【作者】 李润；

【导师】 殷江；

【作者基本信息】 南京大学 ， 凝聚态物理， 2013， 博士

【摘要】 伴随着现代科技的发展,非易失性储器已经遍及到了我们生活的每一个角落。从MP3播放器中的音乐,到数码相机中的照片、再到智能手机中存储的短信和email、 U盘中存储的文档,这些电子设备中程序代码的存储都离不开被称为闪存的非易失性储器。现在非易失性存储器市场上的主流产品还是基于闪存技术,这是因为闪存具有低廉的生产成本和出色的存储密度,然而诸如耐久力差、操作速度慢等致命弱点将严重制约其未来的发展。所以现在需要新一代的非易失性存储器,它应具有比闪存更为优秀的尺寸微缩性能,并且能够适应未来的节点技术从而达到更高的集成密度；同时也应该提供比闪存更出色的擦写耐久力和输出输入特性。近年来相变存储器作为一种新型的非易失性存储器引起了人们广泛关注。相变存储器相比于其他非易失性存储器具有结构简单、尺寸微缩性能出色、擦写读取速度快、存储密度大、操作电压和功耗较低、可操作次数高,寿命长、与现有CMOS工艺兼容等众多优点,被认为是最有前景的下一代非易失性存储器的候选者。目前相变领域研究最多的、最成熟的材料是Ge2Sb2Te5(GST)。 GST应用于相变存储有一系列的优点,但是要用于大规模的商业化生产,其缺点也同样非常明显,例如结晶温度较低使得数据保持能力较差、熔点较高致使功耗较大、结晶过程中成核主导的机制限制了器件擦除的速度。当前,人们在对GST材料性能改进的同时,也在积极地寻找更适合于相变存储的新型材料。另一方面,相变存储器的开关循环失效的机制也有待进一步的明确。在本工作中,我们对二元碲化物薄膜的相变行为和失效机制进行了系统的研究。重点研究了GeTe4材料应用于相变存储器的可行性与适用性,探索了应力对Sb2Te3薄膜的电学性能的影响。主要工作结果如下：1.系统地研究了GeTe4这种二元化合物的基本物性,测定得到了GeTe4薄膜的晶体结构为简单立方结构,结晶温度为234.3℃,熔点为395.5℃,晶态与非晶态的禁带宽度分别为1.59eV和2.09eV,结晶激活能为2.94eV,以及非晶态薄膜的热稳定性为129℃下可以保持10年。GeTe4薄膜具有较高的结晶温度和较大的结晶激活能使得其非晶态薄膜的热稳定性出色,即数据保持能力强；同时GeTe4薄膜具有较大禁带宽度(即电阻率较高)以及较低的熔点,能有效降低相变存储器件的功耗；最后GeTe4具有简单立方结构和晶化过程中晶核生长主导的机制,从理论上保证了其存储器件良好的操作速度。从这几方面看来GeTe4这种材料非常适合用于相变存储方面的应用。2.首次报道了基于GeTe4薄膜的相变存储器的相变开关行为。我们制备了有效尺寸直径为1μm的GeTe4相变存储原型器件,并对其进行了直流模式和脉冲模式下的存储特性的测试。经测试发现所制备的有效尺寸为1μm这样较大尺寸的GeTe4相变原型器件,其展现了较大的高低阻值之比(约为1.8×104)、合适的Set与Reset操作速度(200ns和30ns)和理想的Set与Reset功耗(0.8pJ和2.7nJ)。我们认为GeTe4这种二元化合物是一种很有前景的新型相变存储材料。3.从实验和理论计算两方面探索并分析了应力对相变存储器件电学性能的影响。实验上利用静水压下电阻测试系统对菱方晶相Sb2Te3薄膜进行了电阻随静水压强变化的测试,得到了随着静水压强的逐渐增加,菱方晶相Sb2Te3薄膜电阻连续单调地减小的结果,当施加的静水压强增加到0.377GPa时,菱方晶相Sb2Te3薄膜的电阻减小了24.9%。同时利用第一性原理计算方法从理论上计算得到了菱方晶相Sb2Te3的禁带宽度随静水压强变化的结果,即当静水压强达到0.4GPa的时候,其禁带宽度减小了27%。依据半导体材料的电阻与禁带宽度随压强变化的理论关系,将实验结果和理论计算结果相比较,得到了实验和理论相吻合的结果。最后,计算了不同静水压强下菱方晶相Sb2Te3的态密度图谱和各键长的变化等一系列结果,给出了对于静水压下薄膜电阻减小的解释：由于键长随着静水压增加而逐渐减小,使得各原子间的相互作用逐渐增强、各原子周围未配对的电子态交叠的增强,这导致了能带的展宽,从而减小了禁带宽度,最终表现为材料电阻的降低。更多还原

【Abstract】 As with many modern technologies, the extent to which nonvolatile memory has pervaded our day-to-day lives is truly remarkable. From the music on our MP3players, to the photographs on digital cameras, the stored e-mail and text messages on smart phones, the documents we carry on our USB thumb drives, and the program code that enables everything from our portable electronics to cars, the nonvolatile memory known as Flash memory is everywhere around us. Today, Flash memory represents the most widely nonvolatile memory due to its low fabrication costs and high density. However, the bad endurance and low programming speed restrict its development in future. Thus there is a need for a new next-generation nonvolatile memory that might have an easier scaling path than Flash to reach the higher densities offered by future technology nodes. Simultaneously, there is a need for a memory that could offer better write endurance and input-output performance than Flash. Recently, a new memory concept called phase change memory has attracted considerable attention, due to its merits such as simple structure, fast speed, low cost, high scalability, good endurance, low power and good compatibility with complementary metal-oxide semiconductor technologies as compared with other nonvolatile memories. Based on these advantages, PCM is expected to be the most promising candidates for the next generation nonvolatile memory. Ge2Sb2Te5(GST) is the most widely used material for PCM due to its outstanding electronic character in numerous chalcogenide compounds. In order to become the next generation nonvolatile-memory and achieve commercialization, GST face various challenges. Its date retention ability is poor owing to the relatively low crystallization temperature and its switching speed is limited due to the nucleation-dominated crystallization process. It also consumes relatively high power because of its high melting temperature. At present, researchers are working to improve performance of GST and looking for a new material which is more suitable for phase change application. On the other hand, the switching failure mechanism remains a controversial issue.In this work, the phase change behavior of the binary telluride and its switching failure mechanism are investigated. We have investigated possibility and suitability of GeTe4for phase change memory application, and explored effects of hydrostatic pressure on the electrical properties of rhombohedral Sb2Te3. The main research results are summarized as follows:1. The basic properties of GeTe4were investigated systematically. We obtained that the crystalline GeTe4was simple cubic structure. Its crystallization temperature and melting temperature were234.3℃and395.5℃, respectively. The optical band gaps of the amorphous and crystalline GeTe4films were determined as2.09and1.55eV. The activation energy Ea for GeTe4film is determined to be about2.94eV and the data retention temperature of the GeTe4film for ten years was129℃. The binary compound GeTe4shows a higher crystallization temperature and a larger activation energy, which would improve the thermal stability. The lower melting temperature and larger optical band gaps can decrease the power consumption. The simple cubic structure and the growth-dominated crystallization process of the GeTe4lead to a faster set-operation speed. In summary, GeTe4was quite suitable for phase change memory application.2. For the first time, we reported the switching behavior of GeTe4phase change memory. The prototypical phase-change memory cells were fabricated by using the focused ion beam and magnetron sputtering techniques. GeTe4phase change memory cells with an effective diameter of1μm show good resistance contrast, proper switching speed and low power consumption. The dynamic switching ratio between the OFF and ON states is over than1×104. The Set and Reset operations were achieved by using a200ns-2.0V pulse and a30ns-3.0V pulse, respectively. The Set and Reset power consumption were determined as0.8pJ and2.7nJ. Therefore GeTe4should be a promising candidate for the phase change memory applications in the future.3. We have investigated the effect of the hydrostatic pressure on the electrical properties of the rhombohedral phase change material Sb2Te3both experimentally and theoretically. The resistance can be reduced by24.9%when the applied hydrostatic pressure reaches0.377GPa in the experiment. The electronic band gap can be reduced by27%when the applied hydrostatic pressure reaches0.4GPa in the theoretical calculations. The results of the calculations by using the first principles theoretical method fits quite well with the experimental results.The increase of the conductivity of Sb2Te3under the hydrostatic pressure can be ascribed to the reduction of the electronic band gap. The shortening of all bonds in the rhombohedral Sb2Te3under the hydrostatic pressure causes the band broadening and the band gap decreasing, because the atomic interaction is intensified and the overlap of the lone-pair states are increased by the hydrostatic pressure.更多还原