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Electrical and optical characteristics of the GaN light-emitting diodes with multiple-pair buffer layer
Solid-State Electronics 44 (2000) 1483±1486
Electrical and optical characteristics of the GaN light-emitting diodes with multiple-pair buer layer Chien-Cheng Yang a,b, Meng-Chyi Wu a,*, Chang-Cheng Chuo c, Jen-Inn Chyi c, Chia-Feng Lin d, Gou-Chung Chi d b
a Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Opto-Electronics and Systems Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC c Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan, ROC d Optical Science Center, National Central University, Chung-Li 32054, Taiwan, ROC
Received 31 December 1999; accepted 14 February 2000
Abstract The GaN homo-junction light-emitting diodes (LEDs) with multiple-pair buer layer (MBL) were grown by met thick GaN alorganic vapor phase epitaxy on sapphire substrates. Each pair of the buer layer consists of a 300 A nucleation layer grown at a low temperature of 525°C and a 4 lm thick GaN epitaxial layer grown at a high temperature of 1025°C. As compared to the conventional growth without buer layer, the GaN LEDs with MBL will exhibit a low turn-on voltage, stronger electroluminescence intensity, and higher light output power. It is attributed to the eective reduction in the propagation of defects and dislocations near the p±n junction for the LEDs with MBL. Ó 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction The GaN-based alloys are clearly emerging as the most important materials when the commercial superbrightness blue light-emitting diodes (LEDs) and the room-temperature blue laser diodes (LDs) are demonstrated as the commercial products from Nichia Chemical Industries [1,2]. To further improve these device performance and reliability, reducing the dislocation density remaining in the GaN ®lms becomes the most critical issue at this time. Many researchers attempt to reduce the dislocation density and etch-pit density (EPD) in the GaN epitaxial layers. For example, Sugahara et al. [3] and Vaudo et al. [4] attempted to grow the GaN epitaxial layers on the free-standing GaN by sub-
*
Corresponding author. Fax: +886-35-715971. E-mail address: [email protected] (M.-C. Wu).
limation and hydride vapor phase epitaxial (HVPE) methods, respectively. With regards to more sophistication in growth, Iwaya et al. [5] developed the second nucleation layer to reduce EPD in the GaN ®lms, and Kamp [6] used this technique to increase the light output power of LED to three times higher in magnitude. Usui et al. [7] also introduced the technique of epitaxial laterally overgrown (ELOG) GaN to reduce the dislocation density and then Nakamura et al. [8] used this structure to fabricate the LDs with a lifetime of above 10 000 h. Our previous work reported that the GaN epitaxial layer quality can be improved by growing the multiple-pair buer layer (MBL) at low and high temperatures (550°C and 1025°C) [9]. However, there still lacks detailed electrical and optical properties of the GaN devices to show the eect of MBL on the improvement of device performance. In this article, we investigate the in¯uence of MBL on the current±voltage (I±V) and light output characteristics of the homostructure GaN LEDs. In addition, we also discuss the possible mechanisms of MBL structure on the improved device quality in such a growth.
0038-1101/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 0 5 2 - 6
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C.-C. Yang et al. / Solid-State Electronics 44 (2000) 1483±1486
2. Experimental The p±n homo-junction GaN LEDs with two to three pairs of buer layer were grown on (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The LED structures with and without MBL are shown in Fig. 1(a) and (b), respectively. Here, each thick GaN pair of the buer layer consists of a 300 A nucleation layer grown at a low temperature (LT) of 525°C and a 4 lm thick GaN epitaxial layer grown at high temperature (HT) of 1025°C. The growth procedure of MBL structure is given elsewhere [9]. Trimethylgallium (TMGa), ammonia (NH3 ), disilane (Si2 H6 ), and biscyclopentadienyl magnesium (Cp2 Mg) were used as Ga, N, Si, and Mg sources, respectively. The LED structure grown on the MBL consists of a 1.5 lm Sidoped GaN (n 3±5 1018 cm±3 ), a 0.3 lm Si-doped GaN (n 71017 cm±3 ), and a 0.6 lm Mg-doped GaN. The epitaxial wafer was then thermally annealed for 20 min at 750°C in N2 ambient to obtain a hole concentration of 7 1017 cm±3 in the Mg-doped GaN ®lm. The mesa of 150 lm 150 lm rectangular was ®rst etched on the unmasked part of the p-layer surface until the n layer was exposed by reactive ion etching (RIE) using BCl3 . The p- and n-side contacts were vacuum-deposited and rapid-thermal annealed with Ni/Au (120 A/2000 A) with a 80 lm diameter and Ti/Al/Ni/Au (150 A/1500 A/ 100 A/1000 A) in the 130 lm 130 lm rectangular mesa, respectively. Furthermore, the transparent layer was deposited onto the p layer to of Ni/Au (50 A/50 A) improve the light output eciency. The chip was rectangular size around 350 lm 350 lm and the chip was then attached and bonded to a TO-46 can. The LED characteristics of forward voltage±current (I±V), electroluminescence (EL), and output power (L±I) were
measured under DC-biased conduction at the room temperature. 3. Results and discussion Fig. 2 shows the I±V characteristics for the LEDs with three-pair, two-pair, and without buer layers. The turn-on voltage at 20 mA is 7.4, 8.7 and 11.6 V for the LEDs with three-pair, two-pair, and without buer layers, respectively. The LEDs have the usual I±V behavior represented by I I0 exp (qV/nkT), where q is the electron charge, k, Boltzmann constant, n, ideality factor, and T, the absolute temperature. As shown in the inset of Fig. 2, the LEDs with three-pair buer layer has an ideality factor of 1.7 at low forward-bias region (V < 0:7 V). It indicates that the current transport is not purely dominated by diusion recombination if n 1 or by space-charge recombination if n 2. For the LEDs with two-pair and without buer layer, the n is 4.2 and 5.5, respectively. This means that the current behavior is attributed to the recombination tunneling current. Fedison et al. [10] and Casey et al. [11] reported that the conventional p±n GaN devices having the recombination±tunneling behavior in the low forward-bias region will exhibit heavy deep-level traps and nonradiative recombination centers in the LEDs. However, the diusion current is accompanied with the eective photon emission by carrier recombination across the band-gap transition. Other current components, such as tunneling, space charge, and surface recombination currents, cannot contribute to the eective photon emission. These results show that the LEDs with three-pair buer layer has a better electrical property and should have a higher optical eciency.
Fig. 1. The schematic drawings of p±n junction LEDs (a) without and (b) with the multiple buer layer. Each pair in the multiple buer layer containing a LT-GaN layer grown at 525°C and a HT-GaN layer grown at 1025°C.
C.-C. Yang et al. / Solid-State Electronics 44 (2000) 1483±1486
Fig. 2. The forward I±V characteristics for the LEDs with three-pair, two-pair and without buer layer. The inset of this ®gure shows the corresponding log I±V characteristics at low forward voltages.
In order to further understand the role of MBL structure, we further investigate the optical characteristics of the p±n junction LEDs. Fig. 3 shows the roomtemperature EL spectrum of LEDs with three-pair buer layer operated at a 20 mA forward current. This spectrum is dominated by a stronger peak at 390 nm and a broader and weak peak at 550 nm. The peak at 390 nm is caused by the hole injection from the p-layer to the n-
Fig. 3. EL spectrum of LEDs with the three-pair buer layer operated at 20 mA and 300 K. The transition mechanism of the peaks in the EL spectrum is also shown in this ®gure, where SD means shallow donor, SA means shallow acceptor, and YE means the deep acceptors involved in the yellow luminescence.
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layer and thus UV emission occurred in n-layer through radiative recombination. The broader yellow emission peak observed at 550 nm may be attributed to the traps of diused holes by deep acceptor or tunnel from the deep acceptor in the p-type side [10]. To compare the optical eciency of LEDs with dierent buer structures, the LED with three-pair buer layer has stronger EL intensity than those with two-pair and without buer layer. It means that the probability of eectively radiative recombination increases with pair number of MBL although the estimated diusion length of minority carrier is shorter than the average spacing between defects or dislocations [3]. Fig. 4 shows the light output power (L) as a function of current (I) for the LEDs with dierent pair numbers in the MBL. The LEDs with two- or three-pair buer layer have a higher light output power than that without buer layer. As shown in the inset of Fig. 4, the L±I behavior of LEDs can be generally ®tted by L / Ip , where p is power level and can be accounted for the inference of defects in the characteristic of light emission [11]. If the recombination current is dominant, p becomes 2; if the diusion current is dominant, p becomes 1. For the LEDs with two- or three-pair buer layer, p is larger than 1 (superlinear) in the range of dc current between 0.5 and 1 mA while p becomes approximately 1 at 1±3 mA. Therefore, the recombination current is dominant in the low-current range, and the diusion current becomes dominant in the intermediate-current range [12]. The diusion±recombination current dominating the p±n junction transport means that there are low defects and dislocations occurred in the epitaxial layers. For the LEDs without buer layer, p is 0.3±1.4 between 0.5 and 20 mA. This is due to the high series resistance and heating eect occurred at the junction with heavy defects or dislocations [12].
Fig. 4. The light output power as a function of current for the LEDs with dierent pair numbers in the MBL. The inset of this ®gure shows their corresponding log L±log I at low forward currents.
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an improved light output power in the LEDs with threepair buer layer. The growth of MBL in the LED structure is a feasible way to fabricate the high-performance LEDs and laser devices.
Acknowledgements The ®nancial support from the Opto-Electronics & System Laboratories, Industrial Technology Research Institute, and the National Science Council (Nos. NSC 88-2215-E-007-004 and NSC 88-2112-M-008-006) is deeply appreciated. Fig. 5. The external quantum eciency as a function of current for the LEDs with three-pair and two-pair buer layer.
Fig. 5 shows the external quantum eciency as a function of forward current for the LEDs with two- and three-pair buer layers. On increasing the current, the quantum eciency rises and reaches to a maximum of 0.035% and 0.025% at 4 mA for the LEDs with threeand two-pair buer layers, respectively. Meantime, the LED with three-pair buer layer has higher probability of radiative recombination as compared to the LED with two-pair buer layer. The eciency decreases rapidly with increasing current above 5 mA and drops to 0.014% at 50 mA. 4. Conclusions We have demonstrated that the LEDs with more pairs in the MBL can eectively improve the current± voltage and optical characteristics. The LEDs with three-pair buer layer exhibit a lower turn-on voltage, a stronger EL intensity, and a higher light output power due to more eectively radiative recombinations occurring in the junction. It also appears to a consistent behavior in both the I±V and L±I in which the diusion recombination current can be observed in the low-bias regime. It is inferred that the reduction of both the dislocation in the epitaxial layers and impurity diusion through the dislocations into the junction will result in
References [1] Nakamura S, Senoh M, Iwasa N, Nagahama S, Iwasa N, Yamada T, Mukai T. Jpn J Appl Phys 1995;34: L1332. [2] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Kiyoku H, Sugimoto Y, Kozaki T, Umemoto H, Sano M, Chocho K. J Cryst Growth 1998;189, 190:820. [3] Sugahara T, Asto H, Hao M, Naoi Y, Kurai S, Tottori S, Yamasita K, Nishino K, Sakai S. The Second International Conference on Nitride Semiconductors, Tokushima, Japan, 1997. p. 436. [4] Vaudo RP, Phanse VM, Wu XH, Golan Y, Speck JS. The Second International Conference on Nitride Semiconductors, Tokushima, Japan, 1997. p. 156. [5] Iwaya M, Takeuchi T, Yamaguchi S, Wetzel C, Amano H, Akasaki I. Jpn. J Appl Phys 1998;37:L316. [6] Kamp M. The International Photonics Conference, Taipei, Taiwan, 1998. p. 693. [7] Usui A, Sunakawa H, Sakai A, Yamagucki AA. Jpn J Appl Phys 1997;36:L899. [8] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Kiyoku H, Sugimoto Y, Kozaki T, Umemoto H, Sano M, Chocho K. Appl Phys Lett 1998;72:211. [9] Yang CC, Wu MC, Chang CA, Chi GC. J Appl Phys 1999;85:8427. [10] Fedison JB, Chow TP, Lu H, Bhat IB. Appl Phys Lett 1998;72:2841. [11] Casey Jr. HC, Muth J, Krishnankutty S, Zavada JM. Appl Phys Lett 1996;68:2867. [12] Nakamura S, Senoh M, Mukai T. Jpn J Appl Phys 1991;30:L1708.
Electrical and optical characteristics of the GaN light-emitting diodes with multiple-pair buer layer Chien-Cheng Yang a,b, Meng-Chyi Wu a,*, Chang-Cheng Chuo c, Jen-Inn Chyi c, Chia-Feng Lin d, Gou-Chung Chi d b
a Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30043, Taiwan, ROC Opto-Electronics and Systems Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC c Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan, ROC d Optical Science Center, National Central University, Chung-Li 32054, Taiwan, ROC
Received 31 December 1999; accepted 14 February 2000
Abstract The GaN homo-junction light-emitting diodes (LEDs) with multiple-pair buer layer (MBL) were grown by met thick GaN alorganic vapor phase epitaxy on sapphire substrates. Each pair of the buer layer consists of a 300 A nucleation layer grown at a low temperature of 525°C and a 4 lm thick GaN epitaxial layer grown at a high temperature of 1025°C. As compared to the conventional growth without buer layer, the GaN LEDs with MBL will exhibit a low turn-on voltage, stronger electroluminescence intensity, and higher light output power. It is attributed to the eective reduction in the propagation of defects and dislocations near the p±n junction for the LEDs with MBL. Ó 2000 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction The GaN-based alloys are clearly emerging as the most important materials when the commercial superbrightness blue light-emitting diodes (LEDs) and the room-temperature blue laser diodes (LDs) are demonstrated as the commercial products from Nichia Chemical Industries [1,2]. To further improve these device performance and reliability, reducing the dislocation density remaining in the GaN ®lms becomes the most critical issue at this time. Many researchers attempt to reduce the dislocation density and etch-pit density (EPD) in the GaN epitaxial layers. For example, Sugahara et al. [3] and Vaudo et al. [4] attempted to grow the GaN epitaxial layers on the free-standing GaN by sub-
*
Corresponding author. Fax: +886-35-715971. E-mail address: [email protected] (M.-C. Wu).
limation and hydride vapor phase epitaxial (HVPE) methods, respectively. With regards to more sophistication in growth, Iwaya et al. [5] developed the second nucleation layer to reduce EPD in the GaN ®lms, and Kamp [6] used this technique to increase the light output power of LED to three times higher in magnitude. Usui et al. [7] also introduced the technique of epitaxial laterally overgrown (ELOG) GaN to reduce the dislocation density and then Nakamura et al. [8] used this structure to fabricate the LDs with a lifetime of above 10 000 h. Our previous work reported that the GaN epitaxial layer quality can be improved by growing the multiple-pair buer layer (MBL) at low and high temperatures (550°C and 1025°C) [9]. However, there still lacks detailed electrical and optical properties of the GaN devices to show the eect of MBL on the improvement of device performance. In this article, we investigate the in¯uence of MBL on the current±voltage (I±V) and light output characteristics of the homostructure GaN LEDs. In addition, we also discuss the possible mechanisms of MBL structure on the improved device quality in such a growth.
0038-1101/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 0 ) 0 0 0 5 2 - 6
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C.-C. Yang et al. / Solid-State Electronics 44 (2000) 1483±1486
2. Experimental The p±n homo-junction GaN LEDs with two to three pairs of buer layer were grown on (0 0 0 1) sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The LED structures with and without MBL are shown in Fig. 1(a) and (b), respectively. Here, each thick GaN pair of the buer layer consists of a 300 A nucleation layer grown at a low temperature (LT) of 525°C and a 4 lm thick GaN epitaxial layer grown at high temperature (HT) of 1025°C. The growth procedure of MBL structure is given elsewhere [9]. Trimethylgallium (TMGa), ammonia (NH3 ), disilane (Si2 H6 ), and biscyclopentadienyl magnesium (Cp2 Mg) were used as Ga, N, Si, and Mg sources, respectively. The LED structure grown on the MBL consists of a 1.5 lm Sidoped GaN (n 3±5 1018 cm±3 ), a 0.3 lm Si-doped GaN (n 71017 cm±3 ), and a 0.6 lm Mg-doped GaN. The epitaxial wafer was then thermally annealed for 20 min at 750°C in N2 ambient to obtain a hole concentration of 7 1017 cm±3 in the Mg-doped GaN ®lm. The mesa of 150 lm 150 lm rectangular was ®rst etched on the unmasked part of the p-layer surface until the n layer was exposed by reactive ion etching (RIE) using BCl3 . The p- and n-side contacts were vacuum-deposited and rapid-thermal annealed with Ni/Au (120 A/2000 A) with a 80 lm diameter and Ti/Al/Ni/Au (150 A/1500 A/ 100 A/1000 A) in the 130 lm 130 lm rectangular mesa, respectively. Furthermore, the transparent layer was deposited onto the p layer to of Ni/Au (50 A/50 A) improve the light output eciency. The chip was rectangular size around 350 lm 350 lm and the chip was then attached and bonded to a TO-46 can. The LED characteristics of forward voltage±current (I±V), electroluminescence (EL), and output power (L±I) were
measured under DC-biased conduction at the room temperature. 3. Results and discussion Fig. 2 shows the I±V characteristics for the LEDs with three-pair, two-pair, and without buer layers. The turn-on voltage at 20 mA is 7.4, 8.7 and 11.6 V for the LEDs with three-pair, two-pair, and without buer layers, respectively. The LEDs have the usual I±V behavior represented by I I0 exp (qV/nkT), where q is the electron charge, k, Boltzmann constant, n, ideality factor, and T, the absolute temperature. As shown in the inset of Fig. 2, the LEDs with three-pair buer layer has an ideality factor of 1.7 at low forward-bias region (V < 0:7 V). It indicates that the current transport is not purely dominated by diusion recombination if n 1 or by space-charge recombination if n 2. For the LEDs with two-pair and without buer layer, the n is 4.2 and 5.5, respectively. This means that the current behavior is attributed to the recombination tunneling current. Fedison et al. [10] and Casey et al. [11] reported that the conventional p±n GaN devices having the recombination±tunneling behavior in the low forward-bias region will exhibit heavy deep-level traps and nonradiative recombination centers in the LEDs. However, the diusion current is accompanied with the eective photon emission by carrier recombination across the band-gap transition. Other current components, such as tunneling, space charge, and surface recombination currents, cannot contribute to the eective photon emission. These results show that the LEDs with three-pair buer layer has a better electrical property and should have a higher optical eciency.
Fig. 1. The schematic drawings of p±n junction LEDs (a) without and (b) with the multiple buer layer. Each pair in the multiple buer layer containing a LT-GaN layer grown at 525°C and a HT-GaN layer grown at 1025°C.
C.-C. Yang et al. / Solid-State Electronics 44 (2000) 1483±1486
Fig. 2. The forward I±V characteristics for the LEDs with three-pair, two-pair and without buer layer. The inset of this ®gure shows the corresponding log I±V characteristics at low forward voltages.
In order to further understand the role of MBL structure, we further investigate the optical characteristics of the p±n junction LEDs. Fig. 3 shows the roomtemperature EL spectrum of LEDs with three-pair buer layer operated at a 20 mA forward current. This spectrum is dominated by a stronger peak at 390 nm and a broader and weak peak at 550 nm. The peak at 390 nm is caused by the hole injection from the p-layer to the n-
Fig. 3. EL spectrum of LEDs with the three-pair buer layer operated at 20 mA and 300 K. The transition mechanism of the peaks in the EL spectrum is also shown in this ®gure, where SD means shallow donor, SA means shallow acceptor, and YE means the deep acceptors involved in the yellow luminescence.
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layer and thus UV emission occurred in n-layer through radiative recombination. The broader yellow emission peak observed at 550 nm may be attributed to the traps of diused holes by deep acceptor or tunnel from the deep acceptor in the p-type side [10]. To compare the optical eciency of LEDs with dierent buer structures, the LED with three-pair buer layer has stronger EL intensity than those with two-pair and without buer layer. It means that the probability of eectively radiative recombination increases with pair number of MBL although the estimated diusion length of minority carrier is shorter than the average spacing between defects or dislocations [3]. Fig. 4 shows the light output power (L) as a function of current (I) for the LEDs with dierent pair numbers in the MBL. The LEDs with two- or three-pair buer layer have a higher light output power than that without buer layer. As shown in the inset of Fig. 4, the L±I behavior of LEDs can be generally ®tted by L / Ip , where p is power level and can be accounted for the inference of defects in the characteristic of light emission [11]. If the recombination current is dominant, p becomes 2; if the diusion current is dominant, p becomes 1. For the LEDs with two- or three-pair buer layer, p is larger than 1 (superlinear) in the range of dc current between 0.5 and 1 mA while p becomes approximately 1 at 1±3 mA. Therefore, the recombination current is dominant in the low-current range, and the diusion current becomes dominant in the intermediate-current range [12]. The diusion±recombination current dominating the p±n junction transport means that there are low defects and dislocations occurred in the epitaxial layers. For the LEDs without buer layer, p is 0.3±1.4 between 0.5 and 20 mA. This is due to the high series resistance and heating eect occurred at the junction with heavy defects or dislocations [12].
Fig. 4. The light output power as a function of current for the LEDs with dierent pair numbers in the MBL. The inset of this ®gure shows their corresponding log L±log I at low forward currents.
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C.-C. Yang et al. / Solid-State Electronics 44 (2000) 1483±1486
an improved light output power in the LEDs with threepair buer layer. The growth of MBL in the LED structure is a feasible way to fabricate the high-performance LEDs and laser devices.
Acknowledgements The ®nancial support from the Opto-Electronics & System Laboratories, Industrial Technology Research Institute, and the National Science Council (Nos. NSC 88-2215-E-007-004 and NSC 88-2112-M-008-006) is deeply appreciated. Fig. 5. The external quantum eciency as a function of current for the LEDs with three-pair and two-pair buer layer.
Fig. 5 shows the external quantum eciency as a function of forward current for the LEDs with two- and three-pair buer layers. On increasing the current, the quantum eciency rises and reaches to a maximum of 0.035% and 0.025% at 4 mA for the LEDs with threeand two-pair buer layers, respectively. Meantime, the LED with three-pair buer layer has higher probability of radiative recombination as compared to the LED with two-pair buer layer. The eciency decreases rapidly with increasing current above 5 mA and drops to 0.014% at 50 mA. 4. Conclusions We have demonstrated that the LEDs with more pairs in the MBL can eectively improve the current± voltage and optical characteristics. The LEDs with three-pair buer layer exhibit a lower turn-on voltage, a stronger EL intensity, and a higher light output power due to more eectively radiative recombinations occurring in the junction. It also appears to a consistent behavior in both the I±V and L±I in which the diusion recombination current can be observed in the low-bias regime. It is inferred that the reduction of both the dislocation in the epitaxial layers and impurity diusion through the dislocations into the junction will result in
References [1] Nakamura S, Senoh M, Iwasa N, Nagahama S, Iwasa N, Yamada T, Mukai T. Jpn J Appl Phys 1995;34: L1332. [2] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Kiyoku H, Sugimoto Y, Kozaki T, Umemoto H, Sano M, Chocho K. J Cryst Growth 1998;189, 190:820. [3] Sugahara T, Asto H, Hao M, Naoi Y, Kurai S, Tottori S, Yamasita K, Nishino K, Sakai S. The Second International Conference on Nitride Semiconductors, Tokushima, Japan, 1997. p. 436. [4] Vaudo RP, Phanse VM, Wu XH, Golan Y, Speck JS. The Second International Conference on Nitride Semiconductors, Tokushima, Japan, 1997. p. 156. [5] Iwaya M, Takeuchi T, Yamaguchi S, Wetzel C, Amano H, Akasaki I. Jpn. J Appl Phys 1998;37:L316. [6] Kamp M. The International Photonics Conference, Taipei, Taiwan, 1998. p. 693. [7] Usui A, Sunakawa H, Sakai A, Yamagucki AA. Jpn J Appl Phys 1997;36:L899. [8] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, Kiyoku H, Sugimoto Y, Kozaki T, Umemoto H, Sano M, Chocho K. Appl Phys Lett 1998;72:211. [9] Yang CC, Wu MC, Chang CA, Chi GC. J Appl Phys 1999;85:8427. [10] Fedison JB, Chow TP, Lu H, Bhat IB. Appl Phys Lett 1998;72:2841. [11] Casey Jr. HC, Muth J, Krishnankutty S, Zavada JM. Appl Phys Lett 1996;68:2867. [12] Nakamura S, Senoh M, Mukai T. Jpn J Appl Phys 1991;30:L1708.