Electron emission from deep traps in HfO2 under thermal and optical excitation

The ratio between the energies of optical and thermal ionization depends on the defect nature and the strength of the interaction of trapped electrons with phonons. Knowing this ratio for a certain type of defect allows one to predict, e.g., thermal emission energies from the optically measured values. We present the results of direct empirical extraction of the ratio between optical and thermal electron emission energies for HfO2 bulk electron traps combined with theoretical analysis of the physical mechanism of electron transitions from the trapped state to the mobility edge. We show that, by applying different excitation mechanisms, we affect the same deep traps inside the HfO2 band gap; i.e., these traps are both optically and thermally active and are likely to have similar nature. The extracted empirical optical/thermal ionization energy ratio of 2.2±0.3 is in good agreement with the polaronic nature of the probed electron traps, as shown by the results of theoretical calculations. Our results provide experimental and theoretical methodologies for consistently linking thermal and optical ionization energies of electron traps and describing their distributions in the band gap of amorphous oxides, and can help improve modeling frameworks for reliability issues related to oxide traps. Published by the American Physical Society 2024