Memory properties and charge effect study in Si nanocrystals by scanning capacitance microscopy and spectroscopy
© Lin et al; licensee Springer. 2011
Received: 19 September 2010
Accepted: 22 February 2011
Published: 22 February 2011
In this letter, isolated Si nanocrystal has been formed by dewetting process with a thin silicon dioxide layer on top. Scanning capacitance microscopy and spectroscopy were used to study the memory properties and charge effect in the Si nanocrystal in ambient temperature. The retention time of trapped charges injected by different direct current (DC) bias were evaluated and compared. By ramp process, strong hysteresis window was observed. The DC spectra curve shift direction and distance was observed differently for quantitative measurements. Holes or electrons can be separately injected into these Si-ncs and the capacitance changes caused by these trapped charges can be easily detected by scanning capacitance microscopy/spectroscopy at the nanometer scale. This study is very useful for nanocrystal charge trap memory application.
Recently, the self-assembled silicon nanocrystals (Si-ncs) that are formed within ultrathin SiO2 layer are considered to be a promising replacement of this conventional floating gate [1, 2]. These isolated Si-ncs embedded in between a tunnel and a top dielectric layer serve as the charge storage nodes and exhibit many physical properties even at room temperature such as Coulomb blockade , single-electron transfer  and quantization charges effect  which differ from bulk crystals. It can reduce the problem of charge loss encountered in conventional memories, cause thinner injection oxides and hence smaller operating voltages, better endurance and faster write/erase speeds. So, the characterisation and understanding of its charging mechanism in such nanostructure is of prime importance.
Although the conventional I-V and C-V characterization methods for memory application provide a vast amount of macro information, these methods lack the ability of discriminating structural and material properties on a nanometer scale. Since atomic force microscopy (AFM) was invented by Binning and Rohrer in IBM, 1982 (Nobel Prize awards in 1986), it has become a powerful high-spatial-resolution tool for nanoscale semiconductor analysis or characterization comparing to several conventional methods for such as x-ray, nuclear, electron and ion beam, optical and infrared and chemical technique. It can provide simultaneous topography and various physical feature images with some additional electrical applications such as scanning capacitance microscopy (SCM) [6, 7], electrostatic force microscopy (EFM) , scanning resistance microscopy  and Kelvin probe force microscopy . In amount of these techniques, SCM became one of the most useful methods for the capacitance characterization of semiconductor as its non-destructive detection of varies electrical properties with high resolution such as dopant profiling variation , silicon p-n junction  and carrier injection , etc.
In this letter, scanning capacitance microscopy and spectroscopy (SCS) were used to study the memory properties and charge effect of the Si-ncs materials in ambient temperature.
In this letter, Si-ncs were formed on top of a thermally grown silicon dioxide layer. SCM and SCS were used to study the memory properties and charge effect on the Si-ncs in ambient temperature. Applying DC bias to the conductive tip, charges were injected into the Si-ncs which was recorded by the SCM images. The retention time of these trapped charges injected by different DC bias were evaluated and compared. By ramp process, strong hysteresis window was observed from the SCS signal. Furthermore, the SCS curve shift direction and distance were observed differently for quantitative measurements. This relates to the fact that holes or electrons can be separately injected into these Si-ncs and the capacitance changes caused by these trapped charges could be easily detected by SCM/SCS at the nanometer scale.
Thanks X.Y. Ma for her helpful suggestions and Armel Descamps-Mandine from the CLYM platform facilities for his help and fruitful discussions on AFM measurements.
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