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Defect-induced color-tunable monolithic GaN-based light-emitting diodes
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Appl. Phys. Express 7 102102 (http://iopscience.iop.org/1882-0786/7/10/102102) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.120.242.61 This content was downloaded on 11/11/2014 at 23:20
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Applied Physics Express 7, 102102 (2014) http://dx.doi.org/10.7567/APEX.7.102102
Defect-induced color-tunable monolithic GaN-based light-emitting diodes Yaping Huang1,2, Feng Yun1,2*, Yufeng Li1,2, Wen Ding1,2, Yue Wang2, Hong Wang2, Weihan Zhang2, Ye Zhang2, Maofeng Guo2, Shuo Liu3, and Xun Hou1,2 1 Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Shaanxi Provincial Key Laboratory of Photonics & Information Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China 2 Solid-State Lighting Engineering Research Center, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China 3 Shaanxi Supernova Lighting Technology Co., Ltd., Xi’an, Shaanxi 710077, P. R. China E-mail: [email protected]
Received August 15, 2014; accepted September 2, 2014; published online September 18, 2014 We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor–acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current. © 2014 The Japan Society of Applied Physics
n recent years, tremendous progress has been achieved in the development of InGaN/GaN quantum well (QW) light-emitting diodes (LEDs). This has resulted in a variety of applications such as backlighting, full-color displays, and general illumination.1–4) More extensive applications in the market demand for white and color-tunable LEDs since the fabrication technologies related to LEDs becoming to mature.5) Funato et al. reported color-tunable LEDs with dual-wavelength emission from multifaceted QWs.6) Li et al. demonstrated phosphor-free color-tunable LEDs by inserting an InGaN shallow QW between deep InGaN QWs and GaN barriers.5) Hong et al. fabricated visible-color-tunable LEDs using InGaN/GaN multiple quantum wells (MQWs) formed on GaN nanostructures.7) Wang and Kuo numerically studied the spectral competition of chirped dual-wavelength emission in monolithic InGaN MQW LEDs.8) Furthermore, deep defects that lead to photoluminescence (PL) at energies below the band gap play an important role in the performance of LEDs. An emission peak in red was observed in PL spectra of both Mg-doped and Si-doped GaN, but its origins are unknown.9–12) It was concluded that the red emission in Mg-doped GaN is attributed to nitrogen vacancies (VN)9) and Mg-vacancy complexes (MgGa–VN).10) Recently, red luminescence has been observed in fine-structure undoped GaN. Reshchikov et al. suggested that gallium vacancy (VGa), oxygen, carbon, and complexes related to donor–acceptor pair (DAP) transitions may result in red emission.12) Nevertheless, studies on the practical application of defect-related emission are rare, even though there are so many theoretical studies.9–12,17,18) In this work, we fabricated a defect-induced color-tunable monolithic GaN-based LED. With steady blue InGaN/GaN MQW emission and broadband defect-induced red emission in p-GaN, we obtained wide-emission LEDs covering the spectrum from blue to red. An InGaN/GaN MQW LED epistructure was grown on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD). The structure consists of nine periods of InGaN/GaN QWs (3 nm wells and 9 nm barriers) on a 3 µm n-GaN layer. The QWs are capped by a 20 nm electron-blocking layer (EBL) followed by a 120 nm Mg-doped p-GaN layer. The centroid wavelength of the wafer is ³460 nm. After MOCVD growth, vertical LEDs (VLEDs) with Ag nanorod arrays embedded in p-GaN were fabricated.
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Figure 1 shows a schematic of the fabrication of defectinduced color-tunable VLEDs. First, a hexagonally ordered monolayer template of polystyrene (PS) spheres (200 nm diameter) is coated on p-GaN by the self-assembly method.13) Then, the PS spheres are etched by oxygen plasma until the diameter is reduced to ³130 nm. A titanium thin film is then deposited on the template by electron beam evaporation. After that, a highly ordered two-dimensional (2D) hexagonal Ti nanohole mask on p-GaN is obtained by dissolving PS in trichloromethane. Nanohole arrays of 130 nm depth on p-GaN are fabricated with a Ti mask by BCl3/Cl2 inductively coupled plasma (ICP) etching. Then, Ni (1 nm)/Ag (150 nm) is deposited into the nanoholes by electron beam evaporation after the Ti mask is removed. A Ti (100 nm)/Au (500 nm) layer is then deposited on the Ag layer for bonding to the Cu/W substrate, as shown in Fig. 1. The p-electrode surface of the LED and the Cu/W substrate coated by Ti (50 nm)/Au0.8Sn0.2 (1000 nm) solder are then bonded to each other at 380 °C for 20 min under vacuum. The sapphire substrate is removed using a 248 nm KrF excimer laser lift-off process. Finally, an Al (1500 nm) electrode is deposited on n-GaN after u-GaN is etched using ICP. VLEDs with a chip size of 55 © 55 mil2 were fabricated by this method. Current–voltage (I–V ) measurements were carried out on a MPI LEDA-AT-T200 source meter. The electroluminescence (EL) spectra of the device were measured using an Ocean Optics S2000 spectrometer that employed a fiber optic cable for light collection. The PL spectra were excited by a xenon lamp at room temperature. Figure 2(a) shows a tilted scanning electron microscopy (SEM) image of the hexagonally patterned Ti mask with 200 nm pitch and 130 nm hole diameter at the surface of p-GaN. Figure 2(b) shows an SEM image of the nanoholes on p-GaN with a tilt angle of 60°. The nanohole depth is ³130 nm measured from the tilted SEM image after angle correction. Figure 2(c) shows an SEM image of the sample after Ag deposition, where Ag nanorod arrays embedded in p-GaN can be clearly observed and confirmed by deliberately separating the Ag deposition layer from p-GaN. For the Ag and GaN contact, Ga atoms would out-diffuse into the Ag layer and form a Ag–Ga solid solution at the interface of Ag and GaN during thermal annealing. The out-diffusion of Ga atoms results in the formation of Ga vacancies in the GaN surface region.14–16) Because of the huge contact area of Ag
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Fig. 1. Schematic of the fabrication of defect-induced color-tunable vertical LEDs.
Fig. 2. (a) Tilted SEM image of the Ti mask patterned on the p-GaN with a tilt angle of 60°. (b) SEM image of nanoholes on p-GaN with a depth of 100 nm. (c) SEM image of Ag nanorods after deliberate separation from p-GaN.
(b)
(a)
(c)
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Fig. 3. (a) EL spectrum of VLEDs with Ag nanorods embedded in p-GaN measured at room temperature under continuous current. (b) CIE (1931) chromaticity coordinates of VLEDs at various injection currents. EL images of VLED chips at injection currents of (c) 400, (d) 1000, and (e) 1600 mA.
and p-GaN in the Ag nanorod arrays embedded in the p-GaN case, we can conclude that a large number of Ga vacancies would be generated in the whole p-GaN during the thermal bonding process for fabricating VLEDs. Figure 3(a) shows EL spectra of VLEDs with Ag nanorod arrays embedded in p-GaN at various injection currents.
Under the injection current of 200 mA, only one red emission with a peak wavelength of ³750 nm was observed. As the current increases to 400 and 600 mA, the broadband red emission spectrum exhibits a significant blue shift. Moreover, as the injection current further increases, the red EL emission peaks broaden to the high-energy side, which is ³650 nm at
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(a)
Fig. 4. I–V characteristics of VLEDs with Ag nanorod arrays embedded in p-GaN. The inset shows the reverse I–V characteristics.
1600 mA injection current. As the injection current is increased from 200 to 1600 mA, there is a significant blue shift of the emission peak and the broadening of the full width at half maximum (FWHM). This behavior indicates that the Ga vacancies are related to the red emission probably from DAP recombination.11) Moreover, a blue emission peak centered at ³470 nm in reference to MQW emission emerges when the injection current is above 600 mA. The blue emission intensity increases considerably as the injection current increases from 600 to 1600 mA. Figure 3(b) shows the Commission International de I’Elairage (CIE) 1931 chromaticity coordinates at various currents. The CIE coordinates move from A (0.5906, 0.3759) to H (0.3748, 0.3632) as the injection current increases from 200 to 1600 mA, which corresponds to the red color to white light. The relationship between the injection current and the CIE coordinates obviously demonstrates a current-driven color-tunable characteristic. In this VLED with Ag nanorod arrays embedded in p-GaN, the emission color can be changed from red to white by increasing the injection current. Figure 3 also shows the EL images of VLED chips taken at injection currents of (c) 400, (d) 1000, and (e) 1600 mA. With the injection current increasing from 400 to 1600 mA, the emission color changes from red to yellow-white, and finally shows white emission. Figure 4 shows the I–V characteristics of VLEDs with Ag nanorods embedded in p-GaN. The inset shows the reverse I–V characteristics. Here, we can see that the current leakage is relatively high in the VLED. The ICP process for fabricating the nanohole arrays can cause surface damage and result in current leakage in the VLED. The forward threshold voltage of the VLED with Ag nanorod arrays embedded in pGaN is about 2 V, which is smaller than that of a normal blue MQW GaN-based LED. The emission spectrum in Fig. 3(a) shows that a forward voltage of less than 2 V is too low to drive blue MQW emission and red emission. As the forward voltage and injection current increase to ³2.1 V and ³200 mA, respectively, some electrons tunneling through EBL reach p-GaN by thermally assisted multistep tunneling.17) This results in red emission in p-GaN. Since the forward voltage is still too low to drive blue MQW emission, only red emission is observed in this voltage range. As the forward voltage and injection current further increase to ³2.9 V and ³600 mA, respectively, the MQW emission occurs. The blue emission efficiency markedly increased and exceeded that of the red emission as the injection current further increases. This defect-induced color-tunable VLED shows different
(b)
Fig. 5. (a) Room-temperature PL spectra of InGaN/GaN MQW VLEDs with Ag nanorod arrays embedded in p-GaN. (b) Detailed view of red emission PL spectra with excitation photon energy increasing from 2.18 to 3.18 eV. The inset shows the relationship between the PL peak wavelength and the excitation photon energy.
behaviors for various intensity ratios between long and short emission wavelengths compared with previous multicolor MQW emission18) and emissions of two active regions.19) Matsui et al.19) considered that the holes are built up near the p-side active region with increasing current injection owing to the heavy mass and small diffusion length. This leads to an increase in the intensity ratio of emission from a long wavelength with increasing injection current when a longwavelength active layer is located near the p-type layer. For the defect-induced color-tunable VLED, the intensity ratio of red emission increased as the injection current increased from 200 to 800 mA, which agrees with previous studies. However, the intensity ratio of blue emission increased as the injection current increased from 800 to 1600 mA. A possible reason for this phenomenon is that the hole concentration increased in the MQWs as the injection current further increased. The recombination efficiency in MQWs increased while the defect-related recombination in p-GaN saturated with the further increase in current injection. To better understand the origin of the red emission of the VLEDs, Figs. 5(a) and 5(b) show the room-temperature steady-state PL spectra of VLEDs with different excitation photon energies. As shown in Fig. 5(a), the two emission peaks refer to MQW emission at ³460 nm and defect-related emission at ³720 nm. With increasing excitation photon energy, there is a blue shift of the red emission, whereas there is no shift of the blue emission. A detailed view of the red emission PL spectra is shown in Fig. 5(b). With the excitation photon energy increasing from 2.18 to 3.18 eV, there is a clear blue shift for the red emission PL spectra. The inset figure shows the relationship between the PL peak wavelength and the excitation photon energy. These characteristics reveal that the red PL emission may originate from DAP transitions.11)
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Fig. 6. Schematic of the red DAP recombination process.
The recombination mechanisms related to the red emission are complex. Various transitions from both acceptors and donors to the deep trap are responsible for the obtained band.9) There may be different emission mechanisms under different experimental conditions. Figure 6 shows a schematic of a possible carrier transportation mechanism for the red DAP recombination process in p-GaN under our experimental conditions. In this model, the electrons were transported from n-GaN to p-GaN at forward bias by multistep tunneling or thermally assisted multistep tunneling.17) The localized states produced in EBL and p-GaN during the ICP etching and thermal bonding processes have made it possible to realize this transport. The deep centers enabling the carrier transport could be Ga vacancies,14–16) N vacancies,21) Mgvacancy complexes (MgGa–VN),10) oxygen ion, carbon, and other related complexes.12) Previous experiments showed that the red emission efficiency in GaN decreased with Mg doping because the VGa concentration decreased.20) In our experiment, Ga vacancies are generated in p-GaN during the thermal bonding process. Moreover, N vacancies21) and Mg-vacancy complexes (MgGa–VN)10) produced during ICP etching may be related to the red emission. PL measurements show the blue shift of the red emission with increasing excitation photon energy. The defect-related broad EL emission spectrum extending from green to red shows a blue shift with increasing injection current, suggesting that the red emission originates from DAP transitions. It is possible that the red DAP emission formed from an association with VGa, VN, and Mg-vacancy complexes (MgGa–VN). Owing to current leakage and DAP recombination saturation, the efficiency of this defect-induced VLED is lower than that of the traditional phosphor conversion LED. Many improvements are needed to increase the emission efficiency in future studies. In summary, we have demonstrated the fabrication of defect-induced color-tunable monolithic VLEDs. Large numbers of Ga atoms out-diffuse into Ag and leave Ga vacancies in p-GaN during the bonding process, which resulted in red emission. PL and EL measurements show a blue shift of the red emission with increasing excitation photon energy and injection current, suggesting that the red emission originates from DAP transitions. Since the blue MQW emission shows
a higher emission efficiency and a larger threshold voltage than the red emission, the emission color of VLEDs can be changed from red to white by increasing the injection current. Acknowledgments This project was supported by the National High Technology Research and Development Program of China (Grant No. 2014AA032608) and the Xi’an Jiaotong University State Key Laboratory for Mechanical Behavior of the Material Open Project (Grant No. 20121201).
1) S. Nakamura, Science 281, 956 (1998). 2) H. Li, J. Kang, P. Li, J. Ma, H. Wang, M. Liang, Z. Li, J. Li, X. Yi, and G. Wang, Appl. Phys. Lett. 102, 011105 (2013). 3) Y. Zhang, H. Xie, H. Zheng, T. Wei, H. Yang, J. Li, X. Yi, X. Song, G. Wang, and J. Li, Opt. Express 20, 6808 (2012). 4) Y. P. Huang, F. Yun, W. Ding, Y. Wang, H. Wang, Y. K. Zhao, Y. Zhang, M. F. Guo, X. Hou, and S. Liu, Acta Phys. Sin. 63, 127302 (2014). 5) H. J. Li, P. P. Li, J. J. Kang, Z. Li, Z. C. Li, J. Li, X. Y. Yi, and G. H. Wang, Appl. Phys. Express 6, 102103 (2013). 6) M. Funato, K. Hayashi, M. Ueda, Y. Kawakami, Y. Narukawa, and T. Mukai, Appl. Phys. Lett. 93, 021126 (2008). 7) Y. J. Hong, C. H. Lee, A. Yoon, M. Kim, H. K. Seong, H. J. Chung, C. Sone, Y. J. Park, and G.-C. Yi, Adv. Mater. 23, 3284 (2011). 8) T. H. Wang and Y. K. Kuo, Appl. Phys. Lett. 102, 171112 (2013). 9) M. W. Bayerl, M. S. Brandt, O. Ambacher, M. Stutzmann, E. R. Glaser, R. L. Henry, A. E. Wickenden, D. D. Koleske, T. Suski, I. Grzegory, and S. Porowski, Phys. Rev. B 63, 125203 (2001). 10) Q. Yan, A. Janotti, M. Scheffler, and C. G. Van de Walle, Appl. Phys. Lett. 100, 142110 (2012). 11) M. A. Reshchikov and H. Morkoc, J. Appl. Phys. 97, 061301 (2005). 12) M. A. Reshchikov, A. Usikov, H. Helava, and Y. Makarov, Appl. Phys. Lett. 104, 032103 (2014). 13) Y. Zhao, X. J. Zhang, J. Ye, L. M. Chen, S. P. Lau, W. J. Zhang, and S. T. Lee, ACS Nano 5, 3027 (2011). 14) J. O. Song, J. S. Kwak, Y. Park, and T. Y. Seong, Appl. Phys. Lett. 86, 062104 (2005). 15) H. W. Jang and J. L. Lee, Appl. Phys. Lett. 85, 5920 (2004). 16) J. S. Park, J. Han, J. S. Ha, and T. Y. Seong, Appl. Phys. Lett. 104, 172104 (2014). 17) Q. Shan, D. Meyaard, Q. Dai, J. Cho, E. F. Schubert, J. K. Son, and C. Sone, Appl. Phys. Lett. 99, 253506 (2011). 18) R. Charash, P. P. Maaskant, L. Lewis, C. McAleese, M. J. Kappers, C. J. Humphreys, and B. Corbett, Appl. Phys. Lett. 95, 151103 (2009). 19) K. Matsui, K. Yamashita, M. Kaga, T. Morita, T. Suzuki, T. Takeuch, S. Kamiyama, M. Iwaya, and I. Akasaki, Jpn. J. Appl. Phys. 52, 08JG02 (2013). 20) S. Zeng, G. N. Aliev, D. Wolverson, J. J. Davies, S. J. Bingham, D. A. Abdulmalik, P. G. Coleman, T. Wang, and P. J. Parbrook, Appl. Phys. Lett. 89, 022107 (2006). 21) U. Kaufmann, M. Kunzer, H. Obloh, M. Maier, Ch. Manz, A. Ramakrishnan, and B. Santic, Phys. Rev. B 59, 5561 (1999).
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Defect-induced color-tunable monolithic GaN-based light-emitting diodes
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Appl. Phys. Express 7 102102 (http://iopscience.iop.org/1882-0786/7/10/102102) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.120.242.61 This content was downloaded on 11/11/2014 at 23:20
Please note that terms and conditions apply.
Applied Physics Express 7, 102102 (2014) http://dx.doi.org/10.7567/APEX.7.102102
Defect-induced color-tunable monolithic GaN-based light-emitting diodes Yaping Huang1,2, Feng Yun1,2*, Yufeng Li1,2, Wen Ding1,2, Yue Wang2, Hong Wang2, Weihan Zhang2, Ye Zhang2, Maofeng Guo2, Shuo Liu3, and Xun Hou1,2 1 Key Laboratory for Physical Electronics and Devices of the Ministry of Education and Shaanxi Provincial Key Laboratory of Photonics & Information Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China 2 Solid-State Lighting Engineering Research Center, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China 3 Shaanxi Supernova Lighting Technology Co., Ltd., Xi’an, Shaanxi 710077, P. R. China E-mail: [email protected]
Received August 15, 2014; accepted September 2, 2014; published online September 18, 2014 We have demonstrated defect-induced color-tunable monolithic GaN-based vertical light-emitting diodes (VLEDs). With Ag nanorod arrays embedded in p-GaN, large numbers of Ga vacancies (VGa) were produced during the thermal bonding process in VLED fabrication. VGa-related donor–acceptor pair (DAP) transitions in p-GaN resulted in red emission in photoluminescence (PL) measurements as well as a broad electroluminescence (EL) emission spectrum extending from green to red. In combination with high-emission-efficiency blue InGaN/GaN multiple quantum wells (MQWs), the emission color of VLEDs can be changed from red to white by increasing the injection current. © 2014 The Japan Society of Applied Physics
n recent years, tremendous progress has been achieved in the development of InGaN/GaN quantum well (QW) light-emitting diodes (LEDs). This has resulted in a variety of applications such as backlighting, full-color displays, and general illumination.1–4) More extensive applications in the market demand for white and color-tunable LEDs since the fabrication technologies related to LEDs becoming to mature.5) Funato et al. reported color-tunable LEDs with dual-wavelength emission from multifaceted QWs.6) Li et al. demonstrated phosphor-free color-tunable LEDs by inserting an InGaN shallow QW between deep InGaN QWs and GaN barriers.5) Hong et al. fabricated visible-color-tunable LEDs using InGaN/GaN multiple quantum wells (MQWs) formed on GaN nanostructures.7) Wang and Kuo numerically studied the spectral competition of chirped dual-wavelength emission in monolithic InGaN MQW LEDs.8) Furthermore, deep defects that lead to photoluminescence (PL) at energies below the band gap play an important role in the performance of LEDs. An emission peak in red was observed in PL spectra of both Mg-doped and Si-doped GaN, but its origins are unknown.9–12) It was concluded that the red emission in Mg-doped GaN is attributed to nitrogen vacancies (VN)9) and Mg-vacancy complexes (MgGa–VN).10) Recently, red luminescence has been observed in fine-structure undoped GaN. Reshchikov et al. suggested that gallium vacancy (VGa), oxygen, carbon, and complexes related to donor–acceptor pair (DAP) transitions may result in red emission.12) Nevertheless, studies on the practical application of defect-related emission are rare, even though there are so many theoretical studies.9–12,17,18) In this work, we fabricated a defect-induced color-tunable monolithic GaN-based LED. With steady blue InGaN/GaN MQW emission and broadband defect-induced red emission in p-GaN, we obtained wide-emission LEDs covering the spectrum from blue to red. An InGaN/GaN MQW LED epistructure was grown on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD). The structure consists of nine periods of InGaN/GaN QWs (3 nm wells and 9 nm barriers) on a 3 µm n-GaN layer. The QWs are capped by a 20 nm electron-blocking layer (EBL) followed by a 120 nm Mg-doped p-GaN layer. The centroid wavelength of the wafer is ³460 nm. After MOCVD growth, vertical LEDs (VLEDs) with Ag nanorod arrays embedded in p-GaN were fabricated.
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Figure 1 shows a schematic of the fabrication of defectinduced color-tunable VLEDs. First, a hexagonally ordered monolayer template of polystyrene (PS) spheres (200 nm diameter) is coated on p-GaN by the self-assembly method.13) Then, the PS spheres are etched by oxygen plasma until the diameter is reduced to ³130 nm. A titanium thin film is then deposited on the template by electron beam evaporation. After that, a highly ordered two-dimensional (2D) hexagonal Ti nanohole mask on p-GaN is obtained by dissolving PS in trichloromethane. Nanohole arrays of 130 nm depth on p-GaN are fabricated with a Ti mask by BCl3/Cl2 inductively coupled plasma (ICP) etching. Then, Ni (1 nm)/Ag (150 nm) is deposited into the nanoholes by electron beam evaporation after the Ti mask is removed. A Ti (100 nm)/Au (500 nm) layer is then deposited on the Ag layer for bonding to the Cu/W substrate, as shown in Fig. 1. The p-electrode surface of the LED and the Cu/W substrate coated by Ti (50 nm)/Au0.8Sn0.2 (1000 nm) solder are then bonded to each other at 380 °C for 20 min under vacuum. The sapphire substrate is removed using a 248 nm KrF excimer laser lift-off process. Finally, an Al (1500 nm) electrode is deposited on n-GaN after u-GaN is etched using ICP. VLEDs with a chip size of 55 © 55 mil2 were fabricated by this method. Current–voltage (I–V ) measurements were carried out on a MPI LEDA-AT-T200 source meter. The electroluminescence (EL) spectra of the device were measured using an Ocean Optics S2000 spectrometer that employed a fiber optic cable for light collection. The PL spectra were excited by a xenon lamp at room temperature. Figure 2(a) shows a tilted scanning electron microscopy (SEM) image of the hexagonally patterned Ti mask with 200 nm pitch and 130 nm hole diameter at the surface of p-GaN. Figure 2(b) shows an SEM image of the nanoholes on p-GaN with a tilt angle of 60°. The nanohole depth is ³130 nm measured from the tilted SEM image after angle correction. Figure 2(c) shows an SEM image of the sample after Ag deposition, where Ag nanorod arrays embedded in p-GaN can be clearly observed and confirmed by deliberately separating the Ag deposition layer from p-GaN. For the Ag and GaN contact, Ga atoms would out-diffuse into the Ag layer and form a Ag–Ga solid solution at the interface of Ag and GaN during thermal annealing. The out-diffusion of Ga atoms results in the formation of Ga vacancies in the GaN surface region.14–16) Because of the huge contact area of Ag
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Fig. 1. Schematic of the fabrication of defect-induced color-tunable vertical LEDs.
Fig. 2. (a) Tilted SEM image of the Ti mask patterned on the p-GaN with a tilt angle of 60°. (b) SEM image of nanoholes on p-GaN with a depth of 100 nm. (c) SEM image of Ag nanorods after deliberate separation from p-GaN.
(b)
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(c)
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Fig. 3. (a) EL spectrum of VLEDs with Ag nanorods embedded in p-GaN measured at room temperature under continuous current. (b) CIE (1931) chromaticity coordinates of VLEDs at various injection currents. EL images of VLED chips at injection currents of (c) 400, (d) 1000, and (e) 1600 mA.
and p-GaN in the Ag nanorod arrays embedded in the p-GaN case, we can conclude that a large number of Ga vacancies would be generated in the whole p-GaN during the thermal bonding process for fabricating VLEDs. Figure 3(a) shows EL spectra of VLEDs with Ag nanorod arrays embedded in p-GaN at various injection currents.
Under the injection current of 200 mA, only one red emission with a peak wavelength of ³750 nm was observed. As the current increases to 400 and 600 mA, the broadband red emission spectrum exhibits a significant blue shift. Moreover, as the injection current further increases, the red EL emission peaks broaden to the high-energy side, which is ³650 nm at
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(a)
Fig. 4. I–V characteristics of VLEDs with Ag nanorod arrays embedded in p-GaN. The inset shows the reverse I–V characteristics.
1600 mA injection current. As the injection current is increased from 200 to 1600 mA, there is a significant blue shift of the emission peak and the broadening of the full width at half maximum (FWHM). This behavior indicates that the Ga vacancies are related to the red emission probably from DAP recombination.11) Moreover, a blue emission peak centered at ³470 nm in reference to MQW emission emerges when the injection current is above 600 mA. The blue emission intensity increases considerably as the injection current increases from 600 to 1600 mA. Figure 3(b) shows the Commission International de I’Elairage (CIE) 1931 chromaticity coordinates at various currents. The CIE coordinates move from A (0.5906, 0.3759) to H (0.3748, 0.3632) as the injection current increases from 200 to 1600 mA, which corresponds to the red color to white light. The relationship between the injection current and the CIE coordinates obviously demonstrates a current-driven color-tunable characteristic. In this VLED with Ag nanorod arrays embedded in p-GaN, the emission color can be changed from red to white by increasing the injection current. Figure 3 also shows the EL images of VLED chips taken at injection currents of (c) 400, (d) 1000, and (e) 1600 mA. With the injection current increasing from 400 to 1600 mA, the emission color changes from red to yellow-white, and finally shows white emission. Figure 4 shows the I–V characteristics of VLEDs with Ag nanorods embedded in p-GaN. The inset shows the reverse I–V characteristics. Here, we can see that the current leakage is relatively high in the VLED. The ICP process for fabricating the nanohole arrays can cause surface damage and result in current leakage in the VLED. The forward threshold voltage of the VLED with Ag nanorod arrays embedded in pGaN is about 2 V, which is smaller than that of a normal blue MQW GaN-based LED. The emission spectrum in Fig. 3(a) shows that a forward voltage of less than 2 V is too low to drive blue MQW emission and red emission. As the forward voltage and injection current increase to ³2.1 V and ³200 mA, respectively, some electrons tunneling through EBL reach p-GaN by thermally assisted multistep tunneling.17) This results in red emission in p-GaN. Since the forward voltage is still too low to drive blue MQW emission, only red emission is observed in this voltage range. As the forward voltage and injection current further increase to ³2.9 V and ³600 mA, respectively, the MQW emission occurs. The blue emission efficiency markedly increased and exceeded that of the red emission as the injection current further increases. This defect-induced color-tunable VLED shows different
(b)
Fig. 5. (a) Room-temperature PL spectra of InGaN/GaN MQW VLEDs with Ag nanorod arrays embedded in p-GaN. (b) Detailed view of red emission PL spectra with excitation photon energy increasing from 2.18 to 3.18 eV. The inset shows the relationship between the PL peak wavelength and the excitation photon energy.
behaviors for various intensity ratios between long and short emission wavelengths compared with previous multicolor MQW emission18) and emissions of two active regions.19) Matsui et al.19) considered that the holes are built up near the p-side active region with increasing current injection owing to the heavy mass and small diffusion length. This leads to an increase in the intensity ratio of emission from a long wavelength with increasing injection current when a longwavelength active layer is located near the p-type layer. For the defect-induced color-tunable VLED, the intensity ratio of red emission increased as the injection current increased from 200 to 800 mA, which agrees with previous studies. However, the intensity ratio of blue emission increased as the injection current increased from 800 to 1600 mA. A possible reason for this phenomenon is that the hole concentration increased in the MQWs as the injection current further increased. The recombination efficiency in MQWs increased while the defect-related recombination in p-GaN saturated with the further increase in current injection. To better understand the origin of the red emission of the VLEDs, Figs. 5(a) and 5(b) show the room-temperature steady-state PL spectra of VLEDs with different excitation photon energies. As shown in Fig. 5(a), the two emission peaks refer to MQW emission at ³460 nm and defect-related emission at ³720 nm. With increasing excitation photon energy, there is a blue shift of the red emission, whereas there is no shift of the blue emission. A detailed view of the red emission PL spectra is shown in Fig. 5(b). With the excitation photon energy increasing from 2.18 to 3.18 eV, there is a clear blue shift for the red emission PL spectra. The inset figure shows the relationship between the PL peak wavelength and the excitation photon energy. These characteristics reveal that the red PL emission may originate from DAP transitions.11)
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Fig. 6. Schematic of the red DAP recombination process.
The recombination mechanisms related to the red emission are complex. Various transitions from both acceptors and donors to the deep trap are responsible for the obtained band.9) There may be different emission mechanisms under different experimental conditions. Figure 6 shows a schematic of a possible carrier transportation mechanism for the red DAP recombination process in p-GaN under our experimental conditions. In this model, the electrons were transported from n-GaN to p-GaN at forward bias by multistep tunneling or thermally assisted multistep tunneling.17) The localized states produced in EBL and p-GaN during the ICP etching and thermal bonding processes have made it possible to realize this transport. The deep centers enabling the carrier transport could be Ga vacancies,14–16) N vacancies,21) Mgvacancy complexes (MgGa–VN),10) oxygen ion, carbon, and other related complexes.12) Previous experiments showed that the red emission efficiency in GaN decreased with Mg doping because the VGa concentration decreased.20) In our experiment, Ga vacancies are generated in p-GaN during the thermal bonding process. Moreover, N vacancies21) and Mg-vacancy complexes (MgGa–VN)10) produced during ICP etching may be related to the red emission. PL measurements show the blue shift of the red emission with increasing excitation photon energy. The defect-related broad EL emission spectrum extending from green to red shows a blue shift with increasing injection current, suggesting that the red emission originates from DAP transitions. It is possible that the red DAP emission formed from an association with VGa, VN, and Mg-vacancy complexes (MgGa–VN). Owing to current leakage and DAP recombination saturation, the efficiency of this defect-induced VLED is lower than that of the traditional phosphor conversion LED. Many improvements are needed to increase the emission efficiency in future studies. In summary, we have demonstrated the fabrication of defect-induced color-tunable monolithic VLEDs. Large numbers of Ga atoms out-diffuse into Ag and leave Ga vacancies in p-GaN during the bonding process, which resulted in red emission. PL and EL measurements show a blue shift of the red emission with increasing excitation photon energy and injection current, suggesting that the red emission originates from DAP transitions. Since the blue MQW emission shows
a higher emission efficiency and a larger threshold voltage than the red emission, the emission color of VLEDs can be changed from red to white by increasing the injection current. Acknowledgments This project was supported by the National High Technology Research and Development Program of China (Grant No. 2014AA032608) and the Xi’an Jiaotong University State Key Laboratory for Mechanical Behavior of the Material Open Project (Grant No. 20121201).
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