Non-linear Photoexcited Negative Differential Conductivity in Bulk SrTiO3 Single Crystals

Current density curves for the [100], [110], and [111] crystallographic directions of photoexcited, nominally pure, bulk SrTiO₃ are studied, revealing a strong anisotropy between these crystallographic directions. In the [110] direction two distinct peaks in the current-voltage characteristics with two different negative differential conductivity regions are distinguishable. The two regions can be explained with two different mechanisms, one based on field-enhanced trapping and the second the transferred electron effect based on two-valley conduction band interacting with a trap. Photoluminescence spectra revealed Cr impurities to be the potential origin of the current density anisotropy. In the high temperatures range and low photon fluxes the photoconductivity follows a classical Hecht model with a mu-tau (μτ) value up to 10⁻⁴ cm² V⁻¹.