Supplementary Information for
Isolated nanographene crystals for nano-floating gate in
charge trapping memory
Rong Yang1$, Chenxin Zhu2$, Jianling Meng1, Zongliang Huo2, Meng Cheng1, Donghua Liu1, Wei
Yang1, Dongxia Shi1, Ming Liu2, Guangyu Zhang1‫٭‬
1
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese
Academy of Science, Beijing 100190, China
2
Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of
Microelectronics, Chinese Academy of Sciences, Beijing 100029, China
$
These authors contribute equally to this work.
‫٭‬Corresponding
author. Email: gyzhang@iphy.ac.cn.
1. SKPM measurements
For the charge-trapping memory structures, the CPD induced by electrons or holes trapped in the
electrets structures can be directly measured by SKPM under ambient conditions. This SKPM
technique allows us to directly study the trapping properties of nanographene, including the trap
density, uniformity, capture cross section, and retention, etc., and the charge types (electrons or
holes) by using a simple nanographene/SiO2 structure. The SKPM measurement process includes
two-steps: (1) charges injection. An electrically grounded Pt/Ir-coated Si tip was operated under
the contact mode at room temperature. By applying a charge injection bias (e.g. 3V) to the silicon
substrate, charges can be pulled out from the heavily doped Si, tunnel through the 4-nm oxide
layer, and finally be trapped in nanographene or at the interface. (2) CPD measurements. The
SKPM images were obtained with a twice scan to avoid topographic artifacts. The first scan was
for topography under tapping mode and the second scan was for SKPM imaging under 50-nm
lifted constant height mode. In this step, the CPD between the tip and the sample is obtained by
detecting the real-time cantilever deflection.
2. Comparison of charge trapping effect in Al2O3/SiO2 and Al2O3/nanographene/SiO2 memory
structures
Figure S1. a and c, AFM image of Al2O3/SiO2 and Al2O3/nanographene/SiO2 structures. b and d, Corresponding
SKPM images of the programmed state in these two memory structures.
3. Surface potential measurements of locally charged nanographene.
Figure S2. SKPM images and measured CPD line profiles of nanographene samples (#1-5) with 1X1 um2 low
electric field (-5.8MV/cm) charged cell.
4. Surface potential measurements of a control sample of charged Graphene flake.
Graphene flakes were mechanically cleaved (from HOPG) on to 4 nm thermally grown SiO2
substrates by scotch tape [1].
Figure S3. AFM and SKPM images of a graphene flake after low electric field (-5.8MV/cm) charging.
5. The size and density distribution of Si nanocrystals.
Silicon nanocrystals (Si-NCs) were also formed on 4 nm thermally grown SiO2 substrate with a
two-step low pressure chemical vapor deposition (LPCVD) process [2]. By adjusting the
pre-nucleation waiting time and deposition time, the density, size and uniformity can be
effectively controlled. As shown in Fig. S4, uniform Si-NCs with a density of 8.2×1011 cm−2 with an
average diameter of 5 nm, 4.4×1011 cm−2 with an average diameter of 10 nm, and 1.8×1011 cm−2
with an average diameter of 20 nm were used in our experiment for comparisons.
Figure S4. a, AFM images of silicon nanocrystals (Sample #1-3). b, Size and density distributions of three samples.
6. Surface potential measurements of locally charged silicon nanocrystals.
Figure S5. SKPM images and measured CPD line profiles of silicon nanocrystals (Sizes: 5nm, 10nm, 20nm) with
1X1 um2 negatively (-5.8MV/cm) charged cell.
7. Estimation of charge densities of nanographene from CPD
The measured CPD from surface potential can be relative to the spatial distribution of charges,
which in principle can be calculated by solving the Poisson equation. Therefore, by using
one-dimensional Poisson equation, the approximate total charge density can be estimated from
the measured CPDs as follows:
2 Si kT
qLD
 Si ( I ) 
 Si  CPD   I
exp( Si )   Si  1 
np0
pp0
exp( Si )   Si  1
dOX
 OX
(1)
(2)
where σSi is the static surface charge density of silicon, ψSi is the electrical potential at silicon
surface, εOX and dOX are the permittivity and thickness of the SiO2 layer. From Eqs. (1)-(2) we can
obtain an equation on σI (cm-2), which is the trapped charge density at the nanographene/SiO2
interfaces. The total trapped charge density σt in the sample can then be obtained
L
by  t  1  I dx , where L is the lateral scan distance.
L 0
8. Other device performance characterization of metal/Al2O3/Nanographene/SiO2/Si (MANGOS)
memory structure.
Under a high gate voltage sweeping of 13 V, more than 7 V Vth shift is achieved in the MANGOS
device (as shown in Fig.S6). Such a large memory window might be favorable for multilevel cell
storage if the ten-year retention is allowed.
Figure S6. CV curves of MANGOS device at high sweeping gate voltage.
Fig.S7 shows the cycled retention characteristics of MANGOS device. It shows an extrapolated
10-year 55% loss in memory window after 1000 cycled operations at 85 oC. The performance
degradation could be attributed to the overlap of the nanographene dots as well as the quality of
blocking Al2O3 layer. Further optimization is needed to improve the retention characteristics by
tuning the nanographene sizes and densities and by improving the quality of the high- layers.
Figure S7. Cycled retention characteristics of MANGOS device.
Fig.S8 illustrates the program/erase (P/E) and endurance characteristics of MANGOS device. The
program curve is taken with light-assisted voltage pulses to generate electron carriers. The P/E
curves shows 4.5 V Vth shift with 12 V operation voltage at 10 ms. There is no obvious
degradation in device performance after 1K P/E cycles. It should be pointed out that the capacitor
structure could limit the P/E speed performance and the transistor device structure will be
further studied.
Figure S8. P/E and endurance characteristics of MANGOS device.
Fig.S9 shows the uniformity of MANGOS device. All the cell size is 60*60 µm2 with capacitor
structure. These devices show good uniformity of 4.5 V memory window with 8 V P/E voltages
and 6.2 V memory window with 12 V P/E voltages between 40 cells.
Figure S9. Device uniformity of MANGOS device.
9. Raman spectra of nanographene on the Al2O3 and HfO2 dielectric layers.
Raman spectra of nanographene grown on Al2O3 and HfO2 ALD dielectric layers were listed in Fig.
S6. For both samples, the typical G peak around 1589 cm-1 , high D peak around 1351 cm-1, D’
peak around 1620 cm-1, and 2D peak around 2705 cm-1 were observed, which confirmed the sp2
graphitic bonded nanographene with abundant edge structures. Compared with nanographene
grown on SiO2, the wider and lower D peak intensities may be caused by the amorphous
substrates which have numerous dangling bonds. Besides, the comparable ID’/IG relative
intensities still revealed that the average grain size is small (might be around 8-10 nm).
Figure S10. Raman spectra of nanographene on Al2O3 and HfO2.
Reference:
1. Novoselov, K.S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science 306, 666-669 (2004).
2. Wang, Y. et al. Optimization of silicon nanocrystals growth process by low pressure chemical vapor deposition
for non-volatile memory application. Thin Solid Films 519, 2146-2149 (2011).