Elsevier

Thin Solid Films

Volume 519, Issue 22, 1 September 2011, Pages 8018-8022
Thin Solid Films

Characteristics of sputtered Al-doped ZnO films for transparent electrodes of organic thin-film transistor

https://doi.org/10.1016/j.tsf.2011.06.005Get rights and content

Abstract

Aluminum-doped ZnO (AZO) thin-films were deposited with various RF powers at room temperature by radio frequency (RF) magnetron sputtering method. The electrical properties of the AZO film were improved with the increasing RF power. These results can be explained by the improvement of the crystallinity in the AZO film. We fabricated the organic thin-film transistor (OTFT) of the bottom gate structure using pentacene active and poly-4-vinyl phenol gate dielectric layers on the indium tin oxide gate electrode, and estimated the device properties of the OTFTs including drain current–drain voltage (ID–VD), drain current–gate voltage (ID–VG), threshold voltage (VT), on/off ratio and field effect mobility. The AZO film that grown at 160 W RF power exhibited low resistivity (1.54 × 10 3 Ω·cm), high crystallinity and uniform surface morphology. The pentacene thin-film transistor using the AZO film that's fabricated at 160 W RF power exhibited good device performance such as the mobility of 0.94 cm2/V s and the on/off ratio of ~ 105. Consequently, the performance of the OTFT such as larger field-effect carrier mobility was determined the conductivity of the AZO source/drain (S/D) electrode. AZO films prepared at room temperature by the sputtering method are suitable for the S/D electrodes in the OTFTs.

Introduction

Organic thin film transistors (OTFTs) have attracted much attention and they have been utilized for driving the circuits of active matrix displays, sensors, radio frequency identification tags and smart cards because of their many advantages for flexible electronics such as low cost, low temperature processibility and flexibility compared with the conventional Si-based technology. Pentacene (C22H14) and poly-4-vinyl phenol (PVP) have recently been used as active layers and as insulator layers of the OTFT to fabricate OTFT devices. Most of the research groups have been studied on improving the electrical properties of the source/drain electrodes, semiconductor, and gate insulator materials for OTFTs, however many factors are still lack for understanding. Many materials such as indium tin oxide (ITO) [1], Au [2] and Mo [3] have been studied as an electrode of electronic devices. Specially, the ITO has been widely used as transparent conducting oxide (TCO) in many devices such as thin film transistor [4], sensor [5], solar cell [6], and organic emitting diode [7] because of its high transmittance in visible regions and because its resistivity is close to 1 × 10 4 Ω·cm. However, ITO is an expensive TCO material because indium and tin that are used in ITO are expensive and rare elements. So we proposed the use of AZO thin film as electrodes in OTFTs for replacing the ITO electrodes. Zinc oxide (ZnO)-based materials have recently attracted much interest as a promising alternative to ITO because of those materials' several advantages such as scarcity of indium, low cost for large scale applications and because they improve the thermal and chemical stabilities.

In this study, aluminum-doped ZnO (AZO) thin films were deposited on glass substrate at the room temperature using the radio frequency (RF) magnetron sputtering method. To use the electrode in OTFT devices, the effect of the RF power on the crystalline structural, electrical and optical properties of the AZO films was demonstrated and discussed. Also, we fabricated the pentacene OTFT using AZO source/drain electrodes and estimated the device performance of the OTFT using AZO electrodes prepared at various conditions such as drain current–drain voltage (ID–VD), drain current–gate voltage (ID–VG), threshold voltage (VT), the on/off ratio and field effect mobility.

Section snippets

Experimental details

The AZO films were deposited at room temperature by the RF magnetron sputtering method with AZO target (99.999%, ZnO:Al2O3 = 98:2 wt.%, LTS chem.) pressed on a copper saucer with a diameter of 4 in. Prior to the AZO film deposition, the process chamber was pumped down to a base pressure of 0.13 mPa, by using a turbomolecular, before the plasma generation was activated by the RF power at 13.56 MHz. The flow rate and working pressure of Ar (99.999%) were fixed at 100 sccm and 0.67 Pa, respectively.

Results and discussion

Fig. 1 exhibits the surface images of AZO films prepared at the RF power of 100 W and 160 W, and the variation of the rms surface roughness and the growth rate of the AZO films deposited at room temperature as a function of the RF power. It was shown that the growth rate increased from 7 to 19.5 nm/min and the rms surface roughness of the AZO film was increased from 2.13 to 2.7 nm with increasing RF power. The increase of the sputtering rate with increasing RF power is attributed to the increase of

Conclusion

In summary, we proposed an aluminum doped zinc oxide (AZO) electrode prepared at room temperature by RF sputtering method using a commercial ceramic target with a mixture of ZnO and Al2O3 (2 wt.%) as a source/drain electrodes for organic thin film transistors (OTFTs). We have investigated the influence of the RF power on the structural, electrical and optical properties of the AZO films, and the device performance in the OTFTs using AZO electrode fabricated at various RF powers. As a result, the

Acknowledgments

This work was supported by a grant from the Kyung Hee University in 2010. (KHU-20100183).

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