AVS 52nd International Symposium
    Nanometer-Scale Science and Technology Tuesday Sessions
       Session NS-TuP

Paper NS-TuP27
Formation of Ge Nanocrystals in Hf based High-K Dielectrics for Nonvolatile Flash Memory Device Application

Tuesday, November 1, 2005, 4:00 pm, Room Exhibit Hall C&D

Session: Nanometer Scale Science and Technology Poster Session
Presenter: W.J. Yoo, National University of Singapore
Authors: J. Chen, National University of Singapore
A.-Y. Du, Institute of Microelectronics, Singapore
W.J. Yoo, National University of Singapore
D.S.H. Chan, National University of Singapore
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Nanocrystals (NCs) floating gate has received considerable attention for the future nonvolatile flash memory devices because its discreteness of charge storage suppresses lateral migration of charges, enhancing immunity to oxide defects compared with conventional flash memories using continuous floating gates. High-K dielectrics in place of conventional SiO@sub 2@ can further improve programming efficiency, data retention and read speed of flash memory because of lower F-N tunneling barriers and small equivalent oxide thicknesses. In this work, methods to form Ge nanocrystals embedded in main stream high-K dielectrics, HfO@sub 2@ and (HfO@sub 2@)@sub x@(Al@sub 2@O@sub 3@)@sub 1-x@ (HfAlO) were introduced and compared. Temperature dependence of phase separation process in the formation of Ge NCs in HfO@sub 2@ and HfAlO matrices was studied using X-ray photoelectron spectroscopy. Transmission electron microscopy and energy dispersive X-ray spectroscopy show that Ge NCs can form in either HfAlO or HfO@sub 2@, and the location of Ge NCs varies depending on the high-K films. It is found that, in HfAlO, Ge NCs are well sandwiched in amorphous HfAlO matrix. However, in HfO@sub 2@, Ge NCs are located in grain boundaries of polycrystalline HfO@sub 2@, and most of Ge NCs are in direct contact with Si substrate or gate electrode of memory devices because of the extrusion effect of the growth of HfO@sub 2@ crystal grains on Ge NCs. This results in a significant difference in data retention of memories employing these structures. Flash memories employing Ge NCs embedded in HfAlO can maintain a memory window of 0.5 V in 10 years, whereas flash memories employing Ge NCs embedded in HfO@sub 2@ show a memory window closing only within 2 minutes.