| Co-Investigator(Kenkyū-buntansha) |
KIJIMA Takeshi Sharp Corporation, Corporate Research and Development Group Research Fellow, 技術本部・主任(研究職)
KANASHIMA Takeshi Osaka Univ., Garaduate School of Engineering Science, Assistant Professor, 大学院・基礎工学研究科, 助手 (30283732)
OKUYAMA Masanori Osaka Univ., Garaduate School of Engineering Science, Professor, 大学院・基礎工学研究科, 教授 (60029569)
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| Research Abstract |
1. It is suggested that leakage current through layer of ferroelectric such as SrBi_2TaO_9 and Sr_2 (Ta_<1-x>, Nb_x)_2O_7 has a dominant effect on memory retention properties of Metal-Ferroelectric-Insulator-Semiconductor (MFIS) structure. Furthermore, it is confirmed especially for the. SBT film that suppression of the leakage improved drastically retention properties, then increased the time by an order of magnitude, more than 2x10^4 s. with some post-oxygen annealing process.
2. We proposed a dynamic model where carriers flow though ferroelectric layer and inject into the interface between ferroelectric and insulator layers, thus electric field imposed on the ferroelectric film gradually cancelled and decreased. As a result, experimental results shown above was qualitatively explained.
3. A new analysis instrument utilizing ionization potential measurement was proposed and fabricated, which enables us to evaluate the SBT sutface with and without post-oxygen annealing, then to consider the band diagram of MFIS structure. It reveals that, before the annealing, hole conduction dominates the leakage through the SBT film, and that, after the annealing, barrier height against hole increases from 1.6 to 2.0 eV. This result agrees well with the mentioned above, where a reduction of leakage by the annealing improves the memory retention characteristics.
4. A Rapid Thermal Annealing process with oxygen atmosphere improved greatly the retention time of MFIS diode. With a very high temperature and short time of 1000℃ and 30 s, the retention time became more than 6x10^5 s (about 1 week), and was extrapolated to about 3x10^7 s (about 1 year).
5. A high pressure (7 atm) oxygen annealing was examined in order to improve the retention properties furthermore: It was found that the annealing is effective to reduce the leakage and estimated that Bi_xO_y layer, different from (Bi_2O_2)^<2+> layer, was fermed and suppress the leakage.
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