Admittance Measurements at Epitaxial and Nonepitaxial Silicide Schottky Contacts
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ADMITTANCE MEASUREMENTS AT EPITAXIAL AND NONEPITAXIAL SILICIDE SCHOTTKY CONTACTS
J. WERNER,* R. T. TUNG,** A. F. J. LEVI,** and M. ANZLOWAR** * Max-Planck-Institut fuir Festk6rperforschung, 7000 Stuttgart 80, FRG ** AT&T Bell Laboratories, Murray Hill, N.J. 07974
ABSTRACT We report results of an extensive study examining the usefulness of low frequency capacitance measurements for the characterization of interface states at intimate Schottky contacts. Our measurements on epitaxial as well as on nonepitaxial silicides reveal that the imaginary component of the low frequency ac-admittance is usually inductive. This inductance is caused by minority carriers that are injected by the Schottky contact and modulate the conductivity of the series resistance of the bulk silicon. The frequently reported excess capacitances (instead of inductances) that were ascribed to interface states are only reproducible when we use imperfect back-contacts to the bulk Si that add a contact resistance to the equivalent dc-circuit of the Schottky diode. Excess low frequency capacitances at intimate Schottky contacts are therefore not related to interface states but rather to the contact resistance of the back-contact. INTRODUCTION The basic physics underlying Schottky barrier formation is a challenging problem of solid state research. Schottky[1] predicted the barrier to equal the difference of the work function of the metal and the electron affinity of the semiconductor. Deviations from this early model are usually explained with interface states. These states capture charge from the metal and/or from the semiconductor and create a dipole between the two materials. The mystery of Schottky barrier formation is therefore seemingly reducible to the characterization of interface traps. For the past fifteen years information about interface states has been deduced from the analysis of an 'excess capacitance' that occurred in addition to the space charge capacitance at forward biased Schottky diodes. The so-called Schottky Capacitance Spectroscopy (SCS)[2] and later Accurate Phase Capacitance Spectroscopy (APCS)[3-5] have been applied to measure and to interpret such data. Freeouf[6] was one of the first to question the consistency of the results. Later, we demonstrated[7] that such evalutaions, when applied to Schottky contacts with an interfacial layer, violate charge neutrality as well as at least one Maxwell equation (Gauss's law). Low frequency capacitance measurements alone are not sufficient to gain information about traps at such interfaces.[7,8] Instead, the complete frequency dependence of the capacitance as well as the conductance has to be considered. The admittance and current(I)/voltage(V)-data are then analyzable within a self-consistent transistor-like model.[7,8/ The energy distribution of interface states at Au/oxide/GaAsinterfaces was derived with the help of this Trap Transistor Model (TTM).[7,8] The inconsistent SCS and ASCS-techniques[2-5] have, however, also been applied to contacts without an interfacial layer, i.e. int
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