Transcript Indium Zinc Oxide Thin Films Deposited by Sputtering

Indium Zinc Oxide Thin Films Deposited by Sputtering
at Room Temperature
W. T.
1
Lim ,
Y. L.
1
Wang ,
F.
2
Ren , D.
1Dept.
P.
1
Norton ,
I. I
3
Kravchenko
2Dept
and J. M.
4
Zavada
and Stephen J.
1
Pearton
3Dept.
of Materials Science and Engineering,
of Chemical Engineering,
of Physics, University of Florida, Gainesville, Florida, USA
4Electronics Division, US Army research Office, Research Triangle Park, North Carolina
Abstract
The deposition of amorphous indium zinc oxide (IZO) thin films on glass
substrates with n-type carrier concentrations between 1014 and 3x1020
cm-3 by sputtering from single targets near room temperature was
investigated as a function of power and process pressure. The resistivity
of the films with In/Zn of~ 0.7 could be controlled between 5x10-3-104 Ωcm by varying the power during deposition. The corresponding electron
mobilities were 4-18 cm 2 .V-1 -s -1 .The surface root-mean-square
roughness was < 1nm under all conditions for film thicknesses of 200
nm. Thin film transistors with 1 µm gate length were fabricated on these
IZO layers, showing enhancement mode operation with good pitch-off
characteristics, threshold voltage 2.5V and a maximum
transconductance of 6 mS/mm. These films look promising for
transparent thin film transistor applications.
Experimental
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IZO layers deposited on glass substrate by rf -magnetron sputtering
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High purity In2O3(ZnO)k targets
Deposition temperature : RT
Working pressure : 3-18mTorr
Sputtering power : 50-200 W
Characterization of the IZO films by XRD, X-ray
Microprobe, Hall measurement and AFM
Effect of process pressure on the deposition rate of the IZO films, the resulting
In/Zn ratio in the films and the root-mean-square (RMS) roughness (left) and also
the carrier density, mobility and resistivity (right)
- carrier density was fairly constant at ~1020 cm-3.
- deposition rate was inversely dependent on pressure
Results
XRD θ-2θ scans of IZO films deposited at
room temperature with different pressure
Motivations to TCOs
• Transparent conducting oxides extensively studied for transparent
electrodes in optoelectronic devices
– Application to solar cells, solar heat collectors, gas sensors, liquidcrystal displays, photodetectors, and light-emitting diodes
All films were amorphous
Effect of IZO target power on the deposition rate of the IZO films, the resulting In/Zn
ratio in the films and the root-mean-square (RMS) roughness (left) and also the carrier
density, mobility and resistivity (right)
- Deposition rate depended on IZO target power
- RMS and In/Zn ratio were independent of sputtering power
• SnO2 doped In2O3 (ITO), the most commonly used TCO for transparent
electrodes
– Search for alternatives as the scarcity of In imparts a high cost to ITO
– Numerous studies on novel compound oxides composed of
combinations of In, Zn, Cd, Sn and Ga
• In2O3 : ZnO (IZO), a new alternative to ITO ?
– Absence of toxic cadmium, use of inexpensive zinc
– Larger work function, superior transmission in the 1-1.5 µm range
XRD θ-2θ scans of IZO films deposited at
room temperature with different powers
IZO films were amorphous over the entire
set of deposition conditions
Transmittance > 70%
IDS and gm as a function of VGS for a device with 12.5 nm SiNX gate
E-mode with a threshold votage of ~2V
Maximum transconductance ~ 6mS/mm
On-Off ratio >105
µFE ~ 12.5 cm2/V.s (Hall mobility ~ 15 cm2/V.s)
Conclusions
IZO films with conductivity controlled over a range of more than six
orders of magnitude have been deposited at room temperature on glass
substrates by sputtering from a single IZO target. The electron mobility
in the films is typically in the range 4-18 cm2/V-s. These films looks
promising for TFT applications on low cost substrates.
Indium Zinc Oxide (IZO)
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Good and controllable electrical conductivity
 Wide
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transmittance window(400-2500 nm)
Higher chemical etching rate in comparison with
indium tin oxide (ITO)
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Large work function
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Low deposition temperature
AFM scans on 200nm thick IZO films deposited at different powers
RMS roughnesses were all well below 1 nm
Advantage of amorphous films relative to poly or nanocrystalline films
Acknowledgement
The work is partially supported by DOE under grant DE-FC26-04NT42271
(Ryan Egidi), Army Research Office under grant no. DAAD19-01-1-0603 and
NSF (DMR 0400416, Dr. L. Hess).We thank MAIC staff for their help in the
performance of this work.
Wantae Lim : [email protected]