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GaN multiple quantum well light emitting diodes
Materials Science and Engineering B 138 (2007) 180–183
Influence of AlGaN/GaN superlattice inserted structure on the performance of InGaN/GaN multiple quantum well light emitting diodes Cheng-Liang Wang a , Ming-Chang Tsai b , Jyh-Rong Gong c,∗ , Wei-Tsai Liao a , Ping-Yuan Lin c , Kuo-Yi Yen c , Chia-Chi Chang c , Hsin-Yueh Lin b , Shen-Kwang Hwang b b
a Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan, ROC Institute of Opto-Mechatronics, National Chung Cheng University, Min-Hsiung Chia-Yi 621, Taiwan, ROC c Department of Physics, National Chung Hsing University, Taichung 402, Taiwan, ROC
Received 29 August 2006; received in revised form 17 November 2006; accepted 7 January 2007
Abstract Investigations were conducted to explore the effect of Al0.3 Ga0.7 N/GaN short-period superlattice (SPSL)-inserted structures in the GaN under layer on the performance of In0.2 Ga0.8 N/GaN multiple quantum well (MQW) light emitting diodes (LEDs). The Al0.3 Ga0.7 N/GaN SPSL-inserted LEDs were found to exhibit improved materials and device characteristics including decrements in ideality factor and reverse leakage current. The results of etch pit counts reveal that SPSL-induced threading dislocation density reduction in the SPSL-inserted In0.2 Ga0.8 N/GaN MQW LED structures enables the improved LED performance. © 2007 Elsevier B.V. All rights reserved. Keywords: InGaN/GaN MQW; Light-emitting diode (LED); Short-period superlattice (SPSL)
1. Introduction Recently, GaN-based semiconductors have attracted great interest in several applications including traffic lights, full-color displays, backlights of liquid crystal displays, and solid state lightings [1]. Despite GaN-based light emitting diodes (LEDs) have manifested themselves as the key players for solid state lightings, however, commercially available III-nitride LEDs grown on sapphire substrates usually contain a high density of threading dislocations (TDs). A typical TD density as high as 108 –1011 cm−2 in an epitaxially grown GaN film is due to the poor matching in lattice parameter and thermal expansion coefficient between GaN and sapphire [2]. It is well known that dislocations tend to serve as nonradiative recombination centers in semiconductors, and the reliability of a semiconductor p–n junction is highly sensitive to its dislocation density. With the advances of solid state lightings, high power GaN-based LEDs become important and the influence of TDs remains as an issue of great concern [3].
∗
Corresponding author. Tel.: +886 4 2284 0427x712; fax: +886 4 2285 8583. E-mail address: [email protected] (J.-R. Gong).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.01.005
The use of pseudomorphic III-nitride short-period superlattice (SPSL) structures in the GaN films was previously reported to be efficient in reducing the densities of TDs in the films with improved optical characteristics [4–5]. The effect of SPSL intermediate structures on the properties of GaN-based LEDs is an interesting subject to be explored. With the advent of LED solid state lighting era, lately, there is an increasing demand of high power GaN-based LEDs. Thus, it is of great interest to investigate the influence of SPSL structures on the characteristics of GaN-based LEDs having SPSL intermediate structures. In this paper, we report the effects of AlGaN/GaN SPSLinserted structures on the electrical and optical characteristics of InGaN/GaN MQW LEDs. It was found that SPSL insertion tended to filter TDs inside the LEDs and resulted in improved device characteristics. 2. Experimental procedures In0.2 Ga0.8 N/GaN MQW LED samples having zero-, one-, two-, and three-set five periods Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL structures in the underlying undoped GaN (u-GaN) were grown on (0 0 0 1)sapphire substrates by metalorganic chemical vapor deposition (MOCVD). A typical structure consists of a 1-m thick u-GaN layer which was grown on the sap-
C.-L. Wang et al. / Materials Science and Engineering B 138 (2007) 180–183
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Fig. 1. Semilogarithmic I–V plots of the forward-biased In0.2 Ga0.8 N/GaN MQW LEDs having (a) zero-set, (b) one-set, (c) two-set, and (d) three-set Al0.3 Ga0.7 N/GaN SPSL insertion.
phire substrate followed by a 2.5-m thick n-GaN (GaN:Si) layer and 0.1-m thick five-pair In0.2 Ga0.8 N (2.5 nm)/GaN (17.5 nm) MQWs with a final cap layer of 0.25-m thick p-GaN (GaN:Mg). In this case, the typical In-content in the well regions of the MQWs is ∼20%, and the room temperature (RT) carrier concentrations of n-GaN and p-GaN are n0 ∼ 5 × 1018 cm−3 and p0 ∼ 1 × 1017 cm−3 , respectively. For all the SPSL-inserted LED samples, the five-pair Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL sets were deposited above the 1-m thick u-GaN with a 0.25-m thick u-GaN being sandwiched between two nearest SPSL sets. LED chips having a typical size of 300 m × 300 m was fabricated on the quarter part of a 2-in. epi-wafer by using standard photolithography and dry etch techniques. Conventional Ni/Au and Ti/Al alloys were used to serve as the p-type and n-type contacts, respectively. Current–voltage (I–V) characteristics of the In0.2 Ga0.8 N/GaN MQW LEDs were measured using a Keithley 236 source measurement unit to explore the effect of SPSL-inserted structures on the I–V characteristics and electroluminesence (EL) performance of the LEDs. Wet etch studies were conducted by etching LED in the heated H3 PO4 (160 ◦ C) melt for 30 min. 3. Results and discussion According to the standard Shockley model [6], the I–V relationship of a forward-biased p–n junction can be approximated by qV I = Is exp , ηkT where Is , q, k, η and T, respectively, are saturation current of the diode, electron charge, Boltzmann constant, and ideality factor and absolute temperature of the diode.
Under the logarithmic representation, log I = log Is + 0.434 qV/ηkT, and the ideality factor (η) is inversely proportional to the slope (0.434q/ηkT) of the semilogarithmic I–V plot. It has been reported that the ideality factor (η) of a p–n junction tends to increase when the non-diffusion currents in the p–n junction become important. The high ideality factors (η 2.0) in GaNbased LEDs were attributed to deep-level-assisted tunneling according to the reduced slopes of the semilogarithmic I–V plots of LEDs [7–8]. In this study, typical forward-biased I–V plots of the SPSL-inserted In0.2 Ga0.8 N/GaN MQW LEDs at room temperature are given in Fig. 1a–d. The semilogarithmic I–V plots of the LEDs all show linear dependence of log I versus V over several orders of magnitude of the current injection. The ideality factors for the above-mentioned In0.2 Ga0.8 N/GaN MQW LEDs were evaluated being 3.1, 2.8, 2.6, and 3.2, respectively, for zero-, one-, two-, and three-set SPSL insertion. The decrement of η-value in the In0.2 Ga0.8 N/GaN MQW LED involving twoset (or one-set) SPSL structure implies reduced TD density in the LED due to filtering of TDs by SPSL while the increase of ηvalue in an LED having three-set of SPSL structure is attributed to TD density increment in the LED because of strain relaxation in the SPSL sets. Such an effect was also exhibited by the etch pit density (EPD) counts of the wet-etched LED samples. Fig. 2a–d shows the optical microscopic (OM) micrographs of the surface morphologies of the etched In0.2 Ga0.8 N/GaN MQW LEDs with or without SPSL insertion. An EPD of ∼5.8 × 106 cm−2 for the LED having two-set SPSL insertion is lower than an EPD of ∼8.6 × 106 cm−2 for an LED containing no SPSL. This results clearly exhibit that the insertion of pseudomorphic SPSL inside u-GaN of an In0.2 Ga0.8 N/GaN MQW LED is efficient in the elimination of etch pits in the LED surface.
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Fig. 2. Typical optical surface morphologies of etched In0.2 Ga0.8 N/GaN MQW LEDs having (a) zero-, (b) one-, (c) two-, and (d) three-set Al0.3 Ga0.7 N/GaN SPSL insertion.
Fig. 3 shows typical reverse-biased current–voltage (I–V) plots of the In0.2 Ga0.8 N/GaN MQW LEDs with or without Al0.3 Ga0.7 N/GaN SPSL insertion. Without the insertion of SPSL, the leakage current (IR ) value of the LED was found to increase with the reverse bias and reach 42 A at a reverse bias of −20 V while for an LED inserted with two sets of the five-pair Al0.3 Ga0.7 N/GaN SPSL in u-GaN, a reduced IR value of 19 A was observed at a reverse bias of −20 V. However, the improvement in IR value was not found in the LED inserted with three sets SPSL in u-GaN. It has been reported that the leakage current of a reverse-biased GaN-based LED becomes prominent when the reverse bias increases because of enhanced tunneling probability through the deep-level states which were attributed
Fig. 3. Typical I–V characteristics of the reverse-biased In0.2 Ga0.8 N/GaN MQW LEDs (1) without SPSL insertion, (2) with one set of Al0.3 Ga0.7 N/GaN SPSL insertion, (3) with two sets of Al0.3 Ga0.7 N/GaN SPSL insertion, and (4) with three sets of Al0.3 Ga0.7 N/GaN SPSL insertion, respectively. The inset exhibits plots of the corresponding EL intensity vs. emission wavelength of the two LEDs having no SPSL and two sets of SPSL operated at 20 mA.
to the presence of TDs in the LED [9–10]. Accordingly, the increment of IR value in the In0.2 Ga0.8 N/GaN MQW LED having three sets SPSL is attributed to strain relaxation in the SPSL. As revealed in the inset of Fig. 3, an increment of ∼30% in EL intensity was observed in the GaN-based LED inserted with two sets of Al0.3 Ga0.7 N/GaN SPSL compared to that having no SPSL insertion. Previous report [11] showed that the diffusion length of minority carriers for radiative optical process in III-nitrides was less than 102 nm which is much smaller than the average spacing of 103 nm between two nearest TDs inside an SPSLinserted GaN-base LED. Thus, it is believed that the majority of injected electrons and holes recombine radiatively in spite of the presence of TDs in the LED, and the limited improvement in EL intensity of the Al0.3 Ga0.7 N/GaN SPSL-inserted GaN-based LED is realized. Fig. 4 shows the full-width at half maxima (FWHM) of room temperature EL emissions under 20 mA current injection
Fig. 4. Plots of the ideality factors and FWHM of the EL emissions of the In0.2 Ga0.8 N/GaN MQW LEDs at 20 mA vs. the set numbers of the inserted Al0.3 Ga0.7 N/GaN SPSL.
C.-L. Wang et al. / Materials Science and Engineering B 138 (2007) 180–183
and the ideality factors for In0.2 Ga0.8 N/GaN MQW LEDs with or without the insertion of Al0.3 Ga0.7 N/GaN SPSL. It is evident that the LED having two-set five-pair Al0.3 Ga0.7 N/GaN SPSL insertion exhibits the best performance, which agrees quite well with the improved I–V and structural characteristics of the In0.2 Ga0.8 N/GaN MQW LED. The reduction in TD density of the pseudomorphic Al0.3 Ga0.7 N/GaN SPSL inserted In0.2 Ga0.8 N/GaN MQW LED is considered to enable strong carrier localization and enhanced efficiency of radiative process in the active In0.2 Ga0.8 N/GaN MQWs [12]. 4. Conclusions In summary, we report the improved performance of Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL-inserted In0.2 Ga0.8 N/GaN MQW LEDs. It was found that the insertion of pseudomorphic Al0.3 Ga0.7 N/GaN SPSL result in TD density reduction in an LED, which enable the improvement in the characteristics of the LED. Acknowledgments This work was supported in part by the National Science Council of Taiwan, Republic of China under contract numbers NSC93-2622-E-194-006-CC3 and NSC93-2215-E194-008. The authors are grateful to Dr. Bor-Jen Wu and Mr. Ming-Fa Yeh at Uni Light Technology Inc., Taiwan, Republic of China for their assistance on the epitaxial growth.
183
References [1] P. Waltereit, H. Sato, C. Poblenz, D.S. Green, J.S. Brown, M. McLaurin, T. Katona, S.P. DenBaars, J.S. Speck, J.-H. Liang, M. Kato, H. Tamura, S. Omori, C. Funaoka, Appl. Phys. Lett. 84 (2004) 2748. [2] S.D. Lester, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [3] T. Hino, S. Tomiya, T. Miyajima, K. Yanashima, S. Hashimoto, M. Ikeda, Appl. Phys. Lett. 76 (2000) 3421. [4] S.A. Nikishin, N.N. Faleev, V.G. Antipov, S. Francoeur, L. Grave de Peralta, G.A. Seryogin, T. Temkin, T.I. Prokofyeva, M. Holtz, S.N.G. Chu, Appl. Phys. Lett. 75 (1999) 2073. [5] H.M. Wang, J.P. Zhang, C.Q. Chen, J.W. Yang, M.A. Khan, Appl. Phys. Lett. 81 (2002) 604. [6] F. Capasso, G. Margaritondo, Heterojunction Band Discontinuities: Physics and Device Applications, North-Holland, Netherlands, 1987. [7] J.M. Shah, Y.-L. Li, Th. Gessmann, E.F. Schubert, J. Appl. Phys. 94 (2003) 2627. [8] X.A. Cao, E.B. Stokes, P.M. Sandvik, S.F. Veboeuf, J. Kretchmer, D. Walker, IEEE Electron. Dev. Lett. 23 (2002) 535. [9] H.C. Casey Jr., J. Muth, S. Krishnankutty, J.M. Zavada, Appl. Phys. Lett. 68 (1996) 2867. [10] J.W.P. Hsu, M.J. Manfra, D.V. Lang, S. Richter, S.N.G. Chu, A.M. Sergent, R.N. Kleiman, L.N. Pfeiffer, Appl. Phys. Lett. 78 (2001) 1685. [11] J.C. Gonzalez, K.L. Bunker, P.E. Russell, Appl. Phys. Lett. 79 (2001) 1567. [12] S.M. Sze, Semiconductor Devices: Physics and Technology, Wiley, New York, 1985, p. 95.
Influence of AlGaN/GaN superlattice inserted structure on the performance of InGaN/GaN multiple quantum well light emitting diodes Cheng-Liang Wang a , Ming-Chang Tsai b , Jyh-Rong Gong c,∗ , Wei-Tsai Liao a , Ping-Yuan Lin c , Kuo-Yi Yen c , Chia-Chi Chang c , Hsin-Yueh Lin b , Shen-Kwang Hwang b b
a Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan, ROC Institute of Opto-Mechatronics, National Chung Cheng University, Min-Hsiung Chia-Yi 621, Taiwan, ROC c Department of Physics, National Chung Hsing University, Taichung 402, Taiwan, ROC
Received 29 August 2006; received in revised form 17 November 2006; accepted 7 January 2007
Abstract Investigations were conducted to explore the effect of Al0.3 Ga0.7 N/GaN short-period superlattice (SPSL)-inserted structures in the GaN under layer on the performance of In0.2 Ga0.8 N/GaN multiple quantum well (MQW) light emitting diodes (LEDs). The Al0.3 Ga0.7 N/GaN SPSL-inserted LEDs were found to exhibit improved materials and device characteristics including decrements in ideality factor and reverse leakage current. The results of etch pit counts reveal that SPSL-induced threading dislocation density reduction in the SPSL-inserted In0.2 Ga0.8 N/GaN MQW LED structures enables the improved LED performance. © 2007 Elsevier B.V. All rights reserved. Keywords: InGaN/GaN MQW; Light-emitting diode (LED); Short-period superlattice (SPSL)
1. Introduction Recently, GaN-based semiconductors have attracted great interest in several applications including traffic lights, full-color displays, backlights of liquid crystal displays, and solid state lightings [1]. Despite GaN-based light emitting diodes (LEDs) have manifested themselves as the key players for solid state lightings, however, commercially available III-nitride LEDs grown on sapphire substrates usually contain a high density of threading dislocations (TDs). A typical TD density as high as 108 –1011 cm−2 in an epitaxially grown GaN film is due to the poor matching in lattice parameter and thermal expansion coefficient between GaN and sapphire [2]. It is well known that dislocations tend to serve as nonradiative recombination centers in semiconductors, and the reliability of a semiconductor p–n junction is highly sensitive to its dislocation density. With the advances of solid state lightings, high power GaN-based LEDs become important and the influence of TDs remains as an issue of great concern [3].
∗
Corresponding author. Tel.: +886 4 2284 0427x712; fax: +886 4 2285 8583. E-mail address: [email protected] (J.-R. Gong).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.01.005
The use of pseudomorphic III-nitride short-period superlattice (SPSL) structures in the GaN films was previously reported to be efficient in reducing the densities of TDs in the films with improved optical characteristics [4–5]. The effect of SPSL intermediate structures on the properties of GaN-based LEDs is an interesting subject to be explored. With the advent of LED solid state lighting era, lately, there is an increasing demand of high power GaN-based LEDs. Thus, it is of great interest to investigate the influence of SPSL structures on the characteristics of GaN-based LEDs having SPSL intermediate structures. In this paper, we report the effects of AlGaN/GaN SPSLinserted structures on the electrical and optical characteristics of InGaN/GaN MQW LEDs. It was found that SPSL insertion tended to filter TDs inside the LEDs and resulted in improved device characteristics. 2. Experimental procedures In0.2 Ga0.8 N/GaN MQW LED samples having zero-, one-, two-, and three-set five periods Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL structures in the underlying undoped GaN (u-GaN) were grown on (0 0 0 1)sapphire substrates by metalorganic chemical vapor deposition (MOCVD). A typical structure consists of a 1-m thick u-GaN layer which was grown on the sap-
C.-L. Wang et al. / Materials Science and Engineering B 138 (2007) 180–183
181
Fig. 1. Semilogarithmic I–V plots of the forward-biased In0.2 Ga0.8 N/GaN MQW LEDs having (a) zero-set, (b) one-set, (c) two-set, and (d) three-set Al0.3 Ga0.7 N/GaN SPSL insertion.
phire substrate followed by a 2.5-m thick n-GaN (GaN:Si) layer and 0.1-m thick five-pair In0.2 Ga0.8 N (2.5 nm)/GaN (17.5 nm) MQWs with a final cap layer of 0.25-m thick p-GaN (GaN:Mg). In this case, the typical In-content in the well regions of the MQWs is ∼20%, and the room temperature (RT) carrier concentrations of n-GaN and p-GaN are n0 ∼ 5 × 1018 cm−3 and p0 ∼ 1 × 1017 cm−3 , respectively. For all the SPSL-inserted LED samples, the five-pair Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL sets were deposited above the 1-m thick u-GaN with a 0.25-m thick u-GaN being sandwiched between two nearest SPSL sets. LED chips having a typical size of 300 m × 300 m was fabricated on the quarter part of a 2-in. epi-wafer by using standard photolithography and dry etch techniques. Conventional Ni/Au and Ti/Al alloys were used to serve as the p-type and n-type contacts, respectively. Current–voltage (I–V) characteristics of the In0.2 Ga0.8 N/GaN MQW LEDs were measured using a Keithley 236 source measurement unit to explore the effect of SPSL-inserted structures on the I–V characteristics and electroluminesence (EL) performance of the LEDs. Wet etch studies were conducted by etching LED in the heated H3 PO4 (160 ◦ C) melt for 30 min. 3. Results and discussion According to the standard Shockley model [6], the I–V relationship of a forward-biased p–n junction can be approximated by qV I = Is exp , ηkT where Is , q, k, η and T, respectively, are saturation current of the diode, electron charge, Boltzmann constant, and ideality factor and absolute temperature of the diode.
Under the logarithmic representation, log I = log Is + 0.434 qV/ηkT, and the ideality factor (η) is inversely proportional to the slope (0.434q/ηkT) of the semilogarithmic I–V plot. It has been reported that the ideality factor (η) of a p–n junction tends to increase when the non-diffusion currents in the p–n junction become important. The high ideality factors (η 2.0) in GaNbased LEDs were attributed to deep-level-assisted tunneling according to the reduced slopes of the semilogarithmic I–V plots of LEDs [7–8]. In this study, typical forward-biased I–V plots of the SPSL-inserted In0.2 Ga0.8 N/GaN MQW LEDs at room temperature are given in Fig. 1a–d. The semilogarithmic I–V plots of the LEDs all show linear dependence of log I versus V over several orders of magnitude of the current injection. The ideality factors for the above-mentioned In0.2 Ga0.8 N/GaN MQW LEDs were evaluated being 3.1, 2.8, 2.6, and 3.2, respectively, for zero-, one-, two-, and three-set SPSL insertion. The decrement of η-value in the In0.2 Ga0.8 N/GaN MQW LED involving twoset (or one-set) SPSL structure implies reduced TD density in the LED due to filtering of TDs by SPSL while the increase of ηvalue in an LED having three-set of SPSL structure is attributed to TD density increment in the LED because of strain relaxation in the SPSL sets. Such an effect was also exhibited by the etch pit density (EPD) counts of the wet-etched LED samples. Fig. 2a–d shows the optical microscopic (OM) micrographs of the surface morphologies of the etched In0.2 Ga0.8 N/GaN MQW LEDs with or without SPSL insertion. An EPD of ∼5.8 × 106 cm−2 for the LED having two-set SPSL insertion is lower than an EPD of ∼8.6 × 106 cm−2 for an LED containing no SPSL. This results clearly exhibit that the insertion of pseudomorphic SPSL inside u-GaN of an In0.2 Ga0.8 N/GaN MQW LED is efficient in the elimination of etch pits in the LED surface.
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Fig. 2. Typical optical surface morphologies of etched In0.2 Ga0.8 N/GaN MQW LEDs having (a) zero-, (b) one-, (c) two-, and (d) three-set Al0.3 Ga0.7 N/GaN SPSL insertion.
Fig. 3 shows typical reverse-biased current–voltage (I–V) plots of the In0.2 Ga0.8 N/GaN MQW LEDs with or without Al0.3 Ga0.7 N/GaN SPSL insertion. Without the insertion of SPSL, the leakage current (IR ) value of the LED was found to increase with the reverse bias and reach 42 A at a reverse bias of −20 V while for an LED inserted with two sets of the five-pair Al0.3 Ga0.7 N/GaN SPSL in u-GaN, a reduced IR value of 19 A was observed at a reverse bias of −20 V. However, the improvement in IR value was not found in the LED inserted with three sets SPSL in u-GaN. It has been reported that the leakage current of a reverse-biased GaN-based LED becomes prominent when the reverse bias increases because of enhanced tunneling probability through the deep-level states which were attributed
Fig. 3. Typical I–V characteristics of the reverse-biased In0.2 Ga0.8 N/GaN MQW LEDs (1) without SPSL insertion, (2) with one set of Al0.3 Ga0.7 N/GaN SPSL insertion, (3) with two sets of Al0.3 Ga0.7 N/GaN SPSL insertion, and (4) with three sets of Al0.3 Ga0.7 N/GaN SPSL insertion, respectively. The inset exhibits plots of the corresponding EL intensity vs. emission wavelength of the two LEDs having no SPSL and two sets of SPSL operated at 20 mA.
to the presence of TDs in the LED [9–10]. Accordingly, the increment of IR value in the In0.2 Ga0.8 N/GaN MQW LED having three sets SPSL is attributed to strain relaxation in the SPSL. As revealed in the inset of Fig. 3, an increment of ∼30% in EL intensity was observed in the GaN-based LED inserted with two sets of Al0.3 Ga0.7 N/GaN SPSL compared to that having no SPSL insertion. Previous report [11] showed that the diffusion length of minority carriers for radiative optical process in III-nitrides was less than 102 nm which is much smaller than the average spacing of 103 nm between two nearest TDs inside an SPSLinserted GaN-base LED. Thus, it is believed that the majority of injected electrons and holes recombine radiatively in spite of the presence of TDs in the LED, and the limited improvement in EL intensity of the Al0.3 Ga0.7 N/GaN SPSL-inserted GaN-based LED is realized. Fig. 4 shows the full-width at half maxima (FWHM) of room temperature EL emissions under 20 mA current injection
Fig. 4. Plots of the ideality factors and FWHM of the EL emissions of the In0.2 Ga0.8 N/GaN MQW LEDs at 20 mA vs. the set numbers of the inserted Al0.3 Ga0.7 N/GaN SPSL.
C.-L. Wang et al. / Materials Science and Engineering B 138 (2007) 180–183
and the ideality factors for In0.2 Ga0.8 N/GaN MQW LEDs with or without the insertion of Al0.3 Ga0.7 N/GaN SPSL. It is evident that the LED having two-set five-pair Al0.3 Ga0.7 N/GaN SPSL insertion exhibits the best performance, which agrees quite well with the improved I–V and structural characteristics of the In0.2 Ga0.8 N/GaN MQW LED. The reduction in TD density of the pseudomorphic Al0.3 Ga0.7 N/GaN SPSL inserted In0.2 Ga0.8 N/GaN MQW LED is considered to enable strong carrier localization and enhanced efficiency of radiative process in the active In0.2 Ga0.8 N/GaN MQWs [12]. 4. Conclusions In summary, we report the improved performance of Al0.3 Ga0.7 N (2 nm)/GaN (2 nm) SPSL-inserted In0.2 Ga0.8 N/GaN MQW LEDs. It was found that the insertion of pseudomorphic Al0.3 Ga0.7 N/GaN SPSL result in TD density reduction in an LED, which enable the improvement in the characteristics of the LED. Acknowledgments This work was supported in part by the National Science Council of Taiwan, Republic of China under contract numbers NSC93-2622-E-194-006-CC3 and NSC93-2215-E194-008. The authors are grateful to Dr. Bor-Jen Wu and Mr. Ming-Fa Yeh at Uni Light Technology Inc., Taiwan, Republic of China for their assistance on the epitaxial growth.
183
References [1] P. Waltereit, H. Sato, C. Poblenz, D.S. Green, J.S. Brown, M. McLaurin, T. Katona, S.P. DenBaars, J.S. Speck, J.-H. Liang, M. Kato, H. Tamura, S. Omori, C. Funaoka, Appl. Phys. Lett. 84 (2004) 2748. [2] S.D. Lester, F.A. Ponce, M.G. Craford, D.A. Steigerwald, Appl. Phys. Lett. 66 (1995) 1249. [3] T. Hino, S. Tomiya, T. Miyajima, K. Yanashima, S. Hashimoto, M. Ikeda, Appl. Phys. Lett. 76 (2000) 3421. [4] S.A. Nikishin, N.N. Faleev, V.G. Antipov, S. Francoeur, L. Grave de Peralta, G.A. Seryogin, T. Temkin, T.I. Prokofyeva, M. Holtz, S.N.G. Chu, Appl. Phys. Lett. 75 (1999) 2073. [5] H.M. Wang, J.P. Zhang, C.Q. Chen, J.W. Yang, M.A. Khan, Appl. Phys. Lett. 81 (2002) 604. [6] F. Capasso, G. Margaritondo, Heterojunction Band Discontinuities: Physics and Device Applications, North-Holland, Netherlands, 1987. [7] J.M. Shah, Y.-L. Li, Th. Gessmann, E.F. Schubert, J. Appl. Phys. 94 (2003) 2627. [8] X.A. Cao, E.B. Stokes, P.M. Sandvik, S.F. Veboeuf, J. Kretchmer, D. Walker, IEEE Electron. Dev. Lett. 23 (2002) 535. [9] H.C. Casey Jr., J. Muth, S. Krishnankutty, J.M. Zavada, Appl. Phys. Lett. 68 (1996) 2867. [10] J.W.P. Hsu, M.J. Manfra, D.V. Lang, S. Richter, S.N.G. Chu, A.M. Sergent, R.N. Kleiman, L.N. Pfeiffer, Appl. Phys. Lett. 78 (2001) 1685. [11] J.C. Gonzalez, K.L. Bunker, P.E. Russell, Appl. Phys. Lett. 79 (2001) 1567. [12] S.M. Sze, Semiconductor Devices: Physics and Technology, Wiley, New York, 1985, p. 95.