- Email: [email protected]
GaN multi-quantum well light-emitting diodes
Thin Solid Films 518 (2010) 7437–7440
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Thermal effect of multi-quantum barriers within InGaN/GaN multi-quantum well light-emitting diodes Jiunn-Chyi Lee a, Ya-Fen Wu b,⁎ a b
Department of Electrical Engineering, Technology and Science Institute of Northern Taiwan, Taipei 112, Taiwan Department of Electronic Engineering, Ming Chi University of Technology, Taishan 243, Taiwan
a r t i c l e
i n f o
Available online 10 May 2010 Keywords: InGaN/GaN multi-quantum well Electroluminescence Multi-quantum barrier External quantum efficiency
a b s t r a c t We introduce the InGaN/GaN multi-quantum barriers (MQBs) into InGaN/GaN multi-quantum well (MQW) heterostructures to improve the performance of light-emitting diodes. The temperature and injection current dependent electroluminescence were carried out to study the thermal effect of InGaN/GaN MQWs. We observe the enhancement of carrier confinement in the active layer and the inhibited carrier leakage over the barrier for the sample with MQBs. In addition, the external quantum efficiency of the samples is obtained. It is found that the radiative efficiency of the sample possessing MQBs exhibits less sensitive temperature dependence and leads to an improved efficiency in the high temperature and high injection current range. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growth of GaN-based semiconductors and their heterostructures has been an important issue because of the broad applications to the fabrication of light-emitting diodes (LEDs), laser diodes (LDs), solar cells, and photodetectors, which emit over a broad spectral range from infrared to ultraviolet [1–3]. Much interest has been focused on InGaN/ GaN multiple-quantum wells (MQWs) because of their acting as the active layer in high brightness III-nitride LED and cw blue/green LDs [1,4]. Because of large lattice and thermal expansion coefficient mismatches between the sapphire substrate and the epitaxial layers, it results in the localized states or quantum dots in the InGaN/GaN MQWs due to the presence of indium-rich fluctuations. The emission mechanism that occurs in the InGaN-based light-emitting diodes has been attributed mainly to the localized exciton emission. However, the InGaN-based heterostructures display misfit dislocation in nanocrystalline structures, which lead to nonradiative recombination centers. The performance of such devices is limited by the thermally activated loss of electrons overflowing from the active layer to the nonradiative recombination centers, thus reducing the emission efficiency. A carrier blocking effect can be achieved by an AlGaN thin film layer in the multiquantum wells (MQWs) region. It has been used in high power UV/blue/ green LEDs to achieve high device efficiency [5,6]. An alternative approach is to use multi-quantum barrier (MQB) structures to inhibit hot carrier overflow [7]. In this study, we introduce the structure of MQBs into the InGaN/GaN MQW LEDs to improve the performance of the devices. The MQW
⁎ Corresponding author. Tel.: +886 2 29089899; fax: +886 2 29085247. E-mail address: [email protected] (Y.-F. Wu). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.016
samples with and without MQBs were prepared by metal-organic vapor phase epitaxy system. The electroluminescence (EL) measurements were carried out over a temperature range from 20 to 300 K and an injection current level from 10 to 100 mA. With increasing current, the sample without MQB exhibits considerable blueshift of the peak energy at low temperatures and redshift of the peak energy in the high temperature range. A similar thermal action is conducted for the sample with MQBs, but the thermal-induced variety of peak energy is less evident and the turning temperature is higher. Furthermore, the external quantum efficiency of the samples is also calculated from the EL spectra. It is found that the efficiency is enhanced and exhibits less sensitive temperature dependence, resulting from the increase of effective barrier heights due to MQBs. It is reasonable to conclude that the well-designed MQBs improve the performance of LED significantly. 2. Experimental details The structures of the samples investigated in this study are schematically represented in Fig. 1. Both samples were grown by metal-organic vapor phase epitaxy (MOVPE) on c-plane sapphire substrates. Ammonia (NH3), trimethylgallium (TMG), trimethylindium (TMI), silane (SiH4), and bicyclopentadienyl magnesium (Cp2Mg) were used either as precursors or as dopants. The layer structure of the sample without MQBs consisted of a 20-nm-thick GaN buffer layer, a 3-μm-thick Si-doped n-type GaN layer, an undoped GaN layer possessing five periods of In0.18Ga0.82N/GaN MQWs, and a 100-nm-thick Mg-doped ptype GaN layer. The doping concentrations of n- and p-type GaN were nominally about 5 × 1018 cm− 3 and 1 × 1019 cm− 3, respectively. The thicknesses of the InGaN wells and the GaN barriers in the MQW structures were 2 nm and 11 nm, respectively. For the sample with MQBs, the layer structure was similar, but the MQWs were five periods
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Fig. 1. Schematic representation of the InGaN/GaN multiple-quantum well lightemitting diodes without and with MQBs.
interference fringes seen in the figure are due to the interface reflections. The output powers of the samples without and with MQBs are 1.76 and 2.53 mW, respectively. The MQB structure shown in Fig. 1 (b) is engineered to align a forbidden energy mini-band of the MQB superlattice with the conduction band minimum to enhance the conduction band discontinuity. By using the transfer matrix method to solve the Schrödinger equation, one can obtain the reflectivity probability against incident electron energy. The result illustrates that a band of non-allowed electron states can be created above the classical barrier height [8]. This effectively enhances the reflectivity of electrons incident on the barrier and hence increases the band discontinuity. Thus the higher EL spectra intensity observed for the sample with MQBs in Fig. 2 could be attributed to the improvement of carrier confinement and inhibited carrier leakage in the active layer. Besides, the higher EL peak energy of this sample also gives the evidence of the increased conduction band discontinuity. The temperature dependent EL peak energy of the sample without MQBs is shown in Fig. 3(a). Referring to the curve that measured at 10 mA, it can be seen that the temperature dependence of the peak energy does not follow the Varshni law. When the temperature is low, the carriers gain sufficient thermal energy to overcome small potential barriers and relax down into lower energy tail states caused by the inhomogeneous potential fluctuations where finally the radiative recombination takes place. In the mid-temperature range,
of In0.15Ga0.85N/GaN MQWs and the barrier layer was replaced by a fiveperiod In0.005Ga0.995N/GaN (1 nm/1 nm) heterolayers. Both the LED chips were fabricated over a broad area (350 × 350 μm2) using standard photolithography processes. Ti/Al/Ti/Au and Ni–Au were employed for the n- and p-type electrodes, respectively. The temperature dependent EL measurements were carried out by mounting the sample on a Cu cold stage where the temperature (T) was varied from 20 to 300 K and using a current source operated from 10 to 100 mA. The luminescence signal was dispersed through a 0.5 m monochromator and detected using a Si photodiode by standard lock-in amplification technique. 3. Results and discussion Fig. 2 shows the normalized EL spectra of the samples at temperature 300 K and injection current level 20 mA. We observed InGaN-related main emissions with peak energies 2.68 and 2.93 eV for samples without and with MQBs, respectively. The Fabry–Perot
Fig. 2. Electroluminescence spectra of InGaN/GaN MQW samples without and with MQBs at temperature 300 K. The driving current is 20 mA.
Fig. 3. Temperature dependent EL spectra peak position from the InGaN/GaN MQW samples (a) without MQBs and (b) with MQBs. The solid lines are fitting curves based on the band-tail model with fitting parameters listed in Table 1.
J.-C. Lee, Y.-F. Wu / Thin Solid Films 518 (2010) 7437–7440
7439
the blueshift of the peak energy is a result of the thermal population of higher energy states of the density of states. Here a transition from the localized states in the quantum well takes place. As the temperature is even higher, the blueshift behavior is smaller than the temperature induced band-gap shrinkage, thus the peak position exhibits a redshift behavior. A similar thermal action is conducted for the sample with MQBs, as shown in Fig. 3(b). Observing the experimental results measured at 10 mA, the amounts of variety with temperature are less than those of the sample without MQBs, exhibiting a less temperature-sensitive characteristic of it. The “S-shaped” temperature dependence of the EL peak energy is the fingerprint of the existence of localized states. The degree of localization effect in nitride-based devices could be obtained by the band-filling model for tails of the density of states [9–11]: EðT Þ = Em ð0Þ−
αT 2 σ2 − ; T + β kB T
ð1Þ
where E(T) is the emission energy at temperature T; Em(0) is the energy gap at zero temperature; and α and β are Varshini's fitting parameters. The third term describes the localization effect, in which σ is related with localization effect, and kB is the Boltzmann constant. The value σ gives an estimate of the degree of localization effect; i.e., if the value is larger, the localization effect is stronger. The fitting curves made based on Eq. (1) are also shown in Fig. 3 with the parameters listed in Table 1. Through fitting, the values of σ obtained at injection current level 10 mA for samples without and with MQBs are 34.2 and 37.9 meV, respectively. Since the excitons localized in fluctuation minima prevent them from reaching nonradiative recombination sites, it is thought to be the key to the high luminescence efficiencies observed in InGaN-based LEDs [12], the stronger localization effect of the sample with MQBs implies higher emission intensity. With increased driving-current level, the temperature behavior of the EL spectra undergoes a noticeable alteration. The S-shaped dependence of peak position gradually disappears. It is noticeable that the band energy blueshifts and redshifts at low and high temperatures, respectively; which we attribute to the band-filling effect and the heating effect of the localized energy states. Fig. 4 depicts more details of the driving-current dependent peak energy. For the sample without MQBs shown in Fig. 4(a), considerable blueshift of the luminescence peak with increasing current is visible at low temperatures due to the localized states filling. At somewhat elevated temperature, the peak position is almost independent on the injection currents. In the high temperature range, the increasing currents even lead to a redshift of peak energy. This behavior is explained by the reduced occupation of localized states with increasing temperatures [13]. Thus the localization effect will be cancelled by thermal process at sufficiently high temperatures, yielding a current-independent emission peak. Besides, the heating effect should be taken into account at high injection current level, further enhancing the thermal process. It is reasonable to infer that the localization effect is compensated and overcome by the redshift of peak energy due to the temperature shrinkage of band-gap energy. The blueshift of peak energy shown in Fig. 4(b) for sample with MQBs occurs at a higher temperature, which we attribute to the insensitivity
Table 1 Fitting parameters used in the band-tail model for InGaN/GaN MQW samples without and with MQBs at various current levels. Injection current (mA)
Sample without MQBs
Sample with MQBs
α (meV/K)
β (K)
σ (meV)
α (meV/K)
β (K)
σ (meV)
10 60 100
1 0.63 0.24
1196 1000 350
34.2 19.7 6.8
0.9 0.82 0.46
680 600 200
37.9 28.1 20
Fig. 4. The dependence of the spectral peak position on injection current for the InGaN/ GaN MQWs (a) without MQBs and (b) with MQBs at different temperatures between 20 and 300 K.
to temperature of this sample and the resultant unobvious heating effect. Moreover, the turning temperature is also higher than that of the sample without MQBs. Owing to the band of non-allowed electron states are created above the classical barrier height by MQB structure; it effectively increases the band discontinuity. Hence it is understandable that the sample with MQBs exhibits a higher turning temperature. The effect of MQB structures on the performance of our samples is further investigated by discussing the EL efficiency of the samples. One of the major problems of InGaN/GaN LEDs is the suppression of carrier overflow from the active layer into the p-layer. Because the MQB structure effectively increases the band discontinuity, it is expected to prevent the electrons from overflowing as operated at high temperatures and high driving currents. The detailed variations of the EL quantum efficiency are plotted in Fig. 5 as a function of injection currents under various temperatures for both of the samples. It is clearly seen that the efficiency of the sample with MQBs is greatly improved, especially when working at high temperatures and high current levels. As the injection current is increasing, the EL efficiency increases at low temperatures and decays as the temperature is high. In the low temperature regime, the exciton localization effect is the dominant mechanism [14], the lager σ of sample with MQBs resulting to the better EL efficiency. As the temperature is high enough, the nonradiative recombination of carriers becomes thermally activated; the EL efficiency saturates and then decreases. As shown in Fig. 5, the EL efficiency of the sample with MQBs exhibits less decrement of EL efficiency in the high temperature and high injection current range. It
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4. Conclusions In this article, we investigate the performance of InGaN/GaN MQW LEDs with and without MQBs by the measurements of EL spectra over a broad range of temperature and driving current. From the experimental results, the sample with MQBs exhibits an enhancement of carrier confinement in the active layer and a less temperaturesensitive characteristic, coincides with the increase of effective barrier heights due to the quantum interference of the electrons within MQBs. Moreover, from the evaluation of injection current dependent EL external quantum efficiency in the temperature range from 20 to 300 K for the samples, it is concluded that the EL efficiency is greatly improved by the MQB structures, especially as operating at high temperatures and high injection currents. Acknowledgement This work is supported by the National Science Council of the Republic of China under NSC 98-2112-M-131-001-MY2. References
Fig. 5. EL external quantum efficiency of InGaN/GaN MQW samples (a) without MQBs and (b) with MQBs as a function of injection currents in the temperature range from 20 to 300 K.
is concluded that the MQB structure improves the performance of InGaN/GaN MQW LEDs, especially as working at high temperatures and high injection currents.
[1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [3] C.H. Chen, I. Jin, S.P. Pai, Z.W. Dong, C.J. Lobb, T. Venkatesan, K. Edinger, J. Orloff, J. Melngailis, Appl. Phys. Lett. 73 (1998) 1730. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 2014. [5] S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, J.K. Sheu, T.C. Wen, W.C. Lai, J.F. Chen, J.M. Tsai, IEEE J. Sel. Top. Quantum Electron. 8 (2002) 744. [6] C. Huh, S.J. Park, Electrochem. Solid-State Lett. 7 (2004) G266. [7] K. Iga, H. Uenohara, F. Koyama, Electron. Lett. 22 (1986) 1008. [8] R. Tsu, L. Esaki, Appl. Phys. Lett. 22 (1973) 562. [9] P.G. Eliseev, P. Perlin, J. Lee, M. Osiński, Appl. Phys. Lett. 71 (1997) 569. [10] Q. Li, S.J. Xu, W.C. Cheng, M.H. Xie, S.Y. Tong, C.M. Che, H. Yang, Appl. Phys. Lett. 79 (2001) 1810. [11] J. Lee, P.G. Eliseev, M. Osiński, D. Lee, D.I. Florescu, S. Guo, M. Pophristić, IEEE J. Quantum Electron. 9 (2003) 1239. [12] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 69 (1996) 4188. [13] S. Dhar, U. Jahn, O. Brandt, P. Waltereit, K.H. Ploog, Appl. Phys. Lett. 81 (2002) 673. [14] A. Hori, D. Tasunaga, A. Satake, K. Fujiwara, J. Appl. Phys. 93 (2003) 3152.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Thermal effect of multi-quantum barriers within InGaN/GaN multi-quantum well light-emitting diodes Jiunn-Chyi Lee a, Ya-Fen Wu b,⁎ a b
Department of Electrical Engineering, Technology and Science Institute of Northern Taiwan, Taipei 112, Taiwan Department of Electronic Engineering, Ming Chi University of Technology, Taishan 243, Taiwan
a r t i c l e
i n f o
Available online 10 May 2010 Keywords: InGaN/GaN multi-quantum well Electroluminescence Multi-quantum barrier External quantum efficiency
a b s t r a c t We introduce the InGaN/GaN multi-quantum barriers (MQBs) into InGaN/GaN multi-quantum well (MQW) heterostructures to improve the performance of light-emitting diodes. The temperature and injection current dependent electroluminescence were carried out to study the thermal effect of InGaN/GaN MQWs. We observe the enhancement of carrier confinement in the active layer and the inhibited carrier leakage over the barrier for the sample with MQBs. In addition, the external quantum efficiency of the samples is obtained. It is found that the radiative efficiency of the sample possessing MQBs exhibits less sensitive temperature dependence and leads to an improved efficiency in the high temperature and high injection current range. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The growth of GaN-based semiconductors and their heterostructures has been an important issue because of the broad applications to the fabrication of light-emitting diodes (LEDs), laser diodes (LDs), solar cells, and photodetectors, which emit over a broad spectral range from infrared to ultraviolet [1–3]. Much interest has been focused on InGaN/ GaN multiple-quantum wells (MQWs) because of their acting as the active layer in high brightness III-nitride LED and cw blue/green LDs [1,4]. Because of large lattice and thermal expansion coefficient mismatches between the sapphire substrate and the epitaxial layers, it results in the localized states or quantum dots in the InGaN/GaN MQWs due to the presence of indium-rich fluctuations. The emission mechanism that occurs in the InGaN-based light-emitting diodes has been attributed mainly to the localized exciton emission. However, the InGaN-based heterostructures display misfit dislocation in nanocrystalline structures, which lead to nonradiative recombination centers. The performance of such devices is limited by the thermally activated loss of electrons overflowing from the active layer to the nonradiative recombination centers, thus reducing the emission efficiency. A carrier blocking effect can be achieved by an AlGaN thin film layer in the multiquantum wells (MQWs) region. It has been used in high power UV/blue/ green LEDs to achieve high device efficiency [5,6]. An alternative approach is to use multi-quantum barrier (MQB) structures to inhibit hot carrier overflow [7]. In this study, we introduce the structure of MQBs into the InGaN/GaN MQW LEDs to improve the performance of the devices. The MQW
⁎ Corresponding author. Tel.: +886 2 29089899; fax: +886 2 29085247. E-mail address: [email protected] (Y.-F. Wu). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.016
samples with and without MQBs were prepared by metal-organic vapor phase epitaxy system. The electroluminescence (EL) measurements were carried out over a temperature range from 20 to 300 K and an injection current level from 10 to 100 mA. With increasing current, the sample without MQB exhibits considerable blueshift of the peak energy at low temperatures and redshift of the peak energy in the high temperature range. A similar thermal action is conducted for the sample with MQBs, but the thermal-induced variety of peak energy is less evident and the turning temperature is higher. Furthermore, the external quantum efficiency of the samples is also calculated from the EL spectra. It is found that the efficiency is enhanced and exhibits less sensitive temperature dependence, resulting from the increase of effective barrier heights due to MQBs. It is reasonable to conclude that the well-designed MQBs improve the performance of LED significantly. 2. Experimental details The structures of the samples investigated in this study are schematically represented in Fig. 1. Both samples were grown by metal-organic vapor phase epitaxy (MOVPE) on c-plane sapphire substrates. Ammonia (NH3), trimethylgallium (TMG), trimethylindium (TMI), silane (SiH4), and bicyclopentadienyl magnesium (Cp2Mg) were used either as precursors or as dopants. The layer structure of the sample without MQBs consisted of a 20-nm-thick GaN buffer layer, a 3-μm-thick Si-doped n-type GaN layer, an undoped GaN layer possessing five periods of In0.18Ga0.82N/GaN MQWs, and a 100-nm-thick Mg-doped ptype GaN layer. The doping concentrations of n- and p-type GaN were nominally about 5 × 1018 cm− 3 and 1 × 1019 cm− 3, respectively. The thicknesses of the InGaN wells and the GaN barriers in the MQW structures were 2 nm and 11 nm, respectively. For the sample with MQBs, the layer structure was similar, but the MQWs were five periods
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J.-C. Lee, Y.-F. Wu / Thin Solid Films 518 (2010) 7437–7440
Fig. 1. Schematic representation of the InGaN/GaN multiple-quantum well lightemitting diodes without and with MQBs.
interference fringes seen in the figure are due to the interface reflections. The output powers of the samples without and with MQBs are 1.76 and 2.53 mW, respectively. The MQB structure shown in Fig. 1 (b) is engineered to align a forbidden energy mini-band of the MQB superlattice with the conduction band minimum to enhance the conduction band discontinuity. By using the transfer matrix method to solve the Schrödinger equation, one can obtain the reflectivity probability against incident electron energy. The result illustrates that a band of non-allowed electron states can be created above the classical barrier height [8]. This effectively enhances the reflectivity of electrons incident on the barrier and hence increases the band discontinuity. Thus the higher EL spectra intensity observed for the sample with MQBs in Fig. 2 could be attributed to the improvement of carrier confinement and inhibited carrier leakage in the active layer. Besides, the higher EL peak energy of this sample also gives the evidence of the increased conduction band discontinuity. The temperature dependent EL peak energy of the sample without MQBs is shown in Fig. 3(a). Referring to the curve that measured at 10 mA, it can be seen that the temperature dependence of the peak energy does not follow the Varshni law. When the temperature is low, the carriers gain sufficient thermal energy to overcome small potential barriers and relax down into lower energy tail states caused by the inhomogeneous potential fluctuations where finally the radiative recombination takes place. In the mid-temperature range,
of In0.15Ga0.85N/GaN MQWs and the barrier layer was replaced by a fiveperiod In0.005Ga0.995N/GaN (1 nm/1 nm) heterolayers. Both the LED chips were fabricated over a broad area (350 × 350 μm2) using standard photolithography processes. Ti/Al/Ti/Au and Ni–Au were employed for the n- and p-type electrodes, respectively. The temperature dependent EL measurements were carried out by mounting the sample on a Cu cold stage where the temperature (T) was varied from 20 to 300 K and using a current source operated from 10 to 100 mA. The luminescence signal was dispersed through a 0.5 m monochromator and detected using a Si photodiode by standard lock-in amplification technique. 3. Results and discussion Fig. 2 shows the normalized EL spectra of the samples at temperature 300 K and injection current level 20 mA. We observed InGaN-related main emissions with peak energies 2.68 and 2.93 eV for samples without and with MQBs, respectively. The Fabry–Perot
Fig. 2. Electroluminescence spectra of InGaN/GaN MQW samples without and with MQBs at temperature 300 K. The driving current is 20 mA.
Fig. 3. Temperature dependent EL spectra peak position from the InGaN/GaN MQW samples (a) without MQBs and (b) with MQBs. The solid lines are fitting curves based on the band-tail model with fitting parameters listed in Table 1.
J.-C. Lee, Y.-F. Wu / Thin Solid Films 518 (2010) 7437–7440
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the blueshift of the peak energy is a result of the thermal population of higher energy states of the density of states. Here a transition from the localized states in the quantum well takes place. As the temperature is even higher, the blueshift behavior is smaller than the temperature induced band-gap shrinkage, thus the peak position exhibits a redshift behavior. A similar thermal action is conducted for the sample with MQBs, as shown in Fig. 3(b). Observing the experimental results measured at 10 mA, the amounts of variety with temperature are less than those of the sample without MQBs, exhibiting a less temperature-sensitive characteristic of it. The “S-shaped” temperature dependence of the EL peak energy is the fingerprint of the existence of localized states. The degree of localization effect in nitride-based devices could be obtained by the band-filling model for tails of the density of states [9–11]: EðT Þ = Em ð0Þ−
αT 2 σ2 − ; T + β kB T
ð1Þ
where E(T) is the emission energy at temperature T; Em(0) is the energy gap at zero temperature; and α and β are Varshini's fitting parameters. The third term describes the localization effect, in which σ is related with localization effect, and kB is the Boltzmann constant. The value σ gives an estimate of the degree of localization effect; i.e., if the value is larger, the localization effect is stronger. The fitting curves made based on Eq. (1) are also shown in Fig. 3 with the parameters listed in Table 1. Through fitting, the values of σ obtained at injection current level 10 mA for samples without and with MQBs are 34.2 and 37.9 meV, respectively. Since the excitons localized in fluctuation minima prevent them from reaching nonradiative recombination sites, it is thought to be the key to the high luminescence efficiencies observed in InGaN-based LEDs [12], the stronger localization effect of the sample with MQBs implies higher emission intensity. With increased driving-current level, the temperature behavior of the EL spectra undergoes a noticeable alteration. The S-shaped dependence of peak position gradually disappears. It is noticeable that the band energy blueshifts and redshifts at low and high temperatures, respectively; which we attribute to the band-filling effect and the heating effect of the localized energy states. Fig. 4 depicts more details of the driving-current dependent peak energy. For the sample without MQBs shown in Fig. 4(a), considerable blueshift of the luminescence peak with increasing current is visible at low temperatures due to the localized states filling. At somewhat elevated temperature, the peak position is almost independent on the injection currents. In the high temperature range, the increasing currents even lead to a redshift of peak energy. This behavior is explained by the reduced occupation of localized states with increasing temperatures [13]. Thus the localization effect will be cancelled by thermal process at sufficiently high temperatures, yielding a current-independent emission peak. Besides, the heating effect should be taken into account at high injection current level, further enhancing the thermal process. It is reasonable to infer that the localization effect is compensated and overcome by the redshift of peak energy due to the temperature shrinkage of band-gap energy. The blueshift of peak energy shown in Fig. 4(b) for sample with MQBs occurs at a higher temperature, which we attribute to the insensitivity
Table 1 Fitting parameters used in the band-tail model for InGaN/GaN MQW samples without and with MQBs at various current levels. Injection current (mA)
Sample without MQBs
Sample with MQBs
α (meV/K)
β (K)
σ (meV)
α (meV/K)
β (K)
σ (meV)
10 60 100
1 0.63 0.24
1196 1000 350
34.2 19.7 6.8
0.9 0.82 0.46
680 600 200
37.9 28.1 20
Fig. 4. The dependence of the spectral peak position on injection current for the InGaN/ GaN MQWs (a) without MQBs and (b) with MQBs at different temperatures between 20 and 300 K.
to temperature of this sample and the resultant unobvious heating effect. Moreover, the turning temperature is also higher than that of the sample without MQBs. Owing to the band of non-allowed electron states are created above the classical barrier height by MQB structure; it effectively increases the band discontinuity. Hence it is understandable that the sample with MQBs exhibits a higher turning temperature. The effect of MQB structures on the performance of our samples is further investigated by discussing the EL efficiency of the samples. One of the major problems of InGaN/GaN LEDs is the suppression of carrier overflow from the active layer into the p-layer. Because the MQB structure effectively increases the band discontinuity, it is expected to prevent the electrons from overflowing as operated at high temperatures and high driving currents. The detailed variations of the EL quantum efficiency are plotted in Fig. 5 as a function of injection currents under various temperatures for both of the samples. It is clearly seen that the efficiency of the sample with MQBs is greatly improved, especially when working at high temperatures and high current levels. As the injection current is increasing, the EL efficiency increases at low temperatures and decays as the temperature is high. In the low temperature regime, the exciton localization effect is the dominant mechanism [14], the lager σ of sample with MQBs resulting to the better EL efficiency. As the temperature is high enough, the nonradiative recombination of carriers becomes thermally activated; the EL efficiency saturates and then decreases. As shown in Fig. 5, the EL efficiency of the sample with MQBs exhibits less decrement of EL efficiency in the high temperature and high injection current range. It
7440
J.-C. Lee, Y.-F. Wu / Thin Solid Films 518 (2010) 7437–7440
4. Conclusions In this article, we investigate the performance of InGaN/GaN MQW LEDs with and without MQBs by the measurements of EL spectra over a broad range of temperature and driving current. From the experimental results, the sample with MQBs exhibits an enhancement of carrier confinement in the active layer and a less temperaturesensitive characteristic, coincides with the increase of effective barrier heights due to the quantum interference of the electrons within MQBs. Moreover, from the evaluation of injection current dependent EL external quantum efficiency in the temperature range from 20 to 300 K for the samples, it is concluded that the EL efficiency is greatly improved by the MQB structures, especially as operating at high temperatures and high injection currents. Acknowledgement This work is supported by the National Science Council of the Republic of China under NSC 98-2112-M-131-001-MY2. References
Fig. 5. EL external quantum efficiency of InGaN/GaN MQW samples (a) without MQBs and (b) with MQBs as a function of injection currents in the temperature range from 20 to 300 K.
is concluded that the MQB structure improves the performance of InGaN/GaN MQW LEDs, especially as working at high temperatures and high injection currents.
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