Effect of Mg-doping on the Optical Properties of Vanadium Dioxide

Vanadium dioxide (VO2) is a promising optical phase change material for tunable photonic devices showing large optical contrast, low power consumption and ultrafast modulation. However, the rutile phase of VO2 has excessive optical loss due to the free carrier absorption, limiting its performance in photonics devices. The work aims to demonstrate low loss and high figure of merit in epitaxial VO2 films on c-cut sapphire substrates by Mg2+ doping. The Mg2+ doping can heavily reduce the optical loss in rutile phase, and improve the FOM at mid-infrared wavelength. The epitaxial VO2 films were deposited on c-cut sapphire substrates by pulse laser deposition method. Furthermore, the VO2 epitaxial films with different Mg doping concentrations were deposited by correlative adjustment of the duty-cycle and deposition time. The crystalline structures of Mg-doped VO2 films at different temperatures were confirmed by high-resolution X-ray diffraction (XRD). The surface chemical states of Mg-doped VO2 films were measured by X-ray photoelectron spectroscopy (XPS). The concentrations of Mg doping were measured by inductively coupled plasma optical emission spectrometry (ICP-OES). To understand the atomic configurations of the Mg-doped VO2, the cross-section TEM image and selected area electron diffraction (SAED) pattern were studied by TEM. For optical properties, the Mg-doped VO2 films were measured by spectroscopic ellipsometry. Finally, the first-principles calculations were used to understand the physical mechanism of the improvement of FOM by Mg doping. Four Mg-doped VO2 films with concentrations 0%, 5.6%, 8.6% and 11.9% were successfully deposited. All the films presented well out-of-plane epitaxial relationship to Al2O3 substrate with (020)VO2//(0006)Al2O3. At low doping levels, the Mg-doped VO2 films indicated obvious phase transition process with monoclinic (020)M transforming into metallic rutile (020)R from analysis of XRD spectra. However, the 11.9% Mg-doped VO2 presented no XRD peak shift, demonstrating the disappearance of the first-order phase transition. TEM analysis demonstrated that the 11.9% Mg-doped VO2 film mainly presented rutile phase with high Mg concentration, containing a little precipitated monoclinic phase of low Mg concentration VO2, leading to the disappearance of the phase transition ability. The insulator-to-metal transition properties of Mg-doped VO2 films strongly depended on the valence states of V and the dopant. The valence states of the V, O, and Mg elements in the Mg-doped VO2 films were analyzed by X-ray photoelectron spectroscopy (XPS). The optical constants were measured by spectroscopic ellipsometry. As the Mg2+ doping levels increased from 0% to 11.9%, the extinction coefficient in rutile phase decreased from 10 to 0.1 at around 7.5 μm wavelength, achieving 100 times lower loss, while the refractive index and extinction coefficient of monoclinic phase showed small variations by Mg-doping. Although the refractive index change between monoclinic and rutile phase is also reduced, the 11.9% Mg-doped VO2 still shows 3.7 times higher figure-of-merit (1.32) compared to the undoped film (0.35) in 7.5 μm wavelength. First-principles calculation indicate that the Mg2+ doping can localize and lower the DOS of the V 3d electrons, leading to lower absorption in the mid-infrared wavelength. The study provides an alternative phase change material for active photonics devices applications. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.