Line-focus beam acoustic microscopy has been used to study changes in the elastic constants of GaAs during amorphization produced by implantation with Sif ions at liquid-nitrogen temperature. The distribution of amorphous material was determined by Rutherford backscattering and channeling. Values of cl1 and c44 were estimated by fitting theoretical curves to the measured angular dispersion of surface acoustic waves in the (001) plane. The implanted material was modeled as a statically stressed anisotropic layer on an unmodified GaAs substrate. The values of c, i and cm were found to decrease with increasing ion tluence. At the highest fluence the implanted region was completely amorphous, and it was observed that the softening of c&41%) was significantly greater than that of c I,( 17%). 0 199.5 American Institute of Physics.
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INTRODUCTIONIt is well known that the propagation of both bulk and surface acoustic waves (SAWs) is affected by the presence of static stresses, a phenomenon known as the acoustoelastic effect. Ultrasonic measurements of velocity therefore depend on the stresses within the material, as well the elastic constants and the density. Although the effect of stress on the velocity is small, many ultrasonic methods are sufficiently accurate to detect the changes involved. When inverting such measurements to obtain elastic constants, it is desirable to take the effect of stress into account. Similarly, when using ultrasonic methods to measure stress, it is necessary to have accurate values for the elastic constants. In practice, the material parameters of the 'natural', unstressed state are often either completely unknown or not known with sufficient accuracy (as is usually the case for residual stresses), or else cannot be assumed to be equal to bulk values (as in the case of layered materials). This is a major distinction between situations involving residual as opposed to applied stress, since a reference state of some description is always available in the latter case.In this paper we show how to improve the estimation of the elastic constants of the natural state of a residually stressed layer from SAW measurements. To do this, we incorporate the stress into the forward calculation of the SAW velocity, so that fitting of the measured SAW dispersion then yields the elastic constants of the unstressed state. We then apply the procedure to GaAs wafers implanted with Si+ ions.
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