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 Ω-cm by varying the power during deposition. The corresponding electron mobilities were 4-18 cm2.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 fortransparent thin film transistor applications.

Abstract

Good and controllable electrical conductivity Wide transmittance window(400-2500 nm) Higher chemical etching rate in comparison with indium tin oxide (ITO) Large work function Low deposition temperature

Indium Zinc Oxide (IZO)

• Transparent conducting oxides extensively studied for transparent electrodes in optoelectronic devices– Application to solar cells, solar heat collectors, gas sensors, liquid-crystal displays, photodetectors, and light-emitting diodes

• 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

Motivations to TCOs

IZO layers deposited on glass substrate by rf -magnetron sputtering 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

Experimental

Results

Conclusions

Wantae Lim : plasma95@ufl.edu

Indium Zinc Oxide Thin Films Deposited by Sputtering at Room Temperature

1Dept. of Materials Science and Engineering, 2Dept of Chemical Engineering, 3Dept. of Physics, University of Florida, Gainesville, Florida, USA4Electronics Division, US Army research Office, Research Triangle Park, North Carolina

W. T. Lim1, Y. L. Wang1, F. Ren2, D. P. Norton1, I. I Kravchenko3 and J. M. Zavada4 and Stephen J. Pearton1

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

XRD θ-2θ scans of IZO films deposited at room temperature with different powers

IZO films were amorphous over the entire set of deposition conditionsTransmittance > 70%

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

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

XRD θ-2θ scans of IZO films deposited at room temperature with different pressure

All films were amorphous

IDS and gm as a function of VGS for a device with 12.5 nm SiNX gate

E-mode with a threshold votage of ~2VMaximum transconductance ~ 6mS/mmOn-Off ratio >105

µFE ~ 12.5 cm2/V.s (Hall mobility ~ 15 cm2/V.s)

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.

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.

Acknowledgement