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Journal of the Electron Devices Society 6 V. CONCLUSION With incumbent flash memory devices operated in the range of 15-17 V, it is imperative to pursue green flash memory that can be operated at voltage less than 10 V to realize a sustainable environment. In this paper, two promising CTL types based on crystalline oxide including crystalline high-k dielectric and crystalline oxide semiconductor which possess the capability to achieve this goal are reviewed. For CTL based on crystalline high-k dielectric, the distinct advantage for memory operation is the k value higher than 30 which is much higher than the amorphous counterpart and beneficial to reduce operation voltage since a higher effective electric field can be exerted on the tunnel SiO2 due to electric flux density continuity. The most concerned retention issue of adopting a crystalline high-k CTL has also be circumvented by passivation of grain boundaries related defects through plasma treatment. The concept of employing crystalline high-k dielectric as CTL not only works for Si-channel flash memory but also Ge-channel memory devices which enable superior memory performance to Si-based counterpart due to higher carrier mobility and more desirable band structure. For CTL based on crystalline oxide semiconductor, a CTL formed by stacking n-type and p-type oxide semiconductor demonstrates typical diode characteristics with a built-in internal electric field. It is the unique internal field that enhances P/E speed while reducing the operation voltage. Both kinds of CTL create a differentiating and pioneering technology that gains an outlook on realizing green flash memory devices.

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