Fabrication of Thin-Film Transistor from Amorphous Oxide Semiconductors

Thin-film transistors (TFTs) based on amorphous oxide semiconductor (AOS) are moving towards commercial world as a replacement for hydrogenated amorphous silicon (a:Si:H) TFTs in multiple flat-panel applications. AOS TFTs differ from the conventional complementary metal-oxide semiconductor (CMOS) electronic devices. Instead of using a bulk, crystalline semiconductor, AOS TFTs employ thin amorphous materials in which the channel layer is approximately few tens of nanometers. We have fabricated thin film transistor using a shadow mask technology. Shadow mask technology is a technique that allows a desired pattern to be transferrable onto a substrate. Shadow mask technology is similar to photolithography in patterning; however, it is more environmentally friendly, reusable, and cost effective. Shadow mask technology was used to deposit the active layer and the dielectric layer of our devices. The indium-free oxide-based channel material such as aluminum-doped zinc oxide (AZO) is the active material and can be deposited on glass substrate or silicon for TFT applications. High quality AZO thin films are grown using electron beam evaporation on p-type silicon and on glass for characterization purposes. The effect of Al content on zinc oxide crystal lattice are investigated using various characterization techniques. Atomic force microscopy provided thin film roughness, grain size, and surface morphology. X-ray diffraction provided crystal orientation for both dielectric and active layer deposition. Ultra-violet visible spectroscopy is used for transmission percentage and Keithley 4200 semiconductor characterization system provided dielectric performance and transistor characteristics including on/off ratio, mobility, and threshold voltage. We observed a dielectric capacitance as high as 698 pF and a minimum surface roughness of 0.176 nm. The lowest current density observed for dielectric performance is found to be in the magnitude of 10^-8 A/ cm² and transistor saturation current exceeding 1.5 µA. This present work will provide valuable scientific input of AZO TFTs for the improvement of TFT devices. <br/> <br/>This work is supported by the NSF-CREST Grant number HRD 1547771 and NSF-CREST Grant number HRD 1036494.