed on enhancing the LEE of AlGaN nanowire UV emitters [25,26], emission from the lateral surfaces of nanowire ensembles [27], but the LEE of the TM polarized light emitting from the top surface is still low. We have recently reported the first AlInN nanowire LEDs emitting light in the UV range of 290 nm - 365 nm [28]. Moreover, these AlInN nanowire UV LEDs exhibit high internal quantum efficiency (IQE) and strong TM polarized emission [28]. It is worthwhile to mention that AlInN nanowire based deep UV LEDs show better performance in terms of IQE and output optical power as compared to AlGaN based deep UV LEDs due to enhancement in the carrier transport and reduction of electron leakage into p-region [29]. It is highly desired for the practical application to extract the TM polarized photons from the top surface. However, to the best of our knowledge, investigation on the polarization dependent LEE for AlInN based nanowire UV LEDs is very limited which is of great importance. In recent studies, it is shown that using the nanowire photonic crystals for GaN-based nanowires and by controlling the nanowire radius, spacing between the nanowires and morphology of nanowires through selective area epitaxy, luminescence emission intensity and stability can be enhanced significantly [27,30,31].
In this work, we have demonstrated the epitaxial growth, electrical and optical properties of randomly grown AlInN nanowires on Si (111) substrate by molecular beam epitaxy (MBE). Moreover, we have performed detailed study of ordered AlInN nanowire array UV LEDs using finite difference time domain (FDTD) simulation, which has been widely used in optical property analysis of III-nitride light-emitters [10,16,32]. We have studied the light extraction properties of AlInN nanowire LEDs at 299 nm for different photonic crystal structure arrangement with hexagon and square lattice of nanowires and compared the results with the random arrangement. The simulation results show that the dominant light emission direction is from the nanowire top surface albeit the light is TM polarized and the photonic crystal structures could improve the LEE much more than random structures by controlling the nanowire radius and spacing between the nanowires. The main reason for the lower efficiency in case of random nanowire structures is attributed to the light multipath scattering among adjacent nanowires which may localize the light inside the nanowires or to the increase of the length of light escaping path [33].
2. Experiment and results
The AlInN nanowire LED structure, illustrated in Fig. 1(a), was spontaneously formed on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II MBE system equipped with a radio-frequency plasma-assisted nitrogen source. GaN nanowires were doped n- and p-type using Si and Mg, respectively. The growth conditions for GaN include a growth temperature of ∼ 770 °C, nitrogen flow rate of 1 sccm, and forward plasma power of ∼ 400 W. The device active region consists of a 40 nm undoped Al0.75In0.25N well, sandwiched in between ∼ 100 nm n-Al0.78In0.22N and 100 nm p-Al0.78In0.22N barriers. The active region was grown at relatively low temperature of ∼ 670–700 °C to increase the In incorporation in AlInN segments. During the epitaxial growth of AlInN active region, the nitrogen flow rate was kept at 2.5 sccm, while the plasma power was fixed at 400 W. More details of the growth condition of this AlInN nanowire LED structure can be found in our previous report [28]. After being grown by MBE, the nanowire LED sample was then fabricated using standard lithography method. Detailed fabrication procedure of this type of nanowire UV LEDs is reported elsewhere [28]. The device characterization was performed on LED with area size of 500×500 µm2.
Fig. 1. (a) Schematic structure of AlInN nanowire UV LED on Si substrate. (b) 45o tilted scanning electron microscopy image of randomly grown AlInN nanowire UV LED on Si. (c) Photoluminescence spectrum of AlInN nanowire UV LED. The emission peaks at 280 nm, 299 nm and 368 nm confirms Al0.78In0.22N, Al0.75In0.25N quantum well, and GaN layers.
A 45 degree-titled scanning electron microscope (SEM) image of AlInN nanowire LED sample is illustrated in Fig. 1(b). The nanowires exhibit relatively uniform morphology and size. The wire diameter is in the range of 80 nm to 100 nm. The photoluminescence (PL) experiment was performed using a 266-nm laser (Kimmon Koha) as the excitation source. The signal from the nanowire LEDs is spectrally resolved by a high-resolution spectrometer and detected by a photomultiplier tube. The AlInN nanowire LEDs exhibit strong emission in the UV wavelength range as shown in Fig. 1(c). The peak emission at ∼ 299 nm is responsible for the emission from the AlInN quantum well while the emission at ∼ 280 nm is related to the emission from the AlInN barriers. The emission from GaN segment is recorded at ∼ 368 nm, as presented in Fig. 1(c).
The current-voltage and electroluminescence (EL) of the AlInN nanowire UV LEDs are further studied. Shown in Fig. 2(a), the LED device has low turn-on voltage and leakage current which are at ∼ 5V and less than 1 µA (at -8V), respectively. The inset of Fig. 2(a) shows the optical microscopic image of the fabricated AlInN nanowire UV LEDs. The AlInN nanowire UV LEDs have strong EL intensity with the peak wavelength at ∼ 299 nm, as shown in Fig. 2(b). Moreover, the efficiency droop is not observed up to an injection current density of 350 A/cm2 as shown in the inset of Fig. 2(b) which is consistent to other report for nanowire UV LEDs [34]. The relative external quantum efficiency (EQE) is measured at room temperature and under pulse biasing condition (1% duty cycle) to eliminate the self-heating effect. Due to the effective strain relaxation, nanowire exhibit significantly reduced polarization field and dislocation density, compared to thin-film counterpart. Therefore, efficiency droop related mechanisms such as polarization field, electron and hole wave functions separation in the quantum well, defect assisted Auger recombination, and electron leakage may be greatly reduced or eliminated [34–40].
Fig. 2. (a) I-V characteristic of AlInN nanowire UV LED. The inset shows picture of the fabricated device. (b) Electroluminescence spectrum of AlInN UV nanowire LED under 200 mA injection current and LED exhibit strong emission at 299 nm wavelength. The inset figure shows EQE as a function of current density of AlInN nanowire UV LED.
3. Simulation and results
We have further studied the LEE enhancement of the AlInN nanowire UV LEDs using photonic crystal structures. The FDTD analysis [41–43] is conducted in order to calculate the LEE of AlInN nanowire UV LEDs with different nanowire arrangements. The LED structure used in the simulations consists of a random, square and hexagonal array of nanowires as shown in Fig. 3. Each nanowire in the array consists of 200 nm of n-GaN on Si substrate, 100 nm n-Al0.78In0.22N, 40 nm i-Al0.75In0.25N quantum well, 100 nm p-Al0.78In0.22N and 5 nm of p-GaN layer. The hexagonal and square periodic nanowire arrays can be grown by selective area growth approach, which is a well-established technique [44–47]. The lattice constant and radius of the nanowires are defined as a and r, respectively. The entire device with the side length of 2.5 μm is enclosed with 12 perfectly matched layers (PMLs) boundary conditions in order to avoid reflection of outgoing waves back to the simulation space [48]. In the FDTD simulation, PML parameters such as attenuation factor (σ) and auxiliary attenuation coefficient (κ) are set to 0.25 and 2, respectively [27]. A minimum mesh step size of 0.25 nm and maximum mesh step of 20 nm are used during the simulation. The refractive indices of the n-/p-Al0.78In0.22N, Al0.75In0.25N and n-GaN/p-GaN regions considered in the simulations are 2.275, 2.266 and 2.625, respectively [49,50]. The absorption coefficients of n-/p-Al0.78In0.22N, Al0.75In0.25N and n-GaN/p-GaN regions considered in the simulations are 70000 cm-1, 79000 cm-1 and 170000 cm-1, respectively [49,50]. A single TM-polarized dipole source with the emission wavelength of 299 nm was positioned in the center of the active region for all the simulations, which is appropriate for estimating LEE for a large area LED device [32]. Previous study has shown that use of multiple dipole sources results in non-physical interference pattern [51], which is undesirable for analysis of the optical properties of LEDs. The total power generated in the active region from the dipole source is measured by the source power monitors which are placed around the dipole source, while the total output power radiated out of the LED structure is measured by the output power monitors which are placed around the UV LED nanowire structures. In this study, we have considered the absorption of light from the substrate while calculating the LEE. We used five output power monitors except the bottom monitor. The LEE is calculated as the ratio of the light output power measured by the output power monitors to the total emitted power in the active region measured by the source power monitors. Table 1 summarizes the different parameters and their values used in the simulation.
Fig. 3. Top view of the simulated (a) Random array, (b) Square array, (c) Hexagonal array of AlInN nanowire UV LEDs.
Table 1. Different parameters and their values used during simulation
To get a better understanding of the influence of periodic structures in the nanowire UV LED design, first we have considered the random nanowire UV LEDs, which is a spontaneously grown nanowire array as shown in Fig. 3(a). In this case, structural randomness is introduced by dislocating the nanowires to random positions compared to periodic nanowire UV LEDs. Number of nanowires for random structure is same as the number of nanowires in the hexagonal lattice structure for 2.5 µm of side length. The radius of the randomly grown nanowires varies from 40 nm to 64 nm, which is suitable for the MBE growth experiment. For randomly distributed nanowire array, the average LEE was observed to be around 20% to 35%. For every independent study, LEE from the top surface as well as from the sides are measured and found that more than 70% of total emitted LEE of 100% is being emitted from the top surface of the nanowire. This suggests that, albeit the light is TM polarized, the dominant light emission direction is from the nanowire top surface, due to the strong light scattering effect [23]. However, LEE is very low for the random array of nanowires. Then, we have investigated the variation of LEE for the square and hexagon topologies as shown in Figs. 3(b) and 3(c), respectively with respect to the nanowire radius and lattice constant as these are the key parameters for the periodic array of nanowire LEDs without considering the light absorption from the top p-GaN and metal layer. The radius ranges of 40-64 nm and spacing ranges of 145-300 nm were considered in the simulation. Spacing refers to the center-to-center distance of adjacent nanowires.
Figure 4 shows the contour plots of the LEE vs. nanowire radius and spacing for both the topologies of nanowire arrays in order to gain a complete understanding of the dependence of LEE on nanowires geometry. The maximum LEE of ∼56% was calculated for square array for the spacing of 195 nm and radius of 40 nm as shown in Fig. 4(a) and for hexagonal nanowire array, the maximum LEE was found to be ∼63% for the spacing of 230 nm and radius of 60 nm as shown in Fig. 4(b). It can be understood from Fig. 4 that the nanowires geometry plays an important role in directing the generated photons from the active region to the air. It is also found that the maximum LEE of hexagonal array of nanowire LED is higher than that of the square array. This could be related to the fact that the symmetry of the hexagonal array is higher than that of the square array, due to which guided modes in more directions can be transformed into radiation modes for hexagonal array [31,52]. Therefore, optimized nanowire radius and spacing between the nanowires along with the arrangement of the nanowires could help to overcome the total internal reflection at the semiconductor and air interface, improve the escape probability of the light and consequently increase the LEE. However, in case of random nanowire UV LED design, multiple scattering of light inside the nanowires takes place due to spatially random and high contrast refractive index nanowires, which may localize the light inside the nanowires and causes the reduction of LEE. Therefore, the periodic array of nanowire LEDs are highly desired for light propagation through the structure to the surrounding air.
Fig. 4. Contour plot of the LEE vs. nanowire radius and spacing for (a) square array, (b) hexagonal array of AlInN nanowire UV LEDs.
Based on the above optimized parameters, photonic band structures of both these topologies are calculated using the MIT photonic-bands (MPB), which is an open source, three-dimensional Eigen solver [53]. The photonic band structures of square and hexagonal arrays are plotted in Fig. 5. The black line, known as light line, separates the band diagram into two areas. The gray area above the light line corresponds to the continuum of radiation modes, while modes below the light line are guided modes that are confined within the nanowire arrays. In guided modes, light only propagates in the epitaxial layer because of satisfying the total internal reflection (TIR) condition, however in radiated mode light can be emitted up or down to the outside of the LEDs because it does not satisfy the TIR condition [31,54]. The red line, which represents the frequency of concern (a/λ) in this paper, lies at a location with few guided modes for the square array as shown in Fig. 5(a), however the red line lies in the gray area for the hexagonal array as shown in Fig. 5(b), indicating the existence of radiated mode in the structure. This supports the vertical emission of the extracted light for TM polarized emission.
Fig. 5. Photonic band diagrams of (a) square array and (b) Hexagonal array of AlInN nanowire UV LEDs. Horizontal red lines indicate the frequency of concern.
We have further investigated the percentage of LEE for both the topologies from top and sides of the nanowire and derived the contour plot of the LEE vs the nanowire radius and spacing as shown in Fig. 6. The simulation results in both cases support the vertical emission of the LEE, although the nanowire structure favors highly TM-polarized emission [23]. As an example, for the maximum LEE of ∼56% in case of square array, ∼41% of light extraction can be achieved from top surface as shown in Fig. 6(a), while ∼15% of light extraction is obtained from the side walls of the nanowires as shown in Fig. 6(b). Similarly, for the maximum LEE of ∼63% in hexagonal array, ∼60% of light extraction can be achieved from top surface as shown in Fig. 6(c) while only ∼3% of light extraction is obtained from the side walls of the nanowire as shown in Fig. 6(d). Compared to the lateral side extraction of TM polarized emission, vertical emission of TM polarized light can be affected by the top p-GaN contact layer.
Fig. 6. Contour plot of the LEE from top and sides vs. nanowire radius and spacing for square (a, b) and hexagonal (c, d) array structures of AlInN nanowire UV LEDs.
To further elucidate the impact of the p-GaN layer, we have studied the variation of LEE vs thickness of the p-GaN layer and plotted as shown in Fig. 7. Without considering the p-GaN layer, maximum LEE of the square lattice array is observed to be ∼56% for the radius of 40 nm and spacing of 195 nm and that of the hexagonal lattice array is observed to be ∼63% for the radius of 60 nm and spacing of 230 nm, respectively. With the increase in p-GaN thickness, LEE shows an overall decreasing due to the absorption in the thick p-GaN layer, which is like that observed in conventional planar LED structures. To achieve maximum LEE, it is important to optimize the nanowire height and p-GaN thickness and/or replace p-GaN with a less absorptive and transparent p-AlGaN contact layer [25,55].
4. Summary
In conclusion, high crystalline quality of AlInN nanowire LED structures with stable and strong EL emission at ∼299 nm peak wavelength have been investigated. Further, we have studied the light extraction characteristics for different nanowire photonic crystal structure arrangements including hexagon and square lattices, and compared the results with that of the random nanowire arrangement. Our simulation results show that the LEE of the square array and hexagonal array can reach ∼56% and ∼63%, respectively. However, for random nanowire arrays, the average of LEE is around ∼20% to ∼35% due to light multipath scattering among adjacent nanowires which localizes the light inside the nanowires. Therefore, ordered nanowire arrays exhibit higher LEEs compared to randomly arranged nanowire array. This work also provides promising approach for fabricating high efficiency UV nanowire LEDs using AlInN based semiconductor.
Funding
National Science Foundation (ECCS-1944312); New Jersey Health Foundation (001859-00001A).
Acknowledgments
This work is supported by NJIT, Instrument Usage Seed Grant from Otto H. York Center at NJIT. Part of the work was performed in the Cornell NanoFabrication Center. T. T. Pham acknowledges support from the Ho Chi Minh City University of Technology (HCMUT) under grant number To-KHUD-2019-01.
Disclosures
The authors declare no conflicts of interest.
References
1. M. Mori, A. Hamamoto, A. Takahashi, M. Nakano, N. Wakikawa, S. Tachibana, T. Ikehara, Y. Nakaya, M. Akutagawa, and Y. Kinouchi, “Development of a new water sterilization device with a 365 nm UV-LED,” Med. Biol. Eng. Comput. 45(12), 1237–1241 (2007). [CrossRef]
2. B. de Lacy Costello, R. Ewen, N. M. Ratcliffe, and M. Richards, “Highly sensitive room temperature sensors based on the UV-LED activation of zinc oxide nanoparticles,” Sensors and Actuators B: Chemical 134(2), 945–952 (2008). [CrossRef]
3. S.-W. Fan, A. K. Srivastava, and V. P. Dravid, “UV-activated room-temperature gas sensing mechanism of polycrystalline ZnO,” Appl. Phys. Lett. 95(14), 142106 (2009). [CrossRef]
4. M. Würtele, T. Kolbe, M. Lipsz, A. Külberg, M. Weyers, M. Kneissl, and M. Jekel, “Application of GaN-based ultraviolet-C light emitting diodes–UV LEDs–for water disinfection,” Water Res. 45(3), 1481–1489 (2011). [CrossRef]
5. X. Hai, R. Rashid, S. Sadaf, Z. Mi, and S. Zhao, “Effect of low hole mobility on the efficiency droop of AlGaN nanowire deep ultraviolet light emitting diodes,” Appl. Phys. Lett. 114(10), 101104 (2019). [CrossRef]
6. K. Nam, J. Li, M. Nakarmi, J. Lin, and H. Jiang, “Unique optical properties of AlGaN alloys and related ultraviolet emitters,” Appl. Phys. Lett. 84(25), 5264–5266 (2004). [CrossRef]
7. M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, and T. Mukai, “InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode,” Jpn. J. Appl. Phys. 41(Part 2, No. 12B), L1431–L1433 (2002). [CrossRef]
8. T. Kolbe, A. Knauer, C. Chua, Z. Yang, S. Einfeldt, P. Vogt, N. M. Johnson, M. Weyers, and M. Kneissl, “Optical polarization characteristics of ultraviolet (In)(Al) GaN multiple quantum well light emitting diodes,” Appl. Phys. Lett. 97(17), 171105 (2010). [CrossRef]
9. J. Shakya, K. Knabe, K. Kim, J. Li, J. Lin, and H. Jiang, “Polarization of III-nitride blue and ultraviolet light-emitting diodes,” Appl. Phys. Lett. 86(9), 091107 (2005). [CrossRef]
10. H.-Y. Ryu, I.-G. Choi, H.-S. Choi, and J.-I. Shim, “Investigation of light extraction efficiency in AlGaN deep-ultraviolet light-emitting diodes,” Appl. Phys. Express 6(6), 062101 (2013). [CrossRef]
11. M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodriguez, S. Einfeldt, and Z. Yang, “Advances in group III-nitride-based deep UV light-emitting diode technology,” Semicond. Sci. Technol. 26(1), 014036 (2011). [CrossRef]
12. C. Lee, B. Cheng, Y. Lee, H. Kuo, T. Lu, and S. Wang, “Output power enhancement of vertical-injection ultraviolet light-emitting diodes by GaN-free and surface roughness structures,” Electrochem. Solid-State Lett. 12(2), H44–H46 (2009). [CrossRef]
13. Y.-J. Lee, T. Hsu, H. Kuo, S. Wang, Y. Yang, S. Yen, Y. Chu, Y. Shen, M. Hsieh, and M. Jou, “Improvement in light-output efficiency of near-ultraviolet InGaN–GaN LEDs fabricated on stripe patterned sapphire substrates,” Materials Science and Engineering: B 122(3), 184–187 (2005). [CrossRef]
14. M. Djavid, X. Liu, and Z. Mi, “Improvement of the light extraction efficiency of GaN-based LEDs using rolled-up nanotube arrays,” Opt. Express 22(S7), A1680–A1686 (2014). [CrossRef]
15. J. Shakya, K. Kim, J. Lin, and H. Jiang, “Enhanced light extraction in III-nitride ultraviolet photonic crystal light-emitting diodes,” Appl. Phys. Lett. 85(1), 142–144 (2004). [CrossRef]
16. K. H. Lee, H. J. Park, S. H. Kim, M. Asadirad, Y.-T. Moon, J. S. Kwak, and J.-H. Ryou, “Light-extraction efficiency control in AlGaN-based deep-ultraviolet flip-chip light-emitting diodes: a comparison to InGaN-based visible flip-chip light-emitting diodes,” Opt. Express 23(16), 20340–20349 (2015). [CrossRef]
17. Y. Zhang, Z. Jamal-Eddine, F. Akyol, S. Bajaj, J. M. Johnson, G. Calderon, A. A. Allerman, M. W. Moseley, A. M. Armstrong, and J. Hwang, “Tunnel-injected sub 290 nm ultra-violet light emitting diodes with 2.8% external quantum efficiency,” Appl. Phys. Lett. 112(7), 071107 (2018). [CrossRef]
18. Y. Zhang, S. Krishnamoorthy, F. Akyol, S. Bajaj, A. A. Allerman, M. W. Moseley, A. M. Armstrong, and S. Rajan, “Tunnel-injected sub-260 nm ultraviolet light emitting diodes,” Appl. Phys. Lett. 110(20), 201102 (2017). [CrossRef]
19. Z. Ren, H. Yu, Z. Liu, D. Wang, C. Xing, H. Zhang, C. Huang, S. Long, and H. Sun, “Band engineering of III-nitride-based deep-ultraviolet light-emitting diodes: A review,” J. Phys. D: Appl. Phys. 53(7), 073002 (2020). [CrossRef]
20. H. Sun, J. Yin, E. F. Pecora, L. Dal Negro, R. Paiella, and T. D. Moustakas, “Deep-ultraviolet emitting AlGaN multiple quantum well graded-index separate-confinement heterostructures grown by MBE on SiC substrates,” IEEE Photonics J. 9, 1–9 (2017). [CrossRef]
21. X. Fan, H. Sun, X. Li, H. Sun, C. Zhang, Z. Zhang, and Z. Guo, “Efficiency improvements in AlGaN-based deep ultraviolet light-emitting diodes using inverted-V-shaped graded Al composition electron blocking layer,” Superlattices Microstruct. 88, 467–473 (2015). [CrossRef]
22. Q. Wang, A. Connie, H. Nguyen, M. Kibria, S. Zhao, S. Sharif, I. Shih, and Z. Mi, “Highly efficient, spectrally pure 340 nm ultraviolet emission from AlxGa1− xN nanowire based light emitting diodes,” Nanotechnology 24(34), 345201 (2013). [CrossRef]
23. S. Zhao, M. Djavid, and Z. Mi, “Surface emitting, high efficiency near-vacuum ultraviolet light source with aluminum nitride nanowires monolithically grown on silicon,” Nano Lett. 15(10), 7006–7009 (2015). [CrossRef]
24. S. Zhao, S. Sadaf, S. Vanka, Y. Wang, R. Rashid, and Z. Mi, “Sub-milliwatt AlGaN nanowire tunnel junction deep ultraviolet light emitting diodes on silicon operating at 242 nm,” Appl. Phys. Lett. 109(20), 201106 (2016). [CrossRef]
25. Y. K. Ooi, C. Liu, and J. Zhang, “Analysis of polarization-dependent light extraction and effect of passivation layer for 230-nm AlGaN nanowire light-emitting diodes,” IEEE Photonics J. 9(4), 1–12 (2017). [CrossRef]
26. H. Sun, M. K. Shakfa, M. M. Muhammed, B. Janjua, K.-H. Li, R. Lin, T. K. Ng, I. S. Roqan, B. S. Ooi, and X. Li, “Surface-passivated AlGaN nanowires for enhanced luminescence of ultraviolet light emitting diodes,” ACS Photonics 5(3), 964–970 (2018). [CrossRef]
27. M. Djavid and Z. Mi, “Enhancing the light extraction efficiency of AlGaN deep ultraviolet light emitting diodes by using nanowire structures,” Appl. Phys. Lett. 108(5), 051102 (2016). [CrossRef]
28. R. T. Velpula, B. Jain, M. R. Philip, H. D. Nguyen, R. Wang, and H. P. T. Nguyen, “epitaxial Growth and characterization of Alinn-Based core-Shell nanowire Light emitting Diodes operating in the Ultraviolet Spectrum,” Sci. Rep. 10(1), 2547 (2020). [CrossRef]
29. R. T. Velpula, B. Jain, H. Q. T. Bui, T. T. Pham, H.-D. Nguyen, T. R. Lenka, and H. P. T. Nguyen, “Numerical investigation on the device performance of electron blocking layer free AlInN nanowire deep ultraviolet light-emitting diodes,” Opt. Mater. Express 10(2), 472–483 (2020). [CrossRef]
30. Y. H. Ra, R. T. Rashid, X. Liu, J. Lee, and Z. Mi, “Scalable nanowire photonic crystals: Molding the light emission of InGaN,” Adv. Funct. Mater. 27(38), 1702364 (2017). [CrossRef]
31. P. Du, L. Rao, Y. Liu, and Z. Cheng, “Enhancing the light extraction efficiency of AlGaN LED with nanowire photonic crystal and graphene transparent electrode,” Superlattices Microstruct. 133, 106216 (2019). [CrossRef]
32. P. Zhao, L. Han, M. R. McGoogan, and H. Zhao, “Analysis of TM mode light extraction efficiency enhancement for deep ultraviolet AlGaN quantum wells light-emitting diodes with III-nitride micro-domes,” Opt. Mater. Express 2(10), 1397–1406 (2012). [CrossRef]
33. M. Djavid, “High Efficiency Nanowire-based Phosphor-free White and Deep Ultraviolet Light Sources,” (McGill University Libraries, 2016).
34. B. Janjua, H. Sun, C. Zhao, D. H. Anjum, D. Priante, A. A. Alhamoud, F. Wu, X. Li, A. M. Albadri, and A. Y. Alyamani, “Droop-free AlxGa1-xN/AlyGa1-yN quantum-disks-in-nanowires ultraviolet LED emitting at 337 nm on metal/silicon substrates,” Opt. Express 25(2), 1381–1390 (2017). [CrossRef]
35. W. Guo, M. Zhang, P. Bhattacharya, and J. Heo, “Auger Recombination in III-Nitride Nanowires and Its Effect on Nanowire Light-Emitting Diode Characteristics,” Nano Lett. 11(4), 1434–1438 (2011). [CrossRef]
36. H. P. T. Nguyen, K. Cui, S. Zhang, M. Djavid, A. Korinek, G. A. Botton, and Z. Mi, “Controlling Electron Overflow in Phosphor-Free InGaN/GaN Nanowire White Light-Emitting Diodes,” Nano Lett. 12(3), 1317–1323 (2012). [CrossRef]
37. H. P. T. Nguyen, S. Zhang, K. Cui, X. Han, S. Fathololoumi, M. Couillard, G. A. Botton, and Z. Mi, “p-Type Modulation Doped InGaN/GaN Dot-in-a-Wire White-Light-Emitting Diodes Monolithically Grown on Si(111),” Nano Lett. 11(5), 1919–1924 (2011). [CrossRef]
38. R. T. Velpula, B. Jain, H. Q. T. Bui, and H. P. T. Nguyen, “Full-Color III-Nitride Nanowire Light-Emitting Diodes,” Journal of Advanced Engineering and Computation 3, 551 (2019). [CrossRef]
39. P. Renwick, H. Tang, and J. Bai, “Reduced longitudinal optical phonon-exciton interaction in InGaN/GaN nanorod structures,” Appl Phys Lett 100(18), 182105 (2012). [CrossRef]
40. D. Priante, M. Tangi, J.-W. Min, N. Alfaraj, J. W. Liang, H. Sun, H. H. Alhashim, X. Li, A. M. Albadri, and A. Y. Alyamani, “Enhanced electro-optic performance of surface-treated nanowires: origin and mechanism of nanoscale current injection for reliable ultraviolet light-emitting diodes,” Opt. Mater. Express 9(1), 203–215 (2019). [CrossRef]
41. D. M. Sullivan, Electromagnetic simulation using the FDTD method (John Wiley & Sons, 2013).
42. W. Choi, Q. H. Park, D. Kim, H. Jeon, C. Sone, and Y. Park, “FDTD simulation for light extraction in a GaN-based LED,” J. Korean Phys. Soc. 49, 877–880 (2006).
43. L. Zhang, N. Alexopoulos, D. Sievenpiper, and E. Yablonovitch, “An efficient finite-element method for the analysis of photonic band-gap materials,” in IEEE MTTS Int. Microw. Symp., 1703–1706 (1999).
44. 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]
45. R. Wang, Y.-H. Ra, Y. Wu, S. Zhao, H. P. T. Nguyen, I. Shih, and Z. Mi, “Tunable, full-color nanowire light emitting diode arrays monolithically integrated on Si and sapphire,” Proc. of SPIE9748, 97481S–97489 (2016).
46. K. Kishino, T. Hoshino, S. Ishizawa, and A. Kikuchi, “Selective-area growth of GaN nanocolumns on titanium-mask-patterned silicon (111) substrates by RF-plasma-assisted molecular-beam epitaxy,” Electron. Lett. 44(13), 819 (2008). [CrossRef]
47. S. Ishizawa, K. Kishino, and A. Kikuchi, “Selective-area growth of GaN nanocolumns on Si(111) substrates using nitrided al nanopatterns by rf-plasma-assisted molecular-beam epitaxy,” Appl. Phys. Express 1(1), 015006 (2008). [CrossRef]
48. S. D. Gedney, “An anisotropic perfectly matched layer-absorbing medium for the truncation of FDTD lattices,” IEEE Trans. Antennas Propag. 44(12), 1630–1639 (1996). [CrossRef]
49. A. B. Djurišić and E. H. Li, “Modeling the optical constants of hexagonal GaN, InN, and AlN,” J. Appl. Phys. 85(5), 2848–2853 (1999). [CrossRef]
50. D. Brunner, H. Angerer, E. Bustarret, F. Freudenberg, R. Höpler, R. Dimitrov, O. Ambacher, and M. Stutzmann, “Optical constants of epitaxial AlGaN films and their temperature dependence,” J. Appl. Phys. 82(10), 5090–5096 (1997). [CrossRef]
51. P. Zhu, “Frustrated total internal reflection in organic light-emitting diodes employing sphere cavity embedded in polystyrene,” J. Opt. 18(2), 025403 (2016). [CrossRef]
52. A. David, “High-efficiency GaN-based light-emitting diodes: light extraction by photonic crystals and microcavities,” (Ecole Polytechnique X, 2006).
53. S. G. Johnson and J. D. Joannopoulos, “Block-iterative frequency-domain methods for Maxwell’s equations in a planewave basis,” Opt. Express 8(3), 173–190 (2001). [CrossRef]
54. R. Lin, S. V. Galan, H. Sun, Y. Hu, M. S. Alias, B. Janjua, T. K. Ng, B. S. Ooi, and X. Li, “Tapering-induced enhancement of light extraction efficiency of nanowire deep ultraviolet LED by theoretical simulations,” Photonics Res. 6(5), 457–462 (2018). [CrossRef]
55. X. Liu, K. Mashooq, T. Szkopek, and Z. Mi, “Improving the efficiency of transverse magnetic polarized emission from AlGaN based LEDs by using nanowire photonic crystal,” IEEE Photonics J. 10, 1–11 (2018). [CrossRef]
