Polarization effects of poled ferroelectric substrates upon surface conductivity changes in phthalocyanine and SnO2 films following gas adsorption

Abstract

Organic semiconductors of phthalocyanines (H₂Pc and CuPc) have been deposited as thin films of 15–150 nm on the polarized surfaces of ferroelectric LiNbO₃, and the surface conductive behaviour with the adsorption of gases has been studied. Upon the adsorption of NO₂, the conductivity increases in H₂Pc films as thick as 150 nm were similar between the (+) and (–) polar substrates, where the (–) polar substrate gave rise to a larger conductivity increase than did the (+) polar substrate with decreasing thickness. Differences between oppositely polarized substrates became prominent for gases with larger electron affinity, i.e. NO₂ > NO > O₂. The X-ray diffraction patterns of the thin films showed the growth of crystal morphology with α form, independent of the polarization direction of the substrates. The effect of polarized surfaces is accounted for in terms of a band-bending model. Conductivity changes due to the polar substrates were also demonstrated in the thin films of SnO₂ deposited on LiNbO₃ for the adsorption of C₂H₅OH and in a poled ferroelectric semiconductor, lead strontium zirconate titanate, for the adsorption of H₂O.