Polarized Raman mapping and phase-transition by CW excitation for fast purely optical characterization of VO2 thin films

Abstract

Vanadium dioxide has attracted much interest due to the drastic change of the electrical and optical properties it exhibits during the transition from the semiconductor state to the metallic state, which takes place at a critical temperature of about 68 °C. Much study has been especially devoted to developing advanced fabrication methodologies to improve the performance of VO₂ thin films for phase-change applications in optical devices. Films structural and morphological characterisation is normally performed with expensive and time consuming equipment, as x-ray diffractometers, electron microscopes and atomic force microscopes. Here we propose a purely optical approach which combines Polarized Raman Mapping and Phase-Transition by Continuous Wave Optical Excitation (PTCWE) to acquire through two simple measurements structural, morphological and thermal behaviour information on polycrystalline VO₂ thin films. The combination of the two techniques allows to reconstruct a complete picture of the properties of the films in a fast and effective manner, and also to unveil an interesting stepped appearance of the hysteresis cycles probably induced by the progressive stabilization of rutile metallic domains embedded in the semiconducting monoclinic matrix.

Keywords: Continuous wave excitation; Phase transition; Polarized Raman; VO2 thin films.

Figure 1

Unpolarised Raman spectrum obtained from the thickest VO₂ layer (sample B), and identification of the main vibrational peaks.

Figure 2

Appearance of “Off” (blue coloured) and “On” (green coloured) domains in large area polarized Raman maps of the thickest VO₂ thin film (sample B). The top panel represents the optical image of the analysed area. The two maps are collected on the very same region with X and Y polarized laser excitation and no polarization on the detection (X-none and Y-none), respectively, and are represented as a chemigram referred to the spectral region between 160 and 240 cm⁻¹. The scale marker is the same for the three images.

Figure 3

Crystal grain orientation dependence of the Polarized Raman maps. The maps are collected on the very same region by changing the polarization of the excitation laser and controlling the polarization at the detector. Top Panel: the spectrum collected on the central domain, named 1 in the X-X map and appearing “Off” (blue), is reported on the left. The spectrum collected on the lateral domain, named 2 and appearing “On” (green), is reported on the right. Bottom Panel: the central domain 1 turns “On” when inverting the polarization, in the Y–Y image.

Figure 4

Grain size dependence on film thickness analysed by X-None Raman mapping: (a) sample A (340 nm-thick); (b) sample B (410 nm-thick); (c) histograms of the radius distribution of the domains approximated as circles. The scale bar on the maps is the same.

Figure 5

Raman analysis of the SMT transition. (a) Full spectra acquired on sample A (340 nm); (b) intensity of the band located at about 224 cm⁻¹ as a function of the temperature for sample film A; (c) Full spectra acquired on sample B (410 nm); (d) intensity of the band located at about 224 cm^-1 as a function of the temperature for sample B.

Figure 6

Experimental setup of the experiment: (1) pump laser (Coherent Mira 900); (2) half-waveplate; (3) polarizing beam splitter; (4) telescope; (5) sample; (6) coupling units (7) probe laser (PicoQuant LDH-P–C-1310NM); (8) InGaAs single photon detectors.

Figure 7

Temporal evolution of the relative reflection change ΔR/R induced by optical CW excitation for various average powers and probed by a picosecond diode laser at 1310 nm. (a) sample A, excitation at 532 nm; (b) sample A, excitation at 830 nm; (c) sample B, excitation at 532 nm; (d) sample B, excitation at 830 nm. In all cases, at t = 0, the pump laser is turned on.