Optical bandgap control in Al2O3/TiO2 heterostructures by plasma enhanced atomic layer deposition: Toward quantizing structures and tailored binary oxides

Pallabi Paul; Md Golam Hafiz; Paul Schmitt; Christian Patzig; Felix Otto; Torsten Fritz; Andreas Tünnermann; Adriana Szeghalmi

doi:10.1016/j.saa.2021.119508

Optical bandgap control in Al₂O₃/TiO₂ heterostructures by plasma enhanced atomic layer deposition: Toward quantizing structures and tailored binary oxides

Spectrochim Acta A Mol Biomol Spectrosc. 2021 May 5:252:119508. doi: 10.1016/j.saa.2021.119508. Epub 2021 Jan 29.

Authors

Pallabi Paul¹, Md Golam Hafiz¹, Paul Schmitt², Christian Patzig³, Felix Otto⁴, Torsten Fritz⁴, Andreas Tünnermann², Adriana Szeghalmi⁵

Affiliations

¹ Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany.
² Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany; Fraunhofer Institute for Applied Optics and Precision Engineering, Centre of Excellence in Photonics, Albert-Einstein-Str. 7, 07745 Jena, Germany.
³ Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Walter-Huelse-Str. 1, 06120 Halle (Saale), Germany.
⁴ Institute of Solid-State Physics IFK, Friedrich Schiller University Jena, Helmholtzweg 5, 07743 Jena, Germany.
⁵ Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany; Fraunhofer Institute for Applied Optics and Precision Engineering, Centre of Excellence in Photonics, Albert-Einstein-Str. 7, 07745 Jena, Germany. Electronic address: adriana.szeghalmi@iof.fraunhofer.de.

PMID: 33571739
DOI: 10.1016/j.saa.2021.119508

Abstract

Atomically thin heterostructures and superlattices are promising candidates for various optoelectronic and photonic applications. Different combinations of Al₂O₃/TiO₂ composites are obtained by plasma enhanced atomic layer deposition (PEALD). Their growth, composition, dispersion relation, and optical bandgap are systematically studied by means of UV/VIS spectrophotometry, spectroscopic ellipsometry (SE), x-ray reflectometry (XRR), scanning transmission electron microscopy(STEM) and x-ray photoelectron spectroscopy (XPS). Besides, an effective medium approximation (EMA) approach is applied to model the heterostructures theoretically. The refractive index and the indirect bandgap of the heterostructures depend on the ratio of the two oxides, while the bandgap is very sensitive to the thicknesses of the barrier and quantum well layers. A large blue shift of the absorption edge from 400 nm to 320 nm is obtained by changing the TiO₂ (quantum well) thickness from ~2 nm to ~0.1 nm separated by ~2 nm of Al₂O₃ (barrier) layers. PEALD unfolds the possibility of achieving optical quantizing effects within complex heterostructures enabling control of their structures down to atomic scale. It enables a path towards atomic scale processing of new 'artificial' materials with desired refractive indices and bandgap combinations by precise control of their compositions.

Keywords: Atomic layer deposition; Binary oxides; Dielectrics; Heterostructures; Quantum confinement; Superlattices.