Optical enhancement of dielectric permittivity in reduced lanthanum aluminate

When light is absorbed in solids, electrical conductivity is usually enhanced through generation of photodoped conductive carriers, known as photoconduction. Here we show UV-light absorption restrains photoconduction, but markedly enhances dielectric permittivity in a ceramic sample of $\mathrm{LaAl}{\mathrm{O}}_3$ with defects introduced deliberately by reduction synthesis. Using systematic dielectric measurements under photoirradiation combined with computational studies for defect formation energies, we explain this unconventional photoinduced phenomenon in terms of photoexcited dipoles: The photoexcited electrons are trapped in in-gap states introduced by oxygen vacancies whereas photoexcited holes are localized in a valence-band maximum. Thus the created electron-hole pair acts as an electric dipole to enhance the dielectric permittivity. This unprecedented photoelectric effect does not only provide an alternative functionality for dielectric materials, but also paves the way for next-generation photoelectronic devices.

Figure 1

The frequency dispersion of the dielectric properties observed in (a) reduced $\mathrm{LaAl}{\mathrm{O}}_3$, and (b) $\mathrm{LaAl}{\mathrm{O}}_3$, based on measurements taken shortly after illumination is turned on. Top and bottom panels show a real part and an imaginary part of complex dielectric permittivity, ε′ and ε″, respectively. Circles and squares in the figure denote data observed in the dark state and the photoirradiated state, respectively. The insets in panels (a,b) show the imaginary part of complex dielectric permittivity in a magnified scale.

Figure 2

(Top panel) A transient change in the real part of complex permittivity ε′ for reduced $\mathrm{LaAl}{\mathrm{O}}_3$ observed under the photoirradiation with blinking LED. (Bottom panel) A transient change in the sample temperature measured using an infrared thermometer during the photoirradiation. The shaded regions in the panels denote the photoirradiated states.

Figure 3

The magnitude of PDE at 320 K measured under the photoirradiation of various photon energies.

Figure 4

Optical transition energies calculated for various defect states expected in $\mathrm{LaAl}{\mathrm{O}}_3$ including (a) vacancy, (b) cation antisite, and (c) interstitial-type defects.

Figure 5

Formation energies of various defects in $\mathrm{LaAl}{\mathrm{O}}_3$ plotted as a function of Fermi energy, which were calculated in (a) La-rich and (b) Al-rich conditions. Energy levels for a valence-band maximum (VB-Max) of 0.0 eV and a conduction-band minimum (CB-Min) of 5.5 eV are indicated by arrows in panel (b).