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Operation mechanism of Schottky barrier nonvolatile memory with high conductivity
InGaZnO active layer
Thanh Thuy Trinh, Van Duy Nguyen, Hong Hanh Nguyen, Jayapal Raja, Juyeon Jang, Kyungsoo Jang,
Kyunghyun Baek, Vinh Ai Dao, and Junsin Yi
Citation: Applied Physics Letters 100, 143502 (2012); doi: 10.1063/1.3699221
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APPLIED PHYSICS LETTERS 100, 143502 (2012)

Operation mechanism of Schottky barrier nonvolatile memory with high
conductivity InGaZnO active layer
Thanh Thuy Trinh,1,2 Van Duy Nguyen,3,a) Hong Hanh Nguyen,1 Jayapal Raja,1
Juyeon Jang,1 Kyungsoo Jang,1 Kyunghyun Baek,1 Vinh Ai Dao,1,2 and Junsin Yi1,a)

1
Information and Communication Device Laboratory, School of Information and Communication Engineering,
Sungkyunkwan University, South Korea
2
Faculty of Materials Science, Vietnam National University, Ho Chi Minh City, Vietnam
3
International Training Institute for Materials Science, Hanoi University of Science and Technology, Vietnam

(Received 28 November 2011; accepted 11 March 2012; published online 2 April 2012)
Influence of Schottky contact between source/drain electrodes and high conductivity a-InGaZnO
active layer to the performance of nonvolatile memory devices was first proposed. The Schottky
barrier devices faced to the difficulty on electrical discharging process due to the energy barrier
forming at the interface, which can be resolved by using Ohmic devices. A memory window of
2.83 V at programming/erasing voltage of 613 V for Ohmic and 5.58 V at programming voltage of
13 V and light assisted erasing at À7 V for Schottky devices was obtained. Both memory devices
using SiO2/SiOx/SiOxNy stacks showed a retention exceeding 70% of trapped charges 10 yr with
C 2012 American Institute
operation voltages of 613 V at an only programming duration of 1 ms. V
of Physics. [ />In recent times, amorphous InGaZnO (a-IGZO) film has
been widely studied for using as an active layer in thin-film
transistors (TFTs) because of its inherent advantages, which
include high uniformity, transparence, and high mobility
compared to amorphous silicon.1,2 For next-generation applications, such as system-on-panel (SOP) displays, memoryin-pixel, and transparent memory, an a-IGZO-based nonvolatile memory (NVM) is required, but high operating voltages, retention loss, and cycling decay are challenges that
need to be addressed before arriving at the appropriate
material.3–5
The TFTs using Schottky barrier (SB) at source/drain
(S/D) electrodes (SBTFT) were introduced to improve the
off current in silicon TFTs.6–8 Hence, SBTFTs could be fabricated on the high carrier concentration a-IGZO layer without the problem of high off currents. Otherwise, the
increasing of mobility with carrier concentration is relied on
IGZO system.1,9 Due to these advantages, the IGZO SBTFTs

with high conductivity become potential candidates to
achieve higher field effect mobility (lFE).
In this study, the performance of NVM devices on
a-IGZO is investigated using the memory stack of
SiO2/SiOx/SiOxNy (OOxOn) that has previously been
reported to show some outstanding features, such as low
operating voltages and excellent retention.10,11 The devices
were fabricated in two types of TFT-NVM structures with
Schottky and Ohmic contacts between S/D electrodes and
active layer. The memory behavior of Schottky barrier NVM
(SBNVM) devices is explained in comparison to the
conventional Ohmic contact device (ONVM).
The multi-stack OOxOn-IGZO NVM devices were fabricated in the following steps: OOxOn memory stacks were
deposited using inductive-coupled plasma chemical vapor
a)

Authors to whom correspondence should be addressed. Electronic
addresses: and Tel.:
þ82-31-290-7139. Fax: þ82-31-290-7159.

0003-6951/2012/100(14)/143502/4/$30.00

deposition (IPCVD) process on a low-resistivity c-Si substrate as gate electrode. An SiO2-blocking layer, with a
thickness of 20 nm, and an SiOx storage layer, with a thickness of 20 nm at an SiH4:N2O ratio of 6:1, were sequentially
deposited at 170  C. Subsequently, an ultrathin amorphous
silicon (a-Si) film with an SiH4:H2 gas ratio of 5:20 was deposited with a RF power of 50 W and a temperature of
200  C for 2 min. Then, N2O plasma treatment was carried
out for the creation of the 3.2-nm-thick SiOxNy tunneling
layer. After the deposition of OOxOn memory stacks, an
active layer of 100-nm-thick a-IGZO film was deposited by

DC-pulsed magnetron sputter using a ceramic IGZO target
(Ga2O3:In2O3:ZnO ¼ 1:1:1) at room temperature. The base
vacuum level was maintained at a pressure lower than
5 Â 10À5 Torr while the working pressure and DC power
were maintained at 5 Â 10À3 Torr and 140 W, respectively,
during the sputtering. Before deposition, the presputtering
process was performed for 10 min to remove any contamination that may be present on the target surface. Then, the postannealing process was performed for an as-deposited sample
in an air atmosphere using rapid thermal annealing equipment at a temperature of 250  C for 1 h. After the postannealing process, 150-nm-thick silver (Ag) or aluminum (Al)
films were vaporized to form Schottky or conventional
Ohmic devices, respectively. Finally, S/D regions were patterned by the photolithography method using two maskpatterning processes. The electrical and memory characteristics of the devices were measured by Semiconductor Parameter Analyzer equipment (Model EL 420C).
The bottom gate NVM structures were fabricated on two
kinds of a-IGZO films. The high conductivity layer was deposited in only Ar ambient condition and low conductivity
one was formed at Ar and O2 mixing gases with 80% O2
content. After heat treatment, conductivity of the two 100nm-thick films was found to be stable with values of about
101 and 10À4 S cmÀ1, respectively. The SBNVM was fabricated on high conductivity a-IGZO layer using 150-nm-thick

100, 143502-1

C 2012 American Institute of Physics
V

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Trinh et al.

Appl. Phys. Lett. 100, 143502 (2012)

FIG. 1. (a) Initial transfer characteristics
and lFE of SBNVM and ONVM devices,
(b) energy band diagram of IGZO
Ohmic contact, and (c) Schottky contact.
Work functions of materials are referred
from Refs. 12–14.

Ag electrodes, with a work function estimated to be about
4.74 eV,12 which has already been confirmed by structure analyzed (result is not shown here). The ONVM used a-lowconductivity active layer with 150-nm-thick Al electrodes
with a work function of about 4.26 eV.12 TFT characteristics
of the two types were measured and compared as in Fig. 1.
Note that the high-conductivity IGZO-TFT using Al electrodes could not be modulated by the gate voltage like other
conventional TFTs (not shown here). The advantages of the
SB structure in the on current (ION) and lFE values are
assumed to the high carrier concentration of a-IGZO layer.
The unusual finding of mobility increases when carrier concentration increases up to 1020 cmÀ3 in a-IGZO system,9
which suggests the possible explanation for the earlier
results. While high-conductivity a-IGZO film showed a carrier concentration and Hall mobility of about 1019 cmÀ3 and
15 cm2 VÀ1 sÀ1, the mobility in low-conductivity film could
not be measured using Hall equipment. When study with the
same device dimensions and measurement, SB structure provides an ION approximately 2 orders higher in magnitude

than that of the Ohmic contact structure. Field effect mobility of SBTFT reaches a value of about 6 cm2 VÀ1 sÀ1 in
comparison to that of about 0.1 cm2 VÀ1 sÀ1 for OTFT.
SBTFT operation mechanism has been proposed, in which
the gate voltage modulated the SB height in order to make it
possible to operate the devices (Fig. 1(c)). The OTFT operates using the conventional accumulation process, with
behavior very similar to those of the Si devices, because the
contact at the active and electrodes does not prevent the electron from moving (Fig. 1(b)).

Although the programming of the SBNVM showed
excellent performance, these devices showed difficulty in the
erasing process. As seen in Fig. 2(a), even negative biasing at
À15 V for 10 s did not shift the threshold voltage to the negative region corresponding to the hole-ejection process. Meanwhile, the electron ejection process occurred at the positive
bias voltage of 10 V for 1 ms. In order to discharge the trapped
electron, fluorescent light with an intensity of 10 mW cmÀ2
was introduced during the negative-biasing process, called
light-assisted erasing. The erasing process can be successfully

FIG. 2. Switching characteristics of (a)
the SBNVM device and (c) the ONVM
device; threshold voltage shift of (b)
SBNVM and (d) OTFTNVM.

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Trinh et al.

Appl. Phys. Lett. 100, 143502 (2012)

FIG. 3. (a) Ohmic and Schottky properties of
I–V characteristics of the MSM structure using
Al and Ag electrodes, respectively and (b) diagram of voltage potential distribution over the
structure.

obtained using bias voltages combined with light assisted.

However, the subthreshold swing (SS) of such light-assisted
erasing process degrades when compared to the programming
one. Besides the hole trapping mechanism causing the threshold voltage shift, the degradation of interface state under light
illumination and negative bias stress was supposed to thin film
transistor using a-IGZO material.15,16 During light illumination, the electron hole pairs were generated and they created
trap sites in material. The stretch-out phenomena of SS can be
attributed to photo- generated-traps in a-IGZO layer including
interface and bulk traps. In Fig. 2(b), the switching properties
of the SBNVM indicate the linear dependence of the threshold
voltage shift and bias voltage. For the programming process,
only gate bias voltage was applied to the structure for 1 ms,
from 10 to 13 V causing a threshold voltage shift from about
0.5 to 3 V. Using the light-assisted process, negative bias voltages were applied from À4 to À7 V causing a threshold voltage shift from about À0.6 to À2.6 V. The memory window of
the SBNVM structure was calculated to be higher than about
5.5 V with light assistance.
While using the conventional Ohmic contact structure,
the problem in the erasing process was solved as seen in
Fig. 2(c). The programming and erasing processes were measured from 610 to 613 V and the largest memory window
achieved was about 2.83 V. However, as mentioned earlier,
the active layer for Ohmic contact structure needs to have a
low conductivity for achieving low leakage current. This is
the reason for the lower performance of ONVM compared to
SBNVM. All the threshold voltage shifts at different programming and erasing biases are shown in Fig. 2(d). From this figure, the programming performance seems to be more effective

than the erasing duration as general due to the higher mobility
of electron.
The erasing difficulty phenomenon has also been
reported in some earlier studies,3–5 which could have different explanations. Some reports attributed this phenomenon
to the n-type conductivity behavior of IGZO, due to which
very few holes are available for positive charge injection

into the gate insulator to neutralize the stored negative
charges.4 In this study, we attribute the electrical erasing
problem of the SBNVM to the potential barrier between the
Ag electrodes and a-IGZO layer. The Schottky contact characteristics of IGZO/Ag are shown in Fig. 3(a) by measuring
current-voltage (I–V) characteristics of an Al/a-IGZO/Ag
(metal-semiconductor-metal (MSM)) structure. The I–V plot
proves that current through the structure is blocked when a
negative bias is applied on Ag electrode. Meanwhile, the Ag/
a-IGZO contact seems to open out when a positive bias is
applied on the electrode. This kind of Schottky contact
behavior is the key point for TFT operation using high conductivity a-IGZO as active layer. The leakage current in
reversed bias region is assumed to the trap sites distributed
over metal and semiconductor junction as well as barrier
height of the contact. The improvement in the operation of
TFT using high conductivity a-IGZO is shown in Fig. 1.
Unfortunately, despite the advantages of using SB in the
TFT, it causes problems in performance of the NVM devices.
As illustrated in Fig. 3(b), when the negative bias applied on
the gate electrode, the SB increases with the negative bias voltage because of the blocking current phenomenon. The high
resistance at this barrier redistributes the voltage dropping over
the structure. In this case, an additional V4 component appears

FIG. 4. Retention characteristics of (a)
SB NVM device for the programming
process and (b) Ohmic conventional
NVM structure.

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143502-4

Trinh et al.

significantly reducing the voltage drop, V3, over the tunneling
layer. This is the reason why the threshold voltage did not shift
even when the gate electrode was biased at À15 V for 10 s. The
light-assisted erasing process with recombination and tunneling
mechanism of trapped charges caused by photon energy is the
solution to this problem. This erasing process is not preferred
for modern NVM devices because of complexities in performance. In this study, we attempted to characterize retention
properties of the SBNVM using the electrical bias only for the
programming process.
Figure 4 demonstrates the retention characteristics of the
SBNVM (Fig. 4(a)) and ONVM (Fig. 4(b)). The retention
time was measured for 104 s and extrapolated to 10 yr. The
results show a residue of 87% trapped charges after 10 yr
corresponding to the memory window of 2.67 V for
SBNVM. Meanwhile, with P/E biased at 613 V for 1 ms, the
memory window of the ONVM remains at 2.06 V corresponding to 73% trapped charges. These results are matched
to OOxOn NVM properties using other active materials10,11
and suitable for device applications.
In summary, two types of IGZO TFT/NVM with SB and
Ohmic contacts have been fabricated and compared. The
SBTFT shows better performance compared to the conventional OTFT, which is attributed to the high conductivity.
The difficulty in the erasing process of SBNVM was attributed to the SB height formed at the interface of the active
layer and electrodes. This barrier prevents the electron in the
reverse bias from moving through the electrodes. This disadvantage could be solved by using the ONVM structure,
which shows conventional programming/erasing processes

with a memory window of 2.83 V.

Appl. Phys. Lett. 100, 143502 (2012)

This work was supported by Priority Research Centers
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2011-0018397).
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