Improvements in Sensing Responses to Ammonia Gas for the In-Ga-Zn-O Thin-Film Transistor Using Atomic-Layer-Deposited ZnO Nanoparticles as Gas Sensitizers
Authors: Yun Da-Jeong , Seo Gi-ho , Lee Won-ho , Yoon Sung-Min 
Published: February 2020

Introduction: Gas-sensing technologies have been actively investigated to monitor the quality of air in our daily life. Especially in the industrial environment, detection of toxic gas in the atmosphere is required to ensure the safety of the working place. Among several types of gas sensors categorized with various principles such as optical, electrical, magnetic operations, and so on, the electrical sensors with three-terminal transistor configurations are extensively studied owing to their technical advantages. So-called field-effect transistor (FET)-based sensors are promising candidates for low-cost and portable applications in various fields. Moreover, thanks to their device parameters, such as the field-effect mobility and threshold voltage (VTH), they could be used as sensor devices with many-fold parameters.
In this work, a bottom-gate (BG) structured oxide semiconductor thin-film transistors (TFTs) were proposed as sensor TFTs, in which In-Ga-Zn-O (IGZO) thin films were used as channel layer as well as gas sensing material. In order to improve the device performances such as sensitivity, reliability, and reproducibility in sensing responses, the ZnO nanoparticles (NPs) were introduced on the top of the IGZO channel layer as sensitizers for the sensor TFTs. The ZnO has widely been used as a sensing agent by means of its drastic changes in electrical conductivity at the exposures to various gas molecules. Thus, the ZnO-NPs with island structure formed on IGZO channel can be expected to greatly enhance the sensing response of the IGZO sensing layer thanks to their higher reactivity and larger surface area. For the fabrication of sensor TFTs, the particle size and areal density the ZnO-NPs are optimized by process conditions of atomic-layer deposition (ALD) method. This approach can be very promising for realizing the high-performance gas sensors with three-terminal TFT configurations using the oxide semiconductors.

Experiments: We fabricated sensor TFTs using IGZO channel and ZnO-NP sensitizer on the glass substrate. First, a 150-nm-thick In-Sn-O (ITO) gate electrodes were patterned by wet-etching process and the 100-nm-thick Al2O3 gate insulator films were deposited by ALD. The deposition and patterning of ITO S/D electrodes were followed by the formation of contact holes. Then, IGZO (20 nm) active layers were deposited by rf magnetron sputtering and were patterned without any protection layer, which is conventionally prepared during the IGZO patterning process for protecting the IGZO from the chemical damages. Finally, ZnO NPs with island structure were formed by controlling the ALD cycles and temperatures. The sensing responses of the fabricated sensor TFTs to NH3 were evaluated in vacuum chamber with following four sequential steps; (1) at air ambient to determine reference characteristics; (2) at 1% NH3/N2 gas mixture with various concentrations to obtain the sensing responses; (3) after 100-oC annealing for 1 h in air ambient to confirm the recovery characteristics; and (4) at the identical conditions to those in step (2) to confirm the reproducibility in sensing responses. All measurements were performed for both devices with and without ZnO-NP sensitizer.

Results & Conclusions: First, the BG TFT characteristics were compared between before and after the formation of ZnO-NPs on the IGZO channel layers. The slight improvements in hysteric behaviors in IDS transfer characteristics could be confirmed for the devices using ZnO-NPs sensitizer. Most noteworthy change obtained after the introduction of ZnO-NPs was that both characteristics of the stability in TFT operations and the sensitivity to NH3 were drastically improved. To the contrary, without ZnO-NPs sensitizer, the sensor TFTs showed unstable transfer characteristics at repeatedly-performed measurements even in an air ambient. Eventually, with these unstable TFT operations, the reliable sensing response for the sensor TFTs cannot be guaranteed under the NH3 exposing conditions. The transition of VTH in a negative direction was clearly detected for the sensor TFTs employing the ZnO-NPs sensitizer and the VTH shift width was estimated to be as large as 3.3 V. Second, the recovery and reproducibility in the sensing responses to NH3 were systematically evaluated. After the 100-oC annealing process, the transfer curve and the VTH were almost recovered to initial values, during which the adsorbed NH3 gas molecules were desorbed from the ZnO-NPs sensitizer. Furthermore, it was also confirmed that the second exposure to NH3 after recovery caused almost the same amounts of VTH shift for the sensor TFT.
In conclusion, the introduction of ZnO-NPs sensitizer was sufficiently effective for enhancing both stability and sensitivity of the proposed IGZO gas sensor TFTs. The fabricated devices showed the recovery and reproducibility in sensing responses to NH3.