Ultrafast response of thin film vanadium dioxide grown on titanium dioxide doped with niobium*

We have studied vanadium dioxide (VO₂) films grown on titanium dioxide (TiO₂) substrates to investigate the properties of the heterojunction that forms at the interface between substrate and film with the purpose of applying it as an UV photodetector. The use of niobium as dopant on TiO₂ substrates has been shown to favorably modify the energy levels at the heterojunction thus promoting photocurrent generation when illuminated with UV light. To further investigate this electronic structure modifications we study the ultrafast dynamics of the insulator-metal-transition (IMT) in such samples by using a pump probe configuration. The samples are pumped with ~150 fs pulses of 400nm wavelength light, and the changes in electronic structure of the heterojunction region are detected via change in relative optical reflectance (ΔR/R) of a 800nm probe light. The VO₂ on TiO₂:Nb doped samples generate distinctive ΔR/R effects compared with the VO₂ films deposited on plain TiO₂ substrates samples that are undoped.