Al 2 O 3 /Si 3 N 4 Buffer Layer for High Performance MFIS (Metal-Ferroelectric-Insulator-Semiconductor) Transistors
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Al2O3/Si3N4 Buffer Layer for High Performance MFIS (Metal-Ferroelectric-Insulator-Semiconductor) Transistors Yoshihisa Fujisaki1,2 and Hiroshi Ishiwara1 1 Frontier Collaborative Research Center, Tokyo Institute of Technology, Yokohama 226-8503, Japan 2 Research and Development for Future Electron Devices, Taito, Japan ABSTRACT The Al2O3 thin films were deposited on the Si3N4 (radical- Si3N4) prepared by the direct nitridation of a Si substrate using atomic nitrogen radicals. Because the radical- Si3N4 is highly resistive against the oxidation process, we can completely eliminate the metal phase in Al2O3 that degrades the insulating properties of Al2O3. The 1.3nm thick Al2O3 film was found to have a permittivity of 9.76 in this stacked Al2O3/Si3N4 structure. It was also found that no oxidation took place at the interface of Si3N4 and the Si substrate during the deposition and post-oxidation of the Al2O3 film on the radical-Si3N4/Si substrates. As a result, highly capacitive MIS (Metal-Insulator-Semiconductor) structures were realized using the Al2O3/Si3N4 stacked insulator. The MIS diode with Al2O3/Si3N4 is found to have 105 times less leakage current compared to the diode with Si3N4 insulator film with similar capacitance density. We have shown that the insulating properties of the Al2O3/radical-Si3N4 stack are suitable for the MFIS (Metal-Ferroelectric-Insulator-Semiconductor) transistors. INTRODUCTION The ferroelectric gate transistor is a promising device because it may enable highly integrated memories with low power consumption suitable for mobile devices. Among the various structures proposed for ferroelectric gate transistors, MFIS FET (Field Effect Transistor) is simplest in structure and most suitable for highly integrated memories. Although a lot of work has been done to realize MFIS FETs,[1-3] there still remain serious problems in choosing the materials for the system. Especially, the development of the insulating layer (I-layer) that should have high permittivity and high resistance against the post oxidation process is the biggest issue to be resolved before realizing the MFIS FETs. We have proposed that radical-Si3N4 is one of the most promising candidates for the I-layer because ultra thin radical-Si3N4 can realize both high capacitance density and enough resistance C11.4.1
against oxidation to suppress the emergence of SiO2 at the interface between the Si substrate.[1] We demonstrated that the MFIS with the structure of Pt/(Bi,La)4Ti3O12/radical-Si3N4/Si retains more than 90% of initial charges after 104 sec.[2] However, in realizing the actual transistors, the retention time should be much longer than 108 sec. This means that the current leakage of the I-layer should be smaller than the present radical-Si3N4 at least by a factor of 104. To improve the insulating property of the I-layer, we chose Al2O3 that has larger permittivity and band gap compared to radical-Si3N4. We deposited the Al2O3 thin film on radical-Si3N4/Si to prevent oxidization of the Si substrate. EXPERIMENTAL The nitridation of
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