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 c
Trang 1and spectroscopy
Zhen Lin1*, Georges Bremond1†, Franck Bassani2†
Abstract
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
consid-ered 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
proper-ties even at room temperature such as Coulomb
block-ade [3], single-electron transfer [4] and quantization
charges effect [5] which differ from bulk crystals It can
reduce the problem of charge loss encountered in
con-ventional 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
nanos-tructure is of prime importance
Although the conventional I-V and C-V
characteriza-tion methods for memory applicacharacteriza-tion provide a vast
amount of macro information, these methods lack the
ability of discriminating structural and material
proper-ties on a nanometer scale Since atomic force
micro-scopy (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 chemi-cal technique It can provide simultaneous topography and various physical feature images with some addi-tional electrical applications such as scanning capaci-tance microscopy (SCM) [6,7], electrostatic force microscopy (EFM) [8], scanning resistance microscopy [9] and Kelvin probe force microscopy [10] In amount
of these techniques, SCM became one of the most use-ful methods for the capacitance characterization of semiconductor as its non-destructive detection of varies electrical properties with high resolution such as dopant profiling variation [11], silicon p-n junction [12] and carrier injection [13], 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
Figure 1 shows the formation of these isolated Si-ncs First, a 4-nm-thick thermal oxide was grown as the tun-nelling oxide on an amorphous Si substrate Subse-quently, Si layer was deposited by molecular beam epitaxy over a very thin SiO2 layer, 5 nm in thickness, at ambient temperature and was thermally annealed at
* Correspondence: zhen.lin@insa-lyon.fr
† Contributed equally
1 Institut des Nanotechnologies de Lyon, UMR 5270, Institut National des
Sciences Appliquées de Lyon, Université de Lyon, Bât Blaise Pascal, 20,
avenue Albert Einstein - 69621 Villeurbanne Cedex, France
Full list of author information is available at the end of the article
© 2011 Lin et al; licensee Springer This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium,
Trang 2750°C for 20 min under ultrahigh vacuum The
dewet-ting process leads to the formation of isolated Si
nano-crystals having an average density of 4 × 1010cm-2
Veeco Digital Instruments 3100 Dimensions AFM
employing a Nanoscope V controller was used to
con-duct SCM and SCS measurements The concon-ductive tip
that was selected was commercial Arrow-EFM PtIr
coat-ing tip It has an average tip radius of less than 10 nm,
cantilever spring constant: 2.8 N/m and resonance
fre-quency: 75 kHz SCM images were taken with a fixed
bias frequency of 50 kHz, SCM lock-in phase of 90°and
capacitance sensor frequency of 910 MHz The
ampli-tude of direct current (DC)/direct voltage signal is
strongly dependent on the modulation voltages and the
magnitude of capacitance variation is generally a
non-linear function of the carrier concentration Figure 2
shows the topography and SCM image The contrast
between Si-nc and the oxide layer was clear in SCM
image which indicates that the Si-nc has different
capa-citance from the oxide layer
In order to investigate the effects of DC bias and
alter-nating current (AC) bias to the SCM signal, the
slows-can was disabled and a typical line sslows-can was performed
In Figure 3a, the VAC bias was fixed to 2,500 mV The
SCM image and signal variation with DC bias is shown
in Figure 3a Different DC bias during the scan can
cause different SCM signal The best signal/noise ratio
and highest contrast occurred when -1 or 0.5 V was
applied, which is the same as Ge nanocrystals Too high
DC bias amplitude, such as up to 2 V, will make the SCM signal disappeared Figure 3b illustrates this varia-tion in funcvaria-tion to the DC bias The higher the DC bias amplitude, the stronger SCM signal intensity was How-ever, the lower the contrast between the Si-ncs and dielectric layer was Positive or negative modulation cor-responds to different SCM phase The best resolution and the best signal to noise ratio which correspond to the highest contrast between Si-ncs and dielectric layer
in the image was obtained near -0.5 and 0.5 V with the scan rate of 0.5 Hz
AC bias was also investigated by fixing the DC bias to 0.5 V which is one of the best DC bias as we mentioned above The SCM line scan image with different AC bias and its variation was shown in Figure 4 The contrast between the Si-ncs and dielectric layer changed with AC bias The higher the AC bias, the stronger the SCM sig-nal intensity was However, too high AC voltage can induce charge injection in the sample which will create parasitic capacitance and high noise Here, 2 V AC bias was fixed during the scan
Charge injection was done by separately applying (0.5, 1.0, 2.0, and 3.0 V) to the tip during the contact SCM scan Then DC bias was set back to -0.5 V which was the best as we chose for our signal As the SCM signal
is dependent on the quantity of injected charges, it was monitored for charge retention time study The
non-Figure 1 The formation of isolated Si-ncs.
Figure 2 Si nanocrystal images (a) Topography (b) SCM data image.
Trang 3linear function between the retention time and the DC
bias is shown in Figure 5 The higher the DC bias
(char-ging voltage) was, the longer its discharge time was,
which means more carriers were injected into the Si-nc
Holes are much easier to be injected than electrons as
the retention time of positive charging was longer than
the negative charging with respect to the same DC char-ging intensity When charge injection was done by more than 7 V, the charging process can’t be detected in sev-eral minutes This indicates that the charges were trapped by the Si-nc which made the retention time much longer
Ramp processes between -2 and +2 V were done by SCS separately on and outside an isolated Si-nc without charge injection Strong hysteresis window was observed
on the isolated Si-nc But outside the dot, this effect was too weak (see in Figure 6) Furthermore, SCS was used
to quantitatively investigate trapped charge effect inside the isolated Si-nc From the SCS signals, the curve shift direction and distance were observed differently by applying a DC bias of -10 or +10 V to the tip during charging (see in Figure 7) There is a shift of 0.91 V by +10-V charge while -0.74 V shift by -10-V charge This relates to the fact that different type of carriers can be injected into these Si-ncs and the capacitance changes caused by these trapped charges can be easily detected
by SCM at the nanometer scale It also verified the pre-vious conclusion that holes are much easier to be injected and trapped than electrons
In this letter, Si-ncs were formed on top of a ther-mally 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
Figure 3 SCM image (a) and signal (b) versus different DC bias.
Figure 4 SCM image (a) and signal (b) versus different AC bias.
Figure 5 Charge and discharge with different DC bias.
Trang 4Figure 6 Ramp process for hysteresis window by SCS.
Figure 7 SCS curve shift after charge injection by +10 and -10 V.
Trang 5and the capacitance changes caused by these trapped
charges could be easily detected by SCM/SCS at the
nanometer scale
Acknowledgements
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.
Author details
1 Institut des Nanotechnologies de Lyon, UMR 5270, Institut National des
Sciences Appliquées de Lyon, Université de Lyon, Bât Blaise Pascal, 20,
avenue Albert Einstein - 69621 Villeurbanne Cedex, France 2 Institut Matériaux
Microélectronique Nanosciences de Provence, UMR CNRS 6242, Avenue
Escadrille Normandie-Niemen-Case 142, F-13397 Marseille Cedex 20, France
Authors ’ contributions
ZL carried out the SCM and SCS experiment, studied these results and
drafted the manuscript GB participate the study of experiment results and
manuscript writing FB conducted the sample fabrication and the discussion.
All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 19 September 2010 Accepted: 22 February 2011
Published: 22 February 2011
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