Noboru Kimizuka first synthesized indium gallium zinc oxide (IGZO), an n-type semiconductor, in 1985.¹ The physical properties of IGZO change depending on its composition ratio of constituent metal elements, expressed as InGaO₃(ZnO)_m. This oxide material assumes a layered structure known as a “homologous series,” which exists in a wide range of compositions and has a large solid-solution region. This characteristic reduces the impact of composition variability on the crystal structure and, thus, on electrical characteristics. Therefore, IGZO is a suitable material for mass production of semiconductor devices.

We generally classify crystal structures as single-crystal, polycrystalline, and amorphous—this classification also applies to IGZO. Previously, we discovered new crystal structures that exist between single-crystal and amorphous IGZO, which are c-axis-aligned crystalline (CAAC) and nanocrystalline structures.² Here, we present a novel structure that exhibits functionalities of a composite material: cloud-aligned composite oxide semiconductor (CAC-OS). We synthesized CAC-OS by changing the IGZO composition ratio In:Ga:Zn of InGaO₃(ZnO)_m from 1:1:1 (where m is an integer; m = 1) to 4:2:3 (indium-rich compound, where m is a noninteger).