Stable ZnO thin film transistors by fast open air atomic layer deposition

Article abstract of DOI:10.1063/1.2924768

We report stable, high performance zinc oxide thin film transistors grown by an atmospheric pressure atomic layer deposition system. With all deposition and processing steps kept at or below 200 °C, the alumina gate dielectric shows low leakage (below 10-8 A cm2) and high breakdown fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 108, near zero turn-on voltage, little or no hysteresis, and mobility greater than 10 cm2 V s. With alumina passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in the literature.

Full text of DOI:10.1063/1.2924768

Stable ZnO thin film transistors by fast open air atomic layer deposition
David H. Levy, Diane Freeman, Shelby F. Nelson, Peter J. Cowdery-Corvan, and Lyn M. Irving
Citation: Applied Physics Letters 92, 192101 (2008); doi: 10.1063/1.2924768
View online: http://dx.doi.org/10.1063/1.2924768
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APPLIED PHYSICS LETTERS 92, 192101 ͑2008͒
Stable ZnO thin film transistors by fast open air atomic layer deposition
a͒
David H. Levy, Diane Freeman, Shelby F. Nelson, Peter J. Cowdery-Corvan, and
Lyn M. Irving
Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-2102, USA
͑
Received 2 April 2008; accepted 17 April 2008; published online 12 May 2008͒
We report stable, high performance zinc oxide thin film transistors grown by an atmospheric
pressure atomic layer deposition system. With all deposition and processing steps kept at or below
2
2
00 °C, the alumina gate dielectric shows low leakage ͑below 10−8 A/cm ͒ and high breakdown
8
fields. Zinc oxide thin film transistors in a bottom gate geometry yield on/off ratios above 10 , near
zero turn-on voltage, little or no hysteresis, and mobility greater than 10 cm /V s. With alumina
passivation, shifts in threshold voltage under gate bias stress compare favorably to those reported in
2
the literature. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2924768͔
Oxide-based thin film transistors ͑TFTs͒ have emerged
the substrate see the alternating ALD gas cycles. We oscillate
as a promising technology, particularly for display appli-
the sample to produce alternating gas cycles to achieve the
desired layer thickness. In situations where speed of deposi-
tion is critical, a larger deposition head could enable a single
pass of the substrate to deposit the full thickness. Our current
laboratory transport rates would give an impressively fast
substrate throughput of 3 m/min, which is distinct from the
batch-processing times normally associated with ALD.
To compare with conventional ALD, the film growth
1
cations. ZnO-based devices have been produced by sput-
2
,3
4
tering,
pulsed laser deposition, solution7 process
5
,6
methods, and atomic layer deposition ͑ALD͒. Of these
methods, only the solution methods can be done in an open
atmosphere, although the resulting devices typically exhibit
low mobility. Following initial interest in ZnO semiconduc-
tors, some groups have turned to ternary and quaternary zinc
compounds to ensure an amorphous semiconductor for per-
saturation behavior of Al O was studied. Films were grown
2
3
8
–10
on bare silicon wafers at substrate speeds corresponding
to channel residence times of 37, 56, 112, and 448 ms.
formance and stability.
In this work, we describe ZnO TFTs with device charac-
teristics comparable to the best in the literature and with
improved stability, grown by an open atmospheric pressure
system. This growth system uses a fast ALD method that
could change the manufacturing methods and economics for
oxide-based electronics. While potentially all the electrical
layers in a TFT could be grown by the same system, this
paper focuses on devices with the gate dielectric and semi-
conductor layer deposited by this system.
The oxidizer channels ͑A͒ contained 0.31 mol % water,
3
0
.89 mol % oxygen, and 3.1ϫ10− mol % H O , while the
2
2
metal source channels ͑B͒ contained 0.062 mol % trimethyl-
aluminum. From film thickness measured by ellipsometry as
Atomic layer depositions are typically done in an en-
closed system, where the reactive gases are sequentially
cycled through the chamber in time. The alternative used
here is an ALD process employing spatial isolation of reac-
tive gases ͑spatial ALD͒. In this process, gases are isolated to
regions of the coating head, and the cyclic ALD process
results from movement of these regions over the substrate.
Figure 1͑a͒ depicts the stainless steel ALD coating head po-
sitioned approximately 30 ␮m above a substrate. The head
contains recessed channels facing the substrate that are
closed at one end ͑not shown͒, and gas introduced near the
closed end flows along its channel as shown in the figure. An
inert gas ͑I͒ and two reactive gas streams ͑A: oxidizer gas͒
and ͑B: metal source͒ flow in these channels, as shown by
the designations in Fig. 1͑a͒. In our case, the inert gas is
nitrogen, which also acts as a carrier gas for the metal and
oxidizer precursors. The channel shape and proximity to the
substrate prevent mixing between neighboring channels and
exclude the surrounding air.
FIG. 1. ͑Color online͒ ͑a͒ Schematic of the ALD coating head showing the
gas channels and gas flow. The channels are 0.7 mm wide with a spacing of
1.4 mm. The coating width W is approximately 50 mm. A is the oxidizing
reactant, B is the metal precursor, and I is nitrogen. ͑b͒ Al2O3 film thickness
for 250 ALD cycles as a function of residence time. The squares represent
data from the spatial ALD system operated at 177 °C. The circles represent
data extracted from Ref. 11.
If the head is stationary, no deposition occurs because
each part of the substrate is contacted by only one gas. How-
ever, as the substrate moves relative to the head, regions of
a͒Electronic mail: david.levy@kodak.com.
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003-6951/2008/92͑19͒/192101/3/$23.00 92, 192101-1 © 2008 American Institute of Physics
On: Sun, 23 Nov 2014 04:59:13
0

192101-2
Levy et al.
Appl. Phys. Lett. 92, 192101 ͑2008͒
FIG. 3. ͑Color online͒ XRD of 330-nm-thick ZnO layer with inset of 60 nm
ZnO. For the thicker sample, the primary peak is found at ͑100͒, while in
thinner samples, the ͑002͒ peak becomes equally intense.
FIG. 2. ͑Color online͒ Leakage current as a function of applied electric field
for Al O films grown at several temperatures. The 200 and 250 °C films
2
3
show low leakage and are distinctly better than the 150 °C film.
͑
V =0.25 V͒ and in saturation ͑V =20 V͒. There is no vis-
d d
a function of residence time, we find that the monolayer
ible hysteresis. The turn-on voltage of this device is −0.5 V,
with saturation threshold voltage of 4.8 V. The saturation
mobility, extracted by a fit to the square root of drain current
11
saturation behavior is comparable to chamber-based ALD,
with a maximum growth rate of approximately 1.2 Å/cycle
Fig. 1͑b͔͒.
Current leakage and dielectric breakdown were mea-
sured for Al O films deposited on chromium at 150,
͓
versus gate voltage from the Vd=20 V, sweep is
1
0.25 V curve is 12.5 cm /V s. The close match between
linear and saturation values indicates that excellent contact to
these devices is possible with a simple evaporation process.
The highest mobility we have reproducibly seen is about
20 cm /V s. Mobility of thin film epilayers of ZnO ͑Ref. 15͒
is typically below 100 cm /V s, which can be taken as an
upper limit for these polycrystalline layers. In addition to the
dc performance mentioned above, devices produced by this
method exhibit the highest frequencies to date for oxide
based ring oscillator circuits.
Changes in threshold voltage with bias stress were mea-
sured. Gate bias was held at 20 V during stress, with inter-
2
2.9 cm /V s, while the linear mobility extracted from the
2
2
3
d
2
1
9
00, and 250 °C, with thicknesses of 987, 1243, and
229 Å, respectively. Aluminum metal contacts measuring
0ϫ130 ␮m were deposited by shadow mask evaporation
onto the Al O surface to form test structures.
2
2
3
Figure 2 shows a plot of applied electric field versus
area-corrected leakage current for these devices. The leakage
2
−
9
−9
density at a field of 1 MV/cm is 5.4ϫ10 , 1.89ϫ10 , and
−
9
2
2
.78ϫ10 A/cm for the 150, 200, and 250 °C samples,
respectively. Furthermore, these devices always show break-
down voltages in excess of 5 MV/cm. The leakage currents
are within an order of magnitude of the best of those sum-
marized in Ref. 12 and also compare favorably to ALD alu-
16
1
3
mina films grown as gate dielectrics for zinc oxide TFTs.
The dielectric constant of the alumina was determined from
capacitance-voltage measurements to be between 7.5 and
8
.5, within the range of published values for Al O deposited
2 3
1
2
by ALD.
The TFT devices were made on a glass substrate coated
with chromium or ITO as the gate contact. An Al O film of
2
3
approximately 1100 Å was deposited at 200 °C, followed by
a 250 Å zinc oxide layer produced using 21 mol % NH3,
0.64 mol % water, and 0.74 mol % oxygen in the A channels,
and a 0.52 mol % diethylzinc in the B channels. The micro-
crystalline nature of the ZnO layer has been determined by
cross-sectional transmission electron microscopy ͑TEM͒.
Lattice images ͑not shown͒ of the ZnO layer show a variety
of orientations of sub-100-nm-sized grains. X-ray diffraction
͑
XRD͒ from a 330-nm-thick ZnO ͑Fig. 3͒ shows primarily
͑
100͒ orientation, while XRD from thinner layers show about
equal intensity in the ͑100͒ and ͑002͒ peaks ͑Fig. 3 inset, for
a 60-nm-thick ZnO layer.͒ Grazing incidence XRD confirms
a similar structure in the thin ͑ϳ25 nm͒ layers used for
TFTs. This suggests that ͑100͒ is the preferred orientation for
1
4
this deposition method, although interactions with the sur-
face influence the initial grain orientation.
For TFTs, top aluminum or titanium/gold contacts were
patterned by lift off, and devices were isolated using photo-
lithographically patterned poly͑methyl methacrylate͒ to pro-
tect the channel. Figure 4͑a͒ shows the double-sweep transfer
FIG. 4. ͑Color online͒ ͑a͒ Double sweep transfer characteristics of a zinc
oxide TFT showing no hysteresis at V =0.25 V and V =20 V. Linear and
d
d
2
saturation mobility are 12.5 and 12.9 cm /V s respectively. ͑b͒ Threshold
voltage shift under gate bias stress of 20 V for an unpassivated TFT, and one
passivated with 100 nm Al O . The change in mobility over the 16 h is less
2
3
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characteristics of a typical ZnO TFT in the linear regime
than 5% for both samples.
On: Sun, 23 Nov 2014 04:59:13

192101-3
Levy et al.
Appl. Phys. Lett. 92, 192101 ͑2008͒
F. Gonçalves, A. J. S. Marques, R. F. P. Martins, and L. M. N. Pereira,
Appl. Phys. Lett. 85, 2541 ͑2004͒.
ruptions every 400 s for a linear region I -V sweep. With no
d
g
passivation of the ZnO, the threshold voltage shift is 2.4 V in
6 h. However, a passivation layer of 100 nm of Al O , de-
4
Y. J. Li, Y. W. Kwon, M. Jones, Y. W. Heo, J. Zhou, S. C. Luo, P. H.
Holloway, E. Douglas, D. P. Norton, Z. Park, and S. Li, Semicond. Sci.
Technol. 20, 720 ͑2005͒.
1
2
3
posited using the spatial ALD process, dramatically im-
proves the bias stability. Passivated devices show positive
threshold voltage shifts of 0.6 V for 16 h stress ͓Fig. 4͑b͔͒,
and less than 5% change in mobility. These shifts are some
of the lowest reported for ZnO based TFTs and compare
favorably with values obtained from amorphous multication
5
6
7
D. Levy, L. Irving, and A. Childs, SID Int. Symp. Digest Tech. Papers 38,
2
30 ͑2007͒.
B. J. Norris, J. Anderson, J. F. Wager, and D. A. Keszler, J. Phys. D 36,
L105 ͑2003͒.
E. H. Kim, C. Y. Kim, D. H. Lee, B. H. Chung, J. H. Jeon, H. S. Kim,
J. W. Hyun, Y. Kim, and S. J. Noh, MRS Proceedings, Fall, 2006
͑unpublished͒.
K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono,
Nature ͑London͒ 432, 488 ͑2004͒.
H. Q. Chiang, J. F. Wager, R. L. Hoffman, J. Jeong, and D. A. Keszler,
Appl. Phys. Lett. 86, 013503 ͑2004͒.
H.-N. Lee, J. Kyung, S. K. Kang, D. Y. Kim, M.-C. Sung, S.-J. Kim, C. N.
Kim, H. G. Kim, and S.-T. Kim, SID Int. Symp. Digest Tech. Papers 38,
1
7,18
systems
while maintaining the simplicity of a pure ZnO
8
9
0
semiconductor. Further work to understand the origin of the
shifts is underway.
In summary, we have demonstrated promising ZnO TFTs
grown in an open air ALD deposition system. The alumina
dielectric layer produced has comparable performance to
known dielectrics produced by ALD, while resulting ZnO
TFTs exhibit excellent mobility and stability characteristics.
We believe that this deposition system will be adaptable to
very large format, continuous, high throughput processing.
1
1826 ͑2007͒.
11
A. W. Ott, J. W. Klaus, J. M. Johnson, and S. M. George, Thin Solid Films
92, 135 ͑1997͒.
2
12
M. D. Groner, J. W. Elam, F. H. Fabreguette, and S. M. George, Thin
Solid Films 413, 186 ͑2002͒.
P. F. Carcia, R. S. McLean, and M. H. Reilly, Appl. Phys. Lett. 88,
123509 ͑2006͒.
This is not always the case for ALD ZnO. See S. J. Lim, S.-J. Kwon, H.
Kim, and J.-S. Park, Appl. Phys. Lett. 91, 183517 ͑2007͒.
Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doan, V.
Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 ͑2005͒.
J. Sun, D. A. Mourey, D. Zhao, S. K. Park, S. F. Nelson, D. H. Levy, D.
Freeman, P. Cowdery-Corvan, L. Tutt, and T. N. Jackson, “ZnO thin film
transistor ring oscillators with 31 nsec propagation delay,” IEEE Electron
Device Lett. ͑in press͒.
1
1
1
3
4
5
The authors thank Roger Kerr, Lee Tutt, Thomas Pawlik,
and Professor T. N. Jackson and his students for valuable
conversations, Jeff Carey for technical assistance, Tom
Blanton and Craig Barnes for XRD, and John Minter and
S. Stoker for TEM.
16
1
P. Görrn, M. Sander, J. Meyer, M. Kröger, E. Becker, H.-H. Johannes, W.
Kowalsky, and T. Riedl, Adv. Mater. ͑Weinheim, Ger.͒ 18, 738 ͑2006͒.
R. L. Hoffman, B. J. Norris, and J. F. Wager, Appl. Phys. Lett. 82, 733
2
17
P. Görrn, P. Hölzer, T. Riedl, W. Kowalski, J. Wang, T. Weimann, P. Hinze,
and S. Kipp, Appl. Phys. Lett. 90, 063502 ͑2007͒.
A. Suresh and J. F. Muth, Appl. Phys. Lett. 92, 033502 ͑2008͒.
3͑2003͒.
18
E. M. C. Fortunato, P. M. C. Barquinha, A. C. M. B. G. Pimentel, A. M.
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