| Summary: | 博士 === 國立交通大學 === 電子工程學系 電子研究所 === 102 === With the arrival of Digital Mobile Life, the demands for nonvolatile memory (NVM) have significantly increased, such as mobile phones, digital cameras and portable devices. NVM technology has been developed to obtain high speed, high density and low power consumption. But, when the technology node continuously scaled down, the flash memory faces the challenge of retention drop caused by thin tunneling oxide. During long-term operation, the thickness of tunneling oxide below 10 nm will cause the charge lost in floating gate due to direct tunneling current or defects formed in the tunneling oxide. Therefore, several types of NVMs such as ferroelectric random access memory (FeRAM), magnetic random access memory (MRAM), and resistive random access memory (RRAM) are being investigated. There are many candidate materials for RRAM application including perovskite materials such as Pr0.7Ca0.3MnO3, PbZr0.5Ti0.5O3, SrZrO3, SrTiO3, and binary metal oxides such as NiO, TiO2, HfO2, Al2O3, and HfON. Such a memory application should have the merits of low power consumption, compatibility of the current complementary metal oxide semiconductor (CMOS) process, high-speed operation, high scalability, and simple metal-insulator-metal (MIM) tri-layer structure. The resistive switching mechanisms are attributed to the formation and rupture of the conducting filaments, which are related to trapping/detrapping, reoxidization/reduction, and Mott transition performed by O2− migration. Moreover, the stochastic formation and rupture of the filaments cause the fluctuation of the operation voltages and the memory states during continuous switching cycles, which may lead to severe control and readout problems.
First of all, we have fabricated the Ni/GeOx/PbZr0.5Ti0.5O3/TaN resistive switching memory. Under unipolar-mode operation, the bilayers Ni/GeOx/PZT/TaN RRAM shows a large resistance window of >102, 85℃ retention, and a good DC cycling of 2000 cycles, which are significantly better than those shown by the single-layer Ni/PZT/TaN RRAM without the covalent-bond-dielectric GeOx .
Next, to further improvement the distribution, we use GeOx with TiO2 layer to form Ni/GeOx/TiOx/TaN resistive random access memory for better device-to-device distribution and retention. This RRAM device shows low 30μW switching power (9μA at 3V; −1μA at −3V), 105 cycling endurance and good retention at 85 oC.
To compare the different covalent bond dielectric with GeO, we fabricated AlOx/TiOx and GeOx/TiOx resistive random access memory at room temperature. The AlOx/TiOx and GeOx/TiOx RRAM exhibit similar set/reset powers and switching window, but the AlOx/TiOx RRAM shows much poor data retention and poor switching uniformity as compared to the GeOx/TiOx RRAM. In retention test, AlOx/TiOx RRAM presents poor 60 oC data retention due to high currents for high resistance state (HRS) with low activation energy (Ea) of only 0.4 eV, which is much lower than 0.52 eV of GeOx/TiOx RRAM.
Finally, to avoid the degradation of the GeO layer caused by hydraulic in atmosphere, we fabricated Ge-SiO resistive random access memory to achieve simple structure of single layer, high speed operation of 60s for 104 cycling endurance and good retention at 85 oC.
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