Semiconductor-to-Metallic Phase Transition of VO^sub 2^ by Laser Excitation

Vanadium dioxide is well recognized for its semiconductor-to-metal phase transition (PT) at around 68°C. Recent investigations also show that this PT is ultrafast, as fast as the ultrashort laser pulses employed.1 Materials that exhibit ultrafast PT caused by thermochromic effect or induced by laser excitation are of central interest at present in optoelectronics and nonlinear optics due to their potential application in ultrafast optical switching and passive device applications for optical systems. A number of oxides categorized as Mott insulators exhibit insulator-to-metal transition upon heating to cross the transition point. Most notably, vanadium oxides, such as V^sub 2^O^sub 3^ (TM = 150 K) and VO^sub 2^ (T^sub M^ = 341 K), show increase of conductivity by a factor > 10^sup 4^ across TM and this results in a significant change in optical properties. Vanadium dioxide is widely studied. It is of technological interest as its transition point occurs at the readily accessible temperature of 68°C, which can be easily approached by physical heating, or heat deposition, or by laser excitation at a certain level.1,2 The metallic high-temperature form has a tetragonal rutile structure. Each vanadium ion is situated in the center of an oxygen octahedron with the parameters a = b = 4.55 [Angstrom] and c = 2.88 [Angstrom].3,4 The semiconducting low-temperature form of VO^sub 2^ is a monoclinic distortion of the rutile that involves a pairing between two V^sup 4+^ and off-axis displacement of alternate vanadium ions along the rutile c-axis. The resultant distortion lowers the symmetry with a = 5.75 [Angstrom], b = 5.42 A, and c = 5.38 [Angstrom].3,5 There have been many discussions regarding the laser-induced PT mechanism of VO^sub 2^. Gervais considered the phonon dispersion and the lattice instability at the R point in the Brillouin zone.6 Gup ta et al. calculated the band structure and explained using charge density waves.7 The strong electronphonon interaction was reported by Raman experiment8 and x-ray as well.9 In contrast, by measuring reflectivity versus time delay, Cavalleri et al.1 and Xu et al.10 recently observed that the time scale of PT depends on the excitation level, which may occur in subpicoseconds to nanoseconds They concluded from their results that the structural transition may not be thermally initiated. In this paper, we show the optical and photoluminescence properties of the optical quality PLD-VO^sub 2^ thin film. This is the first time, to our knowledge, that the extremely large polarizability observed in VO^sub 2^ and the excitedstate related lattice dynamical processes have been reported.

PLD-VO^sub 2^ FILM PREPARATION

The VO^sub 2^ films were grown by reactive PLD on fused quartz substrates and MgO substrate as well. A metallic vanadium target was used, which was rotated during laser ablation to avoid crater formation. A Lambda Physik 110 excimer (KrF, 248-nm emission) laser (Lambda Physik USA, Inc.) operating at a repetition rate of 10 pps and the average fluence on the target surface at ~14 J/cm^sup 2^ was used for ablation. Background pressure in the chamber was 10^sup -6^ Torr. Argon and oxygen gases were admitted into the chamber by separate controllers on O2/Ar flow rates. The O2 flow was controlled to be within 0.3-1.5 seem (standard cubic centimeter per minute). Total deposition time for each of the samples was 40 minutes and the film thickness was estimated to be between 01 and 0.5 µm for each sample.

After film growth, all samples appeared smooth. The sample grown with the highest oxygen flow shows a brownish and blue color, while that with moderate flow rate is bronze; the sample with the highest oxygen supplied looks dark gray. The samples were not further treated after deposition. Examination with an optical microscope showed micron-sized particles apparently due to target ejecta, but their number was relatively small, considering the fact that a metallic target was employed for the PLD process. X-ray diffraction scans of all samples showed a very broad diffraction peak under 2θ = 25°C, corresponding to amorphous material. However, since the substrate is amorphous, this is not conclusive. In addition, there was a single sharp reflection, close to the (011) peak for monoclinic VO^sub 2^, which was by far the most intense peak for this material (for example, PDF file 82-0661). It is likely that there is amorphous material in these samples, along with crystalline VO^sub 2^.