Operation mechanism of Schottky barrier nonvolatile memory with high conductivityInGaZnO active layer Thanh Thuy Trinh, Van Duy Nguyen, Hong Hanh Nguyen, Jayapal Raja, Juyeon Jang, Kyung
Trang 1Operation 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
View online: http://dx.doi.org/10.1063/1.3699221
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/14?ver=pdfcov
Published by the AIP Publishing
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Trang 2Operation mechanism of Schottky barrier nonvolatile memory with high
conductivity InGaZnO active layer
Thanh Thuy Trinh,1,2Van Duy Nguyen,3,a)Hong Hanh Nguyen,1Jayapal Raja,1
Juyeon Jang,1Kyungsoo Jang,1Kyunghyun Baek,1Vinh Ai Dao,1,2and 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 at7 V for Schottky devices was obtained Both memory devices
using SiO2/SiOx/SiOxNystacks showed a retention exceeding 70% of trapped charges 10 yr with
operation voltages of 613 V at an only programming duration of 1 ms.V C 2012 American Institute
of Physics [http://dx.doi.org/10.1063/1.3699221]
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.1,2For next-generation
appli-cations, such as system-on-panel (SOP) displays,
memory-in-pixel, and transparent memory, an a-IGZO-based
nonvo-latile memory (NVM) is required, but high operating
vol-tages, retention loss, and cycling decay are challenges that
need to be addressed before arriving at the appropriate
material.35
The TFTs using Schottky barrier (SB) at source/drain
(S/D) electrodes (SBTFT) were introduced to improve the
off current in silicon TFTs.68Hence, SBTFTs could be
fab-ricated on the high carrier concentration a-IGZO layer
with-out the problem of high off currents Otherwise, the
increasing of mobility with carrier concentration is relied on
IGZO system.1,9Due 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)
deposition (IPCVD) process on a low-resistivity c-Si sub-strate as gate electrode An SiO2-blocking layer, with a thickness of 20 nm, and an SiOxstorage layer, with a thick-ness of 20 nm at an SiH4:N2O ratio of 6:1, were sequentially deposited at 170C Subsequently, an ultrathin amorphous silicon (a-Si) film with an SiH4:H2gas ratio of 5:20 was de-posited with a RF power of 50 W and a temperature of
200C 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:ZnO¼ 1:1:1) at room temperature The base vacuum level was maintained at a pressure lower than
5 105 Torr while the working pressure and DC power were maintained at 5 103 Torr and 140 W, respectively, during the sputtering Before deposition, the presputtering process was performed for 10 min to remove any contamina-tion that may be present on the target surface Then, the post-annealing process was performed for an as-deposited sample
in an air atmosphere using rapid thermal annealing equip-ment at a temperature of 250C for 1 h After the postanneal-ing 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 pat-terned by the photolithography method using two mask-patterning processes The electrical and memory characteris-tics of the devices were measured by Semiconductor Param-eter Analyzer equipment (Model EL 420C)
The bottom gate NVM structures were fabricated on two
Trang 3Ag electrodes, with a work function estimated to be about
4.74 eV,12which has already been confirmed by structure
an-alyzed (result is not shown here) The ONVM used
a-low-conductivity active layer with 150-nm-thick Al electrodes
with a work function of about 4.26 eV.12TFT characteristics
of the two types were measured and compared as in Fig.1
Note that the high-conductivity IGZO-TFT using Al
electro-des 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
con-centration increases up to 1020cm3 in a-IGZO system,9
which suggests the possible explanation for the earlier
results While high-conductivity a-IGZO film showed a
car-rier concentration and Hall mobility of about 1019cm3and
15 cm2V1s1, the mobility in low-conductivity film could
not be measured using Hall equipment When study with the
same device dimensions and measurement, SB structure
pro-vides an ION approximately 2 orders higher in magnitude
than that of the Ohmic contact structure Field effect mobil-ity of SBTFT reaches a value of about 6 cm2 V1 s1 in comparison to that of about 0.1 cm2 V1 s1 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 oper-ates 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 elec-tron 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
15 V for 10 s did not shift the threshold voltage to the nega-tive region corresponding to the hole-ejection process Mean-while, 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 cm2 was introduced during the negative-biasing process, called light-assisted erasing The erasing process can be successfully
FIG 1 (a) Initial transfer characteristics and l FE 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
FIG 2 Switching characteristics of (a) the SBNVM device and (c) the ONVM device; threshold voltage shift of (b) SBNVM and (d) OTFTNVM.
Trang 4obtained 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
thresh-old 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
illumina-tion, 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
vol-tages were applied from4 to 7 V causing a threshold
volt-age shift from about0.6 to 2.6 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
meas-ured 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
program-ming and erasing biases are shown in Fig.2(d) From this
fig-ure, 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,35which could have differ-ent 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 char-acteristics 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 con-ductivity 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 volt-age because of the blocking current phenomenon The high resistance at this barrier redistributes the voltage dropping over the structure In this case, an additional V4component appears
FIG 3 (a) Ohmic and Schottky properties of I–V characteristics of the MSM structure using
Al and Ag electrodes, respectively and (b) dia-gram of voltage potential distribution over the structure.
Trang 5significantly 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 at15 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
perform-ance In this study, we attempted to characterize retention
properties of the SBNVM using the electrical bias only for the
programming process
Figure4demonstrates the retention characteristics of the
SBNVM (Fig 4(a)) and ONVM (Fig 4(b)) The retention
time was measured for 104s 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
corre-sponding 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
conven-tional OTFT, which is attributed to the high conductivity
The difficulty in the erasing process of SBNVM was
attrib-uted 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
disad-vantage could be solved by using the ONVM structure,
which shows conventional programming/erasing processes
with a memory window of 2.83 V
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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