Paper TF+EM-MoM9
Nanocomposite Vanadium Oxide Thin Films

Monday, October 18, 2010, 11:00 am, Room Ruidoso

Thin film vanadium oxide is used as the active layer in un-cooled, infrared imaging devices based on microbolometer structures. In this type of imaging device, infrared radiation is detected via a temperature driven resistivity change in the active layer. Underlying readout circuitry and the need for increased detection sensitivity require low electrical resistivity material with high thermal coefficient of resistivity (TCR). Vanadium oxide thin films deposited via reactive pulsed direct current (DC) sputtering have properties in the range of interest with room temperature resistivity varying from 0.01 to 6×10⁴ Ohm cm and TCR’s varying from -0.1 to -4.2 %/°C. Films with resistivity in the range of interest (0.1 to 1.0 Ohm cm) contain the rocksalt structured FCC VO_x phase (0.8 < x < 1.3) accompanied by significant structural disorder, while those with greater resistivity are amorphous. The relationship between TCR and room temperature resistivity is exponential, and throughout the present series of films is fixed, i.e. a film with a given resistivity has a set TCR. Energy filtered electron diffraction patterns collected in the transmission electron microscope (TEM) reveal a diffuse scattering feature at low scattering angle for both amorphous and nanocrystalline films, suggesting that films containing nanocrystals also contain an amorphous phase. Raman spectroscopy results from amorphous films show a broad feature around ~890 cm^-1, while spectra from nanocrystalline films exhibit the “amorphous” feature and a second broad feature at ~300 cm^-1. The feature at ~300 cm^-1 was the only feature present in the most crystalline of the films, suggesting it represents the disordered FCC VO_x phase. Film stoichiometry, as measured with Rutherford backscattering spectroscopy, puts the overall chemistry of the nanocrystalline films outside the FCC VO_x phase field, with many of the samples having a V:O ratio greater than 1.3. A pre-peak feature in the V-K edge was observed with X-ray absorption spectroscopy (XAS), and the intensity of the pre-peak, which is known to result from local octahedral disorder, increased with increasing oxygen content and correlates with diffraction observations of increasing disorder. The combined TEM, RBS, Raman, and XAS analyses suggest vanadium oxide films with properties in the range of interest for microbolometer-based devices consist of a two-phase material containing FCC VO_x nanocrystals embedded in an oxygen-rich amorphous matrix. Films lacking sufficient disorder have resistivity in the range of interest but insufficient TCR magnitude, suggesting the need for both the nano-crystalline phase and the amorphous matrix.