Modeling and Numerical Simulation of Material Science, 2013, 3, 20-22
Published Online January 2013 (http://www.SciRP.org/journal/mnsms)
Copyright © 2013 SciRes. MNSMS
The Charge Storage of Doubly Stacked Nanocrystalline-Si
based Metal Insulator Semiconductor Memory Structure
Xiang Wang, Chao Song, Yanqing Guo, Jie Song, Rui Huang
Department of Physics and Electronic Engineering, Hanshan Normal University, Chaozhou, People’s Republic of China
Doubly stacked nano cr yst alli ne-Si (nc-Si) based metal insulator semiconductor memory structure was fabricated by
plasma enhanced chemical vapor deposition. Capacitance-Voltage (C-V) and capacitance-time (C-t) measurements
were used to investigate electron tunnel, storage and discharging characteristic. The C-V results show that the flatband
voltage increases at first, then decreases and finally increases, exhibiting a clear deep at gate voltage of 9 V. The de-
creasing of flatband voltage at moderate programming bias is attributed to the transfer of electrons from the lower nc-Si
layer to the upper nc-Si layer. The C-t measurement results show that the charges transfer in the structure strongly de-
pends on the hold time and the flatband voltage decreases markedly with increasing the hold time.
Keywords: nc-Si dots; Capaci tance-Voltage Measurement; St or age
In recent years, silicon nanostructures have attracted
enormous attentions.[1 -5] Particularly, metal-i nsula tor -
semiconductor (MIS) structures based on silicon quan-
tum dots are widely studied for their new physical phe-
nomena as well as their potential applications in future
memory device. The first nanocrystals based MIS mem-
ory structure was put forward by Tiwari et al., where Si
nanocrystals were used as storage nodes. Compared to
the conventional nonvolatile memories with polycrystal-
line silicon or silicon nitride floating gate as the charge
storage layer, a nanocrystal based memory has been
suggested as one of the key items for increasing flash
memory stability and decreasing the node size for higher
information density. In such memory device, the Si na-
nocrystals are sufficiently isolated from each other by
insulator. Hence, the electrons are localized in the nano-
crystals and a local leakage path would not discharge all
of the nanocrystals, which means a nanocrystal based
memory should have a better reliability than convention-
al nonvolatile memories.[2-3] However, nanocrystal
based memories have a rather low charge density which
might be a drawback of this approach. In addition, the
charge retention time in single-layer nanocrystal based
memory device is also limited by the fast direct tunneling
process through the ultrathin tunneling layer. To solve
these problems, there are have some approaches, suchas
the introduction of certain tunneling dielcctric materials
and a doubly stacked dot structure. For instance, multi-
layer nc-Si structure in SiO2 matrix was investigated by
some research groups,[6-8] where electrons were injected
into the multilayer structure via a FN tunneling process at
high programming voltages. An ultrahigh stored charge
density is demonstrated in the multilayer nc-Si structure.
A self-aligned stacked double-layer nc-Si structure in
SiNx matrix with enhanced retention time has been re-
In this work, we have prepared the stacked nc-Si
memory structure where double layers of nc-Si dots em-
bedded within triple SiNx barriers. We investigated the
charge storage mechanism in stacked nc-Si memory
structure by using capacitance voltage (C-V) measure-
ment and capacitance-time (C-t) measurements.
2. Experimental Details
The stack nc-Si dots based memory structures were fa-
bricated on n-type crystalline Si substrates (6-8 Ω cm) in
plasma enhanced chemical vapor deposition system. The
doubly stacked layers of nc-Si with the thickness of
about 5 nm were fabricated by the layer-by-layer deposi-
tion technique with silane and hydrogen mixture gas.
SiNx barrier were deposited by a gas mixture of silane
and ammonia with the ratio of 1:5. The thickness of both
lower and upper tunneling SiNx barrier was about 4 nm
and the thickness of control SiNx barrier was about 20
nm. For comparison the sample with single nc-Si dots
layer was also prepared with the same deposition process.
The thickness of nc-Si layer, tunneling layer and the bar-
X. WANG ET AL.
Copyright © 2013 SciRes. MNSMS
Figure 1. The Capacitance-voltage characteristics of the doubly
stacked nc-Si structure.
rier layer was 5nm, 4nm and 20nm, respectively. The
samples were annealed in nitrogen ambient at 900 oC for
30 minute to improve the quality of the nc-Si and to re-
duce the density of interface states and defects in the
structure. The C-V and C-t measurements were per-
formed using Agilent 4284A LCR meter. All measure-
ments were performed at air ambient and at room tem-
perature inside a shield box.
3. Results and discussions
Figure 1 shows the C-V characteristics of the doubly
stacked nc-Si memory structure. The bias was sweeping
from the accumulation to the inversion region for the
MIS device based on the n-type substrates. In order to
obtain saturation of charge storage, before each sweep
the structure was held at a positive gate bias for 120 s.
After each sweep, electrons trapped in the structure were
erased by negative bias of -4 V for 120 s to bring the
structure to the neutral stage. As can seen in figure 1, at
low programming bias, a higher programming bias ap-
plied at the gate results in a bigger shift of the flatband
voltage, which is caused by more and more electrons
trapping in the structure. At moderate programming bias
(6 V to 9 V), the flatband voltage decreases with in-
creasing the bias. At high programming bias (10 V to 13
V), the flatband voltage increases with the bias. Under
fixed erasing bias and varied programming bias, the rela-
tionship between programming bias and flatband voltage
is of special interest.
Figure 2 shows the programming bias dependence of
the flatband voltage. With increasing the gate voltage,
the flatband voltage of the sample with double nc-Si
layers is found to first increase, then decrease, and fi-
nally increase, exhibiting a clear deep at bias of 9 V.
When the programming bias increases from 2 V to 6 V,
the flatband voltage is shift from 0.34 V to 4.47 V. With
Figure 2. The dependence of the flatband voltage on programming
further increasing the programming bias from 6 V to 9 V,
the flatband voltage decrease from 4.47 V to 3.44 V. As
the bias changes from 9 V to 13 V, the flatband voltage
increases to 4.92 V. The flatband voltage of the sample
with single nc-Si layer increases from 0.73V to 3.35V
when the bias changes from 3V to 5V. Then the flat-
band voltage keeps invariably with the increasing the
bias. we thi nk that the decreasing of flatband voltage at
moderate programming bias is attributed to the transfer
of electrons from the lower nc-Si layer to the upper nc-Si
Figure 3 shows the relationship of programming bias
and the flatband voltage under different hold time.
Each sweep measurement was performed after a posi-
tive gate bias was held at the gate for different hold
time of 120 s, 50 s and 20 s, respectively. From the
figure 4, we can see that the flatband voltage strongly
depends on the hold time. When the structure was held
at a positive gate bias for 120 s, we can see clear at
moderate programming bias (9 V). With varying the
hold time to 50 s, the flatband-voltage curve also exhi-
bits a deep, but the deep is smaller than that of the hold
time of 120 s. It demonstrates that the charge transfer is
not sufficient for the hold time of 50 s, resulting that
the amount of transfer charges is less than that of the
hold time of 120 s. Hence, the decrease of flatband
voltage and the deep is smaller than that of the hold
time of 120 s. In the case of the hold time of 20 s, we
can notice that the flatband voltage keeps unchanged
nearly, which demonstrates that few charges exist can
transfer from the lower nc-Si layer to the upper nc-Si
layer. Based on the above experimental result, we con-
clude that the charges transfer in the structure strongly
depends on the hold time and the flatband voltage de-
creases markedly with increasing the hold time.
X. WANG ET AL.
Copyright © 2013 SciRes. MNSMS
In summary, doubly stacked nc-Si based MIS memory
structure was fabricated by plasma enhanced chemical
vapor deposition. C-V and C-t measurement was used to
Figure 3. The programming bias dependence of flatband voltage
under different hold times.
investigate electron storage and discharging in the struc-
ture. The C-V experiment results show that the flatband
voltage first increases, then decreases and finally in-
creases, exhibiting a claer deep at gate voltage of 9 V.
The decreasing of flatband voltage at moderate pro-
gramming bias is attributed to the transfer of electrons
from the lower nc-Si layer to the upper nc-Si layer. The
C-t measurement results show that the charges transfer
in the structure strongly depends on the hold time.
5. Acknowledgemen t s
The authors would like to acknowledge the financial
support of Natural Science Foundation of Guangdong
Province (S2011010001853) and Foundation for Distin-
guished Young Talents in Higher Education of Guang-
dong (LYM09101, LYM11090, LYM10099).
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