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
Fund Project:Project supported by the Natural Science Foundation of Fujian Province, China (Grant No. 2018J05115), the High Level Talent Project of Xiamen University of Technology, China (Grant No. YKJ16012R), the Scientific Research Climbing Plan of Xiamen University of Technology, China (Grant No. XPDKQ18027), and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61704142).
Received Date:08 May 2019
Accepted Date:10 June 2019
Available Online:01 September 2019
Published Online:05 September 2019
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. Keywords:low-temperature annealing/ laser annealing/ germanium/ pn junction diode/ Ohmic contact
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 Table1.Rectification ratio and ideality factor of Ge n+/p junction diodes under different annealing conditions.
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 Figure2. 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. 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 Figure3. 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. 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 Figure4. (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.
图 1 Ge n+/p结二极管制备工艺流程图
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
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
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