Nonvolatile memory MOS capacitors made of CdSe embedded ZrHfO high-k gate dielectric
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Nonvolatile memory MOS capacitors made of CdSe embedded ZrHfO high-k gate dielectric Chi-Chou Lin and Yue Kuo Thin Film Nano & Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843-3122, U.S.A. ABSTRACT MOS capacitor composed of nc-CdSe embedded ZrHfO high-k gate dielectric stack was fabricated and characterized for nonvolatile memory functions. Detailed material and electrical properties have been investigated. With a large charge trapping capability, this kind of device can trap electrons or holes depending on the polarity and magnitude of the applied gate voltage. For the same stress time, the device trapped more holes than electrons under the same magnitude of gate voltage but different polarity. The negative differential resistance peak was observed at the room temperature due to the Coulomb blockade effect. The charge trapping mechanism was delineated with the constant voltage stress test. After 10 years of storage, about 56% of trapped charges still remain in the device. INTRODUCTION The nanocrystalline Si (nc-Si) embedded SiO2 has been used to replace the conventional polycrystalline Si as the gate dielectric layer in the floating-gate nonvolatile memory device [1]. However, during device scaling down, the leakage current becomes very high and the trapped charges are easily tunneled back to the Si substrate [2]. The replacement of SiO2 with a physically thicker layer of high dielectric constant (high-k) film can solve the above problems in the nanosize MOSFET [3]. Previously, it has been demonstrated that the Zr-doped HfO2 (ZrHfO) high-k dielectric has a smaller equivalent oxide thickness (EOT), a lower interface state density, and a higher amorphous to crystalline transition temperature than the undoped HfO2 high-k dielectric [4,5]. Therefore, the ZrHfO high-k film can replace the SiO2 as the tunnel and control oxide layers in the nanocrystals embedded nonvolatile memory device. Many non-Si materials with large work functions can be prepared into nanodots and be embedded into the dielectric as the charge trapping media for the advantage of the large barrier heights with respect to the surrounding dielectrics [2,6,7]. For example, CdSe is a n-type semiconductor with large work function, i.e., 4.8 eV to 5 eV. Therefore, it is a suitable material as the charge trapping medium in the gate dielectric layer [8]. In this paper, MOS capacitors made of the nc-CdSe embedded ZrHfO gate dielectric structure have been fabricated and investigated for nonvolatile memory characteristics. EXPERIMENTAL MOS capacitors composed of nc-CdSe layer embedded in the ZrHfO high-k film was fabricated on the p-type Si (1015cm-3) wafer. The bottom ZrHfO (tunnel oxide)/CdSe/top ZrHfO
(control oxide) trilayer structure was sputter-deposited sequentially in a one pumpdown process without breaking the vacuum. Both ZrHfO layers were deposited from the Zr/Hf (12:88 wt%) target at 60W, 5 mTorr in the Ar/O2 (1:1) atmosphere. The deposition time for the bottom and top ZrHfO high-k layers were 2 min and 10 min, s
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