Comparative Characteristics of High Performance ZnO Thin Film Transistors
Ashwani Kumar, Premlal B. Pillai, and Maria Merlyne De Souza
Introduction:
Amorphous In-Ga-Zn-O (a-IGZO) based TFTs have received considerable attention due to their
high mobility arising from delocalised s-orbitals of heavy metal cations which form a large dispersed conduction
band with small electron effective mass. In contrast, less attention is paid to cheaper and simpler polycrystalline
binary transparent conductive oxides (TCOs) such as ZnO, In
2
O
3
, and SnO
2
which resulted in poor performance
in earlier attempts.
Methodology:
High performance ZnO TFTs on glass with Indium Tin oxide (ITO) as a bottom gate and
Tantalum Oxide (Ta
2
O
5
) as gate insulator are fabricated. The band tail distribution of the DOS is extracted by
employing the MN rule
1
using temperature dependent transfer characteristics:
=
0
⋅ exp(− / )
(1)
0
=
00
⋅ exp( ⋅ )
(2)
where is the activation energy,
0
and
00
are prefactors, and A is the MN parameter. The MN rule is an
intrinsic property that is applicable when the Fermi level position is varied by an applied external electric field.
For evaluating the field-dependent parameters, such as charge density, band bending, and the distribution of tail
and deep state densities, the method of Schropp
et.al
2
based upon a coupled solution of MN analysis and a one
dimensional Poisson equation is adopted.
Results:
As shown in
a), the transfer characteristics show a comparable performance with the reported
results for a-IGZO
3,4
and poly-ZnO
5
TFTs, with a high on-off current ratio in excess of
10
6
, at
of
0.1
, a
positive threshold of
1.6
and a subthreshold swing (S) of
~0.15 /
. The upper limit of interface state
density (
) is calculated from Eqn.
ignoring the contribution of the bulk state density ( ):
= ln(10) ⋅
. [1 + (√ ⋅
+ ⋅
)]
(3)
is estimated to be
1.55 × 10
12 −2 −1
, indicating a high quality interface in ZnO/Ta
2
O
5
. In
(b), the temperature dependent measurements are carried out at
= 10
for a temperature range
of
277.0 − 352.5
. The tail state density of states (DOS) for polycrystalline ZnO is found to be in the range of
1.3 × 10
20 −3 −1
which
is
consistent
with
the
extracted
tail
density
of
1.3 × 10
19
− 1.55 × 10
20
−3 −1
for a-IGZO
6,7
, and
~2.5 × 10
20 −3 −1
in the case of ZnO
8,9
.
Fig. 1: (a) Comparison of transfer characteristics with the reported results for a-IGZO
3,4
and poly-
ZnO
5
TFTs, (b) temperature dependent
−
measurements for MN analysis and DOS extraction.
Conclusion:
High performance ZnO based thin film transistors (TFTs) are demonstrated with an on-off
current ratio in excess of
10
6
, subthreshold swing of
0.15 /
, comparable to high performance a-IGZO
TFTs
3,4
. These results demonstrate the potential of simpler binary system polycrystalline TCOs such as ZnO to
be suitably matched to appropriate gate insulators to provide a cheaper substitute for a-IGZO in many TFT
based applications.
References
1
W. Meyer and H. Neldel, Zeitschrift Fur Tech. Phys.
18
, 588 (1937).
2
R.E.I. Schropp, J. Snijder, and J.F. Verwey, J. Appl.
Phys.
60
, 643 (1986).
3
K. Nomura, T. Kamiya, and H. Hosono, Appl. Phys. Lett.
99
, 10 (2011).
4
G. Yu, C. Shieh, J. Musolf,
F. Foong, T. Xiao, G. Wang, and K. Ottosson, Int. Electron Devices Meet. 2014 26.3.1 (2014).
5
C. Brox-Nilsen, J. Jin, Y.
Luo, P. Bao, and A.M. Song, IEEE Trans. Electron Devices
60
, 3424 (2013).
6
K. Takechi, M. Nakata, T. Eguchi, H.
Yamaguchi, and S. Kaneko, Jpn. J. Appl. Phys.
48
, (2009).
7
T.C. Fung, C.S. Chuang, C. Chen, K. Abe, R. Cottle, M.
Townsend, H. Kumomi, and J. Kanicki, J. Appl. Phys.
106
, (2009).
8
F. Torricelli, E.C.P. Smits, J.R. Meijboom, A.K.
Tripathi, G.H. Gelinck, L. Colalongo, Z.M. Kovács-Vajna, D.M. De Leeuw, and E. Cantatore, IEEE Trans. Electron Devices
58
, 3025 (2011).
9
S. Lee, A. Nathan, Y. Ye, Y. Guo, and J. Robertson, Sci. Rep.
5
, 13467 (2015).
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