Refractive Index and Bandgap Variation in Al₂O₃-ZnO Ultrathin Multilayers Prepared by Atomic Layer Deposition

This research focuses on the study of the refractive index n(λ) and optical bandgap E_g behavior in ultrathin multilayer films of Al₂O₃-ZnO bilayers grown via atomic layer deposition (ALD) technique on Si(100). The multilayer configuration consists in alternate layers of constant thickness Al₂O₃ (2 nm) and varying thickness ZnO films in order to obtain a total thickness of ~100 nm. A set of 10 samples based on bilayers with various 2:X thickness ratios were prepared, where X refers to the ZnO layer thickness. X is proportional to the number of cycles (N) of the ZnO precursor, varying from 1 to 100. The total thickness, n(λ) and E_g values of each multilayer sample were studied via spectroscopic ellipsometry. Scanning electron microscope images verified the multilayer total thickness and corroborated the accuracy of the optical model used. We observe that the n(λ) values varies between 1.63 and 2.3 for λ ~ 370 nm and the E_g values decreases when the bilayer thickness increases, with a maximum variation for ΔE_g ~1.6 eV. These results reveal that the n(λ) and E_g of this material can be modulated systematically as a function of the bilayer thickness and can been suitable for optoelectronics applications.