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Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer
G Model MRB 8705 No. of Pages 4
Materials Research Bulletin xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer Yong Deok Kim, Seung Kyu Oh, Min Joo Park, Joon Seop Kwak* Department of Printed Electronics Engineering, Sunchon National University, Jeonnam 540-742, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 September 2015 Received in revised form 3 March 2016 Accepted 10 March 2016 Available online xxx
GaN-based light emitting diodes (LEDs) with a nitrogen implanted current-blocking layer (CBL) were successfully demonstrated for improving the light extraction efficiency (LEE) and radiant intensity. The LEE and radiant intensity of the LEDs with a shallow implanted CBL with nitrogen was greatly increased by more than 20% compared to that of a conventional LED without the CBL due to an increase in the effective current path, which reduces light absorption at the thick p-pad electrode. Meanwhile, deep implanted CBL with a nitrogen resulted in deterioration of the LEE and radiant intensity because of formation of crystal damage, followed by absorption of the light generated at the multi-quantum well (MQW). These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by optimization of implanted depth. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nitrides A. Optical materials A. Semiconductors B. Optical properties D. Electrical properties
1. Introduction GaN-based materials have attracted considerable attention in optoelectronic devices because of their direct and wide band gap and excellent electronic, optical, and thermal properties [1]. GaNbased light-emitting diodes (LEDs) have found enormous applications in illumination, back-light unit of liquid crystal displays, and projection displays [2]. On the other hand, the external quantum efficiency (EQE) of GaN-based LEDs is insufficient, so that high-power LEDs cannot meet practical needs. The external quantum efficiency is equal to the multiplication of the internal quantum efficiency (IQE) by the light extraction efficiency (LEE). The former relates to the substrate properties and epitaxy quality, and the latter relates to chip processing. Therefore, an increase in the LEE of GaN-based LEDs by chip processing is one of the most important issues for EQE enhancement in solid-state lighting [3,4]. Several methods of current-blocking structures have been studied intensively. The conventional methods for fabricating a current-blocking layer (CBL) are formation of SiO2 layer [5], plasma selective treatment of a p-GaN structure [6], current-blocking hole [7], and selective activated CBL [8]. Among these methods, the CBL with SiO2 layer is widely applied for the formation of CBL. However, it requires the deposition of SiO2 and wet etching process for forming the SiO2 CBL, which is relatively complex and cannot
* Corresponding author. E-mail address: [email protected] (J.S. Kwak).
produce a planar insulating layer. Furthermore, the wet etching process makes it difficult to control the correct pattern size because of over etching. In addition, the plasma selective treatment of the p-GaN structure can increase the p-type conductivity when post-treatment thermal annealing over 600 C is carried out [9]. The current-blocking hole and selective activated CBL methods are relatively complex and impractical. In order to overcome these problems, this paper proposes a method to form an insulating region by nitrogen implantation [10] as a CBL in GaN-based lateral LEDs. The enhanced LEE of the nitrogen implanted CBL LEDs shows that the nitrogen implant CBL effectively prevents shadowing and light absorption in the padelectrode. In addition, the nitrogen implant CBL reduces the need for an additional insulating layer, a SiO2 CBL layer. This decreases the risk of an incorrect pattern size because of the over wet-etching problem. In addition, the nitrogen implanted CBL LED can maintain the high resistance during post-ITO deposition annealing. 2. Experimental All InGaN-GaN samples used in this study were grown by metal organic chemical vapor deposition (MOCVD) on a 2-inch c-plane (0001) sapphire substrate. Ammonia (NH3), trimethylgallium (TMG), trimethylindium (TMI), bis-cyclopentadienyl-magnesium (Cp2Mg), and silane (SiH4) were used as the precursors and dopants. First, the LED consisted of a 2-mm-thick heavily doped ntype GaN, seven-pairs of InxGa1-xN-GaN multiple quantum wells (MQWs) with a total thickness of 90 nm, and a 0.19-mm-thick
http://dx.doi.org/10.1016/j.materresbull.2016.03.012 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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p-type GaN layer. Second, to fabricate LEDs with the nitrogen implanted CBL, the mesa structure was formed using an inductively coupled plasma (ICP-RIE) system, to expose the nGaN layer for n-type ohmic contact formation. Third, a 2-mm- thick positive PR (GXR-601) was deposited by spin coating on the p-GaN layer, where a CBL region with a p-pad electrode pattern was defined by photolithography. Subsequently, nitrogen implantation under three different implanted energy conditions, 18, 120 and 250 KeV, were used to form a CBL in the p-GaN top layers of each of the three different LEDs, where the dose of the nitrogen implantation was maintained as 9.92 1014 cm2. The projected range (Rp) was calculated to be approximately 30, 90 and 200 nm using the SRIM-2008 (Stopping and Range of Ions in Matter-2008) software. We designed to form the nitrogen implanted CBL layer at the surface region (Rp 30 nm), at the middle of the p-GaN (Rp 90 nm), and at the bottom of the p-GaN (Rp 200 nm). After implantation, the positive PR was removed by acetone. An ITObased p-electrode was then deposited onto the p-GaN top layer both as a current spreading and light transmitting layer. Finally, a patterned Cr-Al-Ni-Au electrode was deposited on the exposed nGaN layer and ITO-based p-electrode for the n-contact electrode and p-pad metal, and they were diced into individual chip sizes of 525 525 mm2. All the LED samples were packaged for measuring the typical current-voltage (I–V), radiant intensity-current (L-I), IQE, EQE, and LEE. Fig. 1 shows a schematic diagram of the fabricated InGaN-GaN LEDs with the CBL inserted by nitrogen implantation with three different ion energies for forming three different Rp in the p-GaN top layer. The current and light-emission paths are also illustrated. 3. Results and discussion Fig. 2 shows the I–V, L-I characteristics and beam-profiles at an applied current of 80 mA of the conventional LED and lateral LEDs with a nitrogen implanted CBL with different Rp. Characterization was performed using a Keithley-2635A source-meter, spectroradiometer and 8-inch integrating sphere. As shown in Fig. 2(a), the measured forward voltages at an applied current of 80 mA were 3.26 V, 3.28 V, 3.29 V, and 3.34 V for the conventional LEDs, lateral LEDs with the implant CBL having a Rp of 30, 90, and 200 nm, respectively. The injected current was forced to spread out instead
Fig. 1. Top view of the fabricated chip and a schematic diagram of a cross-section of the A-A0 structure of the GaN-based lateral LEDs with nitrogen implanted CBL which has different Rp. The blue lines located in the LED structure show the current path, and the yellow lines located beside the p-pad show the light-emission path. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) I–V curves of the conventional LED and lateral LEDs with nitrogen implanted CBL, which has different Rp. Shown in the inset are beam-profiles at an applied current 80 mA of LEDs. (b) L-I characteristics of LEDs.
of directly flowing along the shortest path between the p-pad electrode and n-contact metal [5]. Therefore, the effective current path was increased due to the slight increase in series resistance, which resulted in the slight increase in the forward voltage. On the other hands, the radiant intensity of the LEDs with the implant CBL having a Rp of 30 nm was greatly increased by more than 20% compared to those without the CBL, as shown in Fig. 2(b), which can be attributed to the increase in the effective current path, which reduces light absorption at the thick p-pad electrode. This result clearly suggests that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs. Meanwhile, the increase in the Rp to 90 and 200 nm deteriorated the radiant intensity of the LEDs with the implant CBL. These results can also be confirmed in the beam-profiles, which were measured using a ML-3740 beam-profile camera at an applied current of 80 mA, as shown in the inset of Fig. 2(a). The beamprofiles presented that the LEDs with implant CBL having a Rp of 30 nm showed the highest light power, meanwhile, the LEDs with implant CBL having Rp of 90 and 200 nm yielded much lower light power. These results can be related to the formation of crystal damage during the deep implantation of nitrogen with high energy over 100 KeV, followed by the absorption of the light. Fig. 3(a) shows the EQE of LEDs with implant CBL having a Rp of 30, 90 and 200 nm, as well as the EQE of conventional LEDs. The EQE of the LEDs with the implant CBL having a Rp of 30 nm was
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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Fig. 3. (a) EQE, (b) IQE, (c) LEE and (d) comparison of IQE, EQE and LEE for the LEDs with/without the implanted CBL having a different Rp.
Fig. 4. XTEM images of p-GaN and n-GaN in the case of nitrogen implanted CBL LEDs with the implanted CBL having a Rp of 200 nm.
17.3%, which is more than 20% higher value compared to those without the CBL. Meanwhile, the EQE of the LEDs with implant CBL decreased as increase in the Rp to 90 and 200 nm, as shown in Fig. 3(a). The change of EQE can be attributed to change in IQE and/ or change in EXE, since EQE is the product of IQE and LEE. The IQE can be measured using the current dependent electroluminescence (CDEL) method [11], and Fig. 3(b) shows the measured IQE by the CDEL. We can also calculate the maximum IQE, hmax, through
theoretical fit to the measured EQE using the measured maximum EQE and the corresponding current density, Jmax, according to the Eq. (1) [12–14]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 hmax Þ hJ hJJmax ð1 þ h¼1 Þ ð1Þ 2J hmax Jmax hmax
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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We found that the measured IQE by the CDEL and the calculated IQE by the Eq. (1) is identical. As shown in Fig. 3(b), the IQE of the LEDs with/without the implant CBL was 75% at an applied current of 80 mA, which implies that the nitrogen implantation did not deteriorate the IQE due to the precise control of implanted depth. From the measurement of the IQE, we can calculate the LEE through EQE/IQE, as shown in Fig. 3(c) and (d). The LEE of the LEDs with the implant CBL having a Rp of 30 nm was 23.0%, which is more than 20% higher value compared to those without the CBL. Meanwhile, the LEE of the LEDs with implant CBL decreased as increase in the Rp to 90 and 200 nm, as shown in Fig. 3(c) and (d). These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by optimization of implanted depth. Finally, in order to understand the reason for decrease in the LEE as increase in Rp to 200 nm, nitrogen implanted region in the LED with the implant CBL having a Rp of 200 nm was examined by cross-sectional transmission electron microscope (XTEM), as shown Fig. 4. The XTEM images for the top and middle of the pGaN and n-GaN regions showed that crystal destruction was not detected from these regions, which implies that the top and middle of the p-GaN were free from implantation damage. Nevertheless, the crystal destruction was observed in many places in the p-GaN bottom region, as marked with the red-color circle in the p-GaN bottom region. These results can be attributed to the formation of damage during the high energy implantation of 200 KeV. The lower LEE for the LEDs with the implant CBL having a Rp of 200 nm, as shown in Fig. 3(d), can be caused by these implantation-produced damage regions, which can significantly increase the absorption of visible light in GaN. Indeed, the as-implanted GaN grown on sapphire undergoes a color change from yellowish-brown to black depending on the ion dose or the level of implantation-produced damage. This suggests that implantation-induced defects act as strong absorbers of light in the blue part of the visible spectrum [15]. Therefore, the LEEs of the nitrogen implanted CBL LEDs can decrease with increasing implant Rp in the p-GaN layer, because of the easy absorption of blue-light by the implantation damage formed near the MQWs. These results suggest that the precise control of implantation depth is essential for the implementation of ion implantation in the LED fabrication process. 4. Conclusions
radiant intensity. The LEE and radiant intensity of the LEDs with the implant CBL having a Rp of 30 nm were greatly increased by more than 20% compared to those without the CBL, because of the increase in the effective current path, followed by reduction of the light absorption at the thick p-pad electrode. Meanwhile, deep implanted CBL with a nitrogen resulted in deterioration of the LEE and radiant intensity because of formation of crystal damage, followed by absorption of the light generated at the MQWs. These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by the optimized Rp condition. Acknowledgements This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) fund by the Ministry of Education (NRF-2015R1D1A3A01019050 and NRF2014R1A6A1030419). The authors gratefully acknowledge the support of K. Chen and M. Evans from Applied Materials and the support from LG Innotek. References [1] S.J. Chang, C.F. Shen, M.H. Hsieh, C.T. Kuo, T.K. Ko, W.S. Chen, S.C. Shei, J. Light Technol. 26 (2008) 3131–3136. [2] M.R. Krames, O.B. Shchekin, R. Mueller-Mach, G.O. Mueller, L. Zhou, G. Harbers, M.G. Craford, IEEE J. Display Technol. 3 (2007) 160–175. [3] K. McGroddy, A. David, E. Matioli, M. Iza, S. Nakamura, S. DenBaars, J.S. Speck, C. Weisbuch, E.L. Hu, Appl. Phys. Lett. 93 (2008) 103502. [4] M.K. Kwon, J.Y. Kim, I.K. Park, K.S. Kim, G.Y. Jung, S.J. Park, J.W. Kim, Y.C. Kim, Appl. Phys. Lett. 92 (2008) 251110. [5] C. Huh, J.M. Lee, D.J. Kim, S.J. Park, J. Appl. Phys. 92 (2002) 2248–2250. [6] Y.B. Lee, R. Takaki, H. Sato, Y. Naoi, S. Sakai, Phys. Stat. Sol. (A) 200 (2003) 87–90. [7] H.C. Wang, Y.K. Su, C.L. Lin, W.B. Chen, S.M. Chen, Jpn. J. Appl. Phys. 43 (2004) 2006–2007. [8] C.M. Lee, C.C. Chuo, Y.C. Liu, I.L. Chen, J.I. Chyi, Electron Device Lett. 25 (2004) 384–386. [9] S.W. Kim, J.M. Lee, C. Huh, N.M. Park, H.S. Kim, I.H. Lee, S.J. Park, Appl. Phys. Lett. 76 (2000) 3079–3081. [10] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435–1437. [11] J.I. Shim, H. Kim, H.Y. Ryu, D.S. Shin, H. Jung, Proceeding 8th International Conference on Nitride Semiconductor, Jeju, Korea, 2009, pp. 1086–1087. [12] H.-Y. Ryu, H.-S. Kim, J.-I. Shim, Appl. Phys. Lett. 95 (2009) 081114. [13] E. Fred Schubert, Light-Emitting Diodes, second ed., Cambridge, New York, 2006, pp. 86–87. [14] M.J. Park, S.J. Hwang, H.J. Kim, S. Jung, K.H. Bang, H.G. Kim, Y. Chang, Y. Choi, J.S. Kwak, J. Disp. Technol. 9 (2013) 346–352. [15] S.O. Kucheyev, J.S. Williams, S.J. Pearton, Mater. Sci. and Eng. 33 (2001) 85.
In this study, we successfully demonstrated the GaN-based LEDs with a nitrogen implanted CBL for improving the LEE and
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
Materials Research Bulletin xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer Yong Deok Kim, Seung Kyu Oh, Min Joo Park, Joon Seop Kwak* Department of Printed Electronics Engineering, Sunchon National University, Jeonnam 540-742, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 September 2015 Received in revised form 3 March 2016 Accepted 10 March 2016 Available online xxx
GaN-based light emitting diodes (LEDs) with a nitrogen implanted current-blocking layer (CBL) were successfully demonstrated for improving the light extraction efficiency (LEE) and radiant intensity. The LEE and radiant intensity of the LEDs with a shallow implanted CBL with nitrogen was greatly increased by more than 20% compared to that of a conventional LED without the CBL due to an increase in the effective current path, which reduces light absorption at the thick p-pad electrode. Meanwhile, deep implanted CBL with a nitrogen resulted in deterioration of the LEE and radiant intensity because of formation of crystal damage, followed by absorption of the light generated at the multi-quantum well (MQW). These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by optimization of implanted depth. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nitrides A. Optical materials A. Semiconductors B. Optical properties D. Electrical properties
1. Introduction GaN-based materials have attracted considerable attention in optoelectronic devices because of their direct and wide band gap and excellent electronic, optical, and thermal properties [1]. GaNbased light-emitting diodes (LEDs) have found enormous applications in illumination, back-light unit of liquid crystal displays, and projection displays [2]. On the other hand, the external quantum efficiency (EQE) of GaN-based LEDs is insufficient, so that high-power LEDs cannot meet practical needs. The external quantum efficiency is equal to the multiplication of the internal quantum efficiency (IQE) by the light extraction efficiency (LEE). The former relates to the substrate properties and epitaxy quality, and the latter relates to chip processing. Therefore, an increase in the LEE of GaN-based LEDs by chip processing is one of the most important issues for EQE enhancement in solid-state lighting [3,4]. Several methods of current-blocking structures have been studied intensively. The conventional methods for fabricating a current-blocking layer (CBL) are formation of SiO2 layer [5], plasma selective treatment of a p-GaN structure [6], current-blocking hole [7], and selective activated CBL [8]. Among these methods, the CBL with SiO2 layer is widely applied for the formation of CBL. However, it requires the deposition of SiO2 and wet etching process for forming the SiO2 CBL, which is relatively complex and cannot
* Corresponding author. E-mail address: [email protected] (J.S. Kwak).
produce a planar insulating layer. Furthermore, the wet etching process makes it difficult to control the correct pattern size because of over etching. In addition, the plasma selective treatment of the p-GaN structure can increase the p-type conductivity when post-treatment thermal annealing over 600 C is carried out [9]. The current-blocking hole and selective activated CBL methods are relatively complex and impractical. In order to overcome these problems, this paper proposes a method to form an insulating region by nitrogen implantation [10] as a CBL in GaN-based lateral LEDs. The enhanced LEE of the nitrogen implanted CBL LEDs shows that the nitrogen implant CBL effectively prevents shadowing and light absorption in the padelectrode. In addition, the nitrogen implant CBL reduces the need for an additional insulating layer, a SiO2 CBL layer. This decreases the risk of an incorrect pattern size because of the over wet-etching problem. In addition, the nitrogen implanted CBL LED can maintain the high resistance during post-ITO deposition annealing. 2. Experimental All InGaN-GaN samples used in this study were grown by metal organic chemical vapor deposition (MOCVD) on a 2-inch c-plane (0001) sapphire substrate. Ammonia (NH3), trimethylgallium (TMG), trimethylindium (TMI), bis-cyclopentadienyl-magnesium (Cp2Mg), and silane (SiH4) were used as the precursors and dopants. First, the LED consisted of a 2-mm-thick heavily doped ntype GaN, seven-pairs of InxGa1-xN-GaN multiple quantum wells (MQWs) with a total thickness of 90 nm, and a 0.19-mm-thick
http://dx.doi.org/10.1016/j.materresbull.2016.03.012 0025-5408/ ã 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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Y.D. Kim et al. / Materials Research Bulletin xxx (2015) xxx–xxx
p-type GaN layer. Second, to fabricate LEDs with the nitrogen implanted CBL, the mesa structure was formed using an inductively coupled plasma (ICP-RIE) system, to expose the nGaN layer for n-type ohmic contact formation. Third, a 2-mm- thick positive PR (GXR-601) was deposited by spin coating on the p-GaN layer, where a CBL region with a p-pad electrode pattern was defined by photolithography. Subsequently, nitrogen implantation under three different implanted energy conditions, 18, 120 and 250 KeV, were used to form a CBL in the p-GaN top layers of each of the three different LEDs, where the dose of the nitrogen implantation was maintained as 9.92 1014 cm2. The projected range (Rp) was calculated to be approximately 30, 90 and 200 nm using the SRIM-2008 (Stopping and Range of Ions in Matter-2008) software. We designed to form the nitrogen implanted CBL layer at the surface region (Rp 30 nm), at the middle of the p-GaN (Rp 90 nm), and at the bottom of the p-GaN (Rp 200 nm). After implantation, the positive PR was removed by acetone. An ITObased p-electrode was then deposited onto the p-GaN top layer both as a current spreading and light transmitting layer. Finally, a patterned Cr-Al-Ni-Au electrode was deposited on the exposed nGaN layer and ITO-based p-electrode for the n-contact electrode and p-pad metal, and they were diced into individual chip sizes of 525 525 mm2. All the LED samples were packaged for measuring the typical current-voltage (I–V), radiant intensity-current (L-I), IQE, EQE, and LEE. Fig. 1 shows a schematic diagram of the fabricated InGaN-GaN LEDs with the CBL inserted by nitrogen implantation with three different ion energies for forming three different Rp in the p-GaN top layer. The current and light-emission paths are also illustrated. 3. Results and discussion Fig. 2 shows the I–V, L-I characteristics and beam-profiles at an applied current of 80 mA of the conventional LED and lateral LEDs with a nitrogen implanted CBL with different Rp. Characterization was performed using a Keithley-2635A source-meter, spectroradiometer and 8-inch integrating sphere. As shown in Fig. 2(a), the measured forward voltages at an applied current of 80 mA were 3.26 V, 3.28 V, 3.29 V, and 3.34 V for the conventional LEDs, lateral LEDs with the implant CBL having a Rp of 30, 90, and 200 nm, respectively. The injected current was forced to spread out instead
Fig. 1. Top view of the fabricated chip and a schematic diagram of a cross-section of the A-A0 structure of the GaN-based lateral LEDs with nitrogen implanted CBL which has different Rp. The blue lines located in the LED structure show the current path, and the yellow lines located beside the p-pad show the light-emission path. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) I–V curves of the conventional LED and lateral LEDs with nitrogen implanted CBL, which has different Rp. Shown in the inset are beam-profiles at an applied current 80 mA of LEDs. (b) L-I characteristics of LEDs.
of directly flowing along the shortest path between the p-pad electrode and n-contact metal [5]. Therefore, the effective current path was increased due to the slight increase in series resistance, which resulted in the slight increase in the forward voltage. On the other hands, the radiant intensity of the LEDs with the implant CBL having a Rp of 30 nm was greatly increased by more than 20% compared to those without the CBL, as shown in Fig. 2(b), which can be attributed to the increase in the effective current path, which reduces light absorption at the thick p-pad electrode. This result clearly suggests that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs. Meanwhile, the increase in the Rp to 90 and 200 nm deteriorated the radiant intensity of the LEDs with the implant CBL. These results can also be confirmed in the beam-profiles, which were measured using a ML-3740 beam-profile camera at an applied current of 80 mA, as shown in the inset of Fig. 2(a). The beamprofiles presented that the LEDs with implant CBL having a Rp of 30 nm showed the highest light power, meanwhile, the LEDs with implant CBL having Rp of 90 and 200 nm yielded much lower light power. These results can be related to the formation of crystal damage during the deep implantation of nitrogen with high energy over 100 KeV, followed by the absorption of the light. Fig. 3(a) shows the EQE of LEDs with implant CBL having a Rp of 30, 90 and 200 nm, as well as the EQE of conventional LEDs. The EQE of the LEDs with the implant CBL having a Rp of 30 nm was
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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Fig. 3. (a) EQE, (b) IQE, (c) LEE and (d) comparison of IQE, EQE and LEE for the LEDs with/without the implanted CBL having a different Rp.
Fig. 4. XTEM images of p-GaN and n-GaN in the case of nitrogen implanted CBL LEDs with the implanted CBL having a Rp of 200 nm.
17.3%, which is more than 20% higher value compared to those without the CBL. Meanwhile, the EQE of the LEDs with implant CBL decreased as increase in the Rp to 90 and 200 nm, as shown in Fig. 3(a). The change of EQE can be attributed to change in IQE and/ or change in EXE, since EQE is the product of IQE and LEE. The IQE can be measured using the current dependent electroluminescence (CDEL) method [11], and Fig. 3(b) shows the measured IQE by the CDEL. We can also calculate the maximum IQE, hmax, through
theoretical fit to the measured EQE using the measured maximum EQE and the corresponding current density, Jmax, according to the Eq. (1) [12–14]. sffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 hmax Þ hJ hJJmax ð1 þ h¼1 Þ ð1Þ 2J hmax Jmax hmax
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012
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We found that the measured IQE by the CDEL and the calculated IQE by the Eq. (1) is identical. As shown in Fig. 3(b), the IQE of the LEDs with/without the implant CBL was 75% at an applied current of 80 mA, which implies that the nitrogen implantation did not deteriorate the IQE due to the precise control of implanted depth. From the measurement of the IQE, we can calculate the LEE through EQE/IQE, as shown in Fig. 3(c) and (d). The LEE of the LEDs with the implant CBL having a Rp of 30 nm was 23.0%, which is more than 20% higher value compared to those without the CBL. Meanwhile, the LEE of the LEDs with implant CBL decreased as increase in the Rp to 90 and 200 nm, as shown in Fig. 3(c) and (d). These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by optimization of implanted depth. Finally, in order to understand the reason for decrease in the LEE as increase in Rp to 200 nm, nitrogen implanted region in the LED with the implant CBL having a Rp of 200 nm was examined by cross-sectional transmission electron microscope (XTEM), as shown Fig. 4. The XTEM images for the top and middle of the pGaN and n-GaN regions showed that crystal destruction was not detected from these regions, which implies that the top and middle of the p-GaN were free from implantation damage. Nevertheless, the crystal destruction was observed in many places in the p-GaN bottom region, as marked with the red-color circle in the p-GaN bottom region. These results can be attributed to the formation of damage during the high energy implantation of 200 KeV. The lower LEE for the LEDs with the implant CBL having a Rp of 200 nm, as shown in Fig. 3(d), can be caused by these implantation-produced damage regions, which can significantly increase the absorption of visible light in GaN. Indeed, the as-implanted GaN grown on sapphire undergoes a color change from yellowish-brown to black depending on the ion dose or the level of implantation-produced damage. This suggests that implantation-induced defects act as strong absorbers of light in the blue part of the visible spectrum [15]. Therefore, the LEEs of the nitrogen implanted CBL LEDs can decrease with increasing implant Rp in the p-GaN layer, because of the easy absorption of blue-light by the implantation damage formed near the MQWs. These results suggest that the precise control of implantation depth is essential for the implementation of ion implantation in the LED fabrication process. 4. Conclusions
radiant intensity. The LEE and radiant intensity of the LEDs with the implant CBL having a Rp of 30 nm were greatly increased by more than 20% compared to those without the CBL, because of the increase in the effective current path, followed by reduction of the light absorption at the thick p-pad electrode. Meanwhile, deep implanted CBL with a nitrogen resulted in deterioration of the LEE and radiant intensity because of formation of crystal damage, followed by absorption of the light generated at the MQWs. These results clearly suggest that ion implantation method, which is widely applied in the fabrication of Si based devices, can be successfully implemented in the fabrication of GaN based LEDs by the optimized Rp condition. Acknowledgements This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) fund by the Ministry of Education (NRF-2015R1D1A3A01019050 and NRF2014R1A6A1030419). The authors gratefully acknowledge the support of K. Chen and M. Evans from Applied Materials and the support from LG Innotek. References [1] S.J. Chang, C.F. Shen, M.H. Hsieh, C.T. Kuo, T.K. Ko, W.S. Chen, S.C. Shei, J. Light Technol. 26 (2008) 3131–3136. [2] M.R. Krames, O.B. Shchekin, R. Mueller-Mach, G.O. Mueller, L. Zhou, G. Harbers, M.G. Craford, IEEE J. Display Technol. 3 (2007) 160–175. [3] K. McGroddy, A. David, E. Matioli, M. Iza, S. Nakamura, S. DenBaars, J.S. Speck, C. Weisbuch, E.L. Hu, Appl. Phys. Lett. 93 (2008) 103502. [4] M.K. Kwon, J.Y. Kim, I.K. Park, K.S. Kim, G.Y. Jung, S.J. Park, J.W. Kim, Y.C. Kim, Appl. Phys. Lett. 92 (2008) 251110. [5] C. Huh, J.M. Lee, D.J. Kim, S.J. Park, J. Appl. Phys. 92 (2002) 2248–2250. [6] Y.B. Lee, R. Takaki, H. Sato, Y. Naoi, S. Sakai, Phys. Stat. Sol. (A) 200 (2003) 87–90. [7] H.C. Wang, Y.K. Su, C.L. Lin, W.B. Chen, S.M. Chen, Jpn. J. Appl. Phys. 43 (2004) 2006–2007. [8] C.M. Lee, C.C. Chuo, Y.C. Liu, I.L. Chen, J.I. Chyi, Electron Device Lett. 25 (2004) 384–386. [9] S.W. Kim, J.M. Lee, C. Huh, N.M. Park, H.S. Kim, I.H. Lee, S.J. Park, Appl. Phys. Lett. 76 (2000) 3079–3081. [10] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435–1437. [11] J.I. Shim, H. Kim, H.Y. Ryu, D.S. Shin, H. Jung, Proceeding 8th International Conference on Nitride Semiconductor, Jeju, Korea, 2009, pp. 1086–1087. [12] H.-Y. Ryu, H.-S. Kim, J.-I. Shim, Appl. Phys. Lett. 95 (2009) 081114. [13] E. Fred Schubert, Light-Emitting Diodes, second ed., Cambridge, New York, 2006, pp. 86–87. [14] M.J. Park, S.J. Hwang, H.J. Kim, S. Jung, K.H. Bang, H.G. Kim, Y. Chang, Y. Choi, J.S. Kwak, J. Disp. Technol. 9 (2013) 346–352. [15] S.O. Kucheyev, J.S. Williams, S.J. Pearton, Mater. Sci. and Eng. 33 (2001) 85.
In this study, we successfully demonstrated the GaN-based LEDs with a nitrogen implanted CBL for improving the LEE and
Please cite this article in press as: Y.D. Kim, et al., Enhanced light extraction efficiency of GaN-based light-emittng diodes by nitrogen implanted current blocking layer, Mater. Res. Bull. (2016), http://dx.doi.org/10.1016/j.materresbull.2016.03.012