Solution-processed Zinc Tin Oxide Thin-film Transistors Research Articles

Recovery characteristics of solution-processed ZTO thin-film transistors

We investigated the negative-bias illumination stress (NBIS) and the recovery characteristics of solution-processed amorphous zinc-tin-oxide (ZTO)-based thin-film transistors (TFTs). We developed TFTs with a bottom gate structure that used SiO2 films for the gate insulator. The developed TFTs were under NBIS in air at room temperature (RT) with a gate-to-source voltage of −10 V and a white-light power density of 6.2 mW/cm2. For the devices under NBIS, the NBIS-induced instability is attributed to the combined effects of 1) the creation of a low density of subgap states such as ionized oxygen vacancies (V 2+ ) near the Fermi level in ZTO, 2) a fast and large increase in the photo-conductivity at the off-state due to photo-induced band-to-band electron-hole pair generation, and 3) the hole trapping near the ZTO/SiO2 interface. The devices under NBIS also exhibit fast exponential recovery characteristics at RT, which are attributed dominantly to detrapping of holes near the ZTO/SiO2 interface and partially to the weak stabilization of V 2+ near the interface.

Effects of film thickness and Sn concentration on electrical properties of solution-processed zinc tin oxide thin film transistors

This paper investigates the effect of Sn concentration and film thickness on properties of zinc tin oxide (ZTO) thin film transistors (TFTs) fabricated using a solution process. ZTO solution was synthesized using zinc acetate dehydrate and tin chloride dehydrate dissolved in a solvent composed of 2-methoxyethanol and mono-ethanolamine. A ZTO film was obtained for an active channel on a gate oxide layer by spin-coating the solution at room temperature, drying at 300°C for 10min, and annealing at 550°C for 120min. The thickness and Sn concentration affected the material structure and electrical properties of ZTO film. The best solution processed ZTO TFT was obtained at film thickness of 35nm and Sn concentration of 30at.%. The fabricated ZTO TFT exhibited an on/off ratio of 1.88×107, a field effect mobility of 17.02cm2/Vs, a subthreshold swing of 0.77V/decade, and a threshold voltage of 5.01V.

Combined effect of the large ionic radius and low electronegativity of lanthanum additive on solution-processed zinc–tin–oxide thin-film transistors

Solution-processed lanthanum–zinc–tin–oxide thin films were fabricated with varying La content. A peak shift to small angles was observed in X-ray diffractometer spectra due to the expansion of the zinc–tin–oxide lattice originated from the larger ionic radius of La3+, while oxygen vacancies first increased, and then decreased with increasing La molar ratio. These results indicate that La, which has both low electronegativity and a large ionic radius, plays the role of either a generator or a suppressor of oxygen vacancies. Therefore, when choosing an additive to control the electrical properties of an oxide semiconductor, the ionic radius of the added atom should be carefully considered along with its electronegativity.

Effect of Zinc/Tin Composition Ratio on the Operational Stability of Solution-Processed Zinc–Tin–Oxide Thin-Film Transistors

Variation of Zn:Sn atomic composition ratio in zinc-tin-oxide (ZTO) thin films induced a dramatic change in the microstructure and also strongly influenced the device performance and operational stability of ZTO thin-film transistors (TFTs). The large variation of threshold voltage shift under gate bias stress appears to be closely correlated to the excessive or deficient Sn content and the oxidation potentials of the metallic components as well as environmental effects. It is noted that the optimum Zn:Sn atomic composition ratio in ZTO films can improve the device performance and operational stability of the solution-processed ZTO TFTs.

Environmental, Optical, and Electrical Stability Study of Solution-Processed Zinc–Tin–Oxide Thin-Film Transistors

In this paper, we report the environmental, optical, and gate bias stress stability of amorphous zinc-tin-oxide (ZTO) thin-film transistors (TFTs) fabricated by sol-gel spin-coating method. The ZTO TFTs showed excellent environmental and optical stability. The threshold voltage stability of ZTO TFTs was sensitive to both positive and negative gate bias stress. Maximum threshold voltage shifting of +1.9 and -3.2 V was observed under a gate bias stress of +10 and -10 V, respectively, with no significant change to subthreshold swing value.

Investigation of Indium Diffusion into Solution-Processed Oxide TFTs with ZTO Active Layer and IZO Source/Drain Electrodes

We have investigated the indium diffusion in oxide-based thin film transistors (TFTs) fabricated with zinc tin oxide (ZTO) active layer and indium zinc oxide (IZO) source/drain electrode by employing a solution-process. Oxide TFTs were fabricated with spin-coating process using solution with chloride-, acetate- and nitrate- based precursors. After spin-coating, ZTO active layer and IZO source/drain electrodes were annealed sequentially by rapid thermal annealing (RTA). Rutherford backscattering spectrometry (RBS) result shows that 16.8% of indium atoms in IZO source/drain electrodes are diffused into ZTO active layer after annealing process. The In-diffusion into ZTO active layer cause a considerable negative shift of threshold voltage in solution-processed ZTO TFTs.

Charge carrier velocity distributions in field-effect transistors

The measurement of the distribution of charge carrier velocities in a field-effect transistor can provide considerable insight into charge transport mechanisms and structure-property relationships. We have developed such a method and have applied it to study temperature-dependent velocity distributions in solution-processed zinc-tin oxide thin-film transistors. Two distinct transport pathways, each with a different activation energy, have been observed, in contrast to a single activation energy yielded by steady-state measurements. Our results show that more insight into charge transport behavior and phenomena can be obtained with such time-resolved transport measurements.

Solution-processed zinc–tin oxide thin-film transistors with low interfacial trap density and improved performance

Solution-processed amorphous oxide semiconductors are attractive channel materials in thin-film transistors (TFTs) for low-cost electronics. We demonstrate improved performance and uniformity of solution-processed zinc–tin oxide (ZTO) TFTs by optimizing the prebake process for the ZTO precursor film. ZTO prebake process prearranges the dielectric/semiconductor interface and minimizes the performance variation caused by the uneven thermal distribution during annealing process. Prearranging the interface also reduces interfacial trap density and results in improved performance. A mobility of 27.3 cm2/V s, an on/off ratio of ∼107, and a subthreshold swing of 122 mV/decade have been obtained. Significant improvement in operational stability has also been observed.

Self-Aligned Solution-Processed Zinc-Tin Oxide Thin-Film Transistors with High-K Solution-Processed Gate Dielectric

AbstractWe demonstrate high performance zinc-tin oxide (ZTO) thin-film transistors (TFTs) with low operation voltage, small channel length and low parasitic capacitance. Both the zinc tin oxide and the high-k dielectric, ZrO2, were solution processed by sol-gel methods. A self-aligned process was employed to minimize the parasitic capacitance. The transistors with a channel length of 8 μm operate at 5 V and have a saturation mobility of 2.5 cm2/V·s and an on/off ratio of 5.9×106. Gate-induced surface relief has been found to have strong effect on the performance of the active layer.