Charge Trapping Sites in nc-RuO embedded ZrHfO High-k Nonvolatile Memories
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Charge Trapping Sites in nc-RuO embedded ZrHfO High-k Nonvolatile Memories Chen-Han Lin and Yue Kuo Thin Film Nano and Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843, USA ABSTRACT Materials and electrical properties of the MOS capacitor containing nc-RuO embedded in the high-k dielectric ZrHfO film have been studied. The electron- and holetrapping capacities and trapping sites in this kind of device were investigated using the constant voltage stress method, the frequency-dependent C-V measurement, and the retention characteristics. The negligible charge trapping phenomenon in the nonembedded device rules out the possibility of any trapping site in the bulk ZrHfO film or at the Si/ZrHfO interface. The electrical characterization results suggest that electrons are trapped in the bulk nc-RuO. However, holes have two possible trapping sites, i.e., in the bulk nc-RuO or at the nc-RuO/ZrHfO interface. INTRODUCTION The MOS capacitor containing the nanocrytstals embedded high-k dielectric is a promising flash memory device that can be scaled down to a very small geometry. For the conventional polycrystalline Si (poly-Si) floating gate structure, the stored charges could be easily drained off via a single leakage path formed in the tunnel oxide. This is an important reliability issue, which can be improved by replacing the continuous poly-Si film with nanocrystals [1]. Nanocrystals can be made of various materials, e.g., semiconducting elements (Si, Ge [1-3]) and oxides (ZnO [4]), conducting metals (Au, Pt, Ag, W, etc. [5-8]) and oxides (Sn-doped indium oxide, RuO2 [9,10]), and insulating compounds (HfO2 [11]). The memory functions and reliabilities of these devices have been studied. Nanocrystals with large work functions are ideal materials for the memory application because they can serve as the deep charge trapping sites to achieve a long data retention time. Ruthenium oxide (RuO) has a large work function of ~5eV and excellent thermal and chemical stability [12], which is suitable for the above application. High-k dielectrics can replace the thermally grown SiO2 as the tunnel and control oxides in the nanocrystals embedded memory devices [13]. Based on the same equivalent oxide thickness (EOT), the high-k film is physically thicker than SiO2, which favors the charge retention time and is more reliable. HfO2 is a popular high-k for gate dielectric application because it has a large band gap (~6eV) and low conduction and valence band offsets with Si [14]. The nanocrystals embedded HfO2 memory device has been fabricated [15]. However, a high temperature annealing, e.g., >700℃ is required to form nanocrystal and to eliminate defects in the HfO2 film. It causes crystallization of HfO2, which is a reliability concern. Previously, the authors proved that the amorphous-topolycrystalline temperature of the sputter deposited HfO2 could be increased by adding Zr into the film. In addition, compared with the un-doped HfO2 film, the Zr-doped HfO2 film (ZrHfO) can be prepared into
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