Fano resonances in photoconductivity spectra of hydrogen donors in ZnO and rutile

Abstract

The results of photoconductivity studies of hydrogen donors in ZnO and rutile TiO2 are presented. It is shown that local vibrational modes of O–H bonds comprising donors in both semiconductors can be detected in photoconductivity spectra as Fano resonances at 3611 and 3290 cm−1 in the case of ZnO and TiO2, respectively. The frequencies of these features red-shift in energy down to 2668 (ZnO) and 2445 cm−1 (TiO2) if hydrogen is substituted by deuterium. Based on the frequency of the deuterium resonance it is concluded that the ionization energy of the hydrogen donor in TiO2 is less than 300 meV, which is in variance with predictions of theory. The reasons for such a discrepancy are discussed.