A detailed investigation of the Hall mobility has been carried out on a series of [inverted lazy s]2-μ-thick n-type {001} Si/{011̄2} Al2O3 films. A specially designed Hall bridge pattern has been used to obtain independent measurements of mobility as a function of current direction in the plane of the film. The data show an anisotropy in the mobility of approximately 9%, with a maximum in mobility occuring along the 〈100〉 Si direction that is parallel to the 〈21̄1̄0〉 Al2O3 direction in the plane of the substrate. This behavior is found to be a consequence, through the piezoresistance effect, of the anisotropic thermal contraction of Al2O3 on cooling from the deposition temperature, which leads to an anisotropic thermally induced stress in the Si.

The effect of stress on the electrical properties for a number of orientations of Si/Al2O3 has been theoretically determined. The stress model developed and employed is based upon the differential in thermal expansion between Si film and an Al2O3 substrate. The anisotropy in substrate thermal expansion coefficients and in Si film stresses is rigorously included and treated within the framework of a thin-film thick-substrate approach. The phenomenological piezoresistance formalism is used to determine the effect of stress on resistivity which is then in turn related to the carrier mobility. Mobility calculations were performed for the common Si orientations— (001), (111), and (221) —and for a number or orientations not yet grown and investigated experimentally. For n-type Si the mobility is either reduced or enhanced depending upon mode of epitaxy and orientation. For p-type Si the mobility is always enhanced and for some orientations by a significant factor. The theoretical results obtained indicate that the thermal stress effect on mobility has a strong orientation dependence and that, compared with the commonly employed (001) Si/Al2O3 orientation, substantially larger mobilities could be obtained with other Si/Al2O3 orientations.

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The possibility of utilizing guided surface acoustic waves in signal processing devices has generated interest in basic investigations of guidance mechanisms and structures for surface acoustic waves. Surface wave propagation, investigated in detail thus far in guidance structures, has unfortunately proven sufficiently dispersive to rule out many practical device applications. Reported here are theoretical investigations of the basic physical phenomena of a (Δv/v) guidance structure which is substantially nondispersive. Acoustic guidance in the (Δv/v) structure is assumed to be due principally to the velocity reduction Δv of a thin conducting strip on a piezoelectric substrate. With this assumption, major features of (Δv/v) guidance can be predicted theoretically from an isotropic nonpiezoelectric model which incorporates the velocity difference Δv. Numerical results are presented for quartz and lithium niobate. In addition, a generalized description for guided waves is developed which is approximately independent of the (Δv/v) velocity ratio. Thus, with the use of scaling factors, the normalized data are applicable to materials with a range of (Δv/v) ratios. Simply put, our principal conclusion is that for many common piezoelectrics, strong guidance and low dispersion are theoretically predicted. Basic experimental investigations thus appear warranted.