| Summary: | 博士 === 國立中山大學 === 光電工程學系研究所 === 101 === Current requirements of nonvolatile memory (NVM) are the high density cells, low-power consumption, high-speed operation and good reliability for next-generation NVM application. However, all of the charges stored in the floating gate will leak into the substrate if the tunnel oxide has a leakage path in the conventional NVM during endurance test. Therefore, the tunnel oxide thickness is difficult to scale down in terms of charge retention and endurance characteristics. Among the emerging memory structure, resistive random access memory (ReRAM) has attract extensive attention as the promising candidates to substitute for conventional floating gate memory due to its sample structure, superior and scalable properties.
In this thesis, the memory phenomenon and mechanisms for the SiO2 based ReRAM are obtained and investigated. The switching characteristics are improved through the electric post-treatment and structure modulation methods. The ReRAM device was fabricated by depositing a 20 nm-thick Mo:SiO2 film as the active layer. It is discovered that the switching characteristics for the Mo:SiO2 device are dominated by the SiO2. Subsequently, the reasons which cause the switching characteristics for SiO2 are reported rarely are investigated. We find that the oxygen ions generated during the forming process would be limited by the driven ability while the thickness of the SiO2 film gets larger than 20 nm. The extra oxygen ions accumulate at the interface of SiO2/TiN. Afterward, the unexpected reformed SiO2 layer (RFL) is formed to blockade the conductive filament (CF) due to the recombination of the extra oxygen ions and the oxygen vacancies. As a result, the failure in the film activation by the forming process leads to the visible of the switching phenomenon in the SiO2 film. The problem is solved successfully by rising the temperature ambient during the forming process.
In order to achieve the target of low power consumption, reducing the operation current is one of the concerned topics for the research in ReRAM. We discover that the switching characteristics can be affected by the temperature ambient during the forming process. The high resistance state (HRS) current decreases when the temperature ambient increases during the forming process. The phenomenon can be attributed to that more oxygen ions are generated due to the high temperature ambient induced serious damage during the forming process. As a result, the CF can be well repaired by the assistance of the more oxygen ions. The formation of the well-repaired CF leads to the reduction of the HRS current. The low resistance state (LRS) current is also suppression successfully by inserting a 3 nm-thick Si layer between the V:SiO2 active layer and the TiN electrode. Since the Si inserted layer can gather the oxygen ions driven form the V:SiO2 film during the set process, the Si inserted layer transform to the SiO2 layer in LRS. The formation of the SiO2 layer increases the barrier for the carrier transport. Hence, the LRS current is suppressed.
Finally, a -2 V pulse voltage signal is applied to the SiO2 film to simulate the DC condition operated during the reset process. According to utilize B1500 and B1530 semiconductor characterization analyzer, the responding current among the whole voltage pulse period can be obtained and analyzed. The analyzed results present that the reset process can be divided into three parts, i.e. “Ohmic Region”, “Reset Region”, and “Stable Region”. Since the repair behavior of the film (CF) mainly occurs in Reset Region, the analyses of the reset process are focused on this region. When the bias is applied large above the reset voltage (Vreset), the reset process enters the Reset Region. Numerous oxygen ions are suddenly released and driven into the film at the moment of achieving the Vreset. An exponential increase in the resistance is exhibited in the initial of the Reset Region due to the extensive repair caused by these numerous oxygen ions. Afterward, the repair behavior is suppressed by the formation of the SL. The resistance increases linearly instead. Through analyzing the linear relation characteristic, the potential energy of reset is obtained as 0.26 V. By utilizing the parameters to estimate the Vreset value, the validity of these parameters can be confirmed. In addition, breakdown behavior also occurs besides repair behavior for the dielectric film and the SL during the reset process. Oxygen ions can be driven farther and further repairs the film and the CF while the breakdown behavior is induced. Subsequently, the thickness or area of the SL gets larger which leads to the suppression of leakage current and the increase in the resistant state.
The resistive switching phenomenon has been discovered in a number of materials. In order to promote the commercial application of ReRAM, it is important to find the cheap, abound and familiar material as the active layer. The discovery of resistive switching phenomenon in the SiO2 is helpful for the development of the ReRAM device. In addition, the investigation of the switching mechanisms and the improvement of the switching characteristics studied in this thesis are also significant for the promotion of the commercial application of ReRAM.
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