Chapter 3 Results and Discussion
3.3 Electrical and Optical Properties of Devices
3.3.4 Luminance-Current Characteristic of LEDs
Figure 3-17 presents the normal light output as a function of injection current for fabricated LEDs with 300 nm ITO film annealed at different temperatures. Devices with ITO film are all brighter than that with Ni/Au film. Figure 3-18 shows the luminance enhancement as a function of injection current for LEDs with different thicknesses of ITO films. The ITO samples were compared and normalized with Ni/Au samples which were cut from the same wafer to ensure a reliable result. We could achieve a factor of about 1.3-1.5 times luminance enhancement by the difference of optical transmittance between ITO and Ni/Au films. Notice that the enhancement of devices with 60 nm ITO film has a decrease when the injection current increasing. On the other hand, devices with 300 nm ITO film don’t have such a decrease. It is supposed that because the thin ITO film has a heavier thermal effect than thick ITO film and Ni/Au film. The heavy thermal effect is attributed to the poor conductivity which forms a large resistance. Figure 3-19 presents the luminance enhancement with different ITO films at an injection current of 20-100 mA. No matter what the evaporating oxygen flow rate is, the thermal effect is serious with thin
(60 nm) ITO films while thick (300 nm) ITO films almost remain the enhancement factor in constant. Normal luminance of all ITO samples at different injection current is shown in Fig. 3-20. From Fig. 3-13, we have known that the leakage current in condition B is extremely high, especially in the 400 annealing one.℃ It will cause the degradation of output light, as shown in Fig. 3-21. The high density of black dots in Fig. 3-21(a) is caused by the high leakage current. Hence, the circled data in Fig. 3-20 may not be accurate. In addition, it seems that the luminance of each device doesn’t form a regular relationship. Figure 3-22 shows the relationship between calculated current spreading length and forward voltage or normal luminance. From Fig. 3-22(a), we find that LS decreases while the forward voltage increasing, where the straight line is an approximation line fitted by least square method. This may be resulted by the specific contact resistance. A large rC will cause a long current spreading length and a small 20-mA forward voltage. However, in Fig. 3-22(b), it seems that the optical performance is not corresponding to current spreading length. A possible reason is that most of the current spreading lengths are larger than the device size (300 × 300 μm), so almost all devices have a uniform enough current distribution. Figure 3-23 shows the current converting efficiency (dL/dI) of LEDs as a function of injection current. When the injection current becomes larger, a decrease of the converting efficiency could be discovered. It may be a reason of why the luminance has no relationship with LS. Although thicker ITO film has longer current spreading length, the increment in light output is not as much as in the current density. Then, such declined light enhancement may be disrupted because the thickness of evaporated ITO film might not be exact enough as our settings.
Chapter 4 Conclusion
In this experiment, we have fabricated GaN-based LEDs with different thicknesses of ITO films and also found the optical and electrical characteristics of them. First, we observe that the sheet resistance of ITO films on quartz has a decrease when the films grow thicker, which indicates a better conductivity. However, the sheet resistance is also dominated by the annealing temperature due to the change of the amount of oxygen vacancies and grain size of ITO film. From SEM pictures, we can discover that the grain size of ITO film has an obvious increase with its thickness.
Next, the transmittances of these ITO films are all about 80% at a wavelength of 465 nm and have 20% larger than Ni/Au films.
Moreover, the specific contact resistance increases with the annealing temperature, which is attributed to a thin GaO layer formed in the ITO/p-GaN interface. The calculated current distribution and current spreading length are affected by sheet resistance and specific contact resistance, especially the sheet resistance. A small sheet resistance, or a thick ITO film, demonstrates a good current spreading ability. Some of the calculated current spreading lengths are even larger than the size of LED device. The thickness of ITO film is also corresponding to the operating voltage due to the variance of conductivity. However, the film thickness seems no relationship with the normal light output. Compared with the references, all LEDs with different ITO films have a factor of about 1.3-1.5 times luminance enhancement by the difference of optical transmittance. By generalizing the above results, the best thickness of ITO film can be 300 nm.
Chapter 5 Future Work
In the future work, the similar experiment for other TCL structures with an intermediate metal layer, such as Ni/ITO and Ag/ITO, will be carried out. Larger size LEDs can even be fabricated because the current spreading length of a thick ITO film can be in excess of 1000 μm. In addition, after finding the best thickness of ITO film, it will be combined with surface roughening technique to achieve higher light extraction efficiency. This may help fabricating LEDs with an additional luminance enhancement.
Reference
[1] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys.
Lett., vol. 64, pp. 1687-1689, 1994.
[2] S.Nakamuram M. Senoh, N. Iwasa, and S. Nagahama, “High-power InGaN single-quantum-well-structure blue and violet light-emitting diodes,” Appl. Phys.
Lett., vol. 67, pp. 1868-1870, 1995.
[3] T. Mukai and S. Nakamura, “Ultraviolet InGaN and GaN single-quantum-well-structure light-emitting diodes grown on epitaxially laterally overgrown GaN substrates,” Jpn. J. Appl. Phys., vol. 38, pp. 5735-5739, 1999.
[4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y.
Sugimoto, and H. Kiyoku, “Room-temperature continuous-wave operation of InGaN multi-quantum-well-structure laser diodes with a long lifetime,” Appl.
Phys. Lett., vol. 70, pp. 868-870, 1997.
[5] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H.
Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho,
“Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates,” Appl. Phys. Lett., vol. 72, pp. 2014-2016, 1998.
[6] J. J. Wierer, D. A. Steigerwald, M. R. Krames, J. J. O’Shea, M. J. Ludowise, G.
Christenson, Y. -C. Shen, C. Lowery, P. S. Martin, S. Subramanya, W. Gotz, N. F.
Gardner, R. S. Kern, and S. A. Stockman, “High-power AlGaInN flip-chip light-emitting diodes,” Appl. Phys. Lett., vol. 78, pp. 3379-3381, 2001.
[7] C. H. Chen, S. J. Chang, Y. K. Su, J. K. Sheu, J. F. Chen, C. H. Kuo, and Y. C. Lin,
“Nitride-based cascade near white light-emitting diodes,” IEEE Photon. Technol.
Lett., vol. 14, pp. 908-910, 2002.
[8] C. H. Kuo, S. J. Chang, Y. K. Su, J. F. Chen, L. W. Wu, J. K. Sheu, C. H. Chen, a nd G. C. Chi, “InGaN/GaN light emitting diodes activated in O ambient,2 ” IEEE Electron. Dev. Lett., vol. 23, pp. 240-242, 2003.
[9] C. H. Kuo, S. J. Chang, Y. K. Su, L. W. Wu, J. K. Sheu, C. H. Chen and G. C. Chi,
“Low temperature activation of Mg-doped GaN in O2 ambient,” Jpn. J. Appl.
Phys., vol. 41, pp. L112-L114, 2002.
[10] J. K. Ho, C. S. Jong, C. C. Chiu, C. N. Huang, C. Y. Chen, and K. K. Shih,
“Low-resistance ohmic contacts to p-type GaN,” Appl. Phys. Lett., vol. 74, pp.
1275-1277, 1999.
[11] L. Zhou, W. Lanford, A. T. Ping, I. Adesida, J. W. Yang, and A. Khan, “Low resistance Ti/Pt/Au ohmic contacts to p-type GaN,” Appl. Phys. Lett., vol. 76, pp.
3451-3453, 2000.
[12] J. K. Kim, J. L. Lee, J. W. Lee, H. E. Shin, Y. J. Park, and T. Kim, “Low resistance Pd/Au ohmic contacts to p-type GaN using surface treatment,” Appl.
Phys. Lett., vol. 73, pp. 2953-2955, 1998.
[13] J. S. Jang, S. J. Park, and T. Y. Seong, “Interfacial reaction of Ni/Pt/Au contact schemes to p-Type GaN,” J. Electrochem. Soc., vol. 146, pp. 3425-3428, 1999.
[14] D. J. King, L. Zhang, J. C. Ramer, S. D. Hersee, and L. F. Lester, “Temperature behavior of Pt/Au ohmic contacts to p-GaN,” Mater. Res. Soc. Symp. Proc., vol.
468, pp. 421, 1997.
[15] J. K. Sheu, Y. K. Su, G. C. Chi, P. L. Koh, M. J. Jou, C. M. Chang, C. C. Liu, and W. C. Hung, “High-transparency Ni/Au ohmic contact to p-type GaN,” Appl.
Phys. Lett., vol. 74, pp. 2340, 1999.
[16] J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, and Y. K. Su, S. J. Chang, and G. C. Chi, “Low-operation voltage of InGaN/GaN light-emitting diodes with Si-doped In0.3Ga0.7N/GaN short-period superlattice tunneling contact layer,” IEEE Electron. Dev. Lett., vol. 22, pp. 460-462. 2001.
[17] S. J. Chang, C. S. Chang, Y. K. Su, R. W. Chuang, W. C. Lai, C. H. Kuo, Y. P.
Hsu, Y. C. Lin, S. C. Shei, H. M. Lo, J. C. Ke, and J. K. Sheu, “Nitride-based LEDs with an SPS tunneling contact layer and an ITO transparent contact,”
IEEE Photon. Technol. Lett., vol. 16, pp. 1002-1004, 2004.
[18] J. O Song, K. K. Kim, S. J. Park, and T. Y. Seong, “Highly low resistance and transparent Ni/ZnO ohmic contacts to p-type GaN,” Appl. Phys. Lett., vol. 83, pp.
479-481, 2003.
[19] K. Nakahara, K. Tamura, M. Sakai, D. Nakagawa, N. Ito, M. Sonobe, H. Takasu, H. Tampo, P. Fons, K. Matsubara, K. Iwata, A. Yamada, and S. Niki, “Improved external efficiency InGaN-based light-emitting diodes with transparent conductive Ga-doped ZnO as p-electrodes,” Jpn. J. Appl. Phys., vol. 43, pp.
L180-L182, 2004.
[20] D. R. Kammler, T. O. Mason, D. L. Young, and T. J. Coutts, “Thin films of the spinel Cd1 + xIn2–2xSnxO4 transparent conducting oxide solution,” J. Appl. Phys., vol. 90, pp. 3263-3268, 2001.
[21] T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen, S. P.
DenBaars, and L. A. Coldren, “Indium tin oxide contacts to gallium nitride optoelectronic devices,” Appl. Phys. Lett., vol. 74, pp. 3930-3932, 1999.
[22] R. H. Horng, D. S. Wuu, Y. C. Lien, and W. H. Lan, “Low-resistance and high-transparency Ni/indium tin oxide ohmic contacts to p-type GaN,” Appl.
Phys. Lett., vol. 79, pp. 2925-2927, 2001.
[23] Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, S. J. Hsu, C.
H. Liu, U. H. Liaw, S. C. Chen, and B. R. Huang, “Nitride-based light-emitting diodes with Ni/ITO p-type ohmic contacts,” IEEE Photon. Technol. Lett., vol. 14, pp. 1668-1670, 2002.
[24] Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid-State Electron., vol. 47, pp. 849-853, 2003.
[25] Y. C. Lin, S. J. Chang, Y. K. Su, C. S. Chang, S. C. Shei, J. C. Ke, H. M. Lo, S. C.
Chen, and C. W. Kuo, “High power nitride based light emitting diodes with Ni/ITO p-type contacts,” Solid-State Electron., vol. 47, pp. 1565-1568, 2003.
[26] S. M. Pan, R. C. Tu, Y. M. Fan, R. C. Yeh, and J. T. Hsu, “Enhanced output power of InGaN-GaN light-emitting diodes with high-transparency nickel-oxide-indium-tin-oxide ohmic contacts,” IEEE Photon. Technol. Lett., vol.
15, pp. 646-648, 2003.
[27] J. O Song, D. S. Leem, J. S. Kwak, Y. Park, S. W. Chae, and T. Y. Seong,
“Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts,” IEEE Photon. Technol.
Lett., vol. 17, pp. 291-293, 2005.
[28] C. Huh, J. M. Lee, D. J. Kim, and S. J. Park, “Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer,” Appl. Phys. Lett., vol. 92, pp. 2248, 2002.
[29] C. C. Liu, Y. H. Chen, M. P. Houng, Y. H. Wang, Y. K. Su, W. B. Chen, and S. H.
Chen, “Improved light-output power of GaN LEDs by selective region activation,” IEEE Photon. Technol. Lett., vol. 16, pp. 1444, 2004.
[30] S. J. Chang, W. C. Lai, Y. K. Su, J. F. Chen, C. H. Liu, and U. H. Liaw,
“InGaN-GaN multiquantum-well blue and green light-emitting diodes,” IEEE J.
Selected Topics Quantum Electron., vol. 8, pp. 278-283, 2002.
[31] C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. K. Sheu, and J. F. Chen,
“High-efficiency InGaN-GaN MQW green light-emitting diodes with CART and DBR structures,” IEEE J. Selected Topics Quantum Electron., vol. 8, pp.
284-288, 2002.
[32] X. Guo and E. F. Schubert, “Current crowding in GaN/InGaN light emitting diodes on insulating substrates,” J. Appl. Phys., vol. 90, pp. 4191-4195, 2001.
[33] H. Kim, J. M. Lee, C. Huh, S. W. Kim, D. J. Kim, S. J. Park, and H. Hwang,
“Modeling of a GaN-based light-emitting diode for uniform current spreading,”
Appl. Phys. Lett., vol. 77, pp. 1903-1904, 2000.
[34] H. Kim, S. J. Park, and H. Hwang, “Effects of current spreading on the performance of GaN-based light-emitting diodes,” IEEE Trans. Electron.
Devices, vol. 48, pp. 1065-1069, 2001.
[35] H. Kim, S. J. Park, H. Hwang, and N. M. Park, “Lateral current transport path, a model for GaN-based light-emitting diodes: Applications to practical device designs,” Appl. Phys. Lett., vol. 81, pp. 1326-1328, 2002.
[36] T. C. Gorjanc, D. Leong, C. Py, and D. Roth, “Room temperature deposition of ITO using r.f. magnetron sputtering,” Thin Solid Films, vol. 413, pp. 181-185, 2002.
[37] Y. Sadaoka, T. A. Jones, W. Gopel, S. Kimura, and N. Honda, “Effect of NO2 in air on the electrical conductance of In2O3 films with and without added ZnO prepared by R. F. sputtering,” J. Mater. Sci., vol. 25, pp. 2632-2636, 1990.
[38] E. Bertran, J. L. Morenza, J. Esteve, M. Varela, A. Figueras, and J. M. Tura,
“Indium thin films on metal-coated substrates,” Thin Solid Films, vol. 129, pp.
103-109, 1985.
[39] H. Kim, J. S. Horwitz, G. Kushto, A. Pique, Z. H. Kafafi, C. M. Gilmore, and D.
B. Chrisey, “Effect of film thickness on the properties of indium tin oxide thin films,” J. Appl. Phys., vol. 88, pp. 6021-6025, 2000.
[40] J. K. Ho, C. S. Jong, C. C. Chiu, C. N. Huang, K. K. Shih, L. C. Chen, F. R.
Chen, and J. J. Kai, “Low-resistance ohmic contacts to p-type GaN achieved by the oxidation of Ni/Au films,” J. Appl. Phys., vol. 86, pp. 4491-4497, 1999.
[41] T. Maeda, Y. Koide, and M. Murakami, “Effects of NiO on electrical properties of NiAu-based ohmic contacts for p-type GaN,” Appl. Phys. Lett., vol. 75, pp.
4145-4147, 1999.
[42] D. Qiao, L. S. Yu, S. S. Lau, J. Y. Lin, H. X. Jiang, and T. E. Haynes, “A study of the Au/Ni ohmic contact on p-GaN,” J. Appl. Phys., vol. 88, pp. 4196-4200, 2000.
Condition Oxygen flow rate ITO thickness
Table 2-1. Settings of evaporation parameters.
Sheet resistance of ITO (Ω/□)
Annealing temperature Condition O2 flow rate Thickness
400℃ 500℃ 600℃
Table 3-1. The list of sheet resistance of all ITO samples.
Annealing temperature
Table 3-2. Specific contact resistance of all ITO and Ni-Au films on p-GaN.
Annealing temperature
Table 3-3. Sheet resistance of p-GaN of all samples.
Ls (μm)
Annealing temperature Condition O2 flow rate Thickness
400℃ 500℃ 600℃
A 15 sccm 60 nm 120.4533 103.236 115.415 B 15 sccm 180 nm 285.7622 266.282 314.143 C 15 sccm 300 nm 1049.698 532.695 673.864 D 30 sccm 60 nm 121.7222 135.264 226.454 E 30 sccm 180 nm 268.255 517.967 644.056 F 30 sccm 300 nm 391.0211 517.653 618.622
Table 3-4. Simulated current spreading length of all ITO samples.
Dynamic series resistance (Ω)
Annealing temperature Condition O2 flow rate Thickness
400℃ 500℃ 600℃ Ni/Au
A 15 sccm 60 nm 11.93 11.79 11.31 9.53
B 15 sccm 180 nm 10.17 10.66 10.39 10.11
C 15 sccm 300 nm 9.86 10.44 9.87 9.87
D 30 sccm 60 nm 12.86 12.27 12.26 9.45
E 30 sccm 180 nm 11.02 10.44 10.75 9.24
F 30 sccm 300 nm 10.04 9.8 9.84 9.67
Table 3-5. Approximated series resistance of fabricated LEDs.
(a) As-grown GaN on sapphire substrate.
(b) Define mesa region and dry etched by ICP-RIE.
(c) ITO or Ni/Au TCL evaporated and wet etched.
(d) Cr-Au evaporated and partially removed by lift-off procedure to be formed as bonding pad.
Fig. 2-1. Process flow and cross section of nitride-based LEDs.
Fig. 2-2. The scheme of transmittance measurement method.
Fig. 2-3. A transmission line model test structure.
Fig. 2-4. A plot of total resistance as a function of contact spacing, d.
Fig. 2-5. A schematic diagram of a typical SEM.
(a)
(b)
Fig. 3-1. RS of (a) 300 nm, (b) 180 nm ITO films on quartz after different processes.
(a)
(b)
Fig. 3-2. RS of different thicknesses of ITO films evaporated with (a) 15 sccm, (b) 30 sccm O2 flow rate.
(a)
(b)
(c)
Fig. 3-3. SEM pictures of ITO films with thickness of (a) 60 nm, (b) 180 nm, (c) 300 nm.
Fig. 3-4. Transmittance of 300 nm ITO on GaN substrate after annealed at different temperatures (O2 = 15sccm).
Fig. 3-5. Transmittance of different thicknesses of ITO films (O2 = 15 sccm).
Fig. 3-6. I-V characteristics for Ni/Au (4 nm/4 nm) and ITO (300 nm) contacts on p-GaN after annealing at temperatures of 400-600℃ (O2 = 15 sccm), measured between the TLM pads with a spacing of 20 μm.
Fig. 3-7. The fitting diagram of measured data. The ITO film is 300 nm thick and was evaporated with 15 sccm O2 flow rate.
Fig. 3-8. Specific contact resistances of ITO films on p-GaN which were evaporated with different oxygen flow rates and annealed at 400-600 .℃ The thickness of ITO is 300 nm.
(a)
(b)
Fig. 3-9. Calculated current distribution vs. the lateral length x in a LED. The O2 flow rate is (a) 15 sccm and (b) 30 sccm, and annealing temperatures are both 400 .℃
Fig. 3-10. Calculated current spreading length of different thicknesses of ITO films.
The oxygen flow rate during evaporation is 15 sccm.
Fig. 3-11. Calculated current spreading length of 300 nm ITO films which were evaporated with different oxygen flow rates.
Fig. 3-12. Reverse voltage-current characteristics of the fabricated nitride-based LEDs with different annealing temperatures. The thickness of ITO film is 300 nm and evaporated with 15 sccm O2 flow rate.
(a)
(b)
Fig. 3-13. The reverse I-V characteristics of devices on (a) condition B and (b) condition F.
(a)
(b)
Fig. 3-14. Forward current-voltage characteristics of GaN LEDs with different thicknesses of ITO films which were annealed at 400 an℃ d evaporated with (a) O2 = 15 sccm (b) O2 = 30 sccm.
(a)
(b)
Fig. 3-15. Forward current-voltage characteristics of GaN LEDs with 300 nm ITO films annealed at different temperatures and evaporated with (a) O2 = 15 sccm (b) O2 = 30 sccm.
(a)
(b)
Fig. 3-16. Forward voltage vs. dynamic resistance in LEDs with ITO film annealed at 400 and evaporated℃ with (a) 15 sccm and (b) 30 sccm O2 flow rate.
(a)
(b)
Fig. 3-17. The normal luminance vs. injection current of fabricated LEDs with 300 nm ITO film evaporated with (a) 15 sccm and (b) 30 sccm O2 flow rate.
(a)
(b)
Fig. 3-18. The luminance enhancement vs. injection current of fabricated LEDs with (a) 60 nm and (b) 300 nm ITO film evaporated with 15 sccm O2 flow rate.
Fig. 3-19. Luminance enhancement with different ITO films at an injection current of 20-100 mA
(a)
(b)
Fig. 3-20. Normal luminance of all ITO samples at an injection current of (a) 20 mA, and (b) 50 mA. Conditions are referred to table 2-1.
(a)
(b)
Fig. 3-21. Emission microscopy graphs of (a) condition B annealed at 400 and (b℃ ) normal LED at a dc current of 0.2 mA.
(a)
(b)
Fig. 3-22. The relationship between current spreading length and (a) forward voltage at 20 mA and (b) normal luminance.
Fig. 3-23. The current converting efficiency (dL/dI) of LEDs vs. injection current. The ITO films were evaporated with 15 sccm O2 flow rate and annealed at 500 .℃
簡歷
姓 名：郭端祥 性 別：男
出生日期：民國 70 年 5 月 20 日 出 生 地：台灣省台北市
住 址：台北縣汐止市忠三街 99 號 3 樓
學 歷：國立師範大學附屬高級中學 (民國 85 年 9 月~88 年 6 月) 國立交通大學電子工程系 (民國 88 年 9 月~92 年 6 月) 國立交通大學電子所碩士班 (民國 92 年 9 月~94 年 6 月) 碩士論文：氧化銦錫擴散電流層厚度對氮化鎵發光二極體光電特性的影響 Influence on Optical and Electrical Characteristics of GaN-based
Light-emitting Diodes by Varying the Thickness of Indium-tin-oxide Current Spreading Layers