Two-step photoconductivity: determination of impurity levels of Er3+ relative to the host bands of Gd2O3

A two-step photoconductivity measurement in Gd2O3:Er3+ is used to determine the location of the f11 energy levels of Er3+ with respect to the host bands of Gd2O3 . Photoconductivity (PC) experiments were performed with both a tunable UV source and with 488 nm Ar+ excitation, as well as in the presence of both light sources. Using only the UV source, PC features with onsets at 240 and 340 nm are identified as direct band gap excitation and two-step excitation involving excited state absorption on Er3+, respectively; the latter is confirmed by its quadratic dependence on intensity. Under simultaneous Ar+ excitation, which enhances the population of the metastable 4S3/2 Er3+ excited state population, the PC feature with a 340 nm onset increases dramatically and its magnitude depends linearly on both UV and Ar+ laser intensity, supporting its interpretation as resulting from excited state absorption form the 4S3/2 metastable Er3+ excited state.The 340 nm onset of the signal allows one to place the 4I15/2 ground state at 0.7±0.2 eV below the top of the valence band. The lowest 4f105d level then lies close to the bottom of the conduction band and its overlap with the conduction band is thought to enhance the second step in the photoionization process.