A new theoretical model for transverse acoustoelectric voltage measurements on Si/SiO2 structures

Abstract

Acoustoelectric interaction measurements, and particularly transverse acoustoelectric voltage (TAV) measurements, have been extensively used for the determination of a number of semiconductor parameters. These measurements are rapidly becoming a powerful tool to study semiconductor processes. In the last years, they have also been proposed as a tool for testing of microstructure geometries. One of the most relevant characteristics of acoustoelectric measurements, which make them particularly attractive, rely on their nondestructive nature. Neither contacts nor technological processes are required on the surface of the semiconductor under test once the separate-medium structure is used. On the other hand, the separate-medium structure is greatly sensitive to the sample position and to the clean condition of the surface. The grade of coupling between the surface acoustic wave and the semiconductor can dramatically affect the amplitude of the detected TAV signal. In order to obtain quantitative measurement, maintaining the nondestructive capability of the technique, it is always necessary to perform a comparison between experimental results and theoretical predictions versus one external, controlled parameter. From these considerations it is clear that measurement precision relies on the availability of a good theoretical model of the acoustoelectric interaction. In this paper a new theoretical model for the acoustoelectric interaction in the Si/SiO2 structure under field effect is presented, and theoretical expression of TAV versus bias voltage is calculated to be used in interface trap density measurements at a Si/SiO2 interface. Experimental verification of the model is also presented together with a procedure for the calculation of the interface trap density.