On improvements in metal oxide based flexible transistors through systematic evaluation of material properties

Abstract

Thin-film metal oxide (MOx) semiconductors have opened the way to a new generation of electronics based on their unique properties. With mobilities, mu, of up to 80 cm2V-1s-1, metal oxides do not rival crystalline silicon (mu~1000 cm2V-1s-1) for complex applications. But such oxides do have three unique characteristics driving great interest: their mobilities persist in the amorphous form, contrary to the thousandfold drop seen in silicon; they are transparent; and they can be processed at, or near, room temperature.
Most work on MOx semiconductors, in particular indium gallium zinc oxide (IGZO), has focused on display applications, where MOx thin-film transistors (TFTs) are used to drive
individual pixels, reducing power consumption by blocking less light than alternatives, and allowing smaller pixels due to reduced TFT sizes.
Such work has seen great advances in IGZO, but has generally not considered the thermal budget during production. By utilising the low temperature processing possible with MOx, a new world of applications becomes possible: flexible electronics.
This work aims to improve the characteristics of TFTs based on amorphous IGZO (a-IGZO) through detailed study of the thin-film structure in relation to functional performance, looking at the material structure of three critical layers in an a-IGZO TFT. A study of optimisation of a dielectric layer of Al2O3, deposited by atomic layer deposition (ALD), is presented. This dielectric, between the a-IGZO and the gate electrode, shows a three-layer substructure in what has previously been regarded as a single homogeneous layer. A study of the insulating Al2O3 buffer layer below the a-IGZO compared the properties of Al2O3 deposited by ALD and sputtering. Sputtered material has a more complex structure than ALD, consisting of multiple sublayers that correlate with the sputtering process. The structure of the two materials is discussed, and the impact on device performance considered. A detailed systematic study of the effects of annealing of a-IGZO shows a strong dependence of the density on both time and temperature. A two mechanism model is proposed which consists of structural relaxation of the amorphous material followed by absorption of oxygen from the environment.
Finally, investigation of the influence of the buffer material on the a-IGZO, and the structure of this interface showed little difference in the growth of the a-IGZO, but did reveal some changes in the interface, while a systematic study of annealing effects on the a-IGZO-dielectric interface showed some interesting changes in this structure, both of which are likely to significantly impact the operational characteristics of TFT devices.