Negative differential resistance and resistive switching behaviors in Cu2 S nanowire devices

Article abstract of DOI:10.1063/1.3442919

Two-terminal devices of Cu₂ S/ZnO core/shell nanowires were fabricated and measured. Forward bias sweeping produced a rectified I-V characteristic of a diode, with turn-on voltages varying from 150 to 300 mV. The turn-on voltages depended on the rate at which the bias was varied. When the bias scan was reversed, a resistive switching (RS) behavior was observed. A low-resistance state was measured, and the diode characteristic diminished. At -50 to -150 mV, negative differential resistance (NDR) was observed, after which the diode behavior was restored. This phenomenon was explained using the diffusion of Cu⁺ within Cu₂ S. ZnO acted to limit RS to the positive bias range and NDR to the negative bias range.

Full text of DOI:10.1063/1.3442919

Negative differential resistance and resistive switching behaviors in Cu 2 S nanowire
devices
Xiaohua Liu, Matthew T. Mayer, and Dunwei Wang
Citation: Applied Physics Letters 96, 223103 (2010); doi: 10.1063/1.3442919
View online: http://dx.doi.org/10.1063/1.3442919
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/22?ver=pdfcov
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APPLIED PHYSICS LETTERS 96, 223103 ͑2010͒
Negative differential resistance and resistive switching behaviors in Cu S
2
nanowire devices
a͒
Xiaohua Liu, Matthew T. Mayer, and Dunwei Wang
Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street, Chestnut Hill,
Massachusetts 02467, USA
͑
Received 24 March 2010; accepted 10 May 2010; published online 1 June 2010͒
Two-terminal devices of Cu S/ZnO core/shell nanowires were fabricated and measured. Forward
2
bias sweeping produced a rectified I-V characteristic of a diode, with turn-on voltages varying from
150 to 300 mV. The turn-on voltages depended on the rate at which the bias was varied. When the
bias scan was reversed, a resistive switching ͑RS͒ behavior was observed. A low-resistance state was
measured, and the diode characteristic diminished. At Ϫ50 to Ϫ150 mV, negative differential
resistance ͑NDR͒ was observed, after which the diode behavior was restored. This phenomenon was
+
explained using the diffusion of Cu within Cu S. ZnO acted to limit RS to the positive bias range
2
and NDR to the negative bias range. © 2010 American Institute of Physics.
͓
doi:10.1063/1.3442919͔
Cu S is a cation-deficient p-type semiconductor with a
electrical measurement was conducted on a probe station
2
band gap of 1.2 eV. It can potentially be used to interface
with n-type semiconductors such as CdS or ZnO for efficient
solar energy conversion. The high mobility Cu exhibits,
however, has been proven tremendously cumbersome for this
͑Cascade Microtech, M150 Measurement Platform͒. The Cu
support on which Cu S was grown served as one contact and
2
1
,2
+
a tungsten or gold tip ͑radiusϽ2.4 ␮m͒ gently touching the
ZnO from the top was the other contact ͓Fig. 1͑a͔͒. The data
was collected using a source meter ͑Keithley 2400͒ con-
trolled by a computer through a LABVIEW program. The elec-
trode contacting ZnO was grounded for all data presented
here. Control experiments, where ZnO was absent, were per-
formed in the same fashion except that the top contact was
application, which has led to the abandonment of Cu S as a
2
3
photovoltaic material over two decades ago. Recent ad-
vancement in the synthesis of its nanostructures has renewed
4
–7
the interest in Cu S. We have shown that the high mobility
2
+
of Cu vacancies in Cu S is the driving force in the unique
growth of regularly aligned Cu S nanowires, opening up
2
6
2
made to the Cu S nanowires directly.
2
opportunities to rejuvenate research on Cu S for solar energy
2
Also shown in Fig. 1 is a typical I-V plot ͓Fig. 1͑c͔͒.
More detailed data are plotted in Fig. 2. There are three
important features we wish to emphasize. First, the forward
scan produced an I-V plot characteristic to that of p/n diodes.
The turn-on voltage, however, depended on the rate at which
the voltage was varied. Second, the reverse scan yielded a
low-resistance state, which extended to voltages in the nega-
tive range. The I-V curve resulting from the reverse scan was
typified by a good linearity in the low voltage region, mani-
festing the behaviors of a resistor. Third, a NDR behavior
was observed in the negative voltage range. The exact volt-
ages at which the NDR appeared varied with the voltage scan
rates. The NDR diminished at extremely slow scan rates such
as 2 mV/s ͓Fig. 3͑a͔͒.
harvesting. Nevertheless, to reach this goal it is essential to
+
understand the properties of Cu in Cu S ͑Ref. 8͒ and par-
2
ticularly at the interface of the p/n junction. This paper re-
ports our latest discovery toward this end. We found that
when interfaced with ZnO, Cu S exhibits obvious negative
2
differential resistance ͑NDR͒. A resistive switching ͑RS͒
characteristic is also observed on this device. We show that
these observations can be accounted for by the high mobility
+
of Cu . Our results shed light on the nature of the p/n junc-
tion consisting of Cu S and highlight the necessity to control
2
the cation diffusion in Cu S for energy-related applications.
2
The synthesis of vertically aligned Cu S nanowires has
2
6
been described elsewhere. Briefly, a Cu foil ͑Sigma-
Aldrich͒ was electrochemically polished then placed in a
chemical vapor deposition chamber, where a mixture of H S
RS and NDR have been reported earlier in a number of
2
1
0,11
12
13
14
͓
10 SCCM ͑SCCM denotes cubic centimeter per minute
systems, including TiO2,
NiO, ZnO/SiOx, Co O ,
3
4
at STP͔͒, N2 ͑160 SCCM, saturated with H O͒ and O
and Cu S ͑Refs. 15 and 16͒. Compared with the existing
2
2
2
͑
80 SCCM͒ flowed over the surface of the Cu foil. The re-
reports, our result is unique in following two aspects: the I-V
characteristics and the governing mechanism. As described
above, a typical I-V curve measured in our experiments in-
cludes two nonlinear components, a diode and a RS, both of
which are well defined and highly reproducible. The intro-
action took place at room temperature and lasted for 5 h.
Vertical Cu S nanowires, 100 nm wide and 2 ␮m long, were
2
obtained. The resulting material was transferred to an atomic
layer deposition ͑ALD, Savannah 100, Cambridge Nanotech͒
chamber for ZnO deposition. For this purpose, diethyl zinc
duction of ZnO to form a p/n junction with Cu S permits us
2
͑
ZnEt ͒ ͑Sigma-Aldrich͒ and H O served as the precursors,
2 2
9
to restrict the NDR peak to the negative bias range and the
RS behavior to the positive bias range. These features and
the low operating potentials make it possible to utilize the
reported structure for future device fabrications, promising
high reliability, low energy consumption, and low heat dissi-
pation.
and the reaction took place at 100 °C. Thin ZnO films
͑
ϳ50 nm͒ were grown for imaging purposes; and thicker
ZnO films ͑ϳ200 nm͒ were used for the measurements. The
a͒Electronic mail: dunwei.wang@bc.edu. Tel.: 617-552-3121.
0
003-6951/2010/96͑22͒/223103/3/$30.00
96, 223103-1
4.88.136.149 On: Mon, 23 Feb 2015 11:23:54
© 2010 American Institute of Physics
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8

223103-2
Liu, Mayer, and Wang
Appl. Phys. Lett. 96, 223103 ͑2010͒
FIG. 2. I-V plots at various scan rates. With a larger scan rate, the
ZnO/Cu S diode was turned on at a larger bias when scan from “Ϫ” to “+”
2
͑
denoted “n”͒ and a larger NDR peak appeared in the reverse “p” scan.
Inset: semilog scale plot.
ally removed from the junction area, allowing the p/n junc-
tion to reestablish, upon the completion of which the resis-
tance increases to limit backward current flow. The
reinstallment of the diode behavior produces the NDR as
observed in the negative bias region ͓Fig. 1͑c͔͒.
The hypothesis is supported by the dependence of the
I-V plot on the voltage scan rates. As shown in Fig. 2, when
the scan rate is fast ͑2500 mV/s͒, the diode was turned on at
a large bias ͑0.3 V͒. This is because at a fast scan rate, the
+
Cu accumulation lags behind the rapidly increasing bias,
and thus a larger bias is reached before the turn-on state is
achieved. Conversely, at slower scan rates, the turn-on volt-
ages were reduced consistently, and the reduction in the
turn-on voltage depended on the scan rate in a monotonic
fashion. Similarly, the height and the position of the NDR
peaks varied with the scan rate as well. The highest and the
most negative NDR peak was observed when a 2500 mV/s
scan rate was used ͑the fastest scan rate tested in our experi-
ments͒. This phenomenon is related to the rate at which the
+
+
accumulated Cu is dissipated. The dissipation of Cu leads
FIG. 1. ͑Color online͒ ͑a͒ A schematic illustration of the ZnO/Cu S nano-
2
to the reinstallment of the Cu S/ZnO diode, which is the
wires and measurement setup. ͑b͒ A transmission electron microscopy image
showing the contact between the sample and a probe. The insets show the
side view ͑upper͒ and the cross section ͑lower͒ of the core/shell structure.
Thicker ZnO coating ͑ϳ200 nm͒ was used in the measurements. ͑c͒ A
typical I-V curve showing both the NDR and RS features.
2
primary reason for the NDR behavior. Consistent with this
hypothesis, the NDR peak was not observed at the extremely
slow scan rate of 2 mV/s because at such a rate the accumu-
+
lated Cu is provided enough time to dissipate before reach-
ing the NDR region ͓Fig. 3͑a͔͒. Note that at this slow scan
We suggest that the key to understanding the observed
rate, the RS behavior is still pronounced. This observation is
+
6,17
+
NDR and RS lies in the high mobility of Cu .
Without
in line with the hypothesis that Cu is the primary cause for
external bias, the Cu S/ZnO interface represents that of a p/n
the NDR and RS. We note that the ON/OFF switching is
highly reversible, which we attribute to the reversibility of
Cu diffusion under small biases.
2
junction. The initial forward scan indeed generates an I-V
curve resembling that of a diode. The forward bias, however,
+
+
has an additional effect to the interface—it drives Cu to
ZnO plays an important role in the results reported here.
As shown in Fig. 3, the incorporation of ZnO introduces a
diode to the device, which helps suppress the current in the
reverse bias region. The suppressed current makes it possible
to distinguish the different features of the I-V curves in the
forward and reverse bias regions. Although similar NDR and
RS behaviors were observed in the control experiments
where ZnO was absent ͓Fig. 3͑c͔͒, the nature of the proper-
accumulate in Cu S near the Cu S/ZnO interface. In effect,
2
2
this accumulation decreases the p-doping level of Cu S,
2
which weakens the junction as the forward bias is increased
or held for longer durations. The weakening effect is mani-
fested as the abrupt turn-on of the forward I-V curve ͓Fig.
1͑c͔͒. When the forward bias is reduced ͑backward scan from
positive to negative͒, the weakened p/n junction fails to be
reinstalled immediately, maintaining the low-resistance even
below the original turn-on voltage. The low-resistance state
is responsible for the linearity of the I-V curve in the low
bias range. After the bias is swept into the negative range, the
ties is different. When Cu S is contacted by a metal directly,
2
+
the Cu accumulation and dissipation changes the Schottky
barrier height, which is manifested as the conductance
change. The lack of rectification of the I-V curve, however,
can make the NDR difficult to observe ͓Fig. 3͑c͔͒. While it is
+
bias polarity is reversed, and the accumulated Cu is gradu-
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223103-3
Liu, Mayer, and Wang
Appl. Phys. Lett. 96, 223103 ͑2010͒
and/or redox reaction. In particular, the formation and disso-
lution of highly conductive metal filaments has been adopted
1
6
to explain the RS phenomena based on Cu S materials. The
2
governing mechanism that depends on ionic accumulation
we proposed here is distinct from the existing models but is
based on similar considerations of the ionic nature of the
active material, Cu S. While further studies will be needed to
2
compare these models to understand their distinctions and
relations, the result presented here sheds light on the detri-
+
mental effect caused by the high mobility of Cu in utilizing
Cu S for solar energy harvesting. For this purpose, it will be
2
of paramount significance to understand and, more impor-
tantly, to control the interface Cu S forms with the n-type
2
semiconductor.
In conclusion, we observed resistance switching and
NDR in Cu S nanowire-based devices. These behaviors were
2
+
explained using the high ionic mobility of Cu in Cu S. The
2
high reproducibility and low operating potential may be use-
ful for memristor device fabrications. The result also pro-
vides insight to the Cu S-based p/n junctions, paving the way
2
for efficient solar energy harvesting using low-cost materials.
The work was supported by Boston College. The authors
thank S. Shepard and G. McMahon for their technical assis-
tance.
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FIG. 3. ͑Color online͒ ͑a͒ NDR peak diminishes at slow scan rate of 2 mV/s.
͑b͒ and ͑c͔͒ Comparison of I-V plots with or without ZnO.
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͓
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