Download TiOx Memristors with Variable Turn-On Voltage using Field

TiOx Memristors with Variable Turn-On Voltage
using Field-Effect for Non-Volatile Memory
Pradeep Pai, Faisal K. Chowdhury, Tien-Vinh Dang-Tran and Massood Tabib-Azar
Electrical and Computer Engineering
University of Utah
Salt Lake City, USA
[email protected]
Abstract— This work demonstrates the first use of field-effect
based memristors that exhibit variable turn-on voltage. Turn-on
voltage as low as 4V is achieved in this work and the switching
characteristics is reproducible. The novelty of this work
compared to earlier work based on metal-insulator-metal (MIM)
structure is in introducing an auxiliary electrode to the MIM
structure that produces variation in the turn-on voltage of the
device through field effect. A gate bias of ±2V varies the drainsource turn-on voltage by ±2V around the turn-on voltage at
zero gate bias. The simplicity in the device structure can allow
easy integration to form an array of these devices to constitute a
memory module. The leakage current in the off state is less than
10pA, which would enable low power operation and long hours
of data storage.
I.
INTRODUCTION
Technology and consumer needs have created a need for
high package density memory devices. The current CMOS
technology allows high package density but also increases the
power consumption. In the past, non-CMOS based nonvolatile memory devices have been realized with the help of
solid electrolytes (SE) [1], MEMS switches [2,3], MIM
devices [4] etc. The SE and MIM devices have very low
switching voltages less than 5V, compared to the MEMS
switches that require 10s of volts. Also, the absence of any
moving parts in SE and MIM devices make them
mechanically robust and limit their speed only by the ionic
mobility. Various insulators like amorphous silicon [5], TiO2
[6], CuOx [7] and methylsilesquioxane [8] etc. have been used
in the MIM configuration to realize memristors. Out of these,
TiO2 is very popular for its CMOS compatibility and ease of
characterization. Extensive work has been done by various
research groups to understand the switching mechanism in the
TiO2 based memristors [9-12].
This work uses oxidized Ti as the insulating layer. Due to
the coarse nature of the oxidation process used here, the
oxidized Ti is assumed to have a non-stoichiometric structure
and is considered to be TiOx. The objective of this work is to
extend the dynamic switching range of a memristor to suit
different applications. This functionality is realized by
introducing an auxiliary electrode to the MIM stack that
This work was partly supported by DARPA MPD program.
978-1-4673-4642-9/13/$31.00 ©2013 IEEE
controls the switching threshold of the insulator through field
effect.
II.
THEORY OF OPERATION
The memory associated with a memristor is in the state of
its resistance. A memristor can have a high resistance state
(HRS) or a low resistance state (LRS) that defines the two bits
of memory. The resistance states can be reversibly switched
from one to the other by applying suitable voltage. TiO2
exhibits a variety of resistive switching (RS) mechanisms. The
RS mechanism can be unipolar or bipolar depending on how
the material is conditioned. The electrical conduction
mechanism through TiO2 is from the drifting of oxygen
vacancies which act as defects. An initially non-conducting
TiO2 is made conductive by continuously sweeping voltage
across it to form conductive filaments through it. This process
is called electroforming. The filaments are formed by
accumulation and alignment of locally available oxygen
vacancies.
Depending on the current compliance set during the
electroforming stage, the material could be unipolar RS (URS)
or bipolar RS (BRS). In case of URS, the switching between
high and low states can be performed solely using either
positive or negative voltage. In contrast, BRS requires one
voltage polarity to switch from a HRS to LRS, and a reversed
polarity to switch it back from LRS to HRS. This work
focuses on the BRS. There are different theories that explain
the principle behind BRS through formation and breaking of
conductive filaments, or trapping and de-trapping of defects at
interfaces, or sweeping of defects from one electrode to the
other. Details of different BRS switching mechanisms are
provided in [12]. However, the basic underlying principle
behind all these theories is the redox reaction of oxygen
vacancies. Finding the mechanism behind BRS is out of scope
of this work. This work merely utilizes the BRS technique to
perform memristive switching and demonstrates the effect of
introducing an auxiliary electrode that varies the RS voltage.
Use of Pt/TiO2/Pt to perform memristive switching is very
common. Pt is used as the conductive electrodes due to its
high oxygen ion mobility, which readily allows trapping and
releasing of interfacial oxygen ions to atmosphere. This makes
Pt highly resistant to oxidation. TiO2 can be deposited by
various techniques. In this work, TiOx was obtained by the
oxidation of sputtered Ti.
The operating principle of the variable switching voltage
memristor is demonstrated schematically in fig 1. For
simplicity, the memristive switching is assumed to be due to
the sweeping of oxygen vacancies from one electrode to the
other. The oxygen vacancies are drawn towards the anode. In
the presence of a third electrode, the e-field in the TiOx layer
is a vector sum of the e-field due to VDS and VGS. So, the VGS
can aid or inhibit the effect of VDS on the oxygen vacancies. In
the actual device, the gate electrode is not symmetric with
respect to the drain and source electrodes as shown in fig 2.
Thus, the effect of VGS can be expected to be stronger than the
case with electrodes arranged as in fig 1.
region is roughly 15nm which is defined by the thickness of
the oxidized Ti layer.
(a)
Source
Drain
(d)
Gate
(b)
Source
VG = 0V
(e)
VD = 0V
VS = 0V
(c)
Gate
Source
Platinum
Drain
HfO2
(a)
VG = 0V
Ti
Silicon
TiOx
Thermal SiO2
Fig 2. Schematic representation of the fabrication process. Pt/Ti layers are
sputter deposited and patterned by lift-off. TiOx is formed by oxidizing
sandwiched Ti layer at 500°C for 10 minutes. HfO2 is used to insulate the gate
electrode from source-drain electrodes.
VD = +V
VS = 0V
Drain
(b)
Motion of oxygen vacancies
VG = +V
VD = +V
VS = 0V
(c)
G
Motion of oxygen vacancies
Platinum
HfO2
TiOx
Fig 1. Schematic representation of the effect of gate field-effect on the motion
of oxygen vacancies that changes the threshold e-field for turn on.
III.
S
D
FABRICATION
A 4" p-type Si wafer is thermally oxidized to form 100nm
insulating oxide. 5nm/50nm/10nm of Ti/Pt/Ti are sputtered on
top of this and patterned to form the source/drain electrode as
shown in fig 2a. 5nm/50nm of Ti/Pt is again sputtered on and
patterned to form the drain/source electrode, fig 2b. The drain
and source electrodes are re-sputtered with 100nm Pt and
patterned in the electrical connection pad regions to make
them mechanically robust. The wafer is then placed in a
furnace at 500°C for 10 minutes to oxidize the sandwiched Ti
to form TiOx, fig 2c. 50nm HfO2 is deposited by atomic layer
deposition and patterned to form the gate oxide, fig 2d. 100nm
Pt is sputter deposited and patterned to form the gate
electrode, fig 2e.
Fig 3 shows images of the fabricated device. The sourcedrain electrodes are 3µm wide and 10µm long. The overlap
between the source-drain electrodes is less than 1µm and this
defines the width of the active region. The length of the active
10um
Fig 3. Optical microscope image of the tunable memristor with the SEM
image shown in the inset. The source-drain overlap defines the active region
of the device and is less than 1µm wide.
IV.
RESULTS AND DISCUSSION
The devices are tested using the FET characterization
program in the Agilent 4156 C Semiconductor Parameter
Analyzer. Initially the devices exhibit a random switching
behavior with different turn-on voltages. The voltage between
drain-source is swept from 0 to a certain VDS and back to 0.
VDS is swept both in positive and negative directions. The
devices undergo the electroforming phase with repeated
switching cycles until they develop a stable switching
behavior. This typically takes at least 30-40 repeated
switching with a current compliance of 10nA. At smaller
current compliance (sub 100nA), the device shows a
reversible switching behavior by turning-OFF while the
voltage is swept from VDS to 0. Higher current compliance is
needed to latch the device to the ON state.
A. Ohmic switching
Ohmic switching occurrs at current compliance of 1µA
and greater. The switching is considered ohmic because the
device exhibits a linear I-V after switching from HRS to LRS.
The switching voltage varies between 3 and 10 volts for
different devices due to the slight difference in their sourcedrain overlap. Ohmic switching reported in earlier work
occurrs for higher current compliances of the order of a few
hundred micro-amps up to a few milli-amps [12]. The reason
for low current switching in this work is due to the extremely
small volume (1µm×1µm×15nm) of the active region.
The current is limited to 1µA to prevent device damage
from dielectric breakdown of TiOx film. The switching data is
plotted in fig 4. The voltage sweep sequence is numbered for
clarity. The device is initially in the HRS (OFF state),
indicated by (1). At a certain drain-source threshold voltage,
the device switches to the LRS (ON state), indicated by (2).
The device retains the ON state when the voltage is decreased
to 0, indicated by (3-4-5). The device remains in the ON state
for any subsequent voltage sweeps (6-7-8-9). Switching from
ON state to OFF state is performed by sweeping the voltage in
the reverse direction. Similar switching is observed for
negative VDS sweep as shown in the third quadrant of fig 4.
The device retains the OFF and ON state successfully for
voltages less than the turn-ON voltage. This voltage range is
sufficient to determine the state of the memristor, which
constitutes the read operation in a non-volatile memory
element. It must be noted that the gate electrode is connected
to ground during these measurements.
B. Gate control through field effect
The effect of gate bias is tested by sweeping VDS and
recording the ID, while changing VGS in steps. The thick HfO2
gate oxide layer prevents the gate from starting its own
memristive switching with respect to the source-drain
electrodes. Since the measurement focuses only on the effect
of VGS on the turn-on voltage VDS(ON), it is not necessary to
latch the device to the ON state. Hence, the current
compliance is set to a low value of 10nA. The measurements
are repeated 4 times for every value of VGS. The effect of gate
bias on the turn-on voltage is evident from the plot in fig 5.
It can also be noted that the effect of gate bias is not
symmetric. The turn-on voltage increases with gate bias for
positive VDS, whereas it decreases (absolute value) with gate
bias for negative VDS. The cause for this behavior is
understood from the physical arrangement of S-D-G
electrodes in fig 2. For positive values of VDS and VGS, the
effective e-field in the TiOx film increases since they are in the
same direction. This should lower the turn-ON voltage and
vice-versa for opposing VGS and VDS. However, the
measurement data in fig 5 is exactly the opposite. From this, it
can be deduced that the source and drain electrodes are
interchanged for these sets of measurements. Also, the turn-on
voltages are different for positive and negative VDS since the
gate electrode is not equidistant from source and drain
electrodes. The reason for the hysteresis in the switching
behavior is not clearly understood. Further experiments are
necessary to understand the origin of hysteresis, which would
also explain the difference in hysteresis between
measurements.
1.20E-08
7
8
4
I D (A)
6
5.00E-07
9
-10
-5
2
1
0
8.00E-09
4.00E-09
3
5
ON state
ID (A)
1.50E-06
5
OFF state
10
-9
-6
1.00E-24
-3
0
3
-8.00E-09
ON state
9
Vg = -2V
Vg = 0V
Vg = 2V
-4.00E-09
OFF state
-5.00E-07
6
Turn-ON voltage
-1.20E-08
VDS (V)
-1.50E-06
Fig 4. Resistance switching characterization of the memristor. The device
resistance is high in the OFF state indicated by the nearly zero current lines up
to the turn-on voltage. The turn-ON voltage corresponds to the threshold efield required to form the conductive filaments in the TiOx film. The device is
turned OFF by applying a reverse voltage. The device response to positive and
negative voltage is almost symmetric.
VDS (V)
Fig 5. Characterization of the gate field-effect on the memristor switching
behavior. The turn-on voltage of the memristor increases when VDS and VGS
appear in opposing directions in the TiOx film and reduces when they are in
the same direction.
To observe the effect of gate voltage at a finer scale, the
device is tested in the VGS-ID mode at a constant VDS. Fig 6
plots the measurement data obtained for VDS=5V (fig 6a) and
VDS=6V (fig 6b). In both the plots, the effect of VGS is clearly
seen from the drop in ID at higher VGS. The trend is consistent
with theory at both values of VDS where the threshold voltage
increases with VGS. This corroborates with the data obtained in
fig 5.
8.00E-09
ID (A)
1.20E-08
4.00E-09
VGS (V)
-2.5
-2
-1.5
1.00E-24
-1 -0.5 0
0.5
1
-4.00E-09
1.5
2
V.
This work successfully demonstrates the working of a
tunable switching voltage memristor. Different switching
characteristics are observed based on the current compliance,
which corroborates with earlier work. The use of small
volume of TiOx for active region allows memristive switching
at low compliances. But this also makes the device vulnerable
to dielectric breakdown at higher currents. The device can be
equipped to handle larger currents by using thicker and wider
TiOx film. Further tests are needed to characterize the
switching speed of the device.
2.5
VDS = 5V
VDS = -5V
ACKNOWLEDGMENT
The devices were fabricated at Utah Nanofab. The authors
thank Nanofab staff for their technical help.
REFERENCES
-8.00E-09
[1]
-1.20E-08
(a)
ID (A)
1.20E-08
8.00E-09
4.00E-09
-2.5
-2
-1.5
-1
1.00E-24
-0.5
0
-4.00E-09
CONCLUSION
0.5
1
VGS (V)
1.5
2
2.5
VDS = 6V
VDS = -6V
-8.00E-09
-1.20E-08
(b)
Fig 6. Gate effect characterization curves showing the relation between VGS
and ID at a constant VDS. The drop in current is indicative of the switching
threshold voltage VDS(ON). As expected, a higher gate voltage shows a greater
threshold voltage in compliance to the measurements in fig 5.
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