Characteristics of sputtered Al-doped ZnO films for transparent electrodes of organic thin-film transistor
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).
References (20)
- et al.
Org. Electron.
(2008) - et al.
Thin Solid Films
(2010) - et al.
Org. Electron.
(2008) - et al.
Thin Solid Films
(2008) - et al.
Sol. Energy Mater. Sol. Cells
(2005) - et al.
Synth. Met.
(2000) - et al.
Ceram. Int.
(2003) Appl. Surf. Sci.
(2008)- et al.
Thin Solid Films
(2007) - et al.
Thin Solid Films
(2007)
Cited by (23)
Bottom-contact organic thin film transistors with transparent Ga-doped ZnO source-drain electrodes
2019, Materials Science in Semiconductor ProcessingCitation Excerpt :The disadvantages of ITO are rare source in nature and expensive [6]. As an alternative, ZnO-based TCLs have been increasing studied for their advantages of abundant resources, similar optical and electrical properties, and better thermal stability [6,7]. However, ZnO-based TCLs are confronted with several difficulties in the application of S-D electrodes in OTFTs.
Nonthermal crystal bridging of ZnO nanoparticles by nonequilibrium excitation reaction of electrons and plasma without cross-linking agent on plastic substrate
2019, Journal of Alloys and CompoundsCitation Excerpt :However, methods of fabricating such conductive oxide thin films with excellent electrical properties have not yet been established. To fabricate ZnO thin films, various methods, including chemical vapor deposition (CVD) [1–6], atomic layer deposition (ALD) [7–11], and sputtering [12–19], have been attempted, as well as liquid-phase methods for synthesizing ZnO nanowires [20–22] and nanoparticles [23–29]. To use ZnO thin films as highly functional thin films that can process a huge amount of data while maintaining their flexibility, a breakthrough technology for forming conductive oxide thin films with stable conductivity on plastic films is required.
Improved efficiency of aluminum doping in ZnO thin films grown by atomic layer deposition
2014, Applied Surface ScienceCitation Excerpt :Aluminum (Al) is one of the common dopants, which also include gallium, indium, and boron. Pulsed laser deposition (PLD) and sputtering methods have successfully fabricated Al-doped ZnO (AZO) thin films exhibiting resistivity down to ∼7 × 10−4 Ω cm due to increased carrier concentrations up to ∼1021 cm−3 [6–8]. As the controlled synthesis of various nanostructures is achieved, demand rapidly grows for the formation of ultrathin metal-oxides for various applications.
In situ modification of low-cost Cu electrodes for high-performance low-voltage pentacene thin film transistors (TFTs)
2013, Organic ElectronicsCitation Excerpt :At the same time, as discussed previously, the interface dipole is suppressed. In addition to the interfacial properties of S/D electrodes and organic semiconductor, the electric conductivity of the electrode itself is another concern which can also influence the performance of OTFTs drastically [54,55]. One major drawback associated with the use of Cu as S/D electrodes is the inevitable oxidation of Cu by oxygen species and water in air, as evidenced from previous XPS results shown in Fig. 4b and c.
Modification of Indium Tin Oxide Electrodes by Fluorinated Silanes for Transparent Organic Thin-Film Transistors
2024, IEEE Electron Device Letters