Application of surface acoustic waves to nondestructive evaluation of semiconductor surfaces

Abstract

The semiconductor on lithium niobate structure (the SAW convolver) is used to determine the distribution of surface states, and majority carriers capture cross section in the energy gap. This nondestructive testing is done by observing the transient response of the attenuated delay line output during and after a high voltage dc pulse is applied across the semiconductor-delay line structure.

This work presents a study of the electrical properties of semiconductors using the interaction between surface acoustic waves and a semiconductor in the separated medium configuration (the SAW convolver). This study has been conducted towards developing a new technique for nondestructive evaluation of semiconductor surfaces using surface acoustic waves. The semiconductor is placed a small distance above the delay line, with a uniform airgap between the two media. Although there is no mechanical contact between the two media, the electric fields associated with the surface acoustic waves penetrate into the semiconductor and interact with the free carriers. As a result of this nonlinear interaction, the SAW is attenuated, there is a change in the SAW velocity, and dc acousto-electric voltages are developed across the semiconductor. In the case of two oppositely propagating surface waves, voltage proportional to the convolution of the two input voltages is also generated.

A theory for the SAW-semiconductor interaction in flat band condition, taking into account both majority and minority carriers, surface recombination velocity, and the free carriers life time is presented. For the infinite semiconductor thickness approximation, simple analytical expressions are obtained for the propagation loss, dc acoustoelectric voltages, and convolution voltages. For finite semiconductor thickness, numerical solution is used to obtain these quantities. For the off flat-band condition, the bulk conductivity is replaced by an effective surface conductivity. The effective surface conductivity is related to the semiconductor surface potential through the excess charge density, in the space charge region, and the surface mobility of the carriers.

The SAW convolver is used to determine the absorption edge and the location of surface states in the energy gap. The spectral response of the delay line attenuation and the dc transverse acoustoelectric voltage are used to monitor changes in the semiconductor conductivity, and the charge trapped in the surface states due to optical excitation. Transverse acoustoelectric voltage inversion is observed in the spectral response of the dc acoustoelectric voltage for high resistivity semiconductor, which improves the sensitivity of the technique.

Photoconductivity response time for various semiconductors is obtained by observing the transient response of the attenuated delay line output after a pulse of light is applied to the semiconductor.