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Abstract
Color tunable micro light emitting diodes (µLEDs) are proposed and realized, making use of V-grooves to vary the Indium content during growth. The V-grooves make use of semi-polar crystal planes and strain relaxation to provide distinct regions of low to high Indium concentration which are simultaneously integrated. The differing Indium content provides emission from 425 to 640 nm. µLEDs ranging from 2 to 500 µm are demonstrated to showcase the concept.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Micro light emitting diodes (µLEDs) are viewed by the display industry as the replacement over organic light emitting diodes (OLEDs), offering high efficiency, environmental ruggedness, greater scaling, and higher brightness [1]. However, the introduction of µLED displays has been hampered by the issue of integration between the separately fabricated and integrated red, green, and blue µLEDs [2]. The issue of integrating millions of individual µLEDs with a transistor backplane is a daunting challenge, with some leaders in the space implementing µLED redundancy to address issues of poor yields [3].
Separate growths of µLEDs for different colors have been necessary due to the distinct material systems involved. Conventionally, Indium Gallium Nitride (InGaN) of differing Indium content is used for blue and green, while Aluminum Indium Gallium Phosphide (AlInGaP) is adapted for red [1,2]. One problem with AlInGaP is that it suffers from scaling issues due in part to sidewall damage, made more problematic due to its higher surface recombination velocity and minority carrier diffusion length [4–7]. Replacing AlInGaP with an all InGaN system for red, green, and blue µLEDs would lead to a scalable and straightforward approach to µLED displays; even offering a monolithic approach on a single wafer. Using a high Indium content to produce red µLEDs is challenging though, due to the associated lattice strain and temperature differences with GaN [8,9].
Despite these challenges for red InGaN µLEDs, the benefits of an integrated system have driven intense research to find solutions. There have been several approaches pursued to form a multi-color InGaN system, including Eu doping [10,11], use of a porous GaN substrate [12], nanowire growth [13–15], dot-in-nanoparticle µLEDs [16], and selective area growth approaches [17–19]. Of these approaches, Eu doping, use of porous GaN, and nanowire growths have been the most researched. Eu doping forms an optical mid-gap state which allows for electron-hole recombination ∼1/2 the GaN bandgap, producing red emission [10,11]. Porous GaN substrates reduce the strain between the high Indium content InGaN and GaN [4,12]. Nanowire growth techniques can range from strain engineering with the free sidewall [20], to lower temperature growth [14], to core-shell structures [13] and more. Though each of these approaches suffer issues of complex manufacturing, uniformity concerns, and an incompatibility with existing growth methods.
Instead, a novel color tunable approach is used here with V-grooves which are integrated during growth. The reduced strain and modified Indium inclusion along different crystal facets with V-grooves lead to color tunable emission with a variable current injection [21]. V-grooves have previously shown to enhance the performance of LEDs through improved electrical injection and minimizing charge recombination in threading dislocations [22–26]. Here though is the first time V-grooves have reported being driven to achieve color tunable emission, and where also the corresponding mechanisms are discussed.
2. Material background
The novel color tunable µLEDs are made through inclusions of the semi-polar {10$\bar{1}$1} facets. These {10$\bar{1}$1} facets are integrated into the InGaN/GaN multiple quantum well (MQW) region in order to facilitate differing levels of Indium inclusion. The {10$\bar{1}$1} facets, commonly known as ‘V-grooves’ are nucleated at a surface depression due to a threading dislocation [22]. Past works have sought to minimize V-groove inclusion, though a growing body of recent reports suggest the overall benefits to V-groove inclusion due to the ability to provide an energy barrier to minimize carrier recombination in threading dislocations and provide strain relief through a free surface depression [22–26].
How V-grooves tie into color tunable µLEDs is in the use of both locally relaxing the lattice, and the contrasting Indium incorporation along the semi-polar facets. Non-polar and semi-polar facets, off-axis from the conventional c-plane, exhibit a lower Indium sticking coefficient such that a lower amount of Indium is incorporated [27]. This lower Indium inclusion is a key problem which limits the use of core shell GaN nanowires for µLEDs, though is leveraged here. The semi-polar planes of the V-grooves cause the planar MQWs to bend downwards, thin, and incorporate less Indium, Fig. 1. Around each V-groove where it makes a localized ‘tear’, there is also strain relaxation. The strain relaxation allows for additional Indium to be incorporated in the planar MQWs near the edges of the V-groove [22]. Therefore, V-grooves can provide a means to create three distinct regions of Indium inclusion: 1. Planar MQWs, 2. Semi-polar MQWs in the V-groove, 3. Planar MQWs adjacent to the V-groove. The planar MQWs can be tailored for an Indium percentage to favor green emission, MQWs in the V-groove will contain less Indium to emit blue, and the planar MQWs adjacent to each V-groove will contain more Indium to emit red, Fig. 1.
Fig. 1. (a)Schematic view of a V-groove in the color tunable LEDs showing varying Indium content and hole injection pathways, (b)TEM of grown structure showing a V-groove.
A p-AlGaN electron blocking layer (EBL) on top of the MQWs provides a critical modification of the hole injection from the p-type GaN, to engineer current controlled color tunability. The EBL with Aluminum does not have the same variability in the sticking coefficient along differing crystal planes, allowing the EBL to uniformly coat in the V-groove [28]. The higher temperature p-GaN, compared to that of the MQWs, with high surface diffusion rates will then proceed to fill in the V-grooves, planarizing the structure [29]. The polarization between the p-GaN and p-AlGaN EBL creates a slight barrier height to hole injection along the c-plane [30]. The semi-polar planes of the V-grooves experience less polarization and a corresponding reduction in the hole barrier height [30]. At low current densities, and correspondingly low applied biases, holes first inject laterally from the p-GaN inside the V-grooves and are able to populate the Indium rich planar MQWs. With increased current density, and correspondingly higher applied biases, the hole barrier caused by the EBL is reduced allowing holes to flow vertically. The vertical hole injection then allows the planar MQWs away from the V-grooves to be filled, leading to green emission. With further increased current density the holes populate the lower Indium containing wells in the V-groove, combined with a reduced quantum confined stark effect (QCSE), leading to blue emission. By engineering a high density of V-grooves and controlling the level of charge injection, color tunable µLEDs can be realized.
3. Material characterization
To make use of the properties of V-grooves for color tunable µLEDs, the underlying buffer layers beneath an LED structure are modified. The buffer layers can be modified to enhance the formation of V-grooves by modifying lattice strain, combined also with the strain and temperatures associated with a green LED growth [22–26]. Strain engineering for increased threading dislocations for V-grooves will lead to a decreased efficiency compared to a conventional growth stack, though these defects are largely mitigated by the presence of the V-grooves. In this preliminary work these factors are balanced with the benefits and functionality provided by the V-grooves. Where the purpose of this work investigates the novel color tunability of V-grooves for µLEDs which was not previously reported on before.
Conventional metal organic chemical vapor deposition (MOCVD) is utilized for wafer growth on a sapphire substrate. Strain optimized buffer layers are first created followed by conventional LED growth with n-GaN, InGaN/GaN MQWs optimized for green emission, a p-AlGaN EBL, and a top p-GaN layer. The subsequent LEDs had a V-groove density greater than 4*108 cm-2. Characterization of the grown material is done through transmission electron microscopy (TEM), cathodoluminescence (CL), and scanning electron microscopy (SEM).
The lamella prepared for TEM was made use of with CL to gauge the emission spectra. CL was done at 10 K, which leads to a blue shift. The CL signal is compared from an area around a representative V-groove to an area removed from any V-grooves. Figure 2 showcases the CL signals from these two areas along with an overall signal taken from the sample.
Fig. 2. CL spectra from near a V-groove, away from a V-groove, and the entire spectra with peaks identified.
In an area removed from any V-grooves the CL emission shows the expected green ∼535 nm peak corresponding to the planar MQWs. A smaller blue peak at ∼400 nm is also noted which could correspond to the first excited state. Taking the signal from around a V-groove a large blue peak at ∼425 nm is observed, which corresponds to the narrow Indium deficient quantum wells located inside the V-groove. Longer wavelength peaks corresponding to yellow and red are also observed at ∼565 nm and ∼619 nm, respectively. These longer wavelengths are due to the increased Indium composition near the V-groove. A smaller green ∼535 nm peak is also observed as the CL measurement captures some emission as the high Indium content is reduced back to the base level further out along the planar MQWs. Together red, green, and blue peaks are observed using V-grooves.
4. µLED fabrication and results
Conventional µLED fabrication methods are utilized in order to fabricate µLEDs from 500 µm down to 2 µm in diameter. Photoresist masking was used in combination with a Cl2/Ar dry etch to form the µLEDs. Dry etch damage was then removed through a hydroxyl based wet etch [31]. Ti/Al/Ni/Ag was lifted-off for the n-type contact and Ni was lifted-off for the p-type contact. The fabricated devices were characterized through use of an HP4145B semiconductor parameter analyzer, a StellarNet spectrometer, and a Newport 843-R optical powermeter. Representative electrical results are shown in Fig. 3(a-b) for the 500, 35, and 2 µm µLEDs. The linear plot shows a turn on voltage ∼2 V, comparable to other InGaN based µLEDs. The smaller µLEDs have a lower drive current, but overall higher current density [32]. An ideality factor of 2.5 is extracted from the log I-V plots showing optimal behavior that can be further improved with a more optimized p-type metallization.
Optical images of the color tunable electroluminescence (EL) emission from 500, 35, and the 2 µm µLEDs are showcased in Fig. 4. At low current density red is observed which transitions all the way to blue at higher current densities. The applied current densities for the 35 µm µLEDs have an approximate value of ∼6*10−5 mA/µm2 for red and ∼8*10−2 mA/µm2 for blue. Therefore, under constant current the blue with a higher current density than red would have a much higher brightness. The brightness differences are, however, made uniform through tailoring the duty ratio of the applied current. A short pulse at high current for blue would be used to equal the brightness of red at a steady state low current. Similarly, colors in-between blue and red would use a longer pulse at a moderate current to achieve equal brightness.
The EL emission spectra are plotting together in Fig. 5 showing the emission spread from 425 nm to 640 nm. These color tunable µLEDs span the majority of the visible spectrum, promising great potential for display applications.
Selecting three operating points of red, green, and blue, these primary colors can be used to form the basis of a pixel in a display. These operating points are mapped on the 1931 CIE color space to see the range of colors possibly obtainable by the V-groove color tunable µLEDs, Fig. 6. The fabricated larger µLEDs, such as those greater than 200 µm have more difficulty to produce a pure blue color, due to a lower current density, having a mixing effect with green to produce a cyan color. To more readily produce blue, smaller µLEDs with a higher current density are preferred as shown in Figs. 4 and 6. The 1931 CIE color space chart for the µLEDs operating in blue, green, and red, covers a space just larger than the sRGB color space. Both the red and blue operating points are well positioned near the edges of the chart, with green being the limiting factor. With further optimization of the green operating point, it is expected that a larger coverage can be obtained. Comparing to other works, the results here in blue and red match closely with the Eu-LEDs to produce color tunable LEDs [10].
Fig. 6. 1931 CIE color chart showing positions of the blue, green, and red operating points. Smaller µLEDs lead to higher current densities for blue.
Preliminary results are collected for the wall plug efficiency (WPE) on wire bonded 500 µm µLEDs, as seen in Fig. 7. The peak emission efficiency occurs in green at a 3% WPE due to the planar MQWs. On either side of this peak the WPE drops off due to the initial turn-on for red, and the efficiency droop for blue. Despite the lower efficiency for red or blue, the WPE at red is in line with other reported results [4]. The red emission shows a WPE of ∼0.6% while amber shows a WPE of ∼1.15%. These results represent a promising approach compared with reported red porous GaN with an external quantum efficiency (EQE) of 0.2% [12] or compared to some strain-engineered LEDs in amber with an EQE of 0.56% [33]. The emerging use of V-grooves provides a novel method to engineer both red and color tunable emission. It is expected in future works that packaging the smaller µLEDs will lead to higher WPE due to higher light extraction efficiency. Significant gains in efficiency are also expected in future works as this preliminary structure is further optimized, given the record efficiency reported for InGaN red LEDs utilizes V-grooves [34]. This work represents a novel breakthrough for a simple, monolithic, solution for µLED displays.
5. Conclusion
In summary, V-grooves in InGaN µLEDs have been utilized as a novel method to vary the lateral distribution of Indium in the MQW region, leading to color tunable emission. µLEDs from 2 to 500 µm in diameter have been fabricated, showing emission from 425 to 640 nm. Operating the color tunable µLEDs in blue, green, and red, covers an area greater than the sRGB color space. WPE show initial success over other color tunable approaches with 0.6% for red, 3% for green, and 1% for blue. Future works promise greater gains as the novel V-groove structure is optimized beyond these preliminary findings. The use of V-grooves provides a novel method to achieve a simple, monolithic, solution for full color µLED displays.
Funding
Innovation Semiconductor, Inc.
Disclosures
The author declares no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the author upon reasonable request.
References
1. Y. Huang, E. Hsiang, M. Deng, S. Wu, E. Moon, M. Kim, J. Roh, H. Kim, and J. Hahn, “Mini-LED, Micro-LED and OLED displays: present status and future perspectives,” Light: Sci. Appl. 9(1), 105 (2020). [CrossRef] .
2. E. Virey and Z. Bouhamri, “From Lab to Fab: Challenges and Requirements for High-Volume MicroLED Manufacturing Equipment,” Dig. Tech. Pap. - Soc. Inf. Disp. Int. Symp. 52(1), 826–829 (2021). [CrossRef] .
3. R. Cok, M. Meitl, R. Rotzoll, G. Melnik, A. Fecioru, A. J. Trindade, B. Raymond, S. Bonafede, D. Gomez, T. Moore, C. Prevatte, E. Radauscher, S. Goodwin, P. Hines, and C. Bower, “Inorganic light-emitting diode displays using micro-transfer prinint,” J. Soc. Inf. Display 25(10), 589–609 (2017). [CrossRef]
4. P. Li, H. Li, M. Wong, P. Chan, Y. Yang, H. Zhang, M. Iza, J. Speck, S. Nakamura, and S. Denbaars, “Progress of InGaN-Based Red Micro-Light-Emitting Diodes,” Crystals 12(4), 541 (2022). [CrossRef] .
5. M. Wong, J. Kearns, C. Lee, J. Smith, C. Lynsky, G. Lheureux, H. Choi, J. Kim, C. Kim, S. Nakamura, J. Speck, and S. Denbaars, “Improved performance of AlGaInP red micro-light-emitting-diodes with sidewall treatments,” Opt. Express 28(4), 5787–5793 (2020). [CrossRef] .
6. K. Fan, J. Tao, Y. Zhao, P. Li, W. Sun, L. Zhu, J. Lv, Y. Quin, Q. Wang, J. Liang, and W. Wang, “Size effects of AlGaInP red vertical micro-LEDs on silicon substrate,” Results Phys. 36(105449), 105449 (2022). [CrossRef] .
7. J. Park, W. Baek, D. Geum, and S. Kim, “Understanding the Sidewall Passivation Effects in AlGaInP/GaInP Micro-LED,” Nanoscale Res. Lett. 17(1), 29 (2022). [CrossRef] .
8. T. Matsuoka, “Progress in nitride semiconductors from GaN to InN-MOVPE growth and characteristics,” Superlattices Microstruct. 37(1), 19–32 (2005). [CrossRef] .
9. Z. Zhuang, D. Iida, and K. Ohkawa, “InGaN-based red light emitting diodes: from traditional to micro-LEDs,” Jpn. J. Appl. Phys. 61(SA), SA0809 (2022). [CrossRef] .
10. S. Ichikawa, K. Shiomi, T. Morikawa, D. Timmerman, Y. Sasaki, J. Tatebayashi, and Y. Fujiwara, “Eu-doped GaN and InGaN monolithically stacked full-color LEDs with a wide color gamut,” Appl. Phys. Express 14(3), 031008 (2021). [CrossRef] .
11. I. Fragkos, C. Tan, V. Dierolf, Y. Fujiwara, and N. Tansu, “Pathway Towards High-Efficiency Eu-droped GaN Light-Emitting Diodes,” Sci. Rep. 7(1), 14648 (2017). [CrossRef] .
12. S. Pasayat, R. Ley, C. Gupta, M. Wong, C. Lynsky, Y. Wang, M. Gordon, S. Nakamura, S. Denbaars, S. Keller, and U. Mishra, “Color-tunable <10 µm square InGaN micro-LEDs on compliant GaN-on-porous-GaN pseudo-substrates,” Appl. Phys. Lett. 117(6), 061105 (2020). [CrossRef] .
13. K. Ito, W. Lu, S. Katsurom, R. Okuda, N. Nakayama, N. Sone, K. Mizutani, M. Iwaya, T. Takeuchi, S. Kamiyama, and I. Akasaki, “Identification of multi-color emission from coaxial GaInN/GaN multiple-quantum-shell nanowire LEDs,” Nanoscale Adv. 4(1), 102–110 (2021). [CrossRef] .
14. A. Pandey, Y. Malhotra, P. Wang, K. Sun, and Z. Mi, “N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs,” Photonics Res. 10(4), 1107–1116 (2022). [CrossRef] .
15. B. Jain, R. Velpula, H. Bui, H. Nguyen, T. Lenka, T. Nguyen, and H. Nguyen, “High performance electron blocking layer-free InGaN/GaN nanowire white-light-emitting-diodes,” Opt. Express 28(1), 665–675 (2020). [CrossRef] .
16. D. Tuyet, V. Quan, B Bondzior, P. Deren, R. Velpula, H. Nguyen, L. Tuyen, N. Hung, and H. Nguyen, “Deep red fluoride dots-in-nanoparticales for high color quality micro white light-emitting diodes,” Opt. Express 28(18), 26189–26199 (2020). [CrossRef] .
17. Y. Tchoe, J. Jo, M. Kim, J. Heo, G. Yoo, C. Sone, and G. Yi, “Variable-Color Light-Emitting Diodes Using GaN Microdonut arrays,” Adv. Mater. 26(19), 3019–3023 (2014). [CrossRef] .
18. M. Funato, K. Hayashi, M. Ueda, Y. Kawakami, Y. Narukawa, and T. Mukai, “Emission color tunable light-emitting diodes composed of InGaN multifacet quantum wells,” Appl. Phys. Lett. 93(2), 021126 (2008). [CrossRef] .
19. M. Lee, Y. Yeh, S. Tu, P. Chen, W. Lai, and J. Sheu, “White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with tuncated hexagonal pyramids,” Opt. Express 23(7), A401–A412 (2015). [CrossRef] .
20. R. Wang, H. Nguyen, A. Connie, J. Lee, I. Shih, and Z. Mi, “Color-tunable, phosphor-free InGaN nanowire light-emitting diode arrays monolithically integrated on silicon,” Opt. Express 22(S7), A1768 (2014). [CrossRef] .
21. M. Hartensveld, “Monolithic Color-Tunable Light Emitting Diodes and Methods Thereof,” 63/188, 553 (2021).
22. R. Yapparov, Y. C. Chow, C. Lynsky, F. Wu, S. Nakamura, J. Speck, and S. Marcinkevicius, “Variations of light emission and carrier dynamics around V-defects in InGaN quantum wells,” J. Appl. Phys. 128(22), 225703 (2020). [CrossRef] .
23. J. Kim, J. Kim, Y. Tak, S. Chae, J. Kim, and Y. Park, “Effect of V-Shaped Pit Size on the Reverse Leakage Current of InGaN/GaN Light Emitting Diodes,” IEEE Electron Dev. Lett. 34(11), 1409–1411 (2013). [CrossRef] .
24. S. Zhoi, X. Liu, H. Yan, Y. Gao, H. Xu, J. Zhao, Z. Quan, C. Gui, and S. Liu, “The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN-based green light-emitting diodes,” Sci. Rep. 8(1), 11053 (2018). [CrossRef] .
25. S. Marcinkevicius, R. Yapparov, Y. C. Chow, C. Lynsky, S. Nakamura, S. DenBaars, and J. Speck, “High internal quantum efficiency of long wavelength InGaN quantum wells,” Appl. Phys. Lett. 119(7), 071102 (2021). [CrossRef] .
26. Z. Quan, J. Liu, F. Fang, G. Wang, and F. Jiang, “A new interpretation for performance improvement of high-efficiency vertical blue light-emitting diodes by InGaN/GaN superlattices,” J. Appl. Phys. 118(19), 193102 (2015). [CrossRef] .
27. I. Girgel, P. Edwards, E. L. Boulbar, P. Coulon, S. L. Sahonta, D. Allsopp, R. Martin, C. Humphreys, and P. Shields, “Investigation of Indium gallium nitride facet-dependent nonpolar growth rates and composition for core-shell light-emitting diodes,” J. Nanophoton 10(1), 016010 (2016). [CrossRef] .
28. L. Spasevski, G. Kusch, P. Pampili, V. Zubialevich, D. Dinh, J. Bruckbauer, P. Edwards, P. Parbrook, and R. Martin, “A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN conent,” J. Phys. D 54(3), 035302 (2021). [CrossRef] .
29. X. Wu, C. Elsass, A. Abare, M. Mack, S. Keller, P. M. Petroff, S. DenBaars, J. Speck, and S. Rosner, “Structural origin of V-defects and correlation with localized excitonic centers in InGan/GaN multiple quantum wells,” Appl. Phys. Lett. 72(6), 692–694 (1998). [CrossRef] .
30. Z. Quan, L. Wang, C. Zheng, J. Liu, and F. Jiang, “Roles of V-shaped pits on the improvement of quantum efficiency in InGaN/GaN multiple quantum well light-emitting diodes,” J. Appl. Phys. 116(18), 183107 (2014). [CrossRef] .
31. M. Hartensveld, G. Ouin, C. Liu, and J. Zhang, “Effect of KOH passivation for top-down fabricated InGaN nanowire light emitting diodes,” J. Appl. Phys. 126(18), 183102 (2019). [CrossRef] .
32. P. Chen, P. Hsiao, H. Chen, B. Lee, K. Chang, C. Yen, R. Horng, and D. Wuu, “On the mechanism of carrier recobination in downsized blue micro-LEDs,” Sci. Rep. 11(1), 22788 (2021). [CrossRef] .
33. Z. Zhuang, D. Lida, M. Velazquez-Rizo, and K. Ohkawa, “606-nm InGaN Amber Micro-Light-Emitting Diodes with an On-Wafer External Quantum Efficiency of 0.56%,” IEEE Electron Device Lett. 42(7), 1029–1032 (2021). [CrossRef] .
34. S. Zhang, J. Zhang, J. Gaoi, Z. Wang, C. Zheng, M. Zhang, X. Wu, L. Xu, J. Ding, Z. Quan, and F. Jiang, “Efficient emission of InGaN-based light-emitting diodes: toward orange and red,” Photonics Res. 8(11), 1671–1675 (2020). [CrossRef] .
