Optical evidence for quantization in transparent amorphous oxide semiconductor superlattice

We fabricated transparent amorphous oxide semiconductor superlattices composed of In-Ga-Zn-O (a-IGZO) well layers and Ga₂O₃ (a-Ga₂O₃) barrier layers, and investigated their optical absorption properties to examine energy quantization in the a-IGZO well layer. The Tauc gap of a-IGZO well layers monotonically increases with decreasing well thickness at ≤5 nm. The thickness dependence of the Tauc gap is quantitatively explained by a Kr\"onig-Penny model employing a conduction band offset of $1.2$ eV between the a-IGZO and the a-Ga₂O₃, and the effective masses of 0.35m₀ for the a-IGZO well layer and 0.5m₀ for the a-Ga₂O₃ barrier layer, where m₀ is the electron rest mass. This result demonstrates the quantization in the a-IGZO well layer. The phase relaxation length of the a-IGZO is estimated to be larger than $3.5$ nm.