T. Gießel, H. Prima Garcia

Vanadium dioxide VO₂ is one of several oxides of first-row transition metals, which are known to exhibit a semiconductor-to-metal phase transition (SMPT) as a function of the temperature. The SMPT is associated with a crystallographic distortion and is believed to arise from changes in strong electronic correlation mechanisms. The crystallographic changes correspond to an atomic rearrangement from a high-T metallic tetragonal phase to a low-T semiconductive monoclinic phase.

The relative importance of the structural and electronic changes in the opening of the semiconductive gap is still under debate. The investigation of the transient electronic properties and of the transient structural distortion in the laser-excited SMPT of VO₂ might give some important suggestions concerning this point. The abrupt shift in optical, electrical and magnetic properties at the SMPT near room temperature (~340 K) makes VO₂ a potential material for switching devices.

Our approach to investigate transient electronic and structural changes in the SMPT in VO₂ is based on Laser-pump Synchrotron radiation-probe experiments in which the SMPT is triggered by a Laser pulse and the induced electronic (structural) changes are probed by the Synchrotron radiation.

Hereby, the closing of the semiconductive gap at the SMPT manifests itself in an abrupt change in the density of states in the region of the V 3d band close to the Fermi level.

The figure above shows photoelectron spectra of VO₂ films (thickness 200 nm) grown by reactive radio frequency sputtering on glass. We observed a significant change in the shape of the V 3d level above and below the phase transition as a function of the temperature.

The figure below shows V 3d region photoelectron spectra with (red) and without (blue) laser irradiation for two different sample temperature. The laser used is a regeneratively amplified Ti:Sapphire Laser (RegA 9000) (approx. 3 µJ/Puls, 208,3 kHz repetition rate, for more information see MBI-BESSY-Beamline). For laser irradiation of the sample the fundamental wavelength of the RegA of 800 nm has been used. The laser excitation density had been set to 3 mJ/cm² in order to match the focal size of the synchrotron radiation spot.

The sample has been measured at 308 K and 203 K temperature, whereas laser irradiation heats the sample by some degrees Kelvin. After laser excitation while O 2p-band does not show essential changes (not show here), the V 3d-band show the characteristic transformatiom to rutile structure. Time-resolved measurements showed no changes in the valence band spectra within one period of the laser (4.8 µs). Therefore, the contrast between the spectra with and without laser irradiation has to be assigned exclusively to an accumulation of the heat transferred by the laser in the glass substrate of the sample.