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 and bandgap behavior in ultrathin multilayer films of Al2O3-ZnO bilayers grown via atomic layer deposition (ALD) technique on Si(100) substrates. The multilayer configuration stack 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 sample morphology was studied via Atomic Force Microscopy and the results show that the surface roughness of the multilayers varies from 0.2 to 1.2 nm, as the ZnO layer thickness increases. In all cases, the roughness values remain below 2% of the total thickness of the multilayer. The refractive index n(λ) and optical bandgap, Eg, of each multilayer sample were studied via spectroscopic ellipsometry (SE). A General Oscillator optical model was utilized to fit the experimental data in order to obtain the total thickness, refractive index and absorption coefficient. Cross-sectional mode scanning electron microscope images verified the multilayer total thickness and corroborated the accuracy of the optical model. The refractive index varies significantly from values close to the Al2O3 refractive index when the bilayer thickness is small, up to values corresponding closely to ZnO for thicker bilayers. The refractive index, as a function of bilayer thickness, varies between 1.63 and 2.3, for λ = 370 nm (UV region), showing high sensitivity. In addition, the optical bandgap energy, Eg, determined using the Tauc model, decreases when the bilayer thickness increases, with a maximum variation of ΔEg ∼1.6 eV. These results reveal that the refractive index and optical bandgap of Al2O3-ZnO material can be modulated systematically as a function of the bilayer thickness. Such behavior is of great importance for optoelectronics applications, in particular for the development of devices with response in the UV spectral range.