Reference : Admittance spectroscopy of semiconductor structures
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Physical, chemical, mathematical & earth Sciences : Physics
Admittance spectroscopy of semiconductor structures
Nguyen, Ngoc Duy mailto [Université de Liège - ULiège > Département de physique > Physique des solides, interfaces et nanostructures >]
Seminar (Invited)
[en] Admittance spectroscopy
[en] Admittance spectroscopy is a powerful tool for electrical characterization of semiconductor structures such as p-n junctions, Schottky diodes, light-emitting systems or quantum wells. It consists in monitoring the complex admittance of the overall device under test as a function of frequency, applied dc voltage and temperature. This technique gives 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. 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 three different structures are shown in order to illustrate the method.
In the first case, the role of Mg in GaN Schottky diodes is investigated. The results of the study show that this acceptor has simultaneously the functions of dopant and of deep impurity. In order to explain the electrical behavior of Mg, thermal admittance spectroscopy measurements are performed on Schottky structures based on Mg-doped GaN layers grown by metalorganic vapor phase epitaxy on sapphire. The analysis is carried out by simulating the electrical characteristics of the device. The calculated curves fully reproduce the experimental results and the microscopic parameters are determined by an optimized fitting procedure, based on the comparison of the electrical characteristics obtained from the numerical simulations to those of the experiment.
In the second example, charge-carrier mobility in organic materials is evaluated. The frequency- dependent complex admittance and impedance of the structure consisting of the organic layer, grown by thermal evaporation and sandwiched by metallic electrodes, are measured as functions of the dc bias. The capacitance-versus-frequency curve exhibits a minimum, the frequency-position of which increases with the applied dc voltage. Based on numerical calculations, the theoretical analysis shows that the inductive contribution to the capacitance originates from the modulation of the free carrier concentration in the organic material. The finite carrier transit time determines the frequency-response of the structure. Moreover, the low-frequency behavior of the capacitance curves can be explained by the presence of a band of defect states that modifies the charge distribution within the device.
As third application of the admittance spectroscopy method, the electrical characteristics of an organic light-emitting diode are numerically simulated for the dc and the ac regimes. This approach allows to obtain a detailed microscopic description of the dependences of the carrier concentrations and current densities on the applied steady-state voltage and the modulation frequency. The fitting of the resulting admittance and impedance curves to the response of an equivalent electrical circuit shows that each element can be associated with a particular region of the structure. It is then possible to correlate the dependence of each feature of the admittance and impedance curves with one or several parameters of the material system.
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