Indium-free fully transparent electronics deposited entirely by atomic layer deposition
Zhenwei Wang and Husam N. Alshareef
Materials Science and Engineering, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia.
Presenter contact details:
E-mail:
Address: KAUST P.O. Box 4347, 4700 King Abdullah University of Science & Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia
The field of transparent electronics based on metal oxide conductors and
semiconductors has attracted much attention recently, because it is expected that fabrication
of fully transparent devices will not only enable higher performance displays, but will also
usher in a new era of transparent electronics. Fully transparent thin-film transistors (TFTs)
with appreciating switching performance have been achieved using oxides materials for
channel, gates and electrodes. A stable supply for high performance transparent TFTs is of
great importance. However, the limited work on fully transparent circuits has almost
exclusively relied on indium tin oxide (ITO), indium doped zinc oxide (IZO) or other indium-
containing oxides. It is well-known that indium supplies have been a constant concern for the
display and touch screen industries, thus it is necessary to demonstrate high-performance
fully transparent TFTs using alternative transparent oxides. Atomic layer deposition (ALD)
technique has been widely used in a variety of applications and is capable of depositing
conformal and uniform films on a wide range of substrates and geometries. Oxide films
processed by ALD are compatible not only with planar device architecture, but also with
emerging three-dimensional device architectures, which make them very good candidates for
building TFTs on curved screens or displays with even more complicated geometries, that
could give better or novel watching experience.
In light of the above facts, we demonstrate robust processes for fully-transparent
electronics fabrication with the following features: (1) A unique multilayer semiconductor
channel composed of alternating layers of hafnium oxide (HfO
2
) and zinc oxide (ZnO), which
gives significant improvement in the electrical stability of our devices; (2) entirely indium-
free transistors (gate, SD, channel, dielectric are all indium-free); (3) all-oxide, truly fully-
transparent devices and circuits (no metals, only transparent oxide conductors and
semiconductors); (4) single deposition technique (ALD) for all materials, which means
uniform and conformal deposition is possible on both planar and three-dimensional device
architectures; (5) maximum process temperature of 160
C which allowed us to demonstrate
the process on both rigid glass and flexible substrates. Saturation field-effect mobility (
Sat
)
as high as ~10.5 cm
2
V
-1
s
-1
, drain on-current to off-current (I
on
/I
off
) ratio over 10
9
and
subthreshold swing value of 0.194 V·dec
-1
were obtained for the TFTs on glass substrate. The
corresponding device performances for TFTs on flexible substrate were ~8.5 cm
2
V
-1
s
-1
, over
10
9
and 0.201 V·dec
-1
, respectively. The TFTs also showed great stability under both positive
and negative bias gate stress (±10 V for 3000 s), with very small threshold voltage (V
th
) shift
of +0.3 and -0.1 V, respectively. The NMOS inverter shows rectangular shaped voltage
transfer curve (VTC) with a very high gain value of 238 (The supply voltage, V
DD
, is 25 V).
The 11-stage NMOS ring oscillator shows a sinusoidal output waveform with oscillation
frequency of 141 kHz (
V
DD
=20 V), which is equal to a propagation delay less than 321 ns per
stage.
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