Electrical characterization of semiconductor heterostructures by admittance spectroscopy

Electrical characterization by admittance spectroscopy enables the study of interface properties of semiconductor structures such as p-n junctions, Schottky diodes, light-emitting systems, photodiodes, solar cells or quantum well devices. The technique consists in monitoring the complex admittance of the device under test as a function of frequency, applied dc voltage and temperature. This method gives a direct access to the emission-capture processes occurring between an impurity level and the conduction or the valence band and leads to the determination of important electronic properties including the activation energy and the carrier capture cross sections. In the case of organic semiconductors, the field-dependent carrier mobility can be measured as well. However, the interpretation of the admittance curves is straightforward only under restrictive assumptions such as full ionization of the shallow dopant, with a concentration larger than the deep impurity concentration. Numerical simulations, based on the solution of the basic semiconductor equations, allow to carry out a detailed analysis of the steady-state and small-signal electrical characteristics of the systems and thus contribute to a better understanding of the conduction mechanisms and of the microscopic origin of the features in the experimental admittance spectra. In this presentation, the results obtained for different structures are shown in order to illustrate the method.