AOS-based memristive devices towards TFT integration: Materials and challenges

Abstract

Von Neumann’s architecture, currently applied to electronic computing systems, is known to be at its optimization limits since CMOS technology cannot be further miniaturized without performance degradation. Neuromorphic computation is an effective alternative as it allows for power-efficient systems with high density information, in-memory computation and parallel data processing. A key component for this technology is the memristor, whose conductance can be altered upon application of an electric field. These devices have striking similarities to biological synapses, revealing many potential applications in neuromorphic engineering.
To achieve a fully integrated system consisting of memristors and thin-film transistors (TFTs), both should share the same materials. Here, we study two amorphous oxide semiconductors (AOS) as resistive switching (RS) layers on memristors: IGZO and ZTO (both often employed on TFTs due to their great performance and stability) and the engineering of the top and bottom contacts.
In the memristive devices we present, the main intrinsic donors within the AOS, commonly referred to as oxygen vacancies, are responsible for the RS mechanism, as they move homogenously throughout the entire active-matrix area. The current is controlled by a potential barrier and the switching involves changes to the barrier characteristics. To facilitate the oxygen vacancies movement, thus achieving higher ON/OFF ratios, an asymmetry in the AOS structure was established. Pt was chosen for the bottom electrode as it is inert and forms a Schottky barrier when in contact with ZTO and IGZO. For the top electrode, Ti/Au was used since Ti has a high oxygen affinity which leads to the removal of oxygen from the AOS layer, forming a highly conductive region at the interface.
We show these devices can be operated at two distinct modes. Device initialization at reverse polarity leads to an abrupt counter-8-wise filamentary switching and device initialization in the forward direction presents an 8-wise area-dependent switching with ON/OFF ratios up to two orders of magnitude. These devices maintain the diode-type characteristic resulting in analog control of resistance states which provides the means for subsequent high-density processing and storage in a neuromorphic system.[1]
However, Pt and Ti/Au are expensive and, usually, patterned by lift-off, which limits device downsizing, reproducibility and yield, due to particles remaining at the device edges, causing electrical shortcuts. A noble metal-free strategy with easy integration reduces the overall system cost. Therefore, we additionally studied Mo as bottom and top electrodes which, being patterned by dry etching, improves reproducibility and yield. Here, we present an IGZO-based memristor with Mo electrodes ready for integration tests.
With this memristor structure, the IGZO asymmetry was caused by oxidizing the bottom Mo at the interface which reduced the switching oxide in that region. Devices down to 0.5 µm2 were fabricated using conventional photolithography processes, with an extraordinary yield of 100%. We successfully demonstrate the memristor’s synaptic behavior through potentiation and depression, short-term to long-term memory transition and simulation of the learning process.[2]
Finally, for TFT integration another challenge needs to be addressed: the annealing temperature (TA), as this is an essential step for TFTs. TA affects the AOS’s electrical properties and is employed on TFTs to create a more stable with less hysteresis transistor. This may worsen the memristor performance in which a big hysteresis is crucial. Hence, either different steps of AOS depositions should be employed, or a compromise must be met using a low TA, enough to stabilize TFTs, without hindering the memristor’s RS behavior. Passivation only on top of TFTs can also contribute in achieving the desired stability, while maintaining the hysteresis on the memristor. Should this strategies work, TFT integration with these memristive devices is near to accomplishment.
[1] N. Casa Branca et al., “2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes,” Adv. Electron. Mater., vol. 6, no. 2, 2020.
[2] M. Pereira et al., “Noble-Metal-Free Memristive Devices Based on IGZO for Neuromorphic Applications,” vol. 2000242, 2020.

Full text

www.cenimat.fct.pt
Deposition
➢Both Pt/ZTO/Ti/Au and Mo/IGZO/Mo are suitable for TFT integration and compatible with alternative substrates (such as
plastic or paper)- an essential feature for Internet-of-Things (IoT) applications.
➢For TFT integration, the annealing temperature (TA) needs to be addressed. TA affects the AOS’s electrical properties and is
employed on TFTs to create a more stable with less hysteresis transistor. This may worsen the memristor performance in
which a big hysteresis is crucial. Hence, a compromise must be met using a low TA, enough to stabilize TFTs, without
hindering the memristor’s RS behavior.
➢Should this strategies work, TFT integration with these memristive devices is near accomplishment.
•Pt and Au are expensive;
•Patterned by lift-off -> limits device downsizing,
reproducibility and yield
•No critical raw materials!
Critical raw materials such as
indium and galium;
Patterned by dry etching -> high
yield and acurate reproducibility;
IGZO and Mo are often
employed on TFT technology;
Noble metal free!
Top contact Mo
Active RS layer
IGZO
Bottom contact Mo
Etching
Spin-coating
M.E. Pereira*, J. Deuermeier, C. Silva, P.G. Bahubalindruni, P. Barquinha, R. Martins, E. Fortunato
and A. Kiazadeh**
CENIMAT |i3N, DCM, FCT-UNL and CEMOP/UNINOVA, Campus da Caparica,
2829-516 Caparica, Portugal
*mel.pereira@campus.fct.unl.pt; **a.kiazadeh@fct.unl.pt
Conclusions
[1] N. Casa Branca et al., “2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes,” Adv.
Electron. Mater., vol. 6, no. 2, 2020.
[2] M. Pereira et al., “Noble-Metal-Free Memristive Devices Based on IGZO for Neuromorphic Applications,” vol.
2000242, 2020.
This work is funded by National Funds through the FCT –Fundação para a Ciência e a Tecnologia, I.P., under the scope of the doctoral grant
2020.08335.BD. This work also received funding from FEDER funds through the COMPETE 2020 Programme and National Funds through FCT –
Portuguese Foundation for Science and Technology under the scope of the project UIDB/50025/2020-2023, and the project “NeurOxide,”
Reference PTDC/NAN-MAT/30812/2017. This work also received funding from the European Community’s H2020 program under grant
agreements 716510 (ERC-2016-StG TREND), 787410 (ERC-2019-AdG DIGISMART) and 952169 (SYNERGY, H2020-WIDESPREAD-2020-5, CSA).
References Acknowledgments
AOS-based memristive devices towards TFT integration
Materials and challenges
Neuromorphic computation is an exciting alternative to the Von Neumann’s architecture, as it allows
for power-efficient systems with high density, in-memory computation and parallel data processing. A key
component for this technology is the memristor, whose performance is endowed with striking
similarities to biological synapses.
To develop an integrated, analog controlled on-chip technology, for a large-scale energy-efficient
neural network, the memristor should be integrated with supporting electronics such as thin-film
transistors (TFTs). Therefore, the development of acompatible fabrication process where both TFT and
memristor share the same materials is essential.
Here, we study two amorphous oxide semiconductors (AOS) as resistive switching (RS) layers on
memristors: zinc tin oxide (ZTO)[1] and indium-gallium-zinc oxide (IGZO)[2] and the engineering of the
top and bottom contacts. These two materials are widely applied in TFT technology, making them an
ideal choice for memristors in neuromorphic system-on-panel solutions.
Pt/ZTO/Ti/Au Mo/IGZO/Mo
Pulse
I. Negative photoresist deposition and patterning
Hotplate UV exposure
Development
II. Material deposition and lift-off in acetone
Top contact
Ti/Au
123
4
I. Material and positive photoresist deposition and patterning
Abstract
Sputtered deposition
Mo IGZO Mo Spin-coating
Bottom contact Pt
12
Hotplate 3
UV exposure
4
II. Etching and resist cleaning in acetone
Active RS layer
ZTO
Lift-off
5
6
a
b
c
a
c
b
5
6
Cleaning
Development
010 20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
Atomic fraction (%)
Position (nm)
Mo
O
Ga
In
Zn
𝑶𝟐plasma treatment
Material analysis
𝑂2plasma treatment on Mo
bottom contact creates MoOx layer
Analog
switching
220230240250
150
180
210
240
270
300
Normalized intensity (a.u)
Etch time (s)
Etch time (s)
Mo 3d
XPS
STEM
Electrical characterization
Good schottky contact
010 20 30 40 50 60 70 80 90 100
10-6
10-5
Conductance (S)
# Pulse
-3 -2 -1 0 1 2 3 4
10-8
10-7
10-6
10-5
10-4
10-3
|Current| (A)
Voltage (V)
-3 -2 -1 0 1 2 3
10-9
10-8
10-7
10-6
10-5
10-4
|Current| (A)
Voltage (V)
SET
RESET
050 100 150 200 250 300
10-9
10-8
10-7
Conductance (S)
#Pulse
-2 V
100 ms
0.1 V
250 ms
read
pulse
pulse
read
0.1 V
250 ms
2 V
300 ms
Platinum and gold are expensive;
Patterned by lift-off -> limits device
downsizing, reproducibility and yield;
Both abrupt and analog switching
behavior;
No critical raw materials!
-3 -2 -1 0 1 2 3
10-8
10-7
10-6
10-5
10-4
10-3
|Current| (A)
Voltage (V)
SET
RESET
Abrupt
switching
Analog
switching
-2.2 V
3 ms
pulse
2 V
7 ms
pulse
read
read
(…)
(…)
0.1 V
SET RESET Potentiation
Depression
020 40 60 80
0
20
40
60
80
100
Synaptic weight (%)
#Pulse
Learning 1
0200 400 600
Forgetting 1
Time (s)
010 20
Learning 2
#Pulse
200 400 600
Forgetting 2
Time (s)
0 5 10
Learning 3
#Pulse
0200 400 600
Time (s)
Forgetting 3
Learning and forgetting
Materials Challenges for Memory, APL Materials Virtual Conference, April 2021
Neuromorphic applications
Experimental section
ZTO Results
Potentiation
Depression
TFT Memristor
IGZO Results