- Email: [email protected]
GaN blue diode with multi-quantum barrier structure
ARTICLE IN PRESS
Journal of Crystal Growth 278 (2005) 421–425 www.elsevier.com/locate/jcrysgro
Study of electroluminescence quenching in the InGaN/GaN blue diode with multi-quantum barrier structure Ray-Ming Lina,, Chung-Han Lina, Jen-Cheng Wanga, Tzer-En Neea, Bor-Ren Fangb, Ruey-Yu Wangb a
Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan, ROC b Nan Ya Photonics Incorporation, Taipei, Taiwan, ROC Available online 10 February 2005
Abstract This work has demonstrated electroluminescence (EL) quenching of the InGaN/GaN quantum well (QW) diodes with multi-quantum barrier (MQB) structure at several temperatures from 20 to 300 K. Both high- and low-energy bands indicate a surprising tendency when the temperature and injection current are altered. Increasing temperature reduces the high-energy band and clearly increases the quantum efficiency of the low-energy band. The device with MQB improves the quantum efficiency throughout the whole temperature range and displays good agreement with the EL-integrated intensity in this study. The sample with an optimal MQB structure could significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with the sample without the MQB structure. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 81.05.Ea; 81.15.Gh Keywords: A3. Multi-quantum wells; A3. Multi-quantum barrier; B1. InGaN; B3. Light emitting diodes
1. Introduction Super-bright blue light emitting diode (LED) using III-nitride semiconductor quantum structures have been successfully manufactured [1] and have wide applications in full-color display, Corresponding author. Tel.: +886 3 2118800x5790;
fax: +886 3 2118507. E-mail address: [email protected] (R.-M. Lin).
information storage and communication. Previous works have reported that the localized radiative centers which may originate from In-rich regions may be responsible for this phenomenon by using optical and electrical measurements [2–4]. Some investigations have found that InGaN quantum well (QW) LEDs have an unusual temperaturedependent electroluminescence (EL) characteristic and furthermore, have found an additional higher energy band (400 nm) at low temperature, which
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.01.012
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
can be assigned as Mg-related emission [5,6]. Although temperature- and current-dependent EL with various sample parameters have been discussed, the detailed mechanisms remain controversial. Furthermore, the temperature- and current-dependent characteristics of the additional higher energy band are still not well investigated. In this article, we present temperature- and injection current-dependent measurements of multi-quantum well (MQW) blue LEDs with and without multi-quantum barrier (MQB) structures, and discretely analyze them over a wide temperature range. The MQB was calculated [7,8] and is a useful structure for elevating the performance of the red and infrared LEDs and LDs [9–11]. In contrast to the previously reported photoluminescence (PL) result that the reduction of the nonradiative effect in the sample will enhance the PL intensity, the substantial reduction of the capture rate of the MQW will reduce the EL intensity and lead to higher energy band emission at low temperature. The device containing MQBs with appropriate indium composition is found to exhibit not only depressed higher energy band but also improved order of the quantum efficiency at 20 K. Additionally, the correlation between the EL peak energy and the dynamic of the carrier capture and recombination was discussed.
2. Experiment The LED samples investigated here were grown on c-plane sapphire via metal organic chemical vapor deposition (MOCVD). The samples consist of 20-nm-thick low-temperature GaN nucleation layer, a 3 mm Si-doped n-type GaN, and an unintentionally doped active layer with InxGa1 xN/GaN MQWs (0:15oxo0:18) and Mgdoped p-type GaN. The doping level of n- and ptype GaN are nominally 5 1018 and 1 1019 cm 3, respectively. The InxGa1 xN/ GaN MQW layers comprise five periods InxGa1 xN wells (2 nm) and GaN (10 nm) for sample 1 (sign S1). For samples 2, 3 and 4 (sign S2, S3, S4), the wells are the same as for S1, but barriers comprise five pairs InxGa1 xN/GaN rather than the conventional barrier and named
MQB. The MQB effective composition of InxGa1 x with the x values are 0.005, 0.01 and 0.02 for S2, S3 and S4, respectively. For temperaturedependent EL measurements, the samples mounted in a closed-cycle He cryostate were carried out using Keithley 2430 as a current source over a wide temperature range (20–300 K). The luminescence signal dispersed using a 0.5-m monochromator was detected by a Si photodiode employing a standard lock-in technique.
3. Results and discussion Fig. 1 illustrates the EL spectra of the all samples at 20 K with current operated at 20 mA. A leading blue MQW emission band is observed at nearly 440 nm (2.8 eV) with fine structures owing to the Fabry–Perot fringes. Furthermore, an additional band exists at 400 nm (3.1 eV) and is tentatively attributed to the Mg-relative transition [5,12]. The two peaks of 400 and 440 nm are named the high- and low-energy bands in this work. The temperature-dependent integrated intensity of the high-energy band (400 nm) and the low-energy band (440 nm) at a fixed injection level of 20 mA are illustrated in Fig. 2 for all samples. Unlike the generally expected PL tendency of diminished nonradiative recombination decreasing with temperature, the EL intensity of barrier
MQB
S1 S2 S3 S4
Low energy band
1
EL relative intensity (a.u.)
422
QW
QW
p-GaN MQW n-GaN Sapphire
High energy band
0.1
0.01 340
360
380
400
420
440
460
480
500
520
Wavelength (nm)
Fig. 1. EL spectra of InGaN/GaN MQW LEDs with and without MQB structures measured at 20 K and 20 mA injection current.
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425 6
1
4
Low energy band S1 S2 S3 S4 I = 20 mA
0.1
0.01 1.2
×4
High energy band S1 S2 S3 S4 I = 20 mA
1.0 0.8 0.6 × 24
0.4 0.2
λ (T) - λ (20 K) (nm)
Normalized EL Intensity (a.u.)
423
2
Low energy band S1 S2 S3 S4 I = 20 mA
0
-2
-4
-6 20
40
60
80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 2. Normalized integrated EL intensity of the high- and low-energy bands for devices S1, S2, S3, and S4 measured from 20 to 300 K and 20 mA injection current.
the low-energy band was surprisingly observed to decrease with temperature for all of the present samples. As the temperature reduces below 180 K, a marked reduction in the light intensity is observed. As the temperature further decreases to 20 K, the EL intensity clearly reduces less in the sample with the MQB structure. The improvement of the EL intensity may lead to better carrier confinement in the MQW region, and overflow of fewer carriers into the GaN region where they are either radiatively recombined or nonradiatively extinguished after being trapped by defect [4]. This study also observed a high-energy band (400 nm) that indicates a significantly different temperature-dependent behavior to the low-energy band. The EL intensity of the high-energy band increased as temperature decreased from approximately 140 to 20 K. Based on the different tendency of the high- and low-energy band, this study assumed that the EL intensity and spectra characteristic behavior observed at lower temperature cannot be attributed to the heating effect, since the nonradiative recombination process may be less important at relatively low temperatures. Fig. 3 illustrates the temperature-induced shifts of the low-energy band for the samples operating at 20 mA. As the temperature increases, ‘‘Sshaped’’ behavior is seen in all of the samples except S2. An unusual wavelength shift is observed
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 3. EL peak emission wavelength shift of the InGaN/GaN MQW LEDs with and without MQB structures with various temperatures.
in InGaN/GaN by using PL measurement [13] and can be clarified by carrier relaxation. At low temperature, owing to the increase of the carrier lifetime, carriers have considerable opportunity to relax to the lower-lying state before recombination because of a reduction in the nonradiative process. The analytical result below about 80 K displays good agreement with this explanation. Nevertheless, sample S2 exhibits a blueshift trend rather than a redshift below 80 K. The study interprets this behavior as the efficient carrier capture rate in sample S2, and better EL intensity than S1 can be observed in Figs. 1 and 2. As the temperature is further increased from 80 to 200 K, all experiment samples display a blueshift tendency. Besides, Fig. 2 reveals that the EL intensity of the high-energy band emission exhibits decreases while the lowenergy band increases with increasing temperature. This study hypothesizes that the increase in the carrier capture rate with increasing temperature will not only improve the probability that carriers can be radiatively recombined in the MQW, but also depresses the higher energy band emission. Higher numbers of carriers in the QW region will also produce a band-filling effect, and the emission energy may blueshift, as observed in the present samples. When the temperature further exceeds 200 K, the nonradiative center captures the carriers via thermal enhancing and reduces the
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
carriers in the QW region, while the blueshift of the emission energy reduces. At the same time, the blueshift evolves into a redshift associated with standard temperature-dependent narrowing of the band gap. Fig. 4 reveals a similar EL spectra tendency for the samples with and without MQB, measured as a function of temperature from 20 to 300 K given a current injection of 80 mA. However, the reduction in EL intensity was clearly less at lower temperature for all samples. At higher injection current, the EL quenching rate for all samples is found to be approximately 10 times lower than at low injection current level. This enhancement of the EL intensity at lower temperature with higher injection current may result from the depression of the high-energy band in the present sample (not shown here). The explanation of the present experimental result at higher injection current remains poorly understood. Further work on this area will be conducted in the future. Fig. 5 illustrates the quantum efficiency per area operated with current at 20 mA with a various temperature ranges deduced based on the L–I characteristics. The active area is 3.68 10 4 cm2 for all devices. The quantum efficiency of the devices first nearly stabilizes above 180 K, then decreases significantly with temperature down to 20 K. The results are inconsistent with our experi-
Normalized EL Intensity (a.u.)
1
S1 S2 S3 S4 I = 80 mA
0.1
1
(dL /dI)/A (a.u.)
424
S1 S2 S3 S4 I = 20 mA
0.1
0.01
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (k)
Fig. 5. Normalized quantum efficiency for sample S1, S2, S3 and S4 from 20 to 300 K and 20 mA injection current.
mental observations regarding the temperaturedependent EL intensity illustrated in Fig 1. The EL intensity decreases significantly with increasing higher energy band, i.e. quantum efficiency, of the MQW. The reduction of the high-energy band in the device with the MQB structure then prevents the carrier overflow into the p-GaN region from acting as a ‘‘p–n junction LED’’ and keeps the carrier in the MQW region, giving carriers increased opportunity to recombine in the QW. Furthermore, quantum efficiency is nearly stable at higher temperature, probably due to the increase in the capture rate of the nonradiative center saturating the quantum efficiency. The sample with an optimal MQB structure, i.e. sample S2, will significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with S1. Notably, the higher quantum efficacy of sample S2 also may be caused by the lower nonradiative center in the barrier compared with samples S3 and S4. This may be caused by the higher indium content in the MQB region and form the higher density of the nonradiative centers.
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 4. Normalized EL intensity of the low-energy band for device S1, S2, S3 and S4 at 80 mA with several temperatures from 20 to 300 K.
4. Conclusions To summarize, this study has demonstrated EL measurement with various temperature and
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
injection current parameters in four blue LED devices with and without MQB structures. Both high- and low-energy bands indicate a surprising tendency when the temperature and injection current are altered. Increasing temperature reduces the high-energy band and clearly increases the quantum efficiency of the low-energy band. The device with MQB improves the quantum efficiency throughout the whole temperature range and displays good agreement with the EL-integrated intensity in this study. The sample with an optimal MQB structure could significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with the sample without the MQB structure.
Acknowledgments The authors would like to thank Dr. Nie-Chun Chen for his technical assistance. This work was supported by the National Science Council of the Republic of China under Contract No. NSC 922215-E-182-001.
425
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 69 (1996) 4188. [3] P.G. Eliseev, P. Perlin, J. Lee, M. Osinski, Appl. Phys. Lett. 71 (1997) 569. [4] S.W. Feng, Y.C. Cheng, Y.Y. Chung, C.C. Yang, Y.S. Lin, C. Hsu, K.J. Ma, J.I. Chyi, J. Appl. Phys. 92 (2002) 4441. [5] C.M. Lee, C.C. Chuo, J.F. Dai, X.F. Zheng, J.I. Chyi, J. Appl. Phys. 89 (2001) 6554. [6] A. Hori, D. Yasunaga, A. Satake, K. Fujiwara, J. Appl. Phys. 93 (2003) 3152. [7] K. Iga, H. Uenohara, F. Koyama, Electron. Lett. 22 (1986) 1008. [8] Michinori Irukawa, Takura Ishikawa, Yoshitaka Sasaki, Katsumi Iwasawa, Ikuo Suemune, Kenichi Iga, J. Appl. Phys. 84 (1998) 4667. [9] M.R. Islam, R.V. Chelakara, R.D. Dupuis, Appl. Phys. Lett. 67 (1995) 2057. [10] K. Kishino, A. Kikuchi, Y. Kaneko, I. Namura, Appl. Phys. Lett. 58 (1991) 1822. [11] T. Fukushima, H. Shimizu, K. Nishikata, M. Irikawa, Appl. Phys. Lett. 66 (1995) 2025. [12] M.A. Reshchikov, G.-C. Yi, B.W. Wessels, Phys. Rev. B 59 (1999) 13176. [13] Y. Cho, G.H. Gainer, A.F. Fischer, J.J. Song, S. Keller, U.K. Mishra, S.P. DeenBaar, Appl. Phys. Lett. 73 (1998) 1370.
Journal of Crystal Growth 278 (2005) 421–425 www.elsevier.com/locate/jcrysgro
Study of electroluminescence quenching in the InGaN/GaN blue diode with multi-quantum barrier structure Ray-Ming Lina,, Chung-Han Lina, Jen-Cheng Wanga, Tzer-En Neea, Bor-Ren Fangb, Ruey-Yu Wangb a
Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan, ROC b Nan Ya Photonics Incorporation, Taipei, Taiwan, ROC Available online 10 February 2005
Abstract This work has demonstrated electroluminescence (EL) quenching of the InGaN/GaN quantum well (QW) diodes with multi-quantum barrier (MQB) structure at several temperatures from 20 to 300 K. Both high- and low-energy bands indicate a surprising tendency when the temperature and injection current are altered. Increasing temperature reduces the high-energy band and clearly increases the quantum efficiency of the low-energy band. The device with MQB improves the quantum efficiency throughout the whole temperature range and displays good agreement with the EL-integrated intensity in this study. The sample with an optimal MQB structure could significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with the sample without the MQB structure. r 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 81.05.Ea; 81.15.Gh Keywords: A3. Multi-quantum wells; A3. Multi-quantum barrier; B1. InGaN; B3. Light emitting diodes
1. Introduction Super-bright blue light emitting diode (LED) using III-nitride semiconductor quantum structures have been successfully manufactured [1] and have wide applications in full-color display, Corresponding author. Tel.: +886 3 2118800x5790;
fax: +886 3 2118507. E-mail address: [email protected] (R.-M. Lin).
information storage and communication. Previous works have reported that the localized radiative centers which may originate from In-rich regions may be responsible for this phenomenon by using optical and electrical measurements [2–4]. Some investigations have found that InGaN quantum well (QW) LEDs have an unusual temperaturedependent electroluminescence (EL) characteristic and furthermore, have found an additional higher energy band (400 nm) at low temperature, which
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.01.012
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
can be assigned as Mg-related emission [5,6]. Although temperature- and current-dependent EL with various sample parameters have been discussed, the detailed mechanisms remain controversial. Furthermore, the temperature- and current-dependent characteristics of the additional higher energy band are still not well investigated. In this article, we present temperature- and injection current-dependent measurements of multi-quantum well (MQW) blue LEDs with and without multi-quantum barrier (MQB) structures, and discretely analyze them over a wide temperature range. The MQB was calculated [7,8] and is a useful structure for elevating the performance of the red and infrared LEDs and LDs [9–11]. In contrast to the previously reported photoluminescence (PL) result that the reduction of the nonradiative effect in the sample will enhance the PL intensity, the substantial reduction of the capture rate of the MQW will reduce the EL intensity and lead to higher energy band emission at low temperature. The device containing MQBs with appropriate indium composition is found to exhibit not only depressed higher energy band but also improved order of the quantum efficiency at 20 K. Additionally, the correlation between the EL peak energy and the dynamic of the carrier capture and recombination was discussed.
2. Experiment The LED samples investigated here were grown on c-plane sapphire via metal organic chemical vapor deposition (MOCVD). The samples consist of 20-nm-thick low-temperature GaN nucleation layer, a 3 mm Si-doped n-type GaN, and an unintentionally doped active layer with InxGa1 xN/GaN MQWs (0:15oxo0:18) and Mgdoped p-type GaN. The doping level of n- and ptype GaN are nominally 5 1018 and 1 1019 cm 3, respectively. The InxGa1 xN/ GaN MQW layers comprise five periods InxGa1 xN wells (2 nm) and GaN (10 nm) for sample 1 (sign S1). For samples 2, 3 and 4 (sign S2, S3, S4), the wells are the same as for S1, but barriers comprise five pairs InxGa1 xN/GaN rather than the conventional barrier and named
MQB. The MQB effective composition of InxGa1 x with the x values are 0.005, 0.01 and 0.02 for S2, S3 and S4, respectively. For temperaturedependent EL measurements, the samples mounted in a closed-cycle He cryostate were carried out using Keithley 2430 as a current source over a wide temperature range (20–300 K). The luminescence signal dispersed using a 0.5-m monochromator was detected by a Si photodiode employing a standard lock-in technique.
3. Results and discussion Fig. 1 illustrates the EL spectra of the all samples at 20 K with current operated at 20 mA. A leading blue MQW emission band is observed at nearly 440 nm (2.8 eV) with fine structures owing to the Fabry–Perot fringes. Furthermore, an additional band exists at 400 nm (3.1 eV) and is tentatively attributed to the Mg-relative transition [5,12]. The two peaks of 400 and 440 nm are named the high- and low-energy bands in this work. The temperature-dependent integrated intensity of the high-energy band (400 nm) and the low-energy band (440 nm) at a fixed injection level of 20 mA are illustrated in Fig. 2 for all samples. Unlike the generally expected PL tendency of diminished nonradiative recombination decreasing with temperature, the EL intensity of barrier
MQB
S1 S2 S3 S4
Low energy band
1
EL relative intensity (a.u.)
422
QW
QW
p-GaN MQW n-GaN Sapphire
High energy band
0.1
0.01 340
360
380
400
420
440
460
480
500
520
Wavelength (nm)
Fig. 1. EL spectra of InGaN/GaN MQW LEDs with and without MQB structures measured at 20 K and 20 mA injection current.
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425 6
1
4
Low energy band S1 S2 S3 S4 I = 20 mA
0.1
0.01 1.2
×4
High energy band S1 S2 S3 S4 I = 20 mA
1.0 0.8 0.6 × 24
0.4 0.2
λ (T) - λ (20 K) (nm)
Normalized EL Intensity (a.u.)
423
2
Low energy band S1 S2 S3 S4 I = 20 mA
0
-2
-4
-6 20
40
60
80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 2. Normalized integrated EL intensity of the high- and low-energy bands for devices S1, S2, S3, and S4 measured from 20 to 300 K and 20 mA injection current.
the low-energy band was surprisingly observed to decrease with temperature for all of the present samples. As the temperature reduces below 180 K, a marked reduction in the light intensity is observed. As the temperature further decreases to 20 K, the EL intensity clearly reduces less in the sample with the MQB structure. The improvement of the EL intensity may lead to better carrier confinement in the MQW region, and overflow of fewer carriers into the GaN region where they are either radiatively recombined or nonradiatively extinguished after being trapped by defect [4]. This study also observed a high-energy band (400 nm) that indicates a significantly different temperature-dependent behavior to the low-energy band. The EL intensity of the high-energy band increased as temperature decreased from approximately 140 to 20 K. Based on the different tendency of the high- and low-energy band, this study assumed that the EL intensity and spectra characteristic behavior observed at lower temperature cannot be attributed to the heating effect, since the nonradiative recombination process may be less important at relatively low temperatures. Fig. 3 illustrates the temperature-induced shifts of the low-energy band for the samples operating at 20 mA. As the temperature increases, ‘‘Sshaped’’ behavior is seen in all of the samples except S2. An unusual wavelength shift is observed
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 3. EL peak emission wavelength shift of the InGaN/GaN MQW LEDs with and without MQB structures with various temperatures.
in InGaN/GaN by using PL measurement [13] and can be clarified by carrier relaxation. At low temperature, owing to the increase of the carrier lifetime, carriers have considerable opportunity to relax to the lower-lying state before recombination because of a reduction in the nonradiative process. The analytical result below about 80 K displays good agreement with this explanation. Nevertheless, sample S2 exhibits a blueshift trend rather than a redshift below 80 K. The study interprets this behavior as the efficient carrier capture rate in sample S2, and better EL intensity than S1 can be observed in Figs. 1 and 2. As the temperature is further increased from 80 to 200 K, all experiment samples display a blueshift tendency. Besides, Fig. 2 reveals that the EL intensity of the high-energy band emission exhibits decreases while the lowenergy band increases with increasing temperature. This study hypothesizes that the increase in the carrier capture rate with increasing temperature will not only improve the probability that carriers can be radiatively recombined in the MQW, but also depresses the higher energy band emission. Higher numbers of carriers in the QW region will also produce a band-filling effect, and the emission energy may blueshift, as observed in the present samples. When the temperature further exceeds 200 K, the nonradiative center captures the carriers via thermal enhancing and reduces the
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
carriers in the QW region, while the blueshift of the emission energy reduces. At the same time, the blueshift evolves into a redshift associated with standard temperature-dependent narrowing of the band gap. Fig. 4 reveals a similar EL spectra tendency for the samples with and without MQB, measured as a function of temperature from 20 to 300 K given a current injection of 80 mA. However, the reduction in EL intensity was clearly less at lower temperature for all samples. At higher injection current, the EL quenching rate for all samples is found to be approximately 10 times lower than at low injection current level. This enhancement of the EL intensity at lower temperature with higher injection current may result from the depression of the high-energy band in the present sample (not shown here). The explanation of the present experimental result at higher injection current remains poorly understood. Further work on this area will be conducted in the future. Fig. 5 illustrates the quantum efficiency per area operated with current at 20 mA with a various temperature ranges deduced based on the L–I characteristics. The active area is 3.68 10 4 cm2 for all devices. The quantum efficiency of the devices first nearly stabilizes above 180 K, then decreases significantly with temperature down to 20 K. The results are inconsistent with our experi-
Normalized EL Intensity (a.u.)
1
S1 S2 S3 S4 I = 80 mA
0.1
1
(dL /dI)/A (a.u.)
424
S1 S2 S3 S4 I = 20 mA
0.1
0.01
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (k)
Fig. 5. Normalized quantum efficiency for sample S1, S2, S3 and S4 from 20 to 300 K and 20 mA injection current.
mental observations regarding the temperaturedependent EL intensity illustrated in Fig 1. The EL intensity decreases significantly with increasing higher energy band, i.e. quantum efficiency, of the MQW. The reduction of the high-energy band in the device with the MQB structure then prevents the carrier overflow into the p-GaN region from acting as a ‘‘p–n junction LED’’ and keeps the carrier in the MQW region, giving carriers increased opportunity to recombine in the QW. Furthermore, quantum efficiency is nearly stable at higher temperature, probably due to the increase in the capture rate of the nonradiative center saturating the quantum efficiency. The sample with an optimal MQB structure, i.e. sample S2, will significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with S1. Notably, the higher quantum efficacy of sample S2 also may be caused by the lower nonradiative center in the barrier compared with samples S3 and S4. This may be caused by the higher indium content in the MQB region and form the higher density of the nonradiative centers.
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Temperature (K)
Fig. 4. Normalized EL intensity of the low-energy band for device S1, S2, S3 and S4 at 80 mA with several temperatures from 20 to 300 K.
4. Conclusions To summarize, this study has demonstrated EL measurement with various temperature and
ARTICLE IN PRESS R.-M. Lin et al. / Journal of Crystal Growth 278 (2005) 421–425
injection current parameters in four blue LED devices with and without MQB structures. Both high- and low-energy bands indicate a surprising tendency when the temperature and injection current are altered. Increasing temperature reduces the high-energy band and clearly increases the quantum efficiency of the low-energy band. The device with MQB improves the quantum efficiency throughout the whole temperature range and displays good agreement with the EL-integrated intensity in this study. The sample with an optimal MQB structure could significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with the sample without the MQB structure.
Acknowledgments The authors would like to thank Dr. Nie-Chun Chen for his technical assistance. This work was supported by the National Science Council of the Republic of China under Contract No. NSC 922215-E-182-001.
425
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 69 (1996) 4188. [3] P.G. Eliseev, P. Perlin, J. Lee, M. Osinski, Appl. Phys. Lett. 71 (1997) 569. [4] S.W. Feng, Y.C. Cheng, Y.Y. Chung, C.C. Yang, Y.S. Lin, C. Hsu, K.J. Ma, J.I. Chyi, J. Appl. Phys. 92 (2002) 4441. [5] C.M. Lee, C.C. Chuo, J.F. Dai, X.F. Zheng, J.I. Chyi, J. Appl. Phys. 89 (2001) 6554. [6] A. Hori, D. Yasunaga, A. Satake, K. Fujiwara, J. Appl. Phys. 93 (2003) 3152. [7] K. Iga, H. Uenohara, F. Koyama, Electron. Lett. 22 (1986) 1008. [8] Michinori Irukawa, Takura Ishikawa, Yoshitaka Sasaki, Katsumi Iwasawa, Ikuo Suemune, Kenichi Iga, J. Appl. Phys. 84 (1998) 4667. [9] M.R. Islam, R.V. Chelakara, R.D. Dupuis, Appl. Phys. Lett. 67 (1995) 2057. [10] K. Kishino, A. Kikuchi, Y. Kaneko, I. Namura, Appl. Phys. Lett. 58 (1991) 1822. [11] T. Fukushima, H. Shimizu, K. Nishikata, M. Irikawa, Appl. Phys. Lett. 66 (1995) 2025. [12] M.A. Reshchikov, G.-C. Yi, B.W. Wessels, Phys. Rev. B 59 (1999) 13176. [13] Y. Cho, G.H. Gainer, A.F. Fischer, J.J. Song, S. Keller, U.K. Mishra, S.P. DeenBaar, Appl. Phys. Lett. 73 (1998) 1370.