Complex Impedance Studies of Optically Excited Strontium Barium Niobate

摘要

The complex permittivity and impedance of undoped and unpoled single crystal strontium barium niobate (SBN) were measured with a computer interfaced impedance analyzer over a frequency range from 100 Mhz to 10 Khz using small AC signals with millivolt amplitudes. The thin (-2 mm) planar crystal sample was placed between the plates of a modified parallel plate capacitor and illuminated on the side with linearly polarized argon-ion laser light operating at select wavelengths in the blue-green with photon energies that are slightly below the bandgap energy of SBN. Impedance spectra were obtained in the dark between room temperature and 70 deg C and compared with impedance spectra obtained with laser illumination at room temperature. Dark measurements at all temperatures are consistent with studies in the literature that suggest electron hopping as the predominant charge transfer mechanism. Illumination markedly changed the conductivity of the crystal and the charge transfer mechanism by elevating significant numbers of charge carriers into the conduction band. This was found to be independent of the polarization of the light and slightly dependent on its wavelength over our wavelength range of 458 nm to 515 nm. The dielectric constant of the crystal under illumination changed less than twenty per cent and is probably due to the slight heating of the crystal induced by the laser.