| 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. |
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