Solution-processed Zinc Oxide Thin-film Transistors Research Articles

Photocurrent Characteristics of Zinc-Oxide Films Prepared by Using Sputtering and Spin-Coating Methods

The photocurrent characteristics of zinc-oxide (ZnO) thin-film transistors (TFTs) prepared using radio-frequency sputtering and spin-coating methods were investigated. Various characterization methods were used to compare the physical and the chemical properties of the sputtered and the spin-coated ZnO films. X-ray photoelectron spectroscopy was used to investigate the chemical composition and state of the ZnO films. The transmittance and the optical band gap were measured by using UV-vis spectrometry. The crystal structures of the prepared ZnO films were examined by using an X-ray diffractometer, and the surfaces of the films were investigated by using scanning electron microscopy. ZnO TFTs were prepared using both sputter and solution processes, both of which showed photocurrent characteristics when illuminated by light. The sputtered ZnO TFTs had a photoresponsivity of 3.08 mA/W under illumination with 405-nm light while the solution-processed ZnO TFTs had a photoresponsivity of 5.56 mA/W. This study provides useful information for the development of optoelectronics based on ZnO.

Influence of Gate Dielectric and Its Surface Treatment on Electrical Characteristics of Solution-Processed ZnO Transistors.

We report how interface treatments affect electrical performance, including subthreshold characteristics, in solution-processed transparent metal oxide thin-film transistors (TFTs) with SiO2 and SiNx gate dielectrics. Ultra-violet (UV) ozone treatment and O2 plasma treatment are carried out as a surface treatment of the interface between a spin-coated zinc oxide (ZnO) layer and a gate dielectric. With the SiO2 dielectric, UV ozone treatment dominantly affects subthreshold characteristics, while O2 plasma treatment produces enhanced mobility and lower threshold voltage shift. With the SiNx dielectric, every electrical parameter including mobility, threshold voltage shift, and subthreshold characteristics is enhanced by 02 plasma treatment. Our experimental results demonstrate that interface engineering treatments variously influence the electrical performance in solution-processed ZnO TFTs.

Analysis of Structural and Electrical Properties of Solution-Processed Zinc Oxide Films for Thin-Film Transistor Application

We investigated the electrical properties of solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) with varying the size of ZnO crystallites in the film. The TFTs having the bead-shaped ZnO crystallites with the average diameter of 2 μm apparently operated in depletion mode, whereas the devices exhibited an accumulation operation when the crystallite size of ZnO reduced to sub-micron range. Considering the difference between bulk and channel electrical conductivities in solution-processed ZnO TFTs, these results are explained with the interplay between the crystallite size of ZnO and the electrical conduction in the TFT.

Improved Electrical and Thermal Stability of Solution-Processed Li-Doped ZnO Thin-Film Transistors

The effects of lithium (Li) doping on the performance of solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) grown using the sol-gel method are investigated. Li-doped ZnO films as channel layers are chemically prepared by spin coating the gel with an aqueous solution of zinc acetate dihydrate, Li nitrate, and ethanolamine. TFT devices fabricated with 2 at.% Li-doped films show a good field-effect mobility of 3.31 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, a subthreshold slope of 0.82 V/dec, and an on-off current ratio of over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> . These TFT devices also show good thermal and electrical stability, which is mainly attributed to the compact surface morphology of the channel layer, small carrier concentration, and less oxygen deficiency, which reduces the interface electrical trapping at the gate insulator.

Fabrication conditions for solution-processed high-mobility ZnO thin-film transistors

Stable, solution-processed, non-toxic, high-mobility thin-film semiconductors are required for fabricating low-cost thin-film transistor (TFT) arrays and circuits to enable ubiquitous large-area and ultra low-cost electronics. Most thin-film semiconductors reported to date have been unable to meet the mobility, stability, safety, and cost requirements for this emerging technology, thus precluding their adoption in practical applications. Here, we report the preparation of stable, non-toxic, transparent, high performance zinc oxide (ZnO) thin-film semiconductors via thermal processing of solution-deposited precursor thin films in air. The process conditions influence the performance of the TFTs. By optimizing the fabrication conditions, the prepared ZnO thin-film semiconductor has a well-controlled, preferential crystal orientation and densely packed ZnO crystals, exhibiting excellent field-effect performance characteristics with mobility far exceeding those of hydrogenated amorphous silicon (a-Si:H). Consistently reproducible mobility ∼5–6 cm2V−1s−1 and current on-to-off ratio ∼105–106 have been obtained, while the production cost was controlled as low as possible. This potentially opens up application opportunities inaccessible by a-Si:H technology and renders otherwise costly large-area electronics affordable.

Performance improvement for solution-processed high-mobility ZnO thin-film transistors

The fabrication technology of stable, non-toxic, transparent, high performance zinc oxide (ZnO) thin-film semiconductors via the solution process was investigated. Two methods, which were, respectively, annealing a spin-coated precursor solution and annealing a drop-coated precursor solution, were compared. The prepared ZnO thin-film semiconductor transistors have well-controlled, preferential crystal orientation and exhibit superior field-effect performance characteristics. But the ZnO thin-film transistor (TFT) fabricated by annealing a drop-coated precursor solution has a distinctly elevated linear mobility, which further approaches the saturated mobility, compared with that fabricated by annealing a spin-coated precursor solution. The performance of the solution-processed ZnO TFT was further improved when substituting the spin-coating process by the drop-coating process.