Anisotropic Optical Response of Ti-Doped VO2 Single Crystals

Abstract

This study delves into the effects of titanium (Ti) doping on the optical properties of vanadium dioxide (VO₂), a material well known for its metal-to-insulator transition (MIT) near room temperature. By incorporating Ti into VO₂'s crystal lattice, we aim to uncover the resultant changes in its physical properties, crucial for enhancing its application in smart devices. Utilizing polarized infrared micro-spectroscopy, we examined Ti_xV_1-xO₂ single crystals with varying Ti concentrations (x = 0.059, x = 0.082, and x = 0.187) across different crystal phases (the conductive rutile phase and insulating monoclinic phases M1 and M2) from the far-infrared to the visible spectral range. Our findings reveal that Ti doping significantly influences the phononic spectra, introducing absorption peaks not attributed to pure VO₂ or TiO₂. This is especially notable with polarization along the crystal growth axis, mainly in the x = 0.187 sample. Furthermore, we demonstrate that the electronic contribution to optical conductivity in the metallic phase exhibits strong anisotropy, higher along the c axis than the a-b plane. This anisotropy, coupled with the progressive broadening of the zone center infrared active phonon modes with increasing doping, highlights the complex interplay between structural and electronic dynamics in doped VO₂. Our results underscore the potential of Ti doping in fine-tuning VO₂'s electronic and thermochromic properties, paving the way for its enhanced application in optoelectronic devices and technologies.

Keywords: Ti doping; VO2; anisotropy; infrared spectroscopy; vanadium oxide.

Figure 1

Reflectance spectra obtained from polarized measurements on single crystals with different Ti dopings from x = 0.059 to x = 0.187. The left column shows measurements with polarization parallel (perpendicular) to the growth axis, C_R, and the right column shows measurements with polarization perpendicular to that. It is noticeable that the reflectivity of the rutile phase (blue lines) decreases as the doping increases (see text). As discussed previously, the monoclinic M1 phase is absent in the sample with x = 0.187.

Figure 2

Real part of the optical conductivity of Ti_xV_1−xO₂ single crystals in the R phase obtained with polarization (a) parallel and (b) orthogonal to the growth axis, $c_R$. Brown, orange, and yellow lines denote Ti doping with x = 0.059, x = 0.082, and x = 0.187, respectively. (c) Difference between the real part of the optical conductivities obtained with parallel and orthogonal polarization under fixed doping, as defined in Equation (1).

Figure 3

Imaginary part of the dielectric function of Ti_xV_1−xO₂ single crystals in the M1 phase, with polarization (a–c) parallel and (d–f) orthogonal to the growth axis, ${\overrightarrow{c}}_R$. Each black line denotes the total ${ϵ}_2\left(\omega \right)$, while colored sections represent single phononic modes. ${ϵ}_2\left(\omega \right)$ presented in panels (a,d) are included for samples with x = 0 obtained from reference [9].

Figure 4

Imaginary part of the dielectric function of Ti_xV_1−xO₂ single crystals in the M2 phase, with polarization (a–c) parallel and (d–f) orthogonal to the growth axis. Each black line denotes the total ${ϵ}_2\left(\omega \right)$, while colored areas represent single phononic modes. The vertical scale is set to 100 cm⁻¹ to better show the low-intensity phonons.