Defect Characterization by Admittance Spectroscopy Techniques Based on Temperature-Rate Duality

The behavior of charge trapping and detrapping by deep levels depends on both temperature and the rate at which the measurement (e.g., admittance spectroscopy) is conducted. We report recent advances in admittance spectroscopy based on the temperature-rate duality: the temperature derivative technique, and the 2D Arrhenius plot method. The first technique-temperature derivative admittance spectroscopy-can be used to directly determine the defect density of states just as the existing frequency derivative method, but it possesses certain key advantages. Within practical experimental limits, it allows a wider observation window of defect energies, avoiding possible detection failure, and facilitating simultaneous observation of multiple defects. For defect energies of most interest, it also yields more Arrhenius plot data points and therefore enables more accurate parameter extraction. In practice, the temperature derivative method can avoid system noise at low frequency and is more immune to baseline effects due to parasitic circuit effects. The second technique-the 2D Arrhenius plot method- can accurately and unambiguously solve the activation energy E_a, the pre-exponential factor ν₀, and their temperature dependence, especially if the trapping/detrapping process is non-Arrhenius. The 2D Arrhenius plot method measures E_a and ν₀ at any temperature from matching the first and second moments of the data calculated with respect to temperature and rate in the 2D temperature-rate plane. Defects in GaAsN and Cu(In,Ga)Se₂ solar cells are used as case studies for the above techniques. The latter exhibits a temperature-dependent behavior of E_a and ν₀ obeying the Meyer- Neldel's rule.