Admittance spectroscopy of impurity levels in Schottky barriers

Abstract

Measurements of the complex admittance of Schottky-barrier diodes as a function of temperature provide a spectroscopy of deep trapping levels. The measurement conditions are usually close to thermal equilibrium, thus assuring the validity of equilibrium occupation probabilities. An exact (i.e., to arbitrary accuracy) solution for the junction admittance is given. The problem is reduced to solution of a simple initial-value problem, the final integration being carried out by computer. It is shown that the dispersion in capacitance due to slow trapping levels can lead to serious errors in estimates of junction doping concentrations and barrier height determinations. Examples are given for p-type ZnTe where Shockley-Read-Hall (SRH) centers are dominant and for n-type CdTe and Cd1−xZnTe where double-acceptor centers are dominant. For junctions formed on CdTe : Ga it is shown that one of the steps in capacitance observed in a thermally stimulated capacitance survey is actually a high-frequency response step and is not due to slow emission from a trapping center.