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 Kronig-Penny model employing a conduction band offset of 1.2 eV between the a-IGZO and the a-Ga2O3, and the effective masses of 0.35m(0) for the a-IGZO well layer and 0.5m(0) for the a-Ga2O3 barrier layer, where m(0) 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.