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Ultraviolet light-emitting diodes with polarization-doped p-type layer
Accepted Manuscript Ultraviolet light-emitting diodes with polarization-doped p-type layer Wenxiao Hu, Ping Qin, Weidong Song, Chongzhen Zhang, Rupeng Wang, Liangliang Zhao, Chao Xia, Songyang Yuan, Yian Yin, Shuti Li PII:
S0749-6036(16)30269-5
DOI:
10.1016/j.spmi.2016.06.016
Reference:
YSPMI 4377
To appear in:
Superlattices and Microstructures
Received Date: 5 May 2016 Revised Date:
7 June 2016
Accepted Date: 8 June 2016
Please cite this article as: W. Hu, P. Qin, W. Song, C. Zhang, R. Wang, L. Zhao, C. Xia, S. Yuan, Y. Yin, S. Li, Ultraviolet light-emitting diodes with polarization-doped p-type layer, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.06.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Ultraviolet light-emitting diodes with polarization-doped p-type layer Wenxiao Hu a, Ping Qin a, Weidong Song a, Chongzhen Zhang a, Rupeng Wang a, Liangliang Zhao a , Chao Xia a, Songyang Yuan a, Yian Yin a,b and Shuti Li a,b,* a) Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, PR China
Opto-electronic Materials and Technologs, PR China
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b) Guangdong Engineering Research Center of Optoelectronic Functional Materials and Devices, Institute of
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Abstract We report ultraviolet light emitting diode (LEDs) with polarization doped p-type layer. Fabricated LEDs with polarization doped p-type layer exhibited reduced forward voltage and enhanced light output power, compared to those with traditional p-type AlGaN layer. The improvement is attributed to improved hole concentration and the smooth valence band by the polarization enhanced p-type doping. Our simulated results reveal that this p-type layer can further enhance the performance of ultraviolet LEDs by removing the electron blocking layer (EBL). Keywords: Ultraviolet light emitting diode, polarization-induced-doping, metalorganic chemical vapor deposition (MOCVD)
1. Introduction
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In the past decade, ultraviolet (UV) light emitting diodes (LEDs) have been replacing incumbent technologies like mercury UV lighting sources, due to its advantages such as compactness, low cost, long lifetime and environmentally-friendly composition. AlGaN alloys, with direct bandgap tunable from 3.4 to 6.2 eV, are the most essential building blocks for UV LEDs.[1, 2] However, the activation of Mg is known to increase linearly with Al composition, which leads to limited hole concentration and p-type conductivity in p-type AlGaN layer.[3, 4] Subsequently, UV LEDs often suffer from ineffective hole injection and enlarged on-resistance. To circumvent this issue, numerous techniques have been proposed, such as Mg delta doping, AlGaN/GaN superlattice, modulation doping[5-7]. Recently, D. Jena and J. Simon et al.[8, 9] found that a three dimensional (3D) electron/hole gas can be formed in a graded AlGaN layer, induced by the strong spontaneous and piezoelectric polarizations. L. Zhang et al.[10, 11] used this graded AlGaN layer as the p-type layer and enormously enhanced the performance of blue LED both in simulation and experimental results. Polarization induced hole doping on the order of ~1018cm-3 is achieved in linearly graded AlGaN layer by S. Li et al.[12], and they also realized a type of pn-junction by graded AlGaN layer without any impurity-doping.[13] Zhang et al.[14] found that this 3D hole gas can also act as a hole source of LED even with Mg-doping free. We have compared performance of InGaN/GaN blue LEDs with step graded AlGaN electron blocking layer (EBL) and found that the AlGaN EBL with decreasing Al composition will lead to enhanced light output power and reduced droop effect, as compared with traditional bulk AlGaN EBL.[15] In this letter, we demonstrate polarization doped p-type layers with graded p-type AlGaN for 364 nm UV LEDs. UV LEDs with various growth schemes for the p-type layer have been fabricated and investigated. Our experimental results indicate that LEDs with polarization doped p-type layers reduced forward voltage and enhanced light output power, compared to those with traditional p-type AlGaN layer. A device simulation was also carried out to analyze the UV LEDs with different p-type layer designs.
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2. Device structure and fabrication
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FIG. 1. Structures of sample A, B and C
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The UV LED samples used in this work were grown in an Aixtron close coupled showerhead (CCS) metal organic chemical vapor deposition (MOCVD) system. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) were used as Ga, Al and N sources, respectively. Nitrogen and hydrogen were used as carrier gas. From bottom to top, the original UV LED epi structure (labelled as sample A) consists of a 25 nm thick low-temperature nucleation layer, a 2.0 μm unintentionally doped GaN layer, a 2.0 μm n-type GaN layer, 6 period of Al0.15Ga0.85N/GaN MQWs with 3 nm thick wells and 9 nm thick barriers, a 15 nm Mg-doped Al0.3Ga0.7N EBL layer, an 85 nm p-Al0.2Ga0.8N layer, and a 10nm p-GaN contact layer. The other epitaxial structure (labelled as sample B) was composed of the identical structure as sample A except that the 85 nm p-Al0.2Ga0.8N layer was replaced by an Mg-doped AlGaN layer with Al composition linearly graded from 0.3 to 0.1. To verify the capability of the graded AlGaN layer in electron blocking, we also grew another structure without the p-type Al0.3Ga0.7N EBL layer. Correspondingly, a 100 nm linearly graded p-type AlGaN layer was adopted. Encapsulated LED chips on sapphire were then fabricated to a conventional mesa structure with geometry of 12mil×25mil rectangular shape. Electrical properties were measured by on-wafer probing and luminescence properties were collected by calibrated integrating sphere at room temperature.
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3. Results and discussion
* Corresponding author.
E-mail address: [email protected] (S. T. Li).
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FIG. 2. Current-voltage (I-V) curves of sample A, B and C
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Fig. 2 shows the current-voltage (I-V) curves of sample A, B and C. We can find that sample B and C have a much lower forward voltage compare with that of sample A. What’s more, the trend of voltage rising of sample B and C is much milder. The results show that, adopting polarization doped p-type layer can reduce the forward voltage of UV LED effectively, indicating the lower resistivity of it compare with the conventional p-type layer. For these two p-type layers of sample A and B have the same thickness and the same mean Al component, we ascribe the lower resistivity of sample B to the higher hole concentration in polarization doped p-type layer. On the other hand, we can find that the forward voltage of sample C is less than sample B. This indicates the higher forward voltage of sample B comes from the EBL. Like the polarization-doped AlGaN has a relatively lower resistivity than conventional doped one, sample B with a conventional Mg-doped EBL has a higher resistivity than the corresponding p-type layer, leading to a higher forward voltage than sample C.
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The electroluminescence (EL) spectrum under the condition of 20mA and the light output power curve of sample A, B and C are plotted in Fig. 3 and 4. Obviously, sample B and C have a better luminescence than sample A. And the gap between them in light output power become bigger along with the current increasing in the range we have measured. These may indicate that the polarization-doped AlGaN in sample B and C may have a higher hole concentration, which leading to a better performance of LEDs. On the other hand, conventional EBL not only block electron from overflowing through a conduction band barrier ΔEC, but also prevent hole injection efficiency by the unavoidable valence-band offset ΔEV in it like sample A. Correspondingly, our polarization-doped graded layer in sample B begins with Al0.3Ga0.7N, which is same as the EBL. With this p-type layer, the LED has a smooth valence band in the graded AlGaN/EBL interface. This smooth valence band and the increased hole concentration both promote holes injecting into the MQWS, which result a better EL intensity and light output power of sample B. FIG. 3. EL intensity of sample A, B and C FIG. 4. Light output power curve of sample A, B and C Fig. 4 also shows that the light output power of the sample B and C are nearly the same, indicating this graded AlGaN can block electron effectively without an EBL. But we should also point out that the light output power of sample C is slightly lower than sample B. This can be due to the graded AlGaN layer in sample B and C have a different thickness. The hole concentration in the polarization-doped AlGaN is proportional to the degree of AlGaN gradation Δx/d.[8] Here, d is the thickness of the AlGaN (85nm and 100nm respectively for sample B and C) and Δx is 0.2 in our samples. So the degree of AlGaN gradation in sample B is higher, and this make the hole concentration 17.6% higher than sample C according to the theory, leading to a better output power. But we should notice that the distinction of
ACCEPTED MANUSCRIPT the output power is negligible, and this reflects that removing the EBL may play a role in enhancing the output power in other side.
FIG. 6. Hole concentration of sample A, B and C.
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FIG. 5. Electron current densities of sample A, B and C
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4. Numerical study
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For further study the structure that use a graded Al component AlGaN layer without an EBL as the p-type layer, we numerically study on the sample A, B and C by the APSYS simulation program. Similar to our experimental results, the numerical results show that sample B and C have a better performance than sample A. Fig. 5 and 6 shows the electron current densities and the hole concentration of these three samples under the condition of 180mA. And the horizontal position of sample B and C has been shifted slightly for better observation in Fig. 6. It can be found that sample C has the lowest electron leakage current in the p-type layer, illustrating polarization-doped p-type layer can block electron effectively without an EBL. On the other hand, sample C also has the highest hole concentration in the MQWs, attributed to its smooth valence-band by removing the EBL and thus a better hole injecting into the MQWs. These numerically results are coincident with our discussion before, that removing the EBL can enhance the light output power in some way, and the better performance of sample B lists before is ascribed to its higher hole concentration for its higher degree of AlGaN gradation.
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5. Conclusions In summary, using polarization doped p-type layer can enhance the performance of the ultraviolet LED effectively, this can be ascribed to the better Mg doping effect and lower resistivity than the conventional p-type layer. What’s more, using this polarization doped p-type layer without a traditional EBL can further enhance the hole injecting into the MQWs while blocking electron effectively. These features can make the structure of the ultraviolet LED simpler and further enhance the performance of the UV LED by remove the EBL. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.11474105, No. 51172079), the Science and Technology Program of Guangdong Province, China (Grant No. 2015B090903078, No. 2015B010105011), the program for Changjiang Scholars and Innovative Research Team in University (Project No. IRT13064), the Science and Technology Project of Guangzhou City (No. 201510010229), the Science and Technology Planning Project of Guangdong Province (No. 2015A010105025).
ACCEPTED MANUSCRIPT [1] A. Khan, K. Balakrishnan, and T. Katona, Nature Photonics 2 (2008) 77-84. [2] Y. Taniyasu, M. Kasu, and T. Makimoto, Nature 441 (2006) 325-328. [3] J. Li, T. Oder, M. Nakarmi, J. Lin, and H. Jiang, Applied physics letters 80 (2002) 1210-1212. [4] K. Nam, M. Nakarmi, J. Li, J. Lin, and H. Jiang, Applied physics letters 83 (2003) 878-880. [5] J. Li, W. Lin, W. Yang, W. Cai, Q. Pan, X. Lin, S. Li, H. Chen, D. Liu, and J. Cai, Journal of Crystal Growth 311 (2009) 478-481. [6] K. Kumakura, T. Makimoto, and N. Kobayashi, Japanese Journal of Applied Physics 39 (2000)p. 2428. [7] J. Li and J. Kang, Applied Physics Letters 91 (2007)p. 152106.
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[8] J. Simon, V. Protasenko, C. X. Lian, H. L. Xing, and D. Jena, Science 327 (2010) 60-64.
[9] D. Jena, S. Heikman, D. Green, D. Buttari, R. Coffie, H. Xing, S. Keller, S. DenBaars, J. S. Speck, U. K. Mishra, and I. Smorchkova, Applied Physics Letters 81 (2002) 4395-4397.
[10] L. Zhang, K. Ding, N. X. Liu, T. B. Wei, X. L. Ji, P. Ma, J. C. Yan, J. X. Wang, Y. P. Zeng, and J. M. Li, Applied Physics Letters 98 (2011).
[11] L. Zhang, X. C. Wei, N. X. Liu, H. X. Lu, J. P. Zeng, J. X. Wang, Y. P. Zeng, and J. M. Li, Applied Physics Letters 98 (2011).
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[12] S. B. Li, T. Zhang, J. Wu, Y. J. Yang, Z. M. Wang, Z. M. Wu, Z. Chen, and Y. D. Jiang, Applied Physics Letters 102 (2013). [13] S. B. Li, M. Ware, J. Wu, P. Minor, Z. M. Wang, Z. M. Wu, Y. D. Jiang, and G. J. Salamo, Applied Physics Letters 101 (2012). [14] Z.-H. Zhang, S. T. Tan, Z. Kyaw, W. Liu, Y. Ji, Z. Ju, X. Zhang, X. W. Sun, and H. V. Demir, Applied Physics Letters 103 (2013).
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[15] C. Liu, Z. Ren, X. Chen, B. Zhao, X. Wang, Y. Yin, and S. Li, Photonics Technology Letters, IEEE 26 (2014) 134-137.
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Highlights:
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1. We report ultraviolet light emitting diodes (LEDs) with polarization doped p-type layer to improve the low hole concentration and p-type conductivity in p-type AlGaN layer. 2. Both experimental and simulated results reveal that this p-type layer can enhance the performance of ultraviolet LEDs. 3. This p-type layer can block electron effectively and enhance the hole injecting into the MQWs without an EBL.
S0749-6036(16)30269-5
DOI:
10.1016/j.spmi.2016.06.016
Reference:
YSPMI 4377
To appear in:
Superlattices and Microstructures
Received Date: 5 May 2016 Revised Date:
7 June 2016
Accepted Date: 8 June 2016
Please cite this article as: W. Hu, P. Qin, W. Song, C. Zhang, R. Wang, L. Zhao, C. Xia, S. Yuan, Y. Yin, S. Li, Ultraviolet light-emitting diodes with polarization-doped p-type layer, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.06.016. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Ultraviolet light-emitting diodes with polarization-doped p-type layer Wenxiao Hu a, Ping Qin a, Weidong Song a, Chongzhen Zhang a, Rupeng Wang a, Liangliang Zhao a , Chao Xia a, Songyang Yuan a, Yian Yin a,b and Shuti Li a,b,* a) Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-electronic Materials and Technology, South China Normal University, Guangzhou 510631, PR China
Opto-electronic Materials and Technologs, PR China
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b) Guangdong Engineering Research Center of Optoelectronic Functional Materials and Devices, Institute of
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Abstract We report ultraviolet light emitting diode (LEDs) with polarization doped p-type layer. Fabricated LEDs with polarization doped p-type layer exhibited reduced forward voltage and enhanced light output power, compared to those with traditional p-type AlGaN layer. The improvement is attributed to improved hole concentration and the smooth valence band by the polarization enhanced p-type doping. Our simulated results reveal that this p-type layer can further enhance the performance of ultraviolet LEDs by removing the electron blocking layer (EBL). Keywords: Ultraviolet light emitting diode, polarization-induced-doping, metalorganic chemical vapor deposition (MOCVD)
1. Introduction
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In the past decade, ultraviolet (UV) light emitting diodes (LEDs) have been replacing incumbent technologies like mercury UV lighting sources, due to its advantages such as compactness, low cost, long lifetime and environmentally-friendly composition. AlGaN alloys, with direct bandgap tunable from 3.4 to 6.2 eV, are the most essential building blocks for UV LEDs.[1, 2] However, the activation of Mg is known to increase linearly with Al composition, which leads to limited hole concentration and p-type conductivity in p-type AlGaN layer.[3, 4] Subsequently, UV LEDs often suffer from ineffective hole injection and enlarged on-resistance. To circumvent this issue, numerous techniques have been proposed, such as Mg delta doping, AlGaN/GaN superlattice, modulation doping[5-7]. Recently, D. Jena and J. Simon et al.[8, 9] found that a three dimensional (3D) electron/hole gas can be formed in a graded AlGaN layer, induced by the strong spontaneous and piezoelectric polarizations. L. Zhang et al.[10, 11] used this graded AlGaN layer as the p-type layer and enormously enhanced the performance of blue LED both in simulation and experimental results. Polarization induced hole doping on the order of ~1018cm-3 is achieved in linearly graded AlGaN layer by S. Li et al.[12], and they also realized a type of pn-junction by graded AlGaN layer without any impurity-doping.[13] Zhang et al.[14] found that this 3D hole gas can also act as a hole source of LED even with Mg-doping free. We have compared performance of InGaN/GaN blue LEDs with step graded AlGaN electron blocking layer (EBL) and found that the AlGaN EBL with decreasing Al composition will lead to enhanced light output power and reduced droop effect, as compared with traditional bulk AlGaN EBL.[15] In this letter, we demonstrate polarization doped p-type layers with graded p-type AlGaN for 364 nm UV LEDs. UV LEDs with various growth schemes for the p-type layer have been fabricated and investigated. Our experimental results indicate that LEDs with polarization doped p-type layers reduced forward voltage and enhanced light output power, compared to those with traditional p-type AlGaN layer. A device simulation was also carried out to analyze the UV LEDs with different p-type layer designs.
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2. Device structure and fabrication
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FIG. 1. Structures of sample A, B and C
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The UV LED samples used in this work were grown in an Aixtron close coupled showerhead (CCS) metal organic chemical vapor deposition (MOCVD) system. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH3) were used as Ga, Al and N sources, respectively. Nitrogen and hydrogen were used as carrier gas. From bottom to top, the original UV LED epi structure (labelled as sample A) consists of a 25 nm thick low-temperature nucleation layer, a 2.0 μm unintentionally doped GaN layer, a 2.0 μm n-type GaN layer, 6 period of Al0.15Ga0.85N/GaN MQWs with 3 nm thick wells and 9 nm thick barriers, a 15 nm Mg-doped Al0.3Ga0.7N EBL layer, an 85 nm p-Al0.2Ga0.8N layer, and a 10nm p-GaN contact layer. The other epitaxial structure (labelled as sample B) was composed of the identical structure as sample A except that the 85 nm p-Al0.2Ga0.8N layer was replaced by an Mg-doped AlGaN layer with Al composition linearly graded from 0.3 to 0.1. To verify the capability of the graded AlGaN layer in electron blocking, we also grew another structure without the p-type Al0.3Ga0.7N EBL layer. Correspondingly, a 100 nm linearly graded p-type AlGaN layer was adopted. Encapsulated LED chips on sapphire were then fabricated to a conventional mesa structure with geometry of 12mil×25mil rectangular shape. Electrical properties were measured by on-wafer probing and luminescence properties were collected by calibrated integrating sphere at room temperature.
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3. Results and discussion
* Corresponding author.
E-mail address: [email protected] (S. T. Li).
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FIG. 2. Current-voltage (I-V) curves of sample A, B and C
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Fig. 2 shows the current-voltage (I-V) curves of sample A, B and C. We can find that sample B and C have a much lower forward voltage compare with that of sample A. What’s more, the trend of voltage rising of sample B and C is much milder. The results show that, adopting polarization doped p-type layer can reduce the forward voltage of UV LED effectively, indicating the lower resistivity of it compare with the conventional p-type layer. For these two p-type layers of sample A and B have the same thickness and the same mean Al component, we ascribe the lower resistivity of sample B to the higher hole concentration in polarization doped p-type layer. On the other hand, we can find that the forward voltage of sample C is less than sample B. This indicates the higher forward voltage of sample B comes from the EBL. Like the polarization-doped AlGaN has a relatively lower resistivity than conventional doped one, sample B with a conventional Mg-doped EBL has a higher resistivity than the corresponding p-type layer, leading to a higher forward voltage than sample C.
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The electroluminescence (EL) spectrum under the condition of 20mA and the light output power curve of sample A, B and C are plotted in Fig. 3 and 4. Obviously, sample B and C have a better luminescence than sample A. And the gap between them in light output power become bigger along with the current increasing in the range we have measured. These may indicate that the polarization-doped AlGaN in sample B and C may have a higher hole concentration, which leading to a better performance of LEDs. On the other hand, conventional EBL not only block electron from overflowing through a conduction band barrier ΔEC, but also prevent hole injection efficiency by the unavoidable valence-band offset ΔEV in it like sample A. Correspondingly, our polarization-doped graded layer in sample B begins with Al0.3Ga0.7N, which is same as the EBL. With this p-type layer, the LED has a smooth valence band in the graded AlGaN/EBL interface. This smooth valence band and the increased hole concentration both promote holes injecting into the MQWS, which result a better EL intensity and light output power of sample B. FIG. 3. EL intensity of sample A, B and C FIG. 4. Light output power curve of sample A, B and C Fig. 4 also shows that the light output power of the sample B and C are nearly the same, indicating this graded AlGaN can block electron effectively without an EBL. But we should also point out that the light output power of sample C is slightly lower than sample B. This can be due to the graded AlGaN layer in sample B and C have a different thickness. The hole concentration in the polarization-doped AlGaN is proportional to the degree of AlGaN gradation Δx/d.[8] Here, d is the thickness of the AlGaN (85nm and 100nm respectively for sample B and C) and Δx is 0.2 in our samples. So the degree of AlGaN gradation in sample B is higher, and this make the hole concentration 17.6% higher than sample C according to the theory, leading to a better output power. But we should notice that the distinction of
ACCEPTED MANUSCRIPT the output power is negligible, and this reflects that removing the EBL may play a role in enhancing the output power in other side.
FIG. 6. Hole concentration of sample A, B and C.
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FIG. 5. Electron current densities of sample A, B and C
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4. Numerical study
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For further study the structure that use a graded Al component AlGaN layer without an EBL as the p-type layer, we numerically study on the sample A, B and C by the APSYS simulation program. Similar to our experimental results, the numerical results show that sample B and C have a better performance than sample A. Fig. 5 and 6 shows the electron current densities and the hole concentration of these three samples under the condition of 180mA. And the horizontal position of sample B and C has been shifted slightly for better observation in Fig. 6. It can be found that sample C has the lowest electron leakage current in the p-type layer, illustrating polarization-doped p-type layer can block electron effectively without an EBL. On the other hand, sample C also has the highest hole concentration in the MQWs, attributed to its smooth valence-band by removing the EBL and thus a better hole injecting into the MQWs. These numerically results are coincident with our discussion before, that removing the EBL can enhance the light output power in some way, and the better performance of sample B lists before is ascribed to its higher hole concentration for its higher degree of AlGaN gradation.
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5. Conclusions In summary, using polarization doped p-type layer can enhance the performance of the ultraviolet LED effectively, this can be ascribed to the better Mg doping effect and lower resistivity than the conventional p-type layer. What’s more, using this polarization doped p-type layer without a traditional EBL can further enhance the hole injecting into the MQWs while blocking electron effectively. These features can make the structure of the ultraviolet LED simpler and further enhance the performance of the UV LED by remove the EBL. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.11474105, No. 51172079), the Science and Technology Program of Guangdong Province, China (Grant No. 2015B090903078, No. 2015B010105011), the program for Changjiang Scholars and Innovative Research Team in University (Project No. IRT13064), the Science and Technology Project of Guangzhou City (No. 201510010229), the Science and Technology Planning Project of Guangdong Province (No. 2015A010105025).
ACCEPTED MANUSCRIPT [1] A. Khan, K. Balakrishnan, and T. Katona, Nature Photonics 2 (2008) 77-84. [2] Y. Taniyasu, M. Kasu, and T. Makimoto, Nature 441 (2006) 325-328. [3] J. Li, T. Oder, M. Nakarmi, J. Lin, and H. Jiang, Applied physics letters 80 (2002) 1210-1212. [4] K. Nam, M. Nakarmi, J. Li, J. Lin, and H. Jiang, Applied physics letters 83 (2003) 878-880. [5] J. Li, W. Lin, W. Yang, W. Cai, Q. Pan, X. Lin, S. Li, H. Chen, D. Liu, and J. Cai, Journal of Crystal Growth 311 (2009) 478-481. [6] K. Kumakura, T. Makimoto, and N. Kobayashi, Japanese Journal of Applied Physics 39 (2000)p. 2428. [7] J. Li and J. Kang, Applied Physics Letters 91 (2007)p. 152106.
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[8] J. Simon, V. Protasenko, C. X. Lian, H. L. Xing, and D. Jena, Science 327 (2010) 60-64.
[9] D. Jena, S. Heikman, D. Green, D. Buttari, R. Coffie, H. Xing, S. Keller, S. DenBaars, J. S. Speck, U. K. Mishra, and I. Smorchkova, Applied Physics Letters 81 (2002) 4395-4397.
[10] L. Zhang, K. Ding, N. X. Liu, T. B. Wei, X. L. Ji, P. Ma, J. C. Yan, J. X. Wang, Y. P. Zeng, and J. M. Li, Applied Physics Letters 98 (2011).
[11] L. Zhang, X. C. Wei, N. X. Liu, H. X. Lu, J. P. Zeng, J. X. Wang, Y. P. Zeng, and J. M. Li, Applied Physics Letters 98 (2011).
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[12] S. B. Li, T. Zhang, J. Wu, Y. J. Yang, Z. M. Wang, Z. M. Wu, Z. Chen, and Y. D. Jiang, Applied Physics Letters 102 (2013). [13] S. B. Li, M. Ware, J. Wu, P. Minor, Z. M. Wang, Z. M. Wu, Y. D. Jiang, and G. J. Salamo, Applied Physics Letters 101 (2012). [14] Z.-H. Zhang, S. T. Tan, Z. Kyaw, W. Liu, Y. Ji, Z. Ju, X. Zhang, X. W. Sun, and H. V. Demir, Applied Physics Letters 103 (2013).
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[15] C. Liu, Z. Ren, X. Chen, B. Zhao, X. Wang, Y. Yin, and S. Li, Photonics Technology Letters, IEEE 26 (2014) 134-137.
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Highlights:
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1. We report ultraviolet light emitting diodes (LEDs) with polarization doped p-type layer to improve the low hole concentration and p-type conductivity in p-type AlGaN layer. 2. Both experimental and simulated results reveal that this p-type layer can enhance the performance of ultraviolet LEDs. 3. This p-type layer can block electron effectively and enhance the hole injecting into the MQWs without an EBL.