Electric-field–induced electron density response of GaAs and ZnSe

Abstract

The response of the electron densities of GaAs and ZnSe to an external electric field was probed by measuring the variation of the integral intensity of several weak X-ray structure factors due to a field parallel to [1 1 1]. In the range −3 kV/mm < E < +3 kV/mm we found a nearly linear dependence of the relative intensity variation ∆R/R on E for low indexed reflections (2 2 2, 0 6 0) and an additional E 2 -dependence for higher-indexed ones (4 4 2, 6 6 6). The results are interpreted in terms of a bond-charge model. The linear effect can be interpreted by the redistribution of valence charges from bonds parallel to E into bonds lying oblique (bond-charge transfer) whereas the origin of the non-linear effect is still unclear. After the exclusion of lattice effects (piezo-electric effect, internal strain) it ought to be allocated to a charge density response of the core region.

Experimental determination of electric-field-induced differences in structure-factor phases of the order of 2%

“…The phase shifts (Table 1) can be compared with the ®ndings of former experiments probing the amplitude variations (Stahn, Pucher et al, 1998). There are qualitative conformities: The signs of both Á9 and ÁjFj 2 ajFj 2 depend on the direction of the applied ®eld E. For one ®eld direction, Á9222 and Á9442 have the same sign and negative indexed re¯ections show opposite behaviour.…”

“…A better insight into the quality of the effect may be obtained by measuring the dependence of the phase shift on the ®eld strength (Stahn, Pucher et al, 1998). Those measurements were impossible because GaAs will become conducting if higher ®eld strengths or more intense synchrotron radiation are applied to the sample.…”

“…Bond-charge model: the total charge density is a superposition of spherical atomic contributions (core and valence) and a bond charge between the atoms. (Cousins, 1988;Stahn, Pucher et al, 1998) has been applied to detect the scattered radiation from the sample with and without an applied ®eld quasi-simultaneously. The voltage was switched on and off with a frequency of 20 Hz and for different voltages the intensity was collected in separate counter channels.…”

“…In recent years, considerable theoretical work has been performed to understand the mechanism of dielectric screening in crystals, especially in tetrahedrally coordinated insulators and semiconductors (Resta, 1994;Resta & Baldereschi, 1981). The change in the electron density of GaAs due to an externally applied electric ®eld, which is the microscopical manifestation of screening, was measured indirectly by Fujimoto (1980), Pietsch et al (1985) and recently by Stahn, Pucher et al (1998).…”

“…It has been shown that structure-factor amplitudes of low-index re¯ections, which are particularly affected by the covalent bond, vary nearly linearly with the applied electric ®eld. The very small variations of the integral intensity of ÁjFj 2 ajFj 2 % 17 for E 3 kV mm À1 could be determined using a modulation technique (Stahn, Pucher et al, 1998). Unfortunately, these data provide information about the variation of the structure-factor amplitudes jFj only.…”