Fabrication and characterization of sol-gel-derived zinc oxide thin-film transistor
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Thin-film transistors (TFTs) with zinc oxide channel layers were fabricated through a simple and low-cost solution process. Precursor solution concentration, annealing temperature, and the process were controlled for the purpose of improving the electrical properties of ZnO TFTs and analyzed in terms of microstructural scope. The fabricated ZnO films show preferential orientation of the (002) plane, which contributes to enhanced electron conduction and a dense surface. The results show that the TFT characteristics of the film are clearly affected by the microstructure. The optimized TFT operates in a depletion mode, shows n-type semiconductor behavior, and is highly transparent (>90%) within the visible light range. It exhibits a channel mobility of 9.4 cm2/Vs, a subthreshold slope of 3.3 V/decade, and an on-to-off current ratio greater than 105. In addition, the result of N2 annealing shows the possibility of improvement in electrical property of the ZnO TFTs. I. INTRODUCTION
Transparent thin-film transistors (TTFTs) with oxide active channel layers are presently attracting strong interest, from both basic research and industrial fields, in relation to transparent electronics applications, such as flat-panel displays, flexible displays, radiofrequency identification tags, and smart windows.1–3 TTFTs are considered excellent candidates for the backplane of activematrix organic light-emitting diodes (AMOLEDs) and active-matrix liquid-crystal displays (AMLCDs), where Si-based materials, which suffer many problems involving stability and large area uniformity, are currently used.4 Metal oxide semiconductors offer many advantages such as transparency originating from their large band gap, high uniformity, environmental stability, and high mobility compared with conventional amorphous silicon and organic materials. Many transparent oxide semiconductors (TOSs), such as zinc oxide (ZnO), zinc–tin oxide, indium–zinc oxide, and indium–gallium–zinc oxide, have been reported for transparent channel layers in TTFTs.2,5–7 They exhibit mobility and an on-to-off current ratio in the range of 5 to 100 cm2/Vs and 106 to 107, respectively. Traditionally, deposition of TOSs relies on vacuum deposition processes such as RF-magnetron sputtering and pulsed laser deposition, which enable a low-temperature process (even at room temperature), but require expensive equipment and incur high fabrication cost. The sol-gel method, a representative solution-based thin-film deposition technique, offers many benefits over a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0103 J. Mater. Res., Vol. 25, No. 4, Apr 2010
vacuum-based processes: simplicity, high throughput, and low-cost.8–10 In addition, it enables direct patterning, which could replace conventional photolithographic techniques through ink-jet printing and a roll-to-roll process. Recently, several solution-processed oxide TFTs that showed mobility as high as 16 cm2/Vs and ink-jet printability with an on-to-off current ratio of 106 were r
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