ELECTRIC FIELD INDUCED ELECTRON DENSITY RESPONSE OF GaAs AND ZnSe
J. Stahn, A. Pucher, and U. Pietsch
Institut für Physik, Universität Potsdam, D-14469 Potsdam, Germany
Keywords: charge density, electric field,
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 [1,1,1]. In the range -3 kV/mm < E < +3 kV/mm we found a nearly linear dependence of the relative intensity variation D R / R on E for low indexed reflections (2,2,2, 0,6,0) and an additional E2 dependence for higher indexed ones (4,4,2, 6,6,6). The maximum variation (E=3 kV/mm) is of the order of 1 %.
The results are interpreted in terms of a bond-charge model [1] which treats the charge density as a superposition of spherical 'atomic' and bond-charge contributions. The linear effect can be interpreted by the redistribution of bond-charges from bonds parallel to E into bonds lying oblique and by a shift of these bond-charges against the electrical field (bond-charge transfer). This was already found in former experiments [2,3] and confirms theoretical predictions [4]. A new finding is the strong wavelength-dependence of the effect for low-indexed reflections due to anomalous dispersion.
The origin of the non-linear effect is still unclear. Because it appears at higher indexed reflections only, it seems to be affected by the charge density in the core region. The influences of lattice effects (piezo-electric effect, internal strain) are too small in order to explain the measured quantities. A proper description of the polarisation of atomic cores is not possible within the bond-charge model and requires ab initio methods.