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Transparent Amorphous Oxide Semiconductors (TAOS)

Introduction

A Transparent Amorphous Oxide Semiconductor (TAOS) is a double oxide composed of heavy metal cations with an electron configuration of (n-1)d¹⁰s⁰, which are -used to conduct electricity across transparent surfaces such as an LCD display. TAOSs were found following the discovery of Transparent Conductive Oxides (TCO's)_by Rupperecht in 1954. After the discovery of the first TCO In₂O₃Sn, scientists became interested in transparent oxide semiconductors (TOSs), which have a broad use in Ultraviolet light emitting diodes, organic light emitting diodes, solar cells, and transparent film transistors. These TOS carry electrons most effectively in their amorphous state- hence the creation of TAOS from TOS. In 1996, Hideo Hosono, Masahiro Yasukawa, and Hiroshi Kawazoe developed a method to predict the conductive properties of a TAOS by its electronic configuration and its heavy metal cations.

Characteristics

Possible canidates for heavy metal cations in TAOS.

A major characteristic found in all TAOS is a very high electron mobility where the average TAOS has a mobility around 10cm²/V*s. This makes sense as electrical conductivity are the product of electron mobility and carrier concentration. So, in order to increase the conductivity of an amorphous material, the mobility must be increased. For TAOS, this high mobility is obtained through the large overlap of the ns orbitals on the bottom of the molecule’s conduction band. The spatial spreading and overlap of these ns orbitals is insensitive to any variation in the metal-oxygen bonds, an additional feature that is required to effectively increase the mobility of an amorphous material