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Compared to other modulation parameters, this effect complicates the analysis, but increases the diagnostic value of electroreflectance. These nontensorial anisotropies are most pronounced at higher interband edges of the saddle-point type. Their inclusion into the theory of field effects on the dielectric function was stimulated by electroreflectance, which eliminated the previous restriction of the experiment to transmission effectsa t the fundamental edge. 16 We wiIi discuss the basic concepts, however, as far as they are relevant to an analysis of electroreflectance spectra with respect to band structure and to limitations encountered in the determination of experimental parameters such as the effective modulating field.

R. Lettenberger, and M. Cardona, Bull. Amer. Phys. 12, 1049 (1967). B . 0. SERAPHIN c. Magnetoelectrorejlectance Electroreflectance measurements in the presence of a magnetic field have been performed in both the field-effect and the electrolytic version of the technique. 77 The situation is similar to piezoelectroreflectance. The strain-induced changes in the band structure are difficult to detect using the piezoreflectance technique. The modulating electric field simply serves as a probe of greater resolution and sensitivity.

The integration in (19) is throughout the k space of the Brillouin zone. The symmetry-breaking effect on the matrix element is common to all vectorial modulation techniques. In a cubic crystal, for instance, the optical isotropy is destroyed under the action of the electric field or the uniaxial stress, a preferred direction is established, and the tensorial character of the dielectric function becomes important. To the tensorial effects on the matrix element electroreflectance adds peculiar anisotropies of nontensorial character, resulting from the effect of the electric field on the joint density-of-states function.