Suppression in the negative bias illumination instability of Zn-Sn-O
transistor using oxygen plasma treatment
Shinhyuk Yang,
1,2
Kwang Hwan Ji,
3
Un Ki Kim,
4
Cheol Seong Hwang,
4
Sang-Hee Ko Park,
1
Chi-Sun Hwang,
1
Jin Jang,
2
and Jae Kyeong Jeong
3,a)
1
Oxide Electronics Research Team, Electronics and Telecommunications Research Institute,
Daejeon 305-700, Korea
2
Department of Information Display, Kyung Hee University, Seoul 130-701, Korea
3
Department of Materials Science and Engineering, Inha University, Incheon 402-751, Korea
4
Department of Materials Science and Engineering, WCU Hybrid Materials Program, and Inter-university
Semiconductor Research Center, Seoul National University, Seoul 151-742, Korea
(Received 15 July 2011; accepted 18 August 2011; published online 7 September 2011)
This study examined the effect of oxygen plasma treatment on light-enhanced bias instability in
Zn-Sn-O (ZTO) thin film transistors (TFTs). The treated ZTO TFT exhibited only a threshold
voltage (V
th
) shift of À2.05 V under negative bias illumination stress (NBIS) conditions, whereas
the pristine device suffered from a negative V
th
shift of 3.76 V under identical conditions. X-ray
photoelectron spectroscopic analysis revealed that the oxygen vacancy defect density was diminished
via the oxygen plasma treatment. This suggests the V
th
degradation under NBIS is due to photo-transition
of oxygen vacancy defects.
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2011 American Institute of Physics. [doi:10.1063/1.3634053]
Multi-component metal oxide thin film transistors
(TFTs) have been intensively studied for their perspective
applications in novel active-matrix (AM) display, such as a
transparent and/or bendable AM display, because they offer
the advantages of high field-effect mobility, optical clarity,
excellent electrical uniformity, and low processing
temperature.
1–3
In particular, the ZnO-based semiconductor
has a wide band-gap (>3.0 eV) that enables the transparent
electronics or display to be realized. The intriguing transpar-
ent aspect, however, should be carefully exploited, because
sunlight or ambient light through the transparent electronic
devices can cause a significantly reliability concern in con-
junction with the electrical bias and/or thermal stress. The
application of the negative bias illumination stress (NBIS) in
metal oxide TFTs causes the serious threshold voltage (V
th
)
shift in the negative voltage direction, while the positive bias
illumination stress (PBIS)-induced instability is relatively
negligible.
4
This extreme asymmetric deterioration can be
understood by considering the trapping or injection mecha-
nism of the photo-created hole carriers.
5
The validity of the
hole trapping model has been confirmed by the strong gate
dielectric material dependence on the NBIS instability of
the resulting oxide TFTs.
6,7
Recently, an entirely different
degradation mechanism has been proposed, based on the
photon-transition model from the neutral oxygen vacancy
[V
O
] to double positive charged oxygen vacancy [V
O
2þ
].
8,9
Ji et al. reported that the removal of the [V
O
] defect center,
via intentional oxygen diffusion, results in the strong sup-
pression of NBIS instability in the InGaZnO TFTs. This sup-
ports the involvement of the oxygen vacancy defect.
10
Thus,
the origin of NBIS instability in oxide TFTs is still under
debate. In addition, from the viewpoint of the actual applica-
tion of the metal oxide TFTs, it is urgent to develop a practi-
cal process to prevent NBIS instability, as well as its
complete clarification.
In this letter, we report the effect of oxygen plasma
treatment on the NBIS-induced instability of Zn-Sn-O
(ZTO) TFTs. We chose the ZTO semiconductor as a channel
layer of the metal oxide TFTs, because the most popular
InGaZnO system components are rare in the earth crust, as
well as the expensive cost of In and Ga cations. Compared to
the pristine device, the treated ZTO TFTs exhibited more
stable behavior against the application of NBIS. This result
can be explained by the V
O
transition model. This was con-
firmed by x-ray photoelectron spectroscopy (XPS) data.
The fabricated ZTO TFTs have a bottom gate (BG) and
bottom contact configuration. Lithographically patterned in-
dium tin oxide (ITO) 150 nm thick on a glass substrate, with
an area of 100 Â 100 mm
2
, was used as the gate electrode for
the BG TFT. A 176 nm thick Al
2
O
3
film as a gate insulator
was deposited by atomic layer deposition (ALD) at a temper-
ature of 150
C. ZTO films with thicknesses of 20, 40, and
60 nm as a channel layer were grown using metal organic
chemical vapor deposition (MOCVD). The fundamental
details regarding the MOCVD-derived channel layer will be
published elsewhere. Oxygen plasma treatment was
performed on the ZTO/ITO/Al
2
O
3
/ITO/glass substrate. The
plasma power and treated time were 500 W and 60 s, respec-
tively. ALD-derived 9-nm-thick Al
2
O
3
films were deposited
as a passivation (or protective) layer on a ZTO/Al
2
O
3
/ITO/
glass substrate, which was followed by the patterning of the
active layer/Al
2
O
3
stack simultaneously. The thermal contact
annealing was performed at 250
C for 2 h. The electrical
measurements were performed at room temperature in air
using an Agilent B1500A precision semiconductor parameter
analyzer.
Figure 1 shows the transfer characteristics of the pristine
ZTO TFTs (reference device) and oxygen plasma-treated
ZTO TFTs with W/L ¼ 40 lm/20 lm, respectively. The
extraction procedures for device parameters were described
a)
Author to whom correspondence should be addressed. Electronic mail:
jkjeong@inha.ac.kr.
0003-6951/2011/99(10)/102103/3/$30.00
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2011 American Institute of Physics99, 102103-1
APPLIED PHYSICS LETTERS 99, 102103 (2011)