| AVS 58th Annual International Symposium and Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Thursday Sessions |
| Session EL+AS+EM+MS+PS+TF-ThA |
| Session: | Spectroscopic Ellipsometry for Photovoltaics, Metals and Oxide Thin Films |
| Presenter: | Robert Marvel, Vanderbilt University |
| Authors: | D.W. Ferrara, Vanderbilt University R.E. Marvel, Vanderbilt University J. Nag, Vanderbilt University R.F. Haglund, Vanderbilt University |
| Correspondent: | Click to Email |
Vanadium dioxide (VO2) is a strongly-correlated electron material with a well-known semiconductor-to-metal transition (SMT) that can be induced thermally (Tc = 68oC), optically, or electrically. Recently, VO2 films have attracted attention as a component in active metamaterials, especially in conjunction with metal nanostructures. Since these structures are highly sensitive to the dielectric properties of the embedding material, the SMT of VO2 can be used to tune the optical response of the structure. Accurately modeling the behavior of these structures requires detailed knowledge of the dielectric function of VO2 as it undergoes the SMT; however, previous measurements of the optical constants of VO2 reveal significant variations between experiments.
To understand systematic variations due to growth conditions, films of VO2 were deposited on either silicon, glass, or sapphire substrates by pulsed laser ablation of vanadium metal targets in 10 mTorr oxygen (O2) background gas, followed by annealing at 450oC in 250 mTorr of O2. Anneal times were varied from 30 to 90 depending on film thickness; deposition thickness was varied from 20 nm to 200 nm. For each sample, temperature-dependent spectroscopic ellipsometry measurements at optical and near-infrared wavelengths were conducted to determine the dependence of the optical constants on film thickness, substrate and crystallinity, and temperature.
Bruggeman and Maxwell-Garnett effective-medium formulations were used to account for three constituent materials: semiconducting VO2, metallic VO2, and vanadium pentoxide (V2O5). The effective dielectric functions were modeled using Lorentz and Tauc-Lorentz oscillators. Our results show that the contribution of V2O5 to the effective dielectric function increases with annealing time, consistent with previous studies. The results are also substantiated using Rutherford backscattering, X-ray photoelectron spectroscopy and X-ray diffraction.