Application of pulsed laser deposited zinc oxide films to thin film transistor device
Introduction
Recently, thin films of transparent conductive oxide (TCO) have been of great interest for the development of opto-electronic components, flat panel displays and solar cells. Among them, indium tin oxide (ITO) is currently the most prevalent for all of these applications. ITO is, however, chemically unstable, expensive and a scarce raw material. Therefore, an alternative TCO which overcomes the known problems of the ITO has been intensively studied. With regard to this aspect, zinc oxide (ZnO) is one of the most promising candidates [1].
ZnO thin films can be prepared by various techniques: magnetron sputtering [2], spray pyrolysis [3], pulsed laser deposition (PLD) [4], molecular beam epitaxy [5] and sol–gel process [6]. The reported ZnO thin films are highly transparent (λ = 400–700 nm), c-axis oriented [7], [8]. Since ZnO has a wide direct band gap of 3.35 eV, a thin film transistor (TFT) device using ZnO as an active channel would not degrade on exposure to visible light. On the contrary, active channel of currently prevalent amorphous Si (a-Si) and poly-Si TFTs should be shielded in order to avoid extra photo carriers induced by visible light. Moreover, brightness of single pixels decreases due to opaque a-Si and poly-Si, which is not the case of ZnO based TFT.
In this work, ZnO films were deposited on Si3N4/c-Si using a PLD technique in order to prepare a bottom-gate type TFT device with transparent active channel layer. Electrical, optical and structural properties of PLD ZnO films were investigated. Effect of PLD preparation conditions on the TFT properties was discussed.
Section snippets
Pulsed laser deposition of ZnO and preparation of a test TFT
Quartz glasses, sapphires, glasses, and p-type Si (100) wafers were used as substrates for the sample preparation for structural, optical and electrical characterization of ZnO thin films. In order to evaluate the ZnO thin films as an active channel layer of TFT, a bottom gate type test TFT structures were fabricated using an n-type (111) crystalline Si (c-Si, 0.01–1 Ω cm) wafers for metal-like contact: silicon nitride (100 nm) prepared by low pressure chemical vapor deposition as a gate
Results and discussion
Optical transmission of the pulsed laser deposited ZnO films observed by UV visible photometer is shown in Fig. 2. Table 1 shows optical band gap (Eg) of the films. Except for the film prepared at the oxygen pressure (PO2) of 26.68 Pa, all the ZnO films show optical transmission of over 80% (Fig. 2). The ZnO films showed optical band gap of around 3.25 eV which is somewhat lower than that of bulk ZnO (3.35 eV) but still much higher than those of other transparent oxides. Considering the
Summaries and conclusion
It has been confirmed that pulsed laser deposited n-type transparent ZnO can be applied to TFT as an active channel layer for practical applications. Dependent on the PLD preparation conditions, field effect mobility (μFE) and on/off ratio (Ron/off) of the PLD ZnO-TFT ranged from 2.4 to 12.85 cm2/V s and from 2 order to 6 order, respectively. The best TFT performance (μFE = 12.85 cm2/V s, 6 order Ron/off) in this report was obtained on the ZnO film prepared at PO2 = 26.68 Pa and Ts = 400 °C. Based on
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