Search

Article

x

留言板

尊敬的读者、作者、审稿人, 关于本刊的投稿、审稿、编辑和出版的任何问题, 您可以本页添加留言。我们将尽快给您答复。谢谢您的支持!

姓名
邮箱
手机号码
标题
留言内容
验证码

Improved performance of Al/n+Ge Ohmic contact andGe n+/p diode by two-step annealing method

Wang Chen Xu Yi-Hong Li Cheng Lin Hai-Jun Zhao Ming-Jie

Improved performance of Al/n+Ge Ohmic contact andGe n+/p diode by two-step annealing method

Wang Chen, Xu Yi-Hong, Li Cheng, Lin Hai-Jun, Zhao Ming-Jie
PDF
HTML
Get Citation
  • Silicon based germanium devices are crucial parts of optoelectronic integration as CMOS feature size continuously decreases. Germanium has attracted increasing attention due to its higher electron and hole mobility, larger optical absorption coefficient as well as lower processing temperature than those of silicon. However, the high diffusion coefficient and low solid solubility about n-type dopant and relatively high thermal budget required for high n-type doping in Ge make it difficult to achieve high activation n-type doping and excellent n+/p shallow junction for source/drain in the nano-scaled n-MOSFET (here MOSFET stands for). The high activation concentration and shallow junction n-type doping in Ge are greatly beneficial to the scaled Ge n-MOSFET technology. In this work, the ohmic contact of Al/n+Ge and Ge n+/p junction fabricated by a combination of low temperature pre-annealing process and excimer laser annealing for phosphorus-implanted germanium are demonstrated. Prior to excimer laser annealing, the samplesare annealed at a relatively low temperature, which can heal the implantation damages preliminarily. Through the optimization of pre-annealing temperature and time, the low temperature pre-annealing step can play a critical role in annihilating the implantation damages and significantly suppressing phosphorus diffusion in the laser annealing process, resulting in a very small dopant diffusion length at a high activation level of phosphorus. Through the combination of ion implantation and two-step annealing technology, the specific contact resistivity (ρC) of Al/n+Ge Ohmic contact is measured by CTLM structure. The optimized annealing condition is 400 oC-10 min of low temperature annealing and 150 mJ/cm2 of ELA. Under that annealing condition, the ρC of the sample by two-step annealing is reduced to 2.61 × 10–6 Ω·cm2, which is one order of magnitude lower than that by ELA alone (about 3.44 × 10–4 Ω·cm2). The lower value of ρC for the sample with LTPA can contribute to the higher carrier concentration and better crystalline quality thanthat without LTPA, which is confirmed by SRP and TEM. Moreover, the rectification ratio of Ge n+/p junction diode is improved to 8.35 × 106 at ± 1 V, which is two orders of magnitudes higher than that by ELA alone. And a lower ideality factor of about 1.07 is also obtained than that by ELA alone, which indicates that the implantation damages can be repaired perfectly by two-step annealing method.
      Corresponding author: Wang Chen, chenwang@xmut.edu.cn
    [1]

    Chui C O, Ramanathan S, Triplett B, McIntyre P C, Saraswat K C 2002 IEEE Electron Device Lett. 23 473

    [2]

    Park J H, Kuzum D, Jung W S, Saraswat K C 2011 IEEE Electron Device Lett. 32 234

    [3]

    Zhang R, Huang P C, Lin J C, Taoka N, Takenaka M, Takagi S 2013 IEEE Trans. Electron Devices 60 927

    [4]

    Morii K, Iwasaki T, Nakane R, Takenaka M, Takagi S 2010 IEEE Electron Device Lett. 31 1092

    [5]

    Kuzum D, Krishnamohan T, Nainani A 2009 IEEE IEDM Tech. Dig. p1

    [6]

    Martens K, Chui C O, Brammertz G, et al. 2008 IEEE Trans. Electron Devices 55 547

    [7]

    Shang H, Frank M, Gusev E P, Chu J O, Bedell S W, Guarini K W, Ieong M 2006 IBM J. Res. Develop. 50 377

    [8]

    Simoen E, Satta A, D’Amore A, et al. 2006 Mater. Sci. Semicond. Process 9 634

    [9]

    Dimoulas A, Tsipas P, Sotiropoulos A 2006 Appl. Phys. Lett. 89 252110

    [10]

    Kuzum D, Krishnamohan T, Nainani A, Sun Y, Pianetta P A, Wong H, Saraswat K C 2010 IEEE Trans. Electron Devices 58 59

    [11]

    Chui C O, Kulig L, Moran J, Tsai W, Saraswa K 2005 Appl. Phys. Lett. 87 091909

    [12]

    Wundisch C, Posselt M, Schmidt B, Heera V, Schumann T, Mucklich A, Grotzschel R, Skorupa W, Clarysse T, Simoen E, Hortenbach H 2009 Appl. Phys. Lett. 95 252107

    [13]

    Zhang R, Li J, Chen F, Zhao Y 2016 IEEE Trans. Electron. Dev. 63 2665

    [14]

    Yu B, Wang Y, Wang H, Xiang Q, Riccobene C, Talwar S, Lin M 1999 IEDM Tech. Dig. p509

    [15]

    Wang C, Xu Y, Li C, Lin H 2018 Chin. Phys. B 27 018502

    [16]

    Wang C, Li C, Huang S, et al. 2013 Appl. Phys. Exp. 6 106501

    [17]

    Wang C, Li C, Lin G, et al. 2014 IEEE Trans. on Electron Dev. 61 3060

    [18]

    Thareja G, Chopra S, Adamas B, Kim Y, Moffatt S, Saraswat K 2011 IEEE Electron Device Lett. 32 838

    [19]

    Milazzo R, Napolitani E, Impellizzeri G, Fisicaro G, Boninelli S, Cuscuna M, de Salvador D, Mastromatteo M, Italia M, La Magna A 2014 J. Appl. Phys. 115 053501

    [20]

    Tsouroutas P, Tsoukalas D, Florakis A, Zergioti I, Serafetinides A, Cherkashin N, Marty B, Claverie A 2006 Mater. Sci. Semicond. Processing 9 644

    [21]

    Chao Y L, Woo J 2010 IEEE Trans. Electron Dev. 57 665

    [22]

    Koike M, Kamata Y, Ino T, et al. 2008 J. Appl. Phys. 104 023523

    [23]

    Ruan Y, Chen C, Huang S, Huang W, Chen S, Li C, Li J 2013 IEEE Trans. Electron Dev. 60 3741

  • 图 1  Ge n+/p结二极管制备工艺流程图

    Figure 1.  Process flow used for the fabrication of Ge n+/p junction diodes.

    图 2  150 mJ/cm2激光能量密度不同预退火条件下p-n结二极管的I-V特性曲线

    Figure 2.  Room temperature I-V characteristics of Ge n+/p junction diode formed by ELA with one pulse at 150 mJ/cm2 with different pre-annealing conditions.

    图 3  Al/n+-Ge接触的比接触电阻率随不同退火条件的变化曲线, 内插图是CTLM结构的俯视图

    Figure 3.  Change of specific contact resistivity of Al/n+-Ge extracted by CTLM with different annealing conditions. The inset shows the CTLM schematic structure (top view).

    图 4  (a) 不同退火条件下Ge n+/p结二极管的I-V特性曲线; (b) Ge n+/p结二极管的整流比随退火条件变化曲线

    Figure 4.  (a) Room temperature I-V characteristics of Ge n+/p junction diode; (b) rectification ratio of Ge n+/p junction diodes formed by ELA with or without pre-annealing at 400 ℃-10 min.

    表 1  不同退火条件下Ge n+/p结二极管的整流比和理想因子

    Table 1.  Rectification ratio and ideality factor of Ge n+/p junction diodes under different annealing conditions.

    样品编号退火条件整流比(@ ± 1 V)理想因子
    R1350 ℃-10 min&150 mJ/cm22 × 1051.11
    R2400 ℃-10 min&150 mJ/cm28.35 × 1061.08
    R3400 ℃-30 min&150 mJ/cm21.12 × 102 > 2
    R4450 ℃-10 min&150 mJ/cm213 > 2
    DownLoad: CSV
  • [1]

    Chui C O, Ramanathan S, Triplett B, McIntyre P C, Saraswat K C 2002 IEEE Electron Device Lett. 23 473

    [2]

    Park J H, Kuzum D, Jung W S, Saraswat K C 2011 IEEE Electron Device Lett. 32 234

    [3]

    Zhang R, Huang P C, Lin J C, Taoka N, Takenaka M, Takagi S 2013 IEEE Trans. Electron Devices 60 927

    [4]

    Morii K, Iwasaki T, Nakane R, Takenaka M, Takagi S 2010 IEEE Electron Device Lett. 31 1092

    [5]

    Kuzum D, Krishnamohan T, Nainani A 2009 IEEE IEDM Tech. Dig. p1

    [6]

    Martens K, Chui C O, Brammertz G, et al. 2008 IEEE Trans. Electron Devices 55 547

    [7]

    Shang H, Frank M, Gusev E P, Chu J O, Bedell S W, Guarini K W, Ieong M 2006 IBM J. Res. Develop. 50 377

    [8]

    Simoen E, Satta A, D’Amore A, et al. 2006 Mater. Sci. Semicond. Process 9 634

    [9]

    Dimoulas A, Tsipas P, Sotiropoulos A 2006 Appl. Phys. Lett. 89 252110

    [10]

    Kuzum D, Krishnamohan T, Nainani A, Sun Y, Pianetta P A, Wong H, Saraswat K C 2010 IEEE Trans. Electron Devices 58 59

    [11]

    Chui C O, Kulig L, Moran J, Tsai W, Saraswa K 2005 Appl. Phys. Lett. 87 091909

    [12]

    Wundisch C, Posselt M, Schmidt B, Heera V, Schumann T, Mucklich A, Grotzschel R, Skorupa W, Clarysse T, Simoen E, Hortenbach H 2009 Appl. Phys. Lett. 95 252107

    [13]

    Zhang R, Li J, Chen F, Zhao Y 2016 IEEE Trans. Electron. Dev. 63 2665

    [14]

    Yu B, Wang Y, Wang H, Xiang Q, Riccobene C, Talwar S, Lin M 1999 IEDM Tech. Dig. p509

    [15]

    Wang C, Xu Y, Li C, Lin H 2018 Chin. Phys. B 27 018502

    [16]

    Wang C, Li C, Huang S, et al. 2013 Appl. Phys. Exp. 6 106501

    [17]

    Wang C, Li C, Lin G, et al. 2014 IEEE Trans. on Electron Dev. 61 3060

    [18]

    Thareja G, Chopra S, Adamas B, Kim Y, Moffatt S, Saraswat K 2011 IEEE Electron Device Lett. 32 838

    [19]

    Milazzo R, Napolitani E, Impellizzeri G, Fisicaro G, Boninelli S, Cuscuna M, de Salvador D, Mastromatteo M, Italia M, La Magna A 2014 J. Appl. Phys. 115 053501

    [20]

    Tsouroutas P, Tsoukalas D, Florakis A, Zergioti I, Serafetinides A, Cherkashin N, Marty B, Claverie A 2006 Mater. Sci. Semicond. Processing 9 644

    [21]

    Chao Y L, Woo J 2010 IEEE Trans. Electron Dev. 57 665

    [22]

    Koike M, Kamata Y, Ino T, et al. 2008 J. Appl. Phys. 104 023523

    [23]

    Ruan Y, Chen C, Huang S, Huang W, Chen S, Li C, Li J 2013 IEEE Trans. Electron Dev. 60 3741

  • [1] Wang Su-Jie, Li Shu-Qiang, Wu Xiao-Ming, Chen Fang, Jiang Feng-Yi. Study on the effect of thermal annealing process on ohmic contact performance of AuGeNi/n-AlGaInP. Acta Physica Sinica, 2020, 69(4): 048103. doi: 10.7498/aps.69.20191720
    [2] Li Xiao-Jing, Zhao De-Gang, He Xiao-Guang, Wu Liang-Liang, Li Liang, Yang Jing, Le Ling-Cong, Chen Ping, Liu Zong-Shun, Jiang De-Sheng. Influence of different annealing temperature and atmosphere on the Ni/Au Ohmic contact to p-GaN. Acta Physica Sinica, 2013, 62(20): 206801. doi: 10.7498/aps.62.206801
    [3] Guo De-Cheng, Jiang Xiao-Dong, Huang Jin, Xiang Xia, Wang Feng-Rui, Liu Hong-Jie, Zhou Xin-Da, Zu Xiao-Tao. Effect of raster scan number on damage resistance of KDP crystal irradiated by ultraviolet pulse laser. Acta Physica Sinica, 2013, 62(14): 147803. doi: 10.7498/aps.62.147803
    [4] Lu Wu-Yue, Zhang Yong-Ping, Chen Zhi-Zhan, Cheng Yue, Tan Jia-Hui, Shi Wang-Zhou. Effect of different annealing treatment methods on the Ni/SiC contact interface properties. Acta Physica Sinica, 2015, 64(6): 067303. doi: 10.7498/aps.64.067303
    [5] Wu Xue-Ke, Huang Wei-Qi, Dong Tai-Ge, Wang Gang, Liu Shi-Rong, Qin Chao-Jie. Effects of thermal annealing, laser and electron beam on the fabrication of nanosilicon and the emission properties of its localized states. Acta Physica Sinica, 2016, 65(10): 104202. doi: 10.7498/aps.65.104202
    [6] Feng Fei-Fei, Wang Guang-Xu, Liu Jun-Lin, Jiang Feng-Yi, Qiu Chong. N-polar n-type ohmic contact of GaN-based LED on Si substrate. Acta Physica Sinica, 2010, 59(8): 5706-5709. doi: 10.7498/aps.59.5706
    [7] Huang Ya-Ping, Yun Feng, Ding Wen, Wang Yue, Wang Hong, Zhao Yu-Kun, Zhang Ye, Guo Mao-Feng, Hou Xun, Liu Shuo. The reflectivity and ohmic contact resistivity of Ni/Ag/Ti/Au in contact with p-GaN. Acta Physica Sinica, 2014, 63(12): 127302. doi: 10.7498/aps.63.127302
    [8] Wang Xiao-Yong, Chong Ming, Zhao De-Gang, Su Yan-Mei. Two-dimensional hole gas in p-GaN/p-AlxGa1-xN heterojunctions and its influence on Ohmic contact. Acta Physica Sinica, 2012, 61(21): 217302. doi: 10.7498/aps.61.217302
    [9] WANG YIN-YUE, ZHEN CONG-MIAN, GONG HENG-XIANG, YAN ZHI-JUN, WANG YA-FAN, LIU XUE-QIN, YANG YING-HU, HE SHAN-HU. MEASUREMENT OF THE SPECIFIC CONTACT RESISTANCE OF Au/Ti/p-DIAMOND USING TRANSMIS SION LINE MODEL. Acta Physica Sinica, 2000, 49(7): 1348-1351. doi: 10.7498/aps.49.1348
    [10] Halimulati, Abai, Baishan, Aimaiti. Boundary alternating current characteristics of an ideal p-n junction diode. Acta Physica Sinica, 2008, 57(2): 1161-1165. doi: 10.7498/aps.57.1161
    [11] Ding Zhi-Bo, Wang Kun, Chen Tian-Xiang, Chen Di, Yao Shu-De. Investigation on the formation mechanism and diffusion of the electrode metal of oxidized Au/Ni/p-GaN ohmic contact in different alloying time. Acta Physica Sinica, 2008, 57(4): 2445-2449. doi: 10.7498/aps.57.2445
    [12] He Tian-Li, Wei Hong-Yuan, Li Cheng-Ming, Li Geng-Wei. Comparative study of n-GaN transition group refractory metal Ohmic electrode. Acta Physica Sinica, 2019, 68(20): 206101. doi: 10.7498/aps.68.20190717
    [13] Pan Shu-Wan, Qi Dong-Feng, Chen Song-Yan, Li Cheng, Huang Wei, Lai Hong-Kai. Se ultrathin film growth on Si(100) substrate and its application in Ti/n-Si(100) ohmic contact. Acta Physica Sinica, 2011, 60(9): 098108. doi: 10.7498/aps.60.098108
    [14] Shi Wen-Jun, Yi Ying-Yan, Li Min. Pressure dependence of refractive index of Ge near the absorption edge. Acta Physica Sinica, 2016, 65(16): 167801. doi: 10.7498/aps.65.167801
    [15] Zhu Yan-Xu, Cao Wei-Wei, Xu Chen, Deng Ye, Zou De-Shu. Effect of different ohmic contact pattern on GaN HEMT electrical properties. Acta Physica Sinica, 2014, 63(11): 117302. doi: 10.7498/aps.63.117302
    [16] Zhang Xiao-Fu, Li Yu-Dong, Guo Qi, Luo Mu-Chang, He Cheng-Fa, Yu Xin, Shen Zhi-Hui, Zhang Xing-Yao, Deng Wei, Wu Zheng-Xin. 60Coγ-radiation effects on the ideality factor of AlxGa1?xN p-i-n solar-blind detector with high content of aluminum. Acta Physica Sinica, 2013, 62(7): 076106. doi: 10.7498/aps.62.076106
    [17] Lü Ling, Gong Xin, Hao Yue. Properties of p-type GaN etched by inductively coupled plasma and their improvement. Acta Physica Sinica, 2008, 57(2): 1128-1132. doi: 10.7498/aps.57.1128
    [18] Wang Jian-Yuan, Wang Chen, Li Cheng, Chen Song-Yan. Selective area growth of Ge film on Si. Acta Physica Sinica, 2015, 64(12): 128102. doi: 10.7498/aps.64.128102
    [19] Huang Lei, Liu Wen-Liang, Deng Chao-Sheng. First-principles study of optical properties of germanium doped with phosphorus and bismuth. Acta Physica Sinica, 2018, 67(13): 136101. doi: 10.7498/aps.67.20172680
    [20] Yu Wei, He Jie, Sun Yun-Tao, Zhu Hai-Feng, Han Li, Fu Guang-Sheng. Pulse laser crystallization of silicon carbon thin films. Acta Physica Sinica, 2004, 53(6): 1930-1934. doi: 10.7498/aps.53.1930
  • Citation:
Metrics
  • Abstract views:  240
  • PDF Downloads:  2
  • Cited By: 0
Publishing process
  • Received Date:  08 May 2019
  • Accepted Date:  10 June 2019
  • Available Online:  26 November 2019
  • Published Online:  01 September 2019

Improved performance of Al/n+Ge Ohmic contact andGe n+/p diode by two-step annealing method

    Corresponding author: Wang Chen, chenwang@xmut.edu.cn
  • 1. Fujian Provincial Key Laboratory of Optoelectronic Technology and Devices, School of Opti-electronic andCommunication Engineering, Xiamen University of Technology, Xiamen 361024, China
  • 2. Department of Electric and Information Engineering, Xiamen Institute of Technology, Xiamen 361024, China
  • 3. Department of Physics, Semiconductor Photonics Research Center, Xiamen University, Xiamen 361005, China

Abstract: Silicon based germanium devices are crucial parts of optoelectronic integration as CMOS feature size continuously decreases. Germanium has attracted increasing attention due to its higher electron and hole mobility, larger optical absorption coefficient as well as lower processing temperature than those of silicon. However, the high diffusion coefficient and low solid solubility about n-type dopant and relatively high thermal budget required for high n-type doping in Ge make it difficult to achieve high activation n-type doping and excellent n+/p shallow junction for source/drain in the nano-scaled n-MOSFET (here MOSFET stands for). The high activation concentration and shallow junction n-type doping in Ge are greatly beneficial to the scaled Ge n-MOSFET technology. In this work, the ohmic contact of Al/n+Ge and Ge n+/p junction fabricated by a combination of low temperature pre-annealing process and excimer laser annealing for phosphorus-implanted germanium are demonstrated. Prior to excimer laser annealing, the samplesare annealed at a relatively low temperature, which can heal the implantation damages preliminarily. Through the optimization of pre-annealing temperature and time, the low temperature pre-annealing step can play a critical role in annihilating the implantation damages and significantly suppressing phosphorus diffusion in the laser annealing process, resulting in a very small dopant diffusion length at a high activation level of phosphorus. Through the combination of ion implantation and two-step annealing technology, the specific contact resistivity (ρC) of Al/n+Ge Ohmic contact is measured by CTLM structure. The optimized annealing condition is 400 oC-10 min of low temperature annealing and 150 mJ/cm2 of ELA. Under that annealing condition, the ρC of the sample by two-step annealing is reduced to 2.61 × 10–6 Ω·cm2, which is one order of magnitude lower than that by ELA alone (about 3.44 × 10–4 Ω·cm2). The lower value of ρC for the sample with LTPA can contribute to the higher carrier concentration and better crystalline quality thanthat without LTPA, which is confirmed by SRP and TEM. Moreover, the rectification ratio of Ge n+/p junction diode is improved to 8.35 × 106 at ± 1 V, which is two orders of magnitudes higher than that by ELA alone. And a lower ideality factor of about 1.07 is also obtained than that by ELA alone, which indicates that the implantation damages can be repaired perfectly by two-step annealing method.

    • 随着COMS工艺尺寸的不断减小, Ge材料由于具有比硅更高的电子和空穴迁移率、在光纤通信波段具有更大的吸收系数以及更低的工艺处理温度, 成为极具应用前景的材料之一, 受到人们的广泛关注[1,2]. 目前, 高性能Ge p-MOSFET的研究已经取得了不错的进展, 然而Ge n-MOSFET性能的提升仍存在许多问题[3-5], 如随着器件的源/漏接触面积不断缩小, 源/漏的接触电阻增大, 器件性能下降[6-8]. 此外, 金属与锗低比接触电阻率的欧姆接触较难实现, 这主要是由两方面决定的: 一方面, 金属与锗的接触存在强烈的费米钉扎效应, 导致大的电子势垒高度[9]; 另一方面, 由于n型杂质在Ge中具有较大的扩散系数(与掺杂浓度正相关)以及较低的杂质固溶度, 使得在Ge中实现高掺杂浓度、界面陡峭以及低扩散深度的n型掺杂十分困难. 结合离子注入和后退火工艺是实现Ge中n型掺杂的常用手段, 然而要在Ge中获得高激活浓度n型掺杂的同时, 杂质的扩散深度也要尽可能小, 此时就需要一种退火时间短(减小杂质扩散)以及退火温度高(激活杂质离子)的退火技术. 相比于其他退火技术, 如常规热退火[10]、快速热退火[11]、快闪灯照退火[12]、微波退火[13]等, 脉冲激光退火技术拥有独特的优势[14]: 首先, 退火时间极短, 只有几十纳秒, 使得杂质的扩散深度减小, 杂质的损失也减小; 其次, 脉冲激光退火过程是一个亚稳态的热处理过程, 故允许掺杂浓度超过杂质在半导体中的固溶度极限; 再次, 退火温度很高, 一般可以达到半导体材料的熔点, 有利于杂质的激活; 最后, 可对特定区域进行退火处理, 忽略对周围区域的影响. 这些优势使得脉冲激光退火技术成为一种实现Ge中高激活浓度n型掺杂、低扩散深度以及杂质损失少的有效退火方法[15].

      目前, 结合离子注入和激光退火技术在Ge中实现激活浓度大于1020 cm–3的n型掺杂, 已有许多文献报道[16,17], 且激光退火还可有效修复离子注入损伤, 减小器件的暗电流. 但是, 实现高性能Ge n+/p结二极管以及高激活浓度的n+Ge, 通常需要较大的离子注入剂量以及激光退火能量, 而较大的激光退火能量将导致杂质在Ge中扩散深度加大, 不利于Ge中n型浅结的制备, 限制了其在器件中的应用[18-20].

      本文采用低温预退火处理注磷锗样品, 在保证杂质几乎不发生扩散的同时, 初步修复注入损伤, 而后再脉冲激光退火. 采用低温预退火与脉冲激光退火的两步退火工艺, 降低了杂质在Ge中的扩散深度, 提高了Ge中杂质激活浓度, 并提升了金属与锗的欧姆接触以及Ge n+/p结二极管的性能. 结合离子注入和两步退火工艺, Al/n+Ge欧姆接触的比接触电阻率降至2.61 × 10–6 Ω·cm2, Ge n+/p结二极管在 ± 1 V的整流比提高到8.3 × 107.

    2.   实 验
    • 本实验采用电阻率为0.088 Ω·cm, 厚度为500 μm的p-Ge(100)晶片作为衬底材料, 其过程可分为三个步骤. 第一步, 首先对衬底依次进行丙酮和乙醇超声清洗, 冷去离子水冲洗干净后再用冷去离子水超声清洗3遍, 然后浸泡HF∶H2O = 1∶50约15 s, 去除Ge表面氧化物, 接着冷去离子水冲洗干净, 氮气吹干; 而后, 用PECVD在清洗后的衬底表面沉积15 nm SiO2, 再在注入能量为30 keV, 注入剂量为5 × 1014 cm–2条件下注入磷离子(P+), 离子注入后用浓度适中的氢氟酸去除样品表面的SiO2. 第二步, 对注磷p-Ge样品进行不同条件的退火处理, 形成n+-Ge层; 低温预退火采用的是N2气环境下快速热退火工艺, 而脉冲激光退火(ELA)是用248 nm波长KrF准分子激光器以不同能量密度的脉冲激光对注磷Ge样品在N2气环境下进行扫描退火, 脉冲激光光斑为面积4 mm × 3 mm的矩形光斑, 激光分别沿着X轴和Y轴对样品进行单脉冲扫描. 第三步, 将不同条件退火后的样品经图1所示工艺制备得到Ge n+/p结二极管, 二极管的金相显微镜俯视图如图1最后一步所示, 并通过测试它们的I-V特性曲线, 分析p-n结特性; 同时, 采用光刻、溅射以及腐蚀等工艺, 制备Al/n+-Ge的欧姆接触, 并采用了圆点传输线模型(CTLM)来计算得到欧姆接触的比接触电阻率.

      Figure 1.  Process flow used for the fabrication of Ge n+/p junction diodes.

    3.   结果与分析
    • 首先固定脉冲激光退火能量密度为150 mJ/cm2, 改变低温预退火的温度和退火时间, 制备不同退火条件下的Ge n+/p结二极管, 并测试二极管的I-V特性曲线. 预退火温度的选择依据是在保证杂质不发生明显扩散的情况下能够一定程度修复离子注入损伤, 根据文献报道[21-23], 选择的温度范围为从350 ℃变化至450 ℃, 具体退火条件及相应的样品编号如表1所示. 样品R1至R4的p-n结二极管I-V特性测试曲线如图2所示, 从图2中抽取得到它们的理想因子和整流比如表1所示. 测试结果表明: 4种样品中, 样品R2二极管的理想因子最小、反向漏电最小以及整流比最大, 这说明400 ℃-10 min条件下的低温预退火效果最好. 对比样品R2和R3, 仅低温预退火的时间不同, 样品R2二极管的性能明显更好, 说明在相同预退火温度下, 退火时间不能太长, 尽管低温退火杂质扩散非常小, 但是由于离子注入深度很浅, 杂质离子主要分布在靠近表面很薄的一层Ge中, 长时间的低温退火仍然会有较多杂质向外扩散[23], 造成较大的杂质损失, 影响二极管的性能. 此外, 对比样品R1和R2二极管的I-V曲线, 2种二极管正向电流相差不大, 但是在–1 V时R2样品的反向漏电流比R1样品约小了一个数量级, 说明低温预退火温度越高越有利于注入损伤的修复, 从而减小二极管的反向漏电. 但是低温预退火温度又不能太高, 如样品R4, 当低温预退火温度达到450 ℃时, 尽管有利于离子注入损伤的修复, 但这个温度下退火可能造成的杂质外扩散损失比样品R3还要多[23], 故二极管的性能最差.

      样品编号退火条件整流比(@ ± 1 V)理想因子
      R1350 ℃-10 min&150 mJ/cm22 × 1051.11
      R2400 ℃-10 min&150 mJ/cm28.35 × 1061.08
      R3400 ℃-30 min&150 mJ/cm21.12 × 102 > 2
      R4450 ℃-10 min&150 mJ/cm213 > 2

      Table 1.  Rectification ratio and ideality factor of Ge n+/p junction diodes under different annealing conditions.

      Figure 2.  Room temperature I-V characteristics of Ge n+/p junction diode formed by ELA with one pulse at 150 mJ/cm2 with different pre-annealing conditions.

      而后改变脉冲激光退火能量密度(100, 150, 200, 250 mJ/cm2), 分别基于两步退火法和单独激光退火制备了两组Al/n+-Ge的欧姆接触, 两步退火法中的低温预退火温度和时间定为400 ℃-10 min. 本文采用圆形传输线模型(CTLM), 通过测试不同圆环间距的I-V特性, 拟合计算得到Al/n+-Ge欧姆接触的比接触电阻率随退火条件的变化情况(图3). 从图3中可以看到, 样品单独在100 mJ/cm2激光退火后, 由于其测得的I-V特性曲线不是直线(未在此处显示), 表明该条件下无法得到Al与Ge的欧姆接触, 说明此退火条件下不能很好地修复离子注入损伤以及激活杂质离子; 而结合了低温预退火后, 可得到Al/n+-Ge欧姆接触的比接触电阻率为3.44 × 10–4 Ω·cm2. 结合低温预退火, 提高激光退火能量为150 mJ/cm2时, 得到的Al/n+-Ge接触的比接触电阻率最低, 约为2.61 × 10–6 Ω·cm2, 比单独采用脉冲激光退火样品的比接触电阻率 (4.48 × 10–5 Ω·cm2)降低了一个多数量级. 此外, 从图3中还可以看到, 两步退火法可得到比单独激光退火更低的比接触电阻率, 而低的比接触电阻率对应高的掺杂浓度. 为了得到杂质的扩散深度以及激活浓度, 对样品进行二次离子质谱(SIMS)以及扩展电阻探针(SRP)测试[17], 结果发现, 离子注入样品经过两步退火后, 磷在Ge中的扩散深度明显比单独采用脉冲激光退火后要小很多, 说明低温预退火可降低脉冲激光退火时杂质在Ge中的扩散系数, 更容易在Ge中获得更小杂质扩散深度的n型掺杂. 扩展电阻探针测试可以得到磷的最大激活浓度为6 × 1019 cm–3[17], 比采用传统退火方式获得的杂质激活浓度高好几倍[10], 这说明控制调整离子注入条件以及激光退火能量密度, 结合低温预退火和激光退火的两步退火法是实现锗中实现高激活浓度、低扩散深度的n型掺杂的一种有效途径.

      Figure 3.  Change of specific contact resistivity of Al/n+-Ge extracted by CTLM with different annealing conditions. The inset shows the CTLM schematic structure (top view).

      图4(a)所示为不同退火条件下的Ge n+/p结二极管的I-V特性曲线, 计算抽取得到它们的整流比(@ ± 1 V), 如图4(b)所示. 结果表明, 当脉冲激光能量密度小于等于150 mJ/cm2时, 低温预退火对I-V特性的影响作用十分明显, 经过两步退火法制备得到的二极管性能比单独激光退火后制备的二极管性能要好得多; 而当脉冲激光能量密度大于等于200 mJ/cm2时, 低温预退火对二极管I-V特性的影响减弱. 这是因为Ge中注入的杂质离子在两步退火过程中的扩散主要发生在脉冲激光退火过程中, 扩散深度由Ge层熔化深度决定, Ge中低能量离子注入后, 损伤区域非常薄, 低温预退火可部分修复离子注入损伤, 提高Ge的晶体质量, 在低能量密度激光退火时, Ge层熔化深度较浅, 低温预退火对熔化深度的影响较为明显, 而当激光退火能量密度较大时, Ge层熔化深度较深, 此时低温预退火作用可以忽略不计. 此外, 在400 ℃-10 min的低温预退火外加150 mJ/cm2的脉冲激光退火的退火条件下, 制备得到超高性能的Ge n+/p结二极管, 整流比高达8.35 × 106, 相比于未经低温预退火处理得到的PN结二极管, 整流比提高了约两个数量级, 且二极管的理想因子仅为1.07, 说明正向电流以扩散电流为主, PN结势垒区中产生复合中心很少, 缺陷很少, 离子注入损伤得到有效修复. 通过高分辨投射电镜测试分析[17], 发现p-Ge经过磷离子注入后形成了约15 nm左右的注入损伤区, 经400 ℃-10 min的低温预退火后, 注入损伤区的损伤得到了初步修复, 但仍然存在一些残余的注入损伤, 再经过150 mJ/cm2激光退火后, 残余注入损伤得到了良好的修复, 样品中几乎看不到明显缺陷的存在, 这更直观说明结合低温预退火和激光退火的两步退火法可有效修复Ge中的离子注入损伤.

      Figure 4.  (a) Room temperature I-V characteristics of Ge n+/p junction diode; (b) rectification ratio of Ge n+/p junction diodes formed by ELA with or without pre-annealing at 400 ℃-10 min.

    4.   结 论
    • 本文进行了p-Ge衬底中磷离子注入后, 采用两步退火工艺对其退火处理的研究, 即先低温预退火, 初步修复注入损伤, 而后再经过脉冲激光退火, 进一步修复注入损伤以及激活杂质离子, 在此基础上制备了Al/n+Ge的欧姆接触以及Ge n+/p结二极管. I-V特性和SIMS, SRP, TEM等测试的结果表明: 样品先低温预退火, 可降低脉冲激光退火时锗中杂质的扩散深度, 提高杂质激活浓度, Al/n+Ge欧姆接触的比接触电阻率降至2.61 × 10–6 Ω·cm2, Ge n+/p结二极管在 ± 1 V的整流比提高到8.35 × 106, 欧姆接触及二极管性能均得到了显著提升. 结合低温预退火和脉冲激光退火的两步退火法有望运用在高性能Ge n-MOSFET以及其他Ge器件中.

Reference (23)

Catalog

    /

    返回文章
    返回