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In order to enhance the quantum efficiencies of negative electron affinity (NEA) GaN photocathodes, gradient-doping reflection-mode GaN photocathodes are grown by metal organic chemical vapor deposition (MOCVD)at doping concentrations of 1×1018cm-3, 4×1017cm-3, 2×1017cm-3 and 6×1016cm-3 from the body to the surface, with the thickness of each doping region being about 45nm and the total thickness of GaN 180 nm. The gradient-doping GaN photocathodes are activated in an ultra-high vacuum system and are compared with two kinds of uniform-doping GaN photocathodes whose thicknesses are both 150 nm and doping concentrations are 1.6×1017cm-3 and 3×1018cm-3 separately. The results show that both the photocurrent growth rate and the maximum photocurrent of the gradient-doping GaN photocathodes are greater than those of the uniform-doping GaN in the Cs/O activation process, and the multi-test system measured maximum quantum efficiency of the gradient-doping NEA GaN photocathode is about 56% which is as high as the double of the uniform-doping. Calculations show that the energy band bendings of the gradient-doping GaN photocathodes are 0.024eV, 0.018eV and 0.031eV from the body to the surface, a larger electron drift and diffusion length are gained due to the built-in electric field formed by the energy band bending, because of the 0.073eV total energy band bending the photoelectrons reaching the surface have higher energies and pass through the surface barrier more easily. Therefore the gradient-doping NEA GaN photocathodes have greater quantum efficiencies.
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Keywords:
- NEA GaN photocathodes /
- gradient-doping /
- quantum efficiency /
- energy band structure
[1] Xie C K, Xu F Q, Deng R, Xu P S, Liu F Q, Yibulaxin K 2002 Acta Phys. Sin. 51 2606 (in Chinese) [谢长坤、徐法强、邓 锐、徐彭寿、刘凤琴、 Yibulaxin K 2002 物理学报 51 2606]
[2] Peng D S, Feng Y C, Wang W X, Liu X F, Shi W, Niu H B 2006 Acta Phys. Sin. 55 3606 (in Chinese) [彭冬生、冯玉春、王文欣、刘晓峰、施 炜、牛憨笨 2006 物理学报 55 3606]
[3] Qiao J L, Tian S, Chang B K, Du X Q, Gao P 2009 Acta Phys. Sin. 58 5847 (in Chinese) [乔建良、田 思、常本康、杜晓晴、高 频 2009 物理学报 58 5847]
[4] Machuca F, Sun Y, Liu Z, Loakeimidi K, Pianetta P, Pease R F W 2000 J. Vac. Sci. Techn. 18 3042
[5] Machuca F, Liu Z, Maldonado J R, Coyle S T, Pianetta P, Pease R F W 2004 J. Vac. Sci. Techn. 22 3565
[6] Machuca F, Liu Z, Sun Y, Pianetta P, Spicer W E, Pease R F W 2002 J. Vac. Sci. Techn. 20 2721
[7] Qiao J L, Chang B K, Qian Y S, Du X Q, Zhang Y J, Gao P, Wang X H, Guo X Y, Niu J, Gao Y T 2010 Acta Phys. Sin. 59 3577 (in Chinese) [乔建良、常本康、钱芸生、杜晓晴、张益军、高 频、王晓晖、郭向阳、牛 军、高有堂 2010 物理学报 59 3577]
[8] Du X Q, Chang B K, Zong Z Y 2004 J. Vac. Sci. Techn. 24 195 (in Chinese) [杜晓晴、常本康、宗志园 2004 真空科学与技术学报 24 195]
[9] Zhang Y J, Chang B K, Yang Z, Niu J, Zou J J 2009 Chin. Phys. B 18 4541
[10] Yang Z, Chang B K, Zou J J, Qiao J L, Gao P, Zeng Y P, Li H 2007 Appl. Opt. 46 7035
[11] Zong Z Y, Chang B K 1999 Acta.Optica.Sinica.19 1177 (in Chinese) [宗志园、常本康 1999 光学学报 19 1177]
[12] Liu E K, Zhu B S, Luo J S 2006 Semiconductor Physics (Edition.6) (Beijing: Publishing House of Electronics Industry) p175 (in Chinese) [刘恩科、朱秉升、罗晋生 2006 半导体物理学(第6版)(北京:电子工业出版社) 第175页]
[13] Niu J, Yang Z, Chang B K 2009 Chin. Phys. Lett. 26 104202
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[1] Xie C K, Xu F Q, Deng R, Xu P S, Liu F Q, Yibulaxin K 2002 Acta Phys. Sin. 51 2606 (in Chinese) [谢长坤、徐法强、邓 锐、徐彭寿、刘凤琴、 Yibulaxin K 2002 物理学报 51 2606]
[2] Peng D S, Feng Y C, Wang W X, Liu X F, Shi W, Niu H B 2006 Acta Phys. Sin. 55 3606 (in Chinese) [彭冬生、冯玉春、王文欣、刘晓峰、施 炜、牛憨笨 2006 物理学报 55 3606]
[3] Qiao J L, Tian S, Chang B K, Du X Q, Gao P 2009 Acta Phys. Sin. 58 5847 (in Chinese) [乔建良、田 思、常本康、杜晓晴、高 频 2009 物理学报 58 5847]
[4] Machuca F, Sun Y, Liu Z, Loakeimidi K, Pianetta P, Pease R F W 2000 J. Vac. Sci. Techn. 18 3042
[5] Machuca F, Liu Z, Maldonado J R, Coyle S T, Pianetta P, Pease R F W 2004 J. Vac. Sci. Techn. 22 3565
[6] Machuca F, Liu Z, Sun Y, Pianetta P, Spicer W E, Pease R F W 2002 J. Vac. Sci. Techn. 20 2721
[7] Qiao J L, Chang B K, Qian Y S, Du X Q, Zhang Y J, Gao P, Wang X H, Guo X Y, Niu J, Gao Y T 2010 Acta Phys. Sin. 59 3577 (in Chinese) [乔建良、常本康、钱芸生、杜晓晴、张益军、高 频、王晓晖、郭向阳、牛 军、高有堂 2010 物理学报 59 3577]
[8] Du X Q, Chang B K, Zong Z Y 2004 J. Vac. Sci. Techn. 24 195 (in Chinese) [杜晓晴、常本康、宗志园 2004 真空科学与技术学报 24 195]
[9] Zhang Y J, Chang B K, Yang Z, Niu J, Zou J J 2009 Chin. Phys. B 18 4541
[10] Yang Z, Chang B K, Zou J J, Qiao J L, Gao P, Zeng Y P, Li H 2007 Appl. Opt. 46 7035
[11] Zong Z Y, Chang B K 1999 Acta.Optica.Sinica.19 1177 (in Chinese) [宗志园、常本康 1999 光学学报 19 1177]
[12] Liu E K, Zhu B S, Luo J S 2006 Semiconductor Physics (Edition.6) (Beijing: Publishing House of Electronics Industry) p175 (in Chinese) [刘恩科、朱秉升、罗晋生 2006 半导体物理学(第6版)(北京:电子工业出版社) 第175页]
[13] Niu J, Yang Z, Chang B K 2009 Chin. Phys. Lett. 26 104202
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