Nanocrystals embedded in HfO2-based dielectrics as charge storage nodes of nano-floating gate memory

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Nanocrystals embedded in HfO2-based dielectrics as charge storage nodes of nano-floating gate memory

 

Author: Lee, Pui-fai
Title: Nanocrystals embedded in HfO2-based dielectrics as charge storage nodes of nano-floating gate memory
Degree: Ph.D.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Nanocrystals.
Dielectrics.
Semiconductor storage devices.
Department: Dept. of Applied Physics
Pages: xv, 140 leaves : ill. ; 31 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2094036
URI: http://theses.lib.polyu.edu.hk/handle/200/735
Abstract: Nanocrystals (NC) embedded in dielectrics have attracted a great deal of attention recently because they can potentially be applied in nonvolatile, high-speed, high-density and low-power memory devices. Compared with conventional floating gate memories such as flash, a device composed of nanocrystals isolated by dielectrics benefits from a relatively low operating voltage, high endurance, fast write-erase speeds and better immunity to soft errors. The nanocrystal materials suitable for the nanocrystal floating gate memory application can be either metals or semiconductors. Recent studies have shown that high-k dielectrics, instead of SiO2, for the tunneling layer in nanocrystal floating gate memory can improve the trade-off between data retention and program efficiency due to the unique band alignment of high-k dielectrics in the programming and retention modes. Being physically much thicker than SiO2, high-k dielectrics such as HfO2 with leakage current several orders of magnitude smaller than SiO2 with the same equivalent oxide thickness (EOT), result in superior data retention property. The lower electron barrier heights of high-k dielectrics can also reduce the programming voltage and thus the operation power compared to the traditionally used SiO2 gate dielectric. In this project, HfAlO has been selected as the high-k dielectric for the nanocrystal floating gate memory structure, since it has been shown to be a promising high-k material. Two deposition techniques, namely pulsed-laser deposition (PLD) and nanocluster source, have been implemented to fabricate nanocrystals. Structural properties of the nanocrystal floating gate memory trilayer structures were characterized by transmission electron microscopy, and atomic force microscopy. Their memory and charge retention characteristics were characterized by capacitance-voltage (C-V), capacitance-time (C-t) and current-voltage (I-V) measurements. The trilayer structure (HfAlO/Ge-NC/HfAlO) on Si was fabricated by PLD at a relatively low temperature. The effects of deposition temperature and growth rate in forming Ge nanocrystals were investigated and it revealed that relatively low substrate temperature and growth rate are favourable for the formation of smaller-size Ge nanocrystals. Effects of size/density of the Ge NC, the tunneling and control oxide layer thicknesses and the oxygen partial pressure during their growth on the charge storage and charge retention characteristics have also been studied. The island structure of the Ge NC suggests that the growth is based on the Volmer-Webber mode. The self-organized Ge nanocrystals so formed were uniform in size (5-20 nm diameter) and distribution with a density approaching 1012 -1013cm-2. Flat-band voltage shift (VFB) of about 3.6 V and good retention property have been achieved. Another approach is to use a nanocluster source combined with PLD. The nanocluster source has been shown to be capable of generating nanoclusters of semiconductors and metals with the size of a few nanometers, which is the ideal size for nanocrystal floating gate memory application. The implementation of the nanocluster source in fabricating semiconductor nanoclusters is expected to provide a new approach for the fabrication of nanocrystal floating gate memory. By varying aggregation distance, sputtering gas pressure and ionization power, nanoclusters of Ge with different sizes can be formed. Memory effect of Ge nanocluster floating gate memory structure consisting of HfAlO high-k dielectric tunneling and control oxides has been investigated. The memory effect of the trilayer structure (HfAlO/Ge-NC/HfAlO) so formed with 10 nm Ge nanoclusters are manifested by the counter-clockwise hysteresis loop in the C-V curves and a maximum flat-band voltage shift of 5.0 V has been achieved. The major advantages of metal nanocrystals include higher density of states around the Fermi level, stronger coupling with the conduction channel, a wide range of available work functions, and smaller energy perturbation due to carrier confinement. For comparison purposes, metal nanocrystals have also been investigated by utilizing both of the physical deposition methods as mentioned above. Silver (Ag) nanocrystals with size of 10-40 nm have been embedded in HfAlO matrix in the trilayer capacitor structure and a flat-band voltage shift of 2.0 V has been achieved.

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