Conclusions and Future work
In the thesis, we have demonstrated the high efficient near-UV LEDs by replacing AlGaN by InAlGaN barrier in active region and the mechanical lift-off technology for V-LEDs. Therefore, we will summarize the works contributed from the research.
First, we have demonstrated the high efficient near-UV LEDs by replacing AlGaN by InAlGaN barrier in active region. A 50-nm AlGaN and InAlGaN single heteroepitaxial layers grown on n-AlGaN/ud-GaN/Sapphire, and InGaN/AlGaN and InGaN/InAlGaN MQWs LED structures are prepared to investigate the optoelectronic properties of material and devices in this study. PL emission energy of these two samples are very close and the peak intensity of InAlGaN is slightly higher than AlGaN. The strong PL emission is attributed to the better crystal quality. The relatively small pits and smooth morphology of LT InAlGaN layer can be observed in AFM, and this phenomenon can mainly be attributed to the smaller tensile strain in LT AlGaN or LT GaN by inserting the isoelectronic In atoms. HRXRD and TEM measurements show the two barriers are consistent with the lattice, and the compositions of ternary and quaternary barriers were Al0.08Ga0.92N and In0.0085Al0.1112Ga0.8803N, respectively.
The light output power of InGaN-based near-UV LED with the InAlGaN barrier is higher by 25 % and 55 % than the AlGaN barrier at 350 mA and 1000 mA, respectively.
InGaN/InAlGaN near-UV LEDs exhibit only 13 % efficiency droop when we increase the injection current to 1000 mA. The reduction of efficiency droop is quite clear and the current at maximum efficiency shifts from 150 to 400 mA.
Furthermore, simulations show that quaternary LEDs exhibit 62 % higher radiative recombination rate and low efficiency degradation of 13 % at a high injection current of 100 A/cm2. The band-offset ratio from 6:4 to 7:3 will lead higher conduction-band and lower valence-band between well and barrier, and the efficiency curve will nearest to the
experimental result when electron and hole mobility of InAlGaN is about 1.8 and 2.5 times the value of AlGaN. APSYS simulations show the electron and hole concentration increases in the QW by about 26% and 35%, respectively, and the distribution of carrier becomes more uniform than InGaN/AlGaN case. We attribute this improvement to increasing of carrier concentration and more uniform redistribution of carriers in active region.
Second, we have successfully demonstrated the fabrication of mechanical lift-off high quality GaN-based near-UV LED with HIP structures as a sacrificial layer during wafer bonding process for V-LEDs. The HIP GaN/air/Sapphire structures are formed at the GaN/sapphire substrate interface under high temperature during KOH wet etching process.
The density of threading dislocations can be efficiently reduced from 2109 to 1108 cm-2 by applying the regrowth GaN epilayer on HIP structure. Raman spectroscopy analysis revealed that the compressive stress of GaN epilayer was effectively relieved in the GaN-based GaN nucleation layer. The peak wavelength of the V-LED also shows only a slight redshift of about 3.4 nm in the spectrum from 20 mA to 150 mA. The result indicates the relief of compressive strain and better heat dissipation in V-LED. Finally, the overall optical output power has shown significant 100 % enhancement under operating current 20 mA with this mechanical lift-off technique for fabrication the vertical-LED, and the mechanical lift-off process was proved to be available by regarding the HIP structures as a sacrificial layer at
high temperature during the wafer bonding process.
The 0.18 % of substrate mismatch from divergent thermal expansion coefficient during cooling down process from 400 oC to room temperature shows that GaN thin film between Si and Sapphire was under large strain and the HIP structures must play a sacrificial layer for mechanical lift-off process. SEM image of the HIP surface morphology shows only about 18
% of the contact area with the sapphire substrate. It implies that partially attached to the sapphire substrate of LT GaN HIP structures bear 5.5 times of stress. Therefore, the mechanical lift-off process was proved to be available by regarding the HIP structures as a sacrificial layer at high temperature during the wafer bonding process.
In the previous work, the quaternary material has been proved to be helpful to improve the internal quantum efficiency. However, it is more difficult to develop quite low indium composition in deep wavelength such as 365 nm UV LEDs for curing application.
In the future work, InAlGaN will match in optimized AlGaN barrier in low indium content 365 nm QWs for a fair investigation on the light output and efficiency current droop characteristics. The InGaN-based 365 nm UV LEDs structures are shown in Fig. 6.1.
Fig. 6.1. Schematic structures of 365 nm UV LEDs with (a) Al0.15Ga0.85N barrier and (b) In0.02Al0.18Ga0.8N barrier.
In order to achieve mass production of high quality V-LEDs, it is necessary to develop a stable lift-off process. Hence, we will use the porous SiO2/GaN nanorod array as a sacrificial layer for chemical lift-off process. The entire process of flowchart and V-LEDs structure is
shown in Fig. 6.2.
Fig. 6.2. The process flowchart for fabrication of V-LEDs by using the porous SiO2/GaN nanorod array.
According to these advantages of quaternary InAlGaN and creative lift-off process, we expect near-UV LED can finally achieve the higher efficiency without droop in the future.
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Publication list
Journal paper
[1] C. H. Chiu, P. M. Tu, S. P. Chang, C. C. Lin, C. Y. Jang, Z. Y. Li, H. C. Yang, H. W. Zan, H. C. Kuo, T. C. Lu, S. C. Wang, and C. Y. Chang, “Light Output Enhancement of GaN-Based Light-Emitting Diodes by Optimizing SiO2 Nanorod-Array Depth Patterned Sapphire Substrate,” Jpn. J. Appl. Phys. 51, 04DG11 (2012).
[2] C. C. Chen, C. H. Chiu, P. M. Tu, M. Y. Kuo, M. H. Shih, J. K. Huang, H. C. Kuo, H. W.
Zan, and C. Y. Chang,“Large Area of Ultraviolet GaN-Based Photonic Quasicrystal Laser,” Jpn. J. Appl. Phys. 51, 04DG02 (2012).
[3] C. H. Chiu, C. C. Lin, P. M. Tu, S. C. Huang, C. C. Tu, J. C. Li, Z. Y. Li, W. Y. Uen, H.
W. Zan, T. C. Lu, H. C. Kuo, S. C. Wang, and C. Y. Chang, “Improved Output Power of InGaN-Based Ultraviolet LEDs Using a Heavily Si-Doped GaN Insertion Layer Technique,”IEEE J. Quantum Electron. 48, 175 (2012).
[4] P. M. Tu, C. Y. Chang, S. C. Huang, C. H. Chiu, J. R. Chang, W. T. Chang, D. S. Wuu, H.
W. Zan, C. C. Lin, H. C. Kuo, and C. P. Hsu, “Investigation of efficiency droop for InGaN-Based UV Light-Emitting Diodes with InAlGaN Barrier,” Appl. Phys. Lett. 98(21), 211107 (2011).
[5] T. C. Lu, T. T. Wu, S. W. Chen, P. M. Tu, Z. Y. Li, C. K. Chen, C. H. Chen, H. C. Kuo, S. C. Wang, H. W. Zan, and C. Y. Chang, “Characteristics of Current Injected GaN-Based Vertical-Cavity Surface-Emitting Lasers,” IEEE J. Select. Topics Quantum Electron. 17, 1594 (2011). (Invited Paper)
[6] C. H. Chiu, P. M. Tu, C. C. Lin, D. W. Lin, Z. Y. Li, K. L. Chuang, J. R. Chang, T. C. Lu, H. W. Zan, C. Y. Chen, H. C. Kuo, S. C. Wang, and C. Y. Chang, “Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates,” IEEE J. Select. Topics Quantum Electron. 17, 971 (2011).
[7] B. S. Cheng, Y. L. Wu, T. C. Lu, C. H. Chiu, C. H. Chen, P. M. Tu, H. C. Kuo, S. C.
Wang, and C. Y. Chang, “High Q microcavity light emitting diodes with buried AlN current apertures,” Appl. Phys. Lett. 99(4), 041101 (2011).
[8] T. C. Lu, S. W. Chen, T. T. Wu, P. M. Tu, C. K. Chen, C. H. Chen, Z. Y. Li, H. C. Kuo, and S. C. Wang, “Continuous wave operation of current injected GaN vertical cavity surface emitting lasers at room temperature,” Appl. Phys. Lett. 97(7), 071114 (2010).
[9] M. H. Lo, P. M. Tu, C. H. Wang, C. W. Hung, S. C. Hsu, Y. J. Cheng, H. C. Kuo, H. W.
Zan, S. C. Wang, C. Y. Chang, and S. C. Huang, “High efficiency light emitting diode with anisotropically etched GaN-sapphire interface,” Appl. Phys. Lett. 95(4), 041109 (2009).
Y. Chang, “Investigation of efficiency droop for UV LED with N-type AlGaN layer,” San Francisco California USA, SPIE Photonics West 2012, pp. 82781B-82781B-7.
[2] T. C. Hung, P. M. Tu, S. C. Huang, C. H. Shen, and C. P. Hsu, “Power enhancement of 380 nm UV-LED with hexagonal pyramid structures by AlN sacrificial layer,” San Francisco California USA, SPIE Photonics West 2012, pp. 82780S-82780S-7.
[3] S. C. Huang, P. M. Tu, S. K. Yang, Y. W. Lin, and C. P. Hsu, “High performance 375 nm ultraviolet InGaN/AlGaN light-emitting diodes by using a heavily Si-doped GaN growth mode transition layer,” San Francisco California USA, SPIE Photonics West 2012, pp.
826221-826221-7.
[4] C. H. Chiu, P. M. Tu, J. R. Chang, W. T. Chang, H. C. Kuo, and C. Y. Chang, “Reduction of efficiency droop in InGaN-based UV light-emitting diodes with InAlGaN barrier,” San Francisco California USA, SPIE Photonics West 2012, pp. 826222-826222-6.
[5] S. C. Huang, K. C. Shen, P. M. Tu, D. S. Wuu, H. C. Kuo, and R. H. Horng, “Improved performance of 375 nm InGaN/AlGaN light-emitting diodes by incorporating a heavily Si-doped transition layer,” San Francisco California USA, SPIE Photonics West 2012 , pp.
82621L-82621L-7.
[6] Y. C. Chen, C. C. Tu, J. R. Chang, P. M. Tu, S. P. Chang, S. S. Yen, Y. C. Chen, H. C.
Kuo and C. Y. Chang, “Investigation of Efficiency Droop for UV-LED with N-type AlGaN Layer,” International Conference on Solid State Devices and Materials, Nagoya, 2011, pp. 220-221.
[7] S. S. Yen, W. Y. Chen, J. R. Chang, S. P. Chang, P. M. Tu, Y. C. Hsu,Y. J. Li, Y. C.
Chen, K. P. Sou, and C. Y. Chang, “A Novel Chemical Lift-Off Process based on Embedded Nano-rods Template,” International Conference on Solid State Devices and Materials, Nagoya, 2011, pp. 1401-1402.
[8] Y. L. Wu, B. S. Cheng, T. C. Lu, C. H. Chiu, C. H. Chen, P. M. Tu, H. C. Kuo, and S. C.
Wang, “Optical Characteristics Improvement of High Q Microcavity Light Emitting Diodes with Buried AlN Current Blocking Apertures,” 2011 International Conference on Solid State Devices and Materials, Nagoya, 2011, pp. 1143-1144.
[9] C. C. Chen, M. Y. Kuo, C. H. Chiu, P. M. Tu, M. H. Shih, S. P. Chang, J. K. Huang, H. C.
Kuo, H. W. Zan, and C. Y. Chang, “Large Area of Ultraviolet GaN-based Photonic Quasicrystal Laser,” 2011 International Conference on Solid State Devices and Materials, Nagoya, 2011, pp. 1141-1142.
[10] C. H. Chiu, P. M. Tu, C. Y. Chang, S. C. Huang, J. R. Chang, H. W. Zan, H. C. Kuo,
and C. P. Hsu, “Reduction of efficiency droop in InGaN-Based UV Light-Emitting Diodes with InAIGaN Barrier,” The 16th Opto-Electronics And Communications Conference, OECC 2011, Kaohsiung, Taiwan, pp. 733-734.
[11] P. M. Tu, D. W. Lin, C. H. Chiu, C. C. Lin, Z. Y. Li, H. W. Han, K. L. Chuang, J. R.
Chang, T. H. Yang, T.C. Lu, H.W. Zan, C. Y. Chao, H.C. Kuo, S.C. Wang, and C. Y.
Chang, “Emission Efficiency Enhancement of GaN Light-emitting Diodes Grown on GaN Nano-pillar Template,” 2010 Microoptics Conference, MB3. (Best Paper Award) [12] P. M. Tu, S. C. Hsu, M. H. Lo, H. W. Zan, H. C. Kuo, S. C. Wang, Y. J. Cheng, and C.
Y. Chang, “GaN-based Vertical LEDs Fabrication by Mechanical Lift-off Technology,”
2010 Microoptics Conference,WP4.
[13] P. M. Tu, S. C. Hsu, M. H. Lo, H. W. Zan, H. C. Kuo, S. C. Wang, Y. J. Cheng, and C.
Y. Chang, “High Quality Vertical LEDs Fabrication by Means of Mechanical Lift-off,”
2010 International Conference on Solid State Devices and Materials, Tokyo, 2010, pp.
405-406.
[14] K. L. Chuang, J. R. Chang, P. M. Tu, C. H. Chiu, Y. J. Li, H. W. Zan, H. C. Kuo, and C.
Y. Chang, ” An Investigation of GaN-Based LED with MBE Grown Nanopillars by MOCVD,” 2010 International Conference on Solid State Devices and Materials, Tokyo, 2010, pp. 403-404.
[15] K. L. Chuang, J. R. Chang, S. P. Chang, P. M Tu, Y. C. Hsu, W. T. Chen, H. W. Zan, T.
C. Lu, H. C. Kuo, and C. Y. Chang, “Growth Mechanism of Nonpolar A-Plane GaN on Patterned M-Plane Sapphire,” 2010 International Conference on Solid State Devices and Materials, Tokyo, 2010, pp. 433-434.
[16] T. C. Lu, S. W. Chen, T. T. Wu, C. K. Chen, C. H. Chen, P. M. Tu, Z. Y. Li, H. C. Kuo, and S. C. Wang, “CW Current Injection of GaN-based Vertical Cavity Surface Emitting Laser with Hybrid Mirrors at Room Temperature,” Semiconductor Laser Conference
(ISLC), 2010 22nd IEEE International, MB2, pp. 5-6.
Reports
[1] H. C Kuo, T. C. Lu, Z. Y. Li, T. T. Wu, P. M. Tu, C. H. Chiu, S. W. Chen, S. C. Wang, and C. Y. Chang, “Toward a high-performance, low-power microprojector with a surface-emitting blue laser,” SPIE Newsroom, DOI: 10.1117/2.1201205.004195, 14 June (2012).
[2] P. M. Tu et al. “Quaternary barrier cuts droop in UV LEDs, Compound Semiconductor, Volume 17 Number 5, July (2011) pp. 37.
[3] P. M. Tu et al. “Investigation of efficiency droop for InGaN-based UV lightemitting diodes with InAlGaN barrier,” Physics Communications in Taiwan, 11 Aug. (2011).
Vita
Po-Min Tu was born in Chiayi, Taiwan, on July 30, 1980. He received the B.S. degree in physics from Chung Yuan Christian University (CYCU), Chungli, Taiwan, and the M.S. degrees in physics from National Central University (NCU), Jhongli City, Taiwan, in 2002 and 2004, respectively. He is currently pursuing the Ph.D. degree with the Department of Photonics, Institute of Electro-Optical Engineering, National Chiao-Tung University (NCTU), Hsinchu, Taiwan.
In 2008, he joined the Eco-Electronics Research Group, NCTU, where he was engaged in research on III–V semiconductor materials by radio-frequency plasma-assisted molecular-beam epitaxy (RF-MBE) and metal organic chemical vapor deposition (MOCVD).
His current research interests include GaN-based light emitting diodes (LEDs), vertical cavity surface emitting lasers (VCSELs), high electron mobility transistors (HEMTs), epitaxial growth of III–V materials, and optoelectronic devices.