The effect of content and thickness of chitosan thin films on resistive switching characteristics

f devices or “ERASE” process of the
digital memory device. TheHRS retains when sweep-
ing back to 0 V again. The above I–V characteristics
indicate that the Ag/chitosan/FTO device shows re-
versible bipolar resistive switching behavior. The de-
vice switches from theHRS to LRS at the negative bias
and back from HRS at the positive bias. The operat-
ing voltages of Vset –1.24 V and Vreset 1.26 V are
relatively low, and the ON/OFF ratio was larger than
20 times. These characteristics can be considered for
electronic memory device applications.
For the RRAM structure, the dielectric CS layer ex-
hibits an important role in the switching behavior of
the structure. In this part, we have investigated the ef-
fect of CS content in switching behavior in the same
structure. Herein, we have fabricated devices with dif-
ferent CS concentration of 0.2, 0.5, and 0.8 %wt/v.
The I-V characteristics and endurance of 1st – 100th
sweeps of corresponding patterns are depicted in Fig-
ure 6 A and B.The results indicate that all of it shown
the bipolar resistive switching behavior. However, the
effect of 0.2 %wt/v of CS into the device does not
maintain and starts fades away when reaching 80 cy-
cles. At 0.5 wt/v of CS, the resistive switching hold
on with a more stable and higher ON/OFF ratio (~
2x10) than those of 0.8 %wt/v (Figure 8 A). This re-
sult indicates that the content of CS in theAg/CS/FTO
structure is one of the significant factors, which influ-
ence the reproductivity and ON/OFF ratio in resistive
switching properties of devices.
As presented above, the devicewith 0.5%wt/vCS con-
tent showed better switching behavior than the oth-
ers. Thus, we combine this content with CS thickness
of 100, 300, and 500 nm to investigate their effect on
the resistive switching behavior. The I-V character-
istics and endurance of corresponding thickness are
presented in Figure 7 A and B. The device with 100
nm-thickness chitosan tends to reduce the ON/OFF
ratio follow the cycling bias while 500 nm-thickness
of chitosan has a resistance ratio slightly lower than
that of 300 nm (Figure 8 B). This result implies that
the device with 300 nm thickness of the CS layer and
0.5% wt/v exhibits the largest ON/OFF ratio and the
most stable compared to the others.
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Science & Technology Development Journal, 23(3):632-639
Figure 2: XRDpatterns of chitosan in powder and thin film. The diffraction angle of 2theta wasmeasured from
5 to 60o . The chitosan film was coated on the soda-lime glass substrate.
DISCUSSION
The resistive switching characteristics of the
Ag/chitosan/FTO device in this study show the
non-volatile random access memory with bipolar
behavior (Figure 5), which is consistent with the
previous reports based on chitosan material 8,27. The
operating voltages of our device are similar to their
devices with under 3 V. It exhibits the advantages of
low power consumption in electronic devices. The
ON/OFF resistance ratio is the same as the group [8]
with several tens but lower to the other one 27. In our
device, we use undoped chitosan as an insulator layer
in the capacitor structure, while Ag-doped-chitosan,
Mg-doped-chitosan layer are used in other studies.
The chitosan concentration influences on the fluctu-
ation of current-voltage curves of devices after 100
sweeping cycles. Under the same operating voltages,
current compliance, and film thickness, devices ex-
hibit the largest variations at 0.2 wt%, then following
by 0.7 wt%, but they are the most stable at 0.5 wt%.
The different concentrations of chitosan in solution
are related to the dispersion level of polymer chains
in the solvent. In the acetic acid, chitosan may proto-
nate and form polycation chitosan. It may influence
the electrical property of the chitosan layer, or the re-
sistance at HRS and LRS, and vary the ON/OFF ratio.
In our research, the suitable concentration of chitosan
is about 0.5 wt% in 1.0 wt% acetic acid (Figure 8 A).
The effect of chitosan concentration has been inves-
tigated in bioapplication 28,29, The changes in crys-
tallinity, morphology, degree of the acetylation of chi-
tosan, which was sonicated in various concentration
of acetic acid have also reported 30.
The effect of chitosan thickness is strongly on the
ON/OFF ratio during 100 cycles (Figure 8 B). At the
thickness of 100 nm, theON/OFF ratio approaches 10
at the first several cycles but then decrease promptly
with tens of cycles due to the increment of resistance
at LRS.This decrease ofON/OFF ratio completely dis-
regards in the 300 nm – thick – filmwith the stable re-
sistances at both HRS and LRS. In the 500 nm – thick
– film, the resistance at HRS slightly decreases, lead-
ing to the pretty reduction of ON/OFF ratio in this
sample comparing to the 300 nm – thick – sample. In
other reports, the thickness of the insulator layer has
influenced the resistive switching characteristics such
as themultilevel threshold in amorphous BaTiO3 [11]
or the ON/OFF resistance ratio in AlN material [18].
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Science & Technology Development Journal, 23(3):632-639
Figure 3: FTIR spectra of chitosan powder and thin film. The 100 nm-thick chitosan film was coated onto a
silicon substrate.
Figure 4: Cross-section SEM images of chitosan film on the FTO substrate. The high roughness surface and
less dense structure film have been observed.
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Science & Technology Development Journal, 23(3):632-639
Figure 5: The I–V characteristics of the Ag/chitosan/FTO structure. (a) The linear plot and (b) the semi-
logarithmic plot in the sweeping voltage of -1.5 V 2 V.
Figure 6: The I-V characteristics of the Ag/chitosan/FTO structure with different concentrations of CS. (a)
The semi-logarithmic plot of I-V curve and (b) endurance of 1st -100th cycles corresponding to 0.2%, 0.5, and 0.8
%wt/v content of CS.
The thickness of the insulator film varies the rough-
ness and the leakage current as well as the electrical
conducting mechanism of structure [11], [18]. In our
study, the 300 nm – thick – film of chitosan is appli-
cable for memory devices.
CONCLUSIONS
In summary, we fabricated RRAM devices success-
fully using CS as a switching layer in Ag/CS/FTO
structure. The I-V characteristic devices shown bipo-
lar resistive switching behavior in the range of -2 ¸ 2
V. Furthermore, the influence of content and thick-
ness of CS film on the resistive memory characteris-
tics were investigated. Our results indicated that the
RS behavior at 300 nm-thickness and 0.5 wt%/v of CS
thin filmwas shown stable and reliable during 100 cy-
cles and ON/OFF ratio around ~ 2x10.
ABBREVIATIONS
CS: Chitosan
FTO: Fluorine-doped Tin Oxide
HRS: High Resistance State
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Science & Technology Development Journal, 23(3):632-639
Figure 7: The I-V characteristics of the Ag/chitosan/FTO structure with different thicknesses of CS. (a) The
semi-logarithmic plot and (b) endurance of 1st -100th cycles at 100, 300, and 500 nm thickness of CS film.
Figure 8: The resistance ratio of the Ag/chitosan/FTO structure. (a) With CS concentration of 0.2, 0.5 and 0.8
wt%, With CS thickness of 100, 300 and 500 nm.
I-V: Current-Voltage
LRS: Low Resistance State
RRAM: Resistive Switching Random Access Memory
SEM: Scanning Electron Microscopy
AUTHORS’ CONTRIBUTIONS
Dinh Phuc Do and Tu Uyen DoanThi performed ex-
periments under the supervision of Ngoc Kim Pham.
Tran Duy Tap and Van Dung Hoang analyzed data.
Dinh Phuc Do and Ngoc Kim Pham wrote the paper.
CONFLICT OF INTEREST
There are no conflicts of interest to declare.
ACKNOWLEDGMENTS
This research is funded by Vietnam National Uni-
versity Ho Chi Minh City (VNU-HCM) under grant
number C2018-18-27.
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