Abstract
In conventional memory devices, poly-silicon is employed as a floating-gate to store charge. However, the conventional floating-gate non-volatile memory device has met the challenge of reliability owing to the requirement of down-scaling device. The scaled tunneling oxide is difficult to prevent the stored charge in the floating-gate from tunneling back into the Si-substrate. Therefore, in order to improve the retention time of conventional floating-gate memories, some novel memories are proposed to replace the conventional floating-gate memories, such as SONOS or nanocrystal nonvolatile memory which are similar to conventional floating-gate memories, and resistance random access memory. There are two kind of nonvolatile memories that are investigated in this thesis. We will introduce and study the nanocrystal memory in the first half of the thesis and the resistance random access memory in the latter half.Nanocrystal nonvolatile memory devices have been investigated as a method to overcome these drawbacks in recent years. Because discrete trap storage nodes act as the charge center of nanocrystal nonvolatile memory devices, they can effectively avoid data loss in terms of reliability when such devices scale down. For nanoscale devices, the density of nanocrystal is an issue because the memory window is dependent on the nanocrystal density. High density is helpful to scale down device structure. However, if the density of nanocrystals is very high, the quality of the oxide which surrounds the nanocrystals will be critical. Because the electrons stored in nanocrystals will escape easily by trap tunneling if the oxide quality is not sufficient. When nanocrystals are very close to each other, there is a tradeoff between high nanocrystal density and good reliability for nano-NVM application. In order to achieve both high density and good reliability, using high-pressure hydrogen treatment with high temperature annealing (700-900 oC) has been proven to be a valid method to improve the oxide surrounding NCs. Unfortunately, metal nanocrystals and metal control gates cannot endure this temperature treatment. Hence, some of post-plasma treatment such as hydrogen and oxygen plasma treatment are proposed for tungsten nanocrystal nonvolatile memory as an alternative method to passivate the defects in the surrounding oxide and improve its quality. The advantages of this method are a simple fabrication process and a low thermal budget.In the thesis, the influence of a hydrogen plasma treatment on electrical properties of tungsten nanocrystal nonvolatile memory was studied. The X-ray photon emission spectra shows that, after the hydrogen plasma treatment, a change in binding energy occurs such that Six+ and Siy+ peaks appear at a position that is shifted about 2.3 and 3.3 eV from Si0+ in Si 2p spectra. This indicates Si dangling bonds are passivated to form a Si-H bond structure in the SiO2. Electrical measurement analyses show improved data retention because the hydrogen plasma treatment enhances the quality of the oxide surrounding the nanocrystals.In addition, an oxygen plasma treatment was also used to improve the memory effect of nonvolatile tungsten nanocrystal memory, including memory window, retention and endurance. To investigate the role of the oxygen plasma treatment in charge storage characteristics, the X-ray photon-emission spectra (XPS) were performed to analyze the variation of chemical composition for tungsten nanocrystals embedded oxide with and without the oxygen plasma treatment. The transmission electron microscopy (TEM) analyses were also used to identify the microstructure in the thin film and the size and density of tungsten nanocrystals. The device with the oxygen plasma treatment shows a significant improvement of charge storage effect, because the oxygen plasma treatment enhances the quality of silicon oxide surrounding the tungsten nanocrystals. Therefore, the data retention and endurance characteristics are also improved by the passivation.According to the values of other literatures, a in-situ-steam-generation (ISSG) oxidation process can be used to improve the quality of thin oxide. Therefore, the formation of tungsten nanocrystal nonvolatile memory was provided by using ISSG oxidation process. The charge trapping layer of stacked a-Si and WSi2 was deposited by low pressure chemical vapor deposition (LPCVD) and was oxidized by ISSG system to form uniform tungsten nanocrystals embedded in SiO2. Transmission electron microscopy analyses revealed the microstructure in the thin film and X-ray photon-emission spectra indicated the variation of chemical composition under different oxidizing conditions. Electrical measurement analyses showed the different charge storage effects because the different oxidizing conditions influence composition of trapping layer and surrounding oxide quality. The results show the reliability of the structure with 2% hydrogen and 98% oxygen at 950℃ oxidizing condition has the best performance among the samples.Another kind of nonvolatile memory, resistance random access memory, has attracted a numerous of attention, and researched to replace conventional memory devices. This study also investigates the resistance switching characteristics of Cr2O3-based resistance random access memory (RRAM) with Pt/Cr2O3/TiN and Pt/Cr2O3/Pt structures. Only devices with Pt/Cr2O3/TiN structure exhibit bipolar switching behavior after the forming process because TiN was able to work as an effective oxygen reservoir but Pt was not. Oxygen migration between Cr2O3 and TiN was observed clearly before and after resistance switching from Auger electron spectroscopy (AES) analysis. Both low resistance state, ON state, and high resistance state, OFF state, of Pt/Cr2O3/TiN structures are stable and reproducible during a successive resistive switching. The resistance ratio of ON and OFF state is over 102, on top of that, the retention properties of both states are very stable after 104 seconds with a voltage of -0.2V.The carrier transport phenomenon and multi-level switching mechanism of Cr2O3-based resistance random-access memory (RRAM) with Pt/Cr2O3/TiN structure were investigated. Before the forming process, the interfacial Schottky barrier dominates the carrier transport. The barrier heights of Pt/Cr2O3 and Cr2O3/TiN are 0.7 and 0.96 eV. After the forming process, RRAM at a low resistance state follows the Ohmic conduction. While RRAM is switched to high resistance state during the reset process, the Frenkel-Poole emission becomes a dominant conduction mechanism. The multi-level resistance states were achieved by applying corresponding reset voltages to the device for controlling the trap levels of Cr2O3 layer.The multi-level resistance switching characteristics of Cr2O3-based resistance random access memory with Pt/Cr2O3/TiN structures was researched. By controlling compliance current during set process and sweeping voltage range during reset process, multi low-resistance and high-resistance states can be achieved, respectively. Dependence of multi low and high resistance states on the temperature was further investigated. Energy band diagram models in different temperatures and in the low and high resistance states were proposed to explain corresponding carrier transport mechanisms. According to the results, multi-level operation by changing compliance current is suitable because the multi low resistance states are less sensitive to temperature.