The Fermi level can be controlled in semiconductors via impurity doping or by applying an electric field. Amorphous materials have distinct advantages over their crystalline counterparts in terms of processability and homogeneity, i.e., ease of fabrication of large-sized homogenous thin films at low temperatures. Amorphous materials with good controllability of Fermi level would be highly beneficial for large-area electronics, optoelectronics, and flexible applications. These advantages are the major driving force for researching amorphous semiconductors. However, controlling the Fermi level in amorphous semiconductors is impossible because of high-density carrier traps arising from structural randomness. An exception is hydrogenated amorphous silicon (a-Si:H), which is widely used as solar cells and thin-film transistors for liquid crystal displays (LCD). However, band conduction has not been attained in a-Si:H. Thus, the mobility of a-Si:H remains at ~1 cm2/Vs, which is less than that of polycrystalline Si by two orders of magnitude.

 

 

Transparent amorphous oxide semiconductor (TAOS) is a novel class of amorphous semiconductors characterized by its ionic bonding nature. In addition to high optical transparency to visible light, its mobility is greater than that of a-Si:H by one order of magnitude and can be fabricated using conventional DC sputtering at low temperatures. Presently, thin-film transistors with a channel layer of In–Ga–Zn–O (IGZO) are used to drive pixels of high-definition and energy-saving LCDs in smartphones, tablets, PC monitors, and large-sized organic LED TVs and are being studied for several more applications in X-ray imagers and memories.

Although materials science and device physics of TAOS have rapidly advanced in the last decade, the understanding of science and technology is incomplete due to the short research history and the difference in chemical bonding between oxides and covalent-type semiconductors. Consequently, conventional processing used for a-Si:H has caused serious degradation of the resulting devices, such as plasma treatment involving hydrogen. TAOS is the only semiconducting material which can be fabricated by heating the precursor in the ambient atmosphere. Solution-derived processing has been extensively studied for using this unique feature toward flexible electronics.

 

In this lecture I introduce the fundamental of TAOS and their TFT applications along with a novel class of AOS.

 

Reference: Hideo Hosono and Hideya Kumomi（Edited）Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory (Wiley 2022)

Tokyo Institute of Technology

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National Institute for Materials Science

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