Atomic layer deposition ZnO:N flexible thin film transistors and the effects of bending on device properties

Article abstract of DOI:10.1063/1.3577607

ZnO:N flexible thin film transistors were fabricated by atomic layer deposition on polyethylene naphthalate substrates and the effects of bending on the device properties investigated. The threshold voltage and saturation mobility were observed to change with respect to the amount of substrate bending. These modulations can be explained in terms of piezoelectric nature of in ZnO. In comparison with the previously reported single crystal nanowires ZnO field effect transistors, the amount of the electrical property modulation under bent condition is significantly reduced and our report shows a much improved stability for ZnO:N as a flexible device material.

Full text of DOI:10.1063/1.3577607

Atomic layer deposition ZnO:N flexible thin film transistors and the effects of
bending on device properties
Jae-Min Kim, Taewook Nam, S. J. Lim, Y. G. Seol, N.-E. Lee, Doyoung Kim, and Hyungjun Kim
Citation: Applied Physics Letters 98, 142113 (2011); doi: 10.1063/1.3577607
View online: http://dx.doi.org/10.1063/1.3577607
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/14?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Compositional tuning of atomic layer deposited MgZnO for thin film transistors
Appl. Phys. Lett. 105, 202109 (2014); 10.1063/1.4902389
Effect of Al concentration on Al-doped ZnO channels fabricated by atomic-layer deposition for top-gate oxide
thin-film transistor applications
J. Vac. Sci. Technol. B 32, 041202 (2014); 10.1116/1.4880823
Schottky barrier source-gated ZnO thin film transistors by low temperature atomic layer deposition
Appl. Phys. Lett. 103, 253503 (2013); 10.1063/1.4836955
Stable ZnO thin film transistors by fast open air atomic layer deposition
Appl. Phys. Lett. 92, 192101 (2008); 10.1063/1.2924768
High performance thin film transistor with low temperature atomic layer deposition nitrogen-doped ZnO
Appl. Phys. Lett. 91, 183517 (2007); 10.1063/1.2803219
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Tue, 23 Dec 2014 00:16:59

APPLIED PHYSICS LETTERS 98, 142113 ͑2011͒
Atomic layer deposition ZnO:N flexible thin film transistors and the effects
of bending on device properties
Jae-Min Kim,1 Taewook Nam,1 S. J. Lim,2 Y. G. Seol,3 N.-E. Lee,3 Doyoung Kim,1 and
Hyungjun Kim1,a͒
1School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-Gu,
Seoul 120-749, Republic of Korea
2Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea
3School of Advanced Materials Science and Engineering and Center for Advanced Plasma Surface
Technology, Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Republic of Korea
͑Received 9 December 2010; accepted 22 March 2011; published online 7 April 2011͒
ZnO:N flexible thin film transistors were fabricated by atomic layer deposition on polyethylene
naphthalate substrates and the effects of bending on the device properties investigated. The
threshold voltage and saturation mobility were observed to change with respect to the amount of
substrate bending. These modulations can be explained in terms of piezoelectric nature of in ZnO.
In comparison with the previously reported single crystal nanowires ZnO field effect transistors, the
amount of the electrical property modulation under bent condition is significantly reduced and our
report shows a much improved stability for ZnO:N as a flexible device material. © 2011 American
Institute of Physics. ͓doi:10.1063/1.3577607͔
Recently, ZnO based semiconductors have attracted
great interest as the channel layer in flexible displays.1,2 They
have high mobility and high transparency with low growth
temperature, which is a crucial prerequisite to be employed
for flexible/transparent applications. To date, oxide semicon-
ductor thin film transistors ͑TFTs͒ have been mostly depos-
ited by physical vapor deposition but having problems re-
lated with uniformity and resistivity control.3–5 In contrast,
atomic layer deposition ͑ALD͒ has great advantages such as
precise thickness controllability and large area uniformity at
relatively low growth temperatures.6 Such characteristics
make ALD very attractive for the TFT fabrication on large
area flexible substrate.
=125 °C using diethyl zinc as a precursor and diluted am-
monium hydroxide solution ͑0.01%͒ as a single reactant
source for oxygen and the nitrogen dopant. The channel area
͑width/length=40 ␮m/20 ␮m͒ was defined by standard
lithographic process, followed by wet etching with a diluted
HCl solution ͑HCl:H2O=1:40͒. Finally, a 100 nm thick Ti
source/drain layer was patterned by liftoff. The process tem-
perature through the entire device fabrication did not exceed
150 °C, which satisfies plastic substrate compatibility.
The bending equipment was a specially designed bend-
ing tool equipped with a charge coupled device camera to
measure the radius of curvature of the flexible device. Both
ends of the sample were clamped by the probing pins con-
nected to the parameter analyzer ͑HP 4156B͒ during mea-
surement to obtain transfer and output curves with respect to
the amount of bending. The corresponding strain values are
calculated according to the following equation, assuming
that the TFT layers are approximated by a single continuous
film covering on PEN backplane and the elastic modulus of
both PEN and ZnO channel layers have similar values,9
Regarding the reliability issues for flexible displays, the
devices are required to have strong stability under bending
conditions since the substrates are frequently subjected to
large tensile or compressive strain, which would cause sub-
stantial changes in the device properties. Oh et al.7 showed a
complementary inverter using n-channel sputtered ZnO and
p-channel pentacene channels on a polyethersulfone flexible
substrate. Besides, a single crystal ZnO nanowire field effect
transistor ͑FET͒ was fabricated on a flexible substrate by
Thickness of PEN + Thickness of TFT
Strain ͑%͒ =
.
2 ϫ Rc
Kwon et al.8 They reported that the threshold voltage ͑VTH
͒
͑1͒
and channel mobility was significantly changed by substrate
bending, strongly dependent on the direction of bending and
the radius of curvature. However, there has been no previous
report on the bending reliability of ALD ZnO thin film based
flexible TFT devices. In this letter, we fabricated ALD nitro-
gen doped ZnO flexible TFTs on polyethylene naphthalate
͑PEN͒ substrates and investigated the change in device prop-
erties with respect to the amount of substrate bending.
To begin with, 100 nm thick Ti was patterned for a gate
electrode. Subsequently, 130 nm thick ALD Al2O3 was de-
posited as a gate insulator using trimethyl aluminum and
water vapor at a growth temperature ͑Ts͒ of 150 °C. Then,
a 60 nm thick ZnO:N active layer was prepared at Ts
Positive and negative values indicate upward and downward
bending, respectively. We used nitrogen doped ZnO as an
active layer since the electrical properties of ALD ZnO TFTs
can be effectively controlled by in situ N doping.10 At first,
we conducted the electrical property measurements without
device bending to examine the feasibility of flexible TFT
͑FTFT͒ device operation on flexible PEN substrates. As a
result, typical FET behavior was observed from the output
and transfer curves showing source-drain current ͑IDS͒ satu-
ration by drain voltage ͑VD͒ sweep and IDS modulation by
gate voltage ͑VG͒ sweep with VTH=15.2 V as shown in
Figs. 1͑a͒ and 1͑b͒. The IOFF and ION/OFF are measured to be
30 pA and 8ϫ106, and subthreshold voltage swing and satu-
a͒
Electronic mail: hyungjun@yonsei.ac.kr.
ration mobility ͑␮ ͒ are 0.91 V/dec and 20.9 cm2/V s, re-
sat
0003-6951/2011/98͑14͒/142113/3/$30.00
98, 142113-1
© 2011 American Institute of Physics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Tue, 23 Dec 2014 00:16:59

142113-2
Kim et al.
Appl. Phys. Lett. 98, 142113 ͑2011͒
FIG. 2. The changes in ͑a͒ VTH and ͑b͒ ␮sat as a function of 1/Rc, and their
percentage variations.
FIG. 1. ͑a͒ and ͑b͒ represent the modulations of output and transfer curves
when the device is applied to external stress in condition of Ϯ1.2 cm−1 of
1/Rc with reference to unbending condition ͑1/Rc =0͒.
ZnO is a well-known piezoelectric material having the
highest piezoelectric tensor among the tetrahedral bonded
semiconductors.11 Deflection of a ZnO thin film creates a
relative displacement of Zn2+ cations with respect to O2−
anions in the wurzite structure, inducing a strong piezoelec-
tric potential field inside the channel area. He et al.12 re-
ported that positive potential is induced at the stretched sur-
face ͑␧Ͼ0͒ and negative potential is induced at the
compressed surface ͑␧Ͻ0͒ of ZnO material. Likewise, the
compressive strain ͑␧Ͻ0͒ on the region of channel layer
adjacent to gate insulator in ZnO TFT produces positive pi-
ezoelectric potential when the device is bent in an upward
direction, and vice versa. Thus, the formation of two poten-
tials of opposite signs, depending on the bending direction at
the electrical path in channel area, can be the main reason for
the opposite change in the electrical transport terms of VTH
spectively. This result indicates that the operation of TFT
fabricated by the current ALD processes is also extendable to
flexible plastic substrate.
Next, the bending effects on the FTFT characteristics
were investigated according to the amount of substrate bend-
ing. Since the path for electron current flow is along the
channel/bottom oxide interface due to the formation of the
lowest electron potential, the external stresses exerted to this
interfacial layer would have the greatest effect on the elec-
trical properties of TFT. It should be noted that upward bend-
ing induces a compressive strain at the bottom side of active
layer adjacent to the gate insulator, whereas a downward
bending gives rise to a tensile strain. Thus, the influences
caused by device deflection are expected to occur in an op-
posite way depending on whether the substrate is bent up-
ward or downward since the opposite kinds of strain are
applied on the current path. In Figs. 1͑a͒ and 1͑b͒, IDS in-
creases when the device was bent in an upward direction
under the same gate and drain voltages and vice versa. The
output and transfer curves under device bending between
Ϫ1.2 and +1.2 of 1/Rc shows a gradual change in IDS de-
pending on the amount of bending but only the extremes are
shown for clarity.
and ␮ as shown in Figs. 2͑a͒ and 2͑b͒.
sat
Figures 3͑a͒ and 3͑b͒ illustrate the piezoelectric effect on
the band diagram of ZnO:N channel/Al2O3 gate insulator/Ti
to explain the possible mechanism of the modulations on the
electrical transport behavior in flexible TFTs with respect to
substrate bending. As previously discussed, the compressed
side of the ZnO channel that interfaces with the gate insula-
tor is affected by the negative piezoelectric field when
bended upwards ͓Fig. 3͑a͔͒ even without any gate biasing. It
induces a lowering of the ZnO:N channel surface potential
barrier with downward modulation of energy band structure
leading to the negative shift of VTH since less VG is needed
to reach the channel accumulation layer for the formation of
an electrical current path. Whereas, a positive piezoelectric
field is generated at the ZnO channel/gate insulator interface
when bended downwards as shown in Fig. 3͑b͒, resulting in
potential barrier highering, leading to the positive shift of
Figures 2͑a͒ and 2͑b͒ represent the change in VTH and
as a function of 1/Rc, and their percentage changes ref-
␮
sat
erenced to the value of normal condition. The value of VTH’s
systematically decreased from downward to upward bending
in the range of Ϯ5%, when 1/Rc is adjusted from Ϫ1.2 to
1.2 cm−1. Meanwhile, the ␮sat values increased with upward
bending from around Ϫ2% to 2% change. These results in-
dicate that applied strain to the device causes systematic
change in the device properties.
V
TH. This explanation is in excellent accordance with the
change in electrical transport behavior measured in Fig. 2͑a͒.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Tue, 23 Dec 2014 00:16:59

142113-3
Kim et al.
Appl. Phys. Lett. 98, 142113 ͑2011͒
reported results on ZnO nanowire FETs,8 the current ALD
ZnO:N TFT have a much better stability against bending.
For example, under 1.1% substrate strain, the ⌬VTH and
⌬␮ ͑changes in the values by bending͒ of the current TFT
sat
devices are measured to be 0.5 V of and 0.2 cm2/V s, re-
spectively, which are much smaller than 4.2 V and
1450 cm2/V s for previously reported ZnO nanowire FET.
We attribute this mainly to the difference in microstructure.
While ZnO nanowires are single crystal, ALD ZnO:N thin
films are composed of nanocrystalline grains. We have pre-
viously shown that the incorporation of N in ZnO thin film
significantly reduces the crystallinity of the ͑002͒ oriented
crystal structure, resulting in the formation of a nanocrystal-
line or amorphous phase.10 The piezoelectric field in ZnO is
critically dependent upon the crystal orientation, showing the
strongest piezoelectric response along the c-axis ͑001͒. This
implies that the ͑001͒ oriented single crystalline ZnO has the
strongest piezoelectric field when subjected to strain. In con-
trast, the piezoelectric coefficient in a polycrystalline ZnO
thin film is degraded since the polar ͑001͒ orientation of each
crystalline grain exists with some spread of angles, which
does not align with the direction of the electric field.16 Thus,
an amorphous microstructure for the active layer is expected
to have a benefit in terms of device reliability under a bend-
ing environment.
FIG. 3. ZnO:N channel/Al2O3 gate insulator/Ti gate band diagrams to ex-
plain the results of modulation of electrical transport behaviors when ͑a͒
upward and ͑b͒ downward bending through the possible piezoelectricity
mechanism occurred in flexible TFTs in response to external stresses.
This research was supported by Future-based Technol-
ogy Development Program ͑Nano Fields͒ and Basic Re-
search Program through the National Research Foundation of
Korea ͑NRF͒ funded by the Ministry of Education, Science,
and Technology ͑Grant Nos. 2010-0020230 and 2010-
0024066͒.
The ␮ for ZnO TFTs can be calculated according to
sat
the following equation:13
1/2
2Lm2
dI
DS
1E. Fortunato, P. Barquinha, A. Pimentel, A. Gonçalves, A. Marques, L.
Pereira, and R. Martins, Thin Solid Films 487, 205 ͑2005͒.
2H. Hosono, Thin Solid Films 515, 6000 ͑2007͒.
3Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V.
Avrutin, S. J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 ͑2005͒.
4H. Ohsaki, Y. Tachibana, A. Mitsui, T. Kamiyama, and Y. Hayashi, Thin
Solid Films 392, 169 ͑2001͒.
␮
=
,
m =
,
͑2͒
sat
WCox
dVG
where L and W indicate channel length and width, respec-
tively. COX defines the capacitance of the alumina dielectric
layer with a dielectric constant of 8.8 and the value of which
remains a constant value of 8.2 nF/cm2 regardless of sub-
5P. F. Carcia, R. S. McLean, M. H. Reilly, and J. G. Nunes, Appl. Phys.
Lett. 82, 1117 ͑2003͒.
strate bending. It should be noted that the ␮ is directly
sat
related to the value of transconductance ͑gm=dIDS/dVG͒ ex-
tracted from the slope of IDS versus VG in the current satu-
rated region ͑VGϾ20 V͒ of the transfer curve ͓Fig. 1͑b͔͒.
The estimated gm for ZnO TFT under upward bending is
higher, whereas that for TFT under downward bending is
lower than the normal state. Therefore, the mobility changes
are mainly due to the variations in gm with respect to the
device deflection. This modulation tendency of the electrical
transport characteristics under the device bending conditions
corresponds to other reports on ZnO material based devices
such as ZnO nanowire FETs and flexible ZnO strain sensor
from other research groups.8,14 Also, similar reports on bend-
ing experiments considering flexible FETs but using penta-
cene ͑p-type͒ channel instead of oxide semiconductor have
shown a similar mobility change depending on the induced
substrate strain.15
6H. Kim, H. Lee, and W. Maeng, Thin Solid Films 517, 2563 ͑2009͒.
7M. S. Oh, W. Choi, K. Lee, D. K. Hwang, and S. Im, Appl. Phys. Lett. 93,
033510 ͑2008͒.
8S. Kwon, W. Hong, G. Jo, J. Maeng, T. Kim, S. Song, and T. Lee, Adv.
Mater. ͑Weinheim, Ger.͒ 20, 4557 ͑2008͒.
9K. Lee, J. Lee, H. Hwang, Z. Reitmeier, R. Davis, J. Rogers, and R.
Nuzzo, Small 1, 1164 ͑2005͒.
10S. J. Lim, J.-M. Kim, D. Kim, S. Kwon, J.-S. Park, and H. Kim, J. Elec-
trochem. Soc. 157, H214 ͑2010͒.
11A. Dal Corso, M. Posternak, R. Resta, and A. Baldereschi, Phys. Rev. B
50, 10715 ͑1994͒.
12J. He, C. Hsin, J. Liu, L. Chen, and Z. Wang, Adv. Mater. ͑Weinheim,
Ger.͒ 19, 781 ͑2007͒.
13D. Neamen, Semiconductor Physics and Devices ͑McGraw-Hill, New
York, 1997͒.
14J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao, and Z. Wang,
Nano Lett. 8, 3035 ͑2008͒.
15T. Sekitani, Y. Kato, S. Iba, H. Shinaoka, T. Someya, T. Sakurai, and S.
Takagi, Appl. Phys. Lett. 86, 073511 ͑2005͒.
Although the modulations of IDS, VTH, and ␮ caused
sat
16J. Gardeniers, Z. Rittersma, and G. Burger, J. Appl. Phys. 83, 7844
͑1998͒.
by bending in our experiment are similar to the previously
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
129.105.215.146 On: Tue, 23 Dec 2014 00:16:59