Charge Storage Characteristics Of Ni Nanocrystals Embedded HfO₂ Films

Since mid-1990 s, nano-floating gate nonvolatile memory（NFG-NVM） devices based on complementary metal-oxide-semiconductor（MOS） field effect transistor structure have been extensively studied for potential application in next generation flash memory device. Compared to semi-conductive（Si,Ge） and organic（C, graphene） charge storage candidates, noble metal nano-particles（Pt, Auand Ag） embedded in oxide dielectric films（Al2O3, Hf Ox, Zr O2, Ti O2, Hf Al Ox） have been highlighted due to their high work function and excellent charge properties, but they are not suitable for large-scale industrial applications due to their high cost. Ni nano-crystals（Ni-NCs） has a high work function of 5.35 e V, stable chemical properties, and have been reported to exhibit a memory window width（flat-band-voltage shift（35）VFB） ranging from 1 to 20 V, showing excellent charge storage capacity, but its endurance and retention properties need to be further improved. Moreover, oxidation of Ni in oxide dielectric matrix is another big problem needs to be solved.A trilayer capacitor composed by Ni nanocrystals as memory cells and HfO₂ films as control/tunneling layers has been fabricated on n-type Si（100） substrates by radio-frequency magnetron sputtering. Its surface topography, microstructures and chemical component have been investigated by means of Atomic Force Microscope（AFM）, Transmission electron microscope（TEM） and Xray Photoelectron Spectroscopy（XPS）, respectively; its capacitorvoltage and leakage current-voltage curves have been measured by using Agilent 4294 A impedance analyzer and Keithley2400 source meter. Experimental results reveal that, Ni nanocrystals embedded HfO₂ films exhibit a memory window of 1.5V, corresponding to a charge storage density of 6.0×1012/cm2. These capacitor memory cells exhibit good endurance characteristic up to 4′104 cycles. The capacitance loss was monitored at zero gate voltage after the sample was applied one pulse of ±6 V（electron and hole charging） with a width of 10 s. The loss of electron charge is less than 2%, and the loss of hole charge is less than 5% after waiting for 105 s. After programming the memory cell in the write or erase state, the flatband voltage shift（VFB） was monitored for at least 30 h. Positive/negative VFB has a slight decrease of 20% after waiting for 105 s. Even after extrapolating to the 10 years time, the two states still remain distinguished, which completely meets the retention requirement of next generation non-volatile memory devices. The plotting line fits well with the Schottky tunneling mechanism in the electric field ranging from 2.5′107V/m to 1.75 ′108V/m.On the other hand, we have also investigated the light effect on the charge storage characteristics of Ni nanocrystals embedded HfO₂ films. The comparative study shows that the structure of the Si substrate covered with native silicon oxide layer and the pure HfO₂ film still have no memory window under illumination. However the memory window of Ni nanocrystals embedded HfO₂ film capacitors under illumination increase obviously. When applying ±6V scanning voltage, the memory window of the film have increased from 1.5V to 2.76 V, corresponding to a charge storage density about: 9.9×1012/cm2.In summary, Ni nanocrystals embedded HfO₂ films has a considerable charge storage characteristics, as the NFG-NVM device candidate materials; at the same time, light field assisted NFG-NVM lead the new way for the next generation of flash memory devices.