When 3:30 PM - 5:00 PM Jan 17, 2014
Where 1670 Beyster Building
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Amorphous Oxide Ssemiconductors (a-IZO) for Thin Film Transistor (and more) Applications

David Paine
Alpert Medical School, Brown University

Amorphous oxide semiconductor (AOS) thin film transistors (TFT’s) offer an optically transparent alternative to amorphous Si (a-Si) TFT’s for display and other large area electronics technologies. High mobility (10-50 for IZO vs. 0.1-1 cm2/Vsec for Si), room temperature processing, high surface planarity, and isotropic etch characteristics make the amorphous indium zinc oxide (a-IZO) materials and related ternary cation oxides such as indium gallium zinc oxide (a-IGZO) particularly attractive for these applications. However, many fundamental properties and performance characteristics of these amorphous materials are not yet well understood particularly in the very thin film (10-50 nm), low carrier density (≅1015/cm3) form required for TFT channel material. Even relatively fundamental properties such as the coefficient of thermal expansion, diffusivity of oxygen, pseudo-equilibrium carrier density, dependence of mobility on carrier density, and amorphous oxide structural stability are not well established and, yet, are important parameters that will be needed to enable the full integration of a-IZO-based TFT’s into future display technologies. In this talk, critical materials issues that affect TFT device performance considerations will be reviewed. For example, TFT threshold voltage device stability is, at least in part, dependent on the stability of the carrier density in the 10-30 nm a-IZO channel. Work at Brown University has shown that the a-IZO carrier density is dependent on two competing processes: irreversible structural relaxation of as-deposited material which creates point-defects that provides carriers and oxidation (in air) which consumes them. We have used an unique ultra high pressure oxidation technique to explore the thermodynamics of defect equilibrium in the a- IZO system. The formation of reliable metal contacts to a-IZO channel materials is another important consideration which affects channel carrier density and structural stability of the a-IZO. Review of the literature shows that Ti metallization is widely used in AOS-based TFT fabrication. In-situ TEM combined with resistivity measurements during annealing in the range 200-400° will be presented that show remarkably fast Ti/IZO reaction kinetics which, in part, explains threshold voltage shifts reported by many labs for TFT devices.

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