Low temperature amorphous ZTO TFTs on flexible substrates using a combinatorial
approach
C. Fernandes, A. Kiazadeh, E. Fortunato, R. Martins, P. Barquinha
i3N/CENIMAT, Department of Materials Science, Faculty of Science and Technology,
Universidade NOVA de Lisboa and CEMOP/UNINOVA, Campus de Caparica, 2829-516
Caparica, Portugal
*email:
The level of performance required nowadays in large area and low-cost systems is turning
amorphous oxides semiconductors (AOSs) into essential materials in this field. AOS are
recognized for their remarkable features such as good uniformity even when produced at
room temperature, enabling applications in large area and flexible electronics; wide band gap
(consequently high transparency), making them suitable for transparent electronics; and good
electrical performance despite their amorphous structure, enabling flexible circuitry operating
at high-kHz to MHz range. The potential of these materials as active channel layers in thin-
film transistors (TFTs) started to be recognized in 2004 with the work by Nomura et
al.
[1],
where Indium Gallium Zinc Oxide (IGZO) transistors were produced by pulsed laser
deposition (PLD) in a near room temperature environment, on flexible substrates. The
capabilities of IGZO TFTs have aroused worldwide interest of the display industry in recent
years, starting to replace the well-known a-Si:H and even poly-Si TFTs, particularly when a
combination of low power consumption, high refresh rates and high resolutions is desired.
Despite the establishment of IGZO TFTs as a key technology for large area electronics,
indium and gallium are critical raw materials, thus imposing important constrains regarding
the sustainability of this approach. Therefore, the ideal route for the next generation AOS-
based transistor technology should comprise an indium- and gallium-free semiconductor
material, providing at least comparable performance and processing temperature to IGZO.
Zinc Tin Oxide (ZTO) has been recognized as a likely choice and it was applied as active
channel layer in TFTs for the first time in 2005 [2]. Since then a large number of works on
ZTO TFTs has been published, following both physical and solution-processing routes. As a
generic trend, ZTO TFTs typically exhibit slightly inferior performance levels compared to
IGZO TFTs, even if processed at higher temperatures (e.g., mobility, µ
FE
<10 cm
2
/Vs, even
after >300 °C processing).
In this work we show that by following a combinatorial approach with ZnO and SnO
2
sputtering targets and by proper adjustment of Ar:O
2
:H
2
flow ratio during deposition we
could achieve ZTO TFTs with comparable performance to IGZO ones, after a maximum
processing temperature of only 180 °C. Properties such as V
on
=-0.8V, On/Off ratio >10
6
and
µ
FE
=11 cm
2
/Vs are obtained with a sputtered Ta
2
O
5
-based multilayer dielectric. Devices on
PEN substrates do not show any significant change on their properties when measured under
bending radius from 15 to 45 mm (concave and convex). In addition, a promising ΔV
th
<-0.5
V after 15 min under negative bias illumination stress (NBIS, 420 nm) was also recorded for
improved devices. This work constitutes a major step toward fabrication of low temperature
amorphous indium-free oxide TFTs, fulfilling the expected requirements of next generation
sustainable, flexible and transparent electronics.
[1]
K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication
of transparent flexible thin-film transistors using amorphous oxide semiconductors,”
Nature
, vol. 432,
no. November, pp. 3383–3386, 2004.
[2]
H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong, and D. a. Keszler, “High mobility transparent thin-
film transistors with amorphous zinc tin oxide channel layer,”
Appl. Phys. Lett.
, vol. 86, no. 1, p. 13503,
2005.
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