InGaN-GaN multiquantum-well blue and green light-emitting diodes ...

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IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 8, NO. 2, MARCH/APRIL 2002

InGaN–GaN Multiquantum-Well Blue and Green Light-Emitting Diodes S. J. Chang, W. C. Lai, Y. K. Su, Senior Member, IEEE, J. F. Chen, Member, IEEE, C. H. Liu, and U. H. Liaw

Abstract—InGaN–GaN multiquantum-well (MQW) blue and green light-emitting diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and the properties of these LEDs were evaluated by photoluminescence (PL), double crystal X-ray diffraction, and electroluminescence (EL) measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of quantum well (QW) increased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well was high for the green MQW LEDs. The forward voltage of green MQW LEDs was also found to be larger than that of blue MQW LDDs due to the same reason. Index Terms—DCXRD, EL, InGaN–GaN, MQW, PL.

I. INTRODUCTION

T

HE III–V NITRIDE semiconductor materials have a wurtzite crystal structure and a direct energy bandgap. At room temperature, the bandgap energy of AlInGaN varies from 1.95 to 6.2 eV depends on its composition ratio. Therefore, III–V nitride semiconductors are useful for light-emitting device in the short wavelength region [1]–[3]. Indeed, III–V nitride-based blue and green light-emitting diodes (LEDs) are now commercially available. Currently, these blue and green LEDs have already been extensively used in traffic light source and full color display. The active region of a typical III–V nitride-based blue and green LED consists an unintentionally doped InGaN–GaN multiquantum-well (MQW) structure sandwiched in between the (Al)GaN n-type and p-type cladding layers. However, due to the lattice mismatch between InGaN and GaN, relaxation might occur as the number of quantum well (QW) increases. Furthermore, piezoelectric field-induced quantum confined Stark effect (QCSE) [4], [5] might also influence the luminescence properties of these III–V nitride-based blue and green LEDs. In this work, we report the study of Manuscript received July 11, 2001; revised December 12, 2001. This work was supported in part by the National Science Council under Contract NSC-892215-E-006-005. S. J. Chang, W. C. Lai, Y. K. Su, and J. F. Chen are with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, 70101 Tainan, Taiwan, R.O.C. C. H. Liu is with the Department of Electronic Engineering, Nan-Jeon Institute Technology, Yan-Hsui, Taiwan 737, R.O.C. U. H. Liaw is with the Department of Electronic Engineering, Chin-Min College, To-Fen, 351 Taiwan, R.O.C. Publisher Item Identifier S 1077-260X(02)03778-4.

the InGaN–GaN MQW blue and green LEDs with different numbers of QW pairs. The relaxation induced effects of these LEDs will be discussed in detail. II. EXPERIMENT Samples used in this study were all grown on (0001) sapphire (Al O ) substrates in a vertical low-pressure organometallic vapor phase epitaxy (OMVPE) reactor with a high-speed rotation disk [3]. The gallium, indium, aluminum and nitrogen sources were trimethylgallium (TMGa), trimethylindium (TMI), trimethylaluminum (TMA), and ammonia (NH ), respectively. Bicyclopentadienyl magnesium (Cp Mg) and silane (SiH ) were used as the p-type and n-type doping sources, respectively. After annealing the sapphire substrate at 1100 C in H ambient to remove surface contamination, a 30-nm-thick low-temperature GaN nucleation layer was deposited onto the sapphire substrate at 525 C. The temperature was then raised to 1050 C to grow a 3- m-thick Si-doped GaN n-type cladding layer. We then lowed down the substrate temperature to grow the InGaN–GaN MQW active region. Mg doped AlGaN p-type cladding layer and Mg-doped p-type GaN contact layer were subsequently grown on top of the MQW active region at 1050 C. In order to increase the In incorporation rate, nitrogen was used as the carrier gas when we grew the InGaN–GaN MQW active regions [6]. On the other hand, hydrogen was used as the carrier gas when we grew the other parts of the samples. The growth pressure was kept at 350 mtorr throughout the growth. The MQW active region consists of p-pairs of 3-nm-thick In Ga N well layers and 10-nm-thick GaN barrier layers for the blue LEDs. For the green LEDs, the MQW active region consists of p-pairs of 2.5-nm-thick In Ga N well layers and 8-nm-thick GaN barrier layers. The growth temperature was kept at 760 C and 700 C when we grew the MQW active regions of the blue LEDs and green LEDs, respectively. The as-grown samples were then annealed at 760 C for 25 min in N ambient to activate the Mg-doped p-type layers. With this thermal annealing process, we could achieve uniformly doped highly conductive p-type layers. The crystal qualities of these epitaxial layers were evaluated by room-temperature photoluminescence (PL) and double crystal X-ray diffraction (DCXRD). A Bio-Rad rpm2000 HeCd laser was used for the PL measurement and a Bede QC2A system was used for the DCXRD measurement. After PL and DCXRD measurement, the surface of the p-type GaN layers were partially etched until the p-type GaN layers were exposed [7]. Ni–Au contacts were subsequently evaporated onto the p-type GaN surfaces to serve as the p-electrodes.

1077-260X/02$17.00 © 2002 IEEE

CHANG et al.: InGaN–GaN MULTIQUANTUM-WELL BLUE AND GREEN LIGHT-EMITTING DIODES

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(a) Fig. 1. The DCXRD spectra of the blue MQW LEDs with different numbers of InGaN–GaN MQW pairs.

On the other hand, Ti–Al contacts were deposited onto the exposed p-type GaN layers to serve as the p-type electrodes, to complete the fabrication of the blue and green LEDs. Roomtemperature electroluminescence (EL) characteristics of these fabricated LEDs were evaluated by injecting a different amount of dc current into these LEDs. III. RESULTS AND DISCUSSIONS A. Blue-LED Fig. 1 shows the DCXRD spectra of the blue MQW LEDs with different numbers of InGaN–GaN MQW pairs. Although the zeroth-order peaks of the MQW structures were all merged into the peaks of the GaN buffer layers, we could observe the “ ”, “ ”, “ ,” and “ ” order satellite peaks from these samples. The fact that we could clearly observe these satellite peaks indicates high crystal qualities and reasonably good interfaces of these blue MQW LEDs. From the positions of these satellite peaks, we found that the periodicity of each InGaN–GaN pair was about 13 nm, which is exactly the same as what we have designed. Also, we found that the full-width at half-maximum (FWHM) of the first order satellite peak of the 16-pair blue MQW LED is larger than that of the 11-pair sample. Such an observation suggests some degradation had occurred in the 16-pair blue MQW LED [6], [8]. We believe such degradation is due to strain relaxation in that particular sample. As the number of MQW increases, the effective thickness of the lattice-mismatched MQW layer will also increase. When the effective thickness of the lattice-mismatched MQW relayer is larger than its equivalent critical layer thickness laxation will unavoidably occur. As a result, relaxation induced dislocation will be generated in the sample, as can be seen from the larger DCXRD FWHM of the 16-pair blue MQW LED. Fig. 2(a) and (b) show, respectively, the room-temperature PL spectra and PL FWHM of the blue MQW LEDs. We could see, that although the peak positions of these PL spectra were all located at 464 nm, their PL FWHM were quite different. It was found that the PL FWHM decreases initially as the number of MQW pair increases. The minimum FWHM of 32.2 nm occurred for the 11-pair blue MQW LED. However, as the number of pair was further increased, the PL FWHM

(b) Fig. 2. The room-temperature (a) PL spectra and (b) PL FWHM of the blue MQW LEDs with different pairs of QWs.

became larger again. Such an observation agrees well with the DCXRD result that some strain relaxation induced crystal quality degradation had occurred for the 16-pair blue MQW LED. Fig. 3(a) and (b) show the room-temperature EL spectra and external quantum efficiencies of the blue MQW LEDs, respectively. The injection current was 20 mA for all samples. It could be seen that although the EL spectra were almost the same for all samples with peak positions all located at around 460 nm, samples with 11-pair MQW had the highest external quantum efficiency. Such a result again agrees well with those obtained from PL and DCXRD measurements. Fig. 4(a) and (b) show the for blue forward current–voltage ( – ) characteristics and MQW LEDs with different pairs of QWs, where the forward is defined as the operation voltage with a 20-mA involtage increases jection current. It was found that forward voltage as the number of MQW pair increases. Such an increase is partially attributed to the increased serial resistance and partially attributed to the built-in voltage of the entire heterostructure for samples with a larger number of MQW pairs. Fig. 5(a) and (b) show the EL spectra of the 11-pair and 16-pair blue MQW LEDs with different amount of injection currents. From this figure, we clearly observed a large blue shift in peak EL wavelength of the 11-pair MQW LED from 458.8 to 451.2 nm when the injected current increased from 20 to 200 mA, It should be noted that the 11-pair blue MQW LED is coherently strained with a large piezoelectric filed-induced QCSE. Thus, the large blue shift observed in 11-pair blue MQW LED can be attributed to the fact

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(a)

(a)

(b) (b) Fig. 3. The room-temperature (a) EL spectra and (b) external quantum efficiencies of the blue MQW LEDs with different pairs of QWs.

that the injection current will weaken the QCSE. As a result, the transition energy will become larger with a high injection current. On the other hand, the 16-pair blue MQW LED is partially relaxed with a smaller piezoelectric filed induced QCSE. Thus, the amount of EL blue shift is also smaller. Table I lists the measured EL peak positions with different amount of injection currents for the four blue LEDs. Again, the EL blue shift is much smaller for the partially relaxed 16-pair blue LED. Life test of these blue MQW LEDs were also performed with a 30-mA injection current operated at 55 C. We found that the output power decreased gradually in the first 100 h. After 100 h, the output power then became a constant, which was about 86% of the initial value (i.e., a 14% degradation). We did not observe under the same test condition. any change in forward voltage B. Green-LED Fig.6 shows the DCXRD spectra of the green-LED samples with different numbers of InGaN–GaN MQW pairs. We found that the satellite peaks of green MQW LEDs were not as clear as those observed from the blue MQW LEDs shown in Fig. 1. We also found that FWHM of the green MQW LEDs were larger in general. This could be understood by the fact that green MQW LEDs with a higher In composition ratio will relax much easier than the blue MQW LEDs. Fig.7 shows the room-temperature PL spectra of the green MQW LEDs. It was found that the main PL peak wavelength of the green MQW LED shifted from 526.6 to 501.2 nm when the MQW pair number increased

Fig. 4. The (a) forward I –V characteristics and (b) V for blue MQW LEDs with different pairs of QWs.

from two to eight [9], [10]. The small 365-nm PL peak observed from the two- and four-pair green MQW LEDs was originated from the GaN bandedge emission. It is known that III–V nitride materials have a large piezoelectric effect. When the number of QW was small, the InGaN well layers were under a large biaxial stress. In such a case, a piezoelectric field is induced and the optical properties of InGaN layers will be strongly affected by the QCSE. QCSE will result in a spatial separation of electrons and holes, and thus, the carrier recombination energy will become smaller. As a result, the PL peak position of the two-pair green MQW LED was red shifted by more than 25 nm, as compared to the eight-pair green MQW LED. On the other hand, as the number of QW pair increases, the active region will relax rapidly since the In composition ratio in the InGaN well is high for the green MQW LEDs. With a smaller QCSE, the PL emission wavelength of the eight-pair MQW LED will thus blue shift toward short wavelength side, as can be seen from Fig. 7. Fig.8 shows the room-temperature EL spectra of the green MQW LEDs under the same 20-mA injection current. We found that the EL peak position was 495.7, 504.2, 506, and 519.2 nm for eight-, six-, four-, and two-pair green MQW LEDs, respectively. With the same In composition ratio in the InGaN well layers, we again observed a blue shift in EL spectra when the number of QW increases. Fig. 9 shows a comparison in normalized EL spectra for blue MQW LED and green MQW LED. It can be seen that the EL FWHM of green MQW LED (i.e., 35 nm) was much larger than that of the blue MQW LED (i.e., 26 nm). These experimental observations were again due to


(a)

(b) Fig. 5. The EL spectra of (a) the 11-pair blue MQW LED and (b) 16-pair blue MQW LED with different amount of injection current.

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Fig. 6. The DCXRD spectra of the green MQW LEDs with different pairs of QWs.

Fig. 7. The room-temperature PL spectra of the green MQW LEDs with different pairs of QWs.

TABLE I MEASURED EL PEAK POSITIONS WITH DIFFERENT AMOUNT OF INJECTION CURRENTS FOR THE 4 BLUE LEDS

the relaxation of InGaN–GaN MQW in the green MQW LEDs. Fig. 10(a) and (b) show the forward – characteristics, and for green MQW LEDs with different pairs of QWs. Similar into blue MQW LEDs, it was found that forward voltage creases as the number of MQW pair increases. Again, such an increase is partially attributed to the increased serial resistance and partially attributed to the built-in voltage of the entire heterostructure for samples with a larger number of MQW pairs. of the green LEDs was much Furthermore, we found that larger than those of the blue LEDs. The larger is probably due to the higher resistivity of the MQW active layers in the green LEDs, since these layers were grown at a much lower temperature. The other possibility is due to the degradation of p-AlGaN cladding layer and p-GaN contact layer, since these layers were

Fig. 8. The room-temperature EL spectra of the green MQW LEDs under the same 20-mA injection current.

grown on top of the low quality MQW active region. Fig. 11(a) and (b) show the EL spectra of the six-pair and two-pair green MQW LEDs with different amount of injection currents. Similar to Fig. 5(a) and (b), it was found that the EL peak position of the six-pair green MQW LED only blue shifted slightly from 504.2 to 503.5 nm and the EL peak position of the two-pair green MQW LED blue shifted from 519.2 to 506.4 nm as the injection current increased from 20 to 150 mA. Table II lists the measured EL peak positions with different amount of injection currents for the four green LEDs. Due to their high In composition ratio, green MQW LEDs relaxed rapidly as the number of

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(a) Fig. 9. The comparison in normalized EL spectra for blue MQW LED and green MQW LED.

(b) Fig. 11. The EL spectra of the (a) six-pair green MQW LED (b) two-pair green MQW LED with different amount of injection current. (a) TABLE II MEASURED EL PEAK POSITIONS WITH DIFFERENT AMOUNT OF INJECTION CURRENTS FOR THE FOUR GREEN LEDS

(b) Fig. 10. The (a) forward I –V characteristics and (b) V for green MQW LEDs with different pairs of QWs.

QW increased. As a result, the residual strain remaining in the six-pair green MQW LED is much smaller than that of the coherently strained two-pair green MQW LED and so is the strain induced piezoelectric field. With a smaller QCSE, the EL peak position of the six-pair green MQW LED was thus much less sensitive to the amount of injection currents. In summary, InGaN–GaN MQW blue and green LEDs were prepared by MOVPE, and the properties of these LEDs were evaluated by PL, DCXRD, and EL measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of QW in-

creased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well of was high for the green MQW LEDs. The forward voltage green MQW LEDs was also found to be larger than that of blue MQW LEDs due to the same reason. REFERENCES [1] S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High brightness blue, green and yellow light emitting diodes with quantum well structure,” Jpn. J. Appl. Phys., vol. 34, pp. L797–L799, July 1995.


[2] I. Akasaki and H. Amano, “Crystal growth and conductivity control of group III-nitride semiconductors and their applications to short wavelength light emitters,” Jpn. J. Appl. Phys., vol. 36, pp. 5393–5408, Sept. 1997. [3] W. C. Lai, S. J. Chang, M. Yokoyama, J. K. Sheu, and J. F. Chen, “InGaN/AlInGaN light emitting diodes,” IEEE Photon. Technol. Lett., vol. 13, pp. 559–561, June 2001. [4] D. A. Meller, D. S. Chemla, T. C. Damen, A. C. Gross, W. Wiegmann, T. H. Wood, and C. A. Burrus, “Band-edge electroabsorption in quantum well structure: The quantum-confined stark effects,” Phys. Rev. Lett., vol. 53, pp. 2173–2176, Nov. 1981. [5] D. A. B. Miller, D. S. Chemla, and S. Schmitt-Rink, “Relation between electroabsorption in bulk semiconductors and in quantum wells: The quantum-confined Franz-Keldysh effect,” Phys. Rev. B, vol. 33, pp. 6976–6982, May 1986. [6] M. J. Reed, N. A. El-MasryC, A. Parker, J. C. Roberts, and S. M. Bedair, “Critical layer thickness determination of GaN/InGaN/GaN double heterostructures,” Appl. Phys. Lett., vol. 77, pp. 4121–4123, Dec. 2000. [7] C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. K. Sheu, and I. C. Lin, “Vertical high quality mirror-like facet of GaN-based devices by reactive ion etching,” Jpn. J. Appl. Phys., vol. 40, pp. 2762–2764, Apr. 2001. [8] C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. Y. Chi, C. A. Chang, J. K. Sheu, and J. F. Chen, “GaN metal-semiconductor-metal ultraviolet photodetectors with transparent indium-tin-oxide Schottky contacts,” IEEE Photon. Technol. Lett., vol. 13, pp. 848–850, Aug. 2001. [9] T. Wang, D. Nakagawa, J. Wang, T. Sugahara, and S. Sakai, “Photoluminescence investigation of InGaN/GaN single quantum well and multiple quantum wells,” Appl. Phys. Lett., vol. 73, pp. 3571–3573, Dec. 1998. [10] M. S. Minsky, S. B. Fleischer, A. C. Abare, J. E. Bowers, E. L. Hu, S. Keller, and S. P. Denbaars, “Characterization of high-quality InGaN/GaN multiquantum wells with time-resolved photoluminescence,” Appl. Phys. Lett., vol. 72, pp. 1066–1068, Mar. 1998.

S. J. Chang was born in Taipei, Taiwan in January 17, 1961. He received the MSEE degree from the National Cheng Kung University (NCKU), Tainan, Taiwan, the BSEE degree from the State University of New York, Stony Brook, NY, and the Ph.D. EE from the University of California, Los Angeles, Los Angeles, CA, in 1983, 1985 and 1989, respectively. From 1989 to 1992, he was a research scientist in NTT Basic Research Laboratories. In 1992, he became an Associate Professor in electrical engineering department at NCKU and was promoted to full Professor in 1998. Currently, he also served as the Director of Semiconductor Research Center in NCKU. He was a Visiting Scholar in Research Center for Advanced Science and Technology, University of Tokyo, Japan from July 1999 to February 2000, and a Royal Society Visiting Scholar in University of Wales, Sawasea, UK from January 1999 to March 1999. His current research interests include semiconductor physics and optoelectronic devices.

W. C. Lai was born in Taiwan. He received the B.S. degree in electrical engineering from the Feng Chia University, Taiwan, and the M.S. and Ph.D. degrees in electrical engineering from National Cheng Kung University in 1993 and 2001, respectively. In 2001, he was a Postdoctoral Associate in the Department of Electrical Engineer, National Cheng Kung University. His research interest includes the III–V nitride lighting device and UV detectors.

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Y. K. Su (S’77–M’84–M’90–SM’91) was born in Kaohsiung, Taiwan, R.O.C., on Auguest 23, 1948. He received the B.S. and Ph.D. degrees in electrical engineering from National Cheng Kung University (NCKU), Taiwan. From 1979 to 1983, he was with the Department of Electrical Engineering, NCKU, as an Associate Professor and was engaged in research on compound semiconductors and optoelectronic materials. In 1983, he was promoted to full professor in the Department of Electrical Engineering. From 1979 to 1980 and 1986 to 1987, he was on leave, working at the University of Southern California, Los Angeles, and AT&T Bell Laboratories as a visiting scholar. He was also a visiting professor at Stuttgart University, Germany, in 1993. In 1991, he became an Adjunct professor at the State University of New York, Binghamton. Now, he is a professor in the Department of Electrical Engineering at NCKU and Director General of the Department of Engineering and Applied Science, National Science Council. His research activities have been in compound semiconductors, integrated optics, and microwave devices. He has published over 200 papers in the area of thin-film materials and devices and optoelectronic devices. Dr. Su is a member of SPIE, the Materials Research Society, and Phi Tau Phi. He received the Outstanding Research Professor Fellowship from the National Science Council (NSC), R.O.C., during 1986–1992 and 1994–1995. He also received the Best Teaching Professor Fellowship from the Ministry of Education, R.O.C., in 1992. In 1995, he received the Excellent Engineering Professor Fellowship from the Chinese Engineering Association. In 1996 and 1998, he received the Award from the Chinese Electrical Engineering Association. In 1998, he also received the Academy Member of Asia-Pacific Academy of Materials (APAM).

J. F. Chen (S’93–M’98) received the B.S. degree in electrical engineering from the National Cheng Kung University, Tainan, Taiwan, R.O.C., and the M.S. and Ph.D. degrees in electrical engineering from the University of California, Berkeley, Berkeley, CA, in 1990, 1995, and 1998, respectively. Currently, he is with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C., as an assistant Professor. His main field of research includes optoelectronic devices and reliability of integrated circuits.

C. H. Liu received the M.S. and Ph.D. degrees in electrical engineering from the National Cheng Kung University, Tainan, Taiwan, R.O.C., in 1991 and 1997, respectively. Currently, he is with the Department of Electronic Engineering of Nan-Joen Institute of Technology, Tainan, Taiwan, R.O.C., as assistant Professor. His main field of research includes optoelectronic devices and compound semiconductors.

U. H. Liaw received the Ph.D. degree from the Department of Electrical Engineering, the National Cheng Kung University, Tainan, Taiwan, R.O.C. He is currently an associate professor of the Department of Electronic Engineering, Chin-Min College, To-Fen, Taiwan.