next up previous
Next: FEFF analysis for InGaAs Up: WAVEVECTOR SELECTIVITY Previous: DAFS measurements for InGaAs

The virtual crystal model and the local structural model for InGaAs

To interpret the DAFS measurements, we used two standard models of the InGaAs crystal structure [26]. In the virtual crystal model (VCM), the atoms are located without local distortions at the ``ideal'' lattice sites of the average unit cell. The atoms remain on these ideal sites as the average unit cell expands or contracts. In the local structure model (LSM), the structure distorts locally to accomodate the average lattice constant changes, but keeps the near-neighbor distances roughly constant. This model attempts to keep the bond lengths fixed because it is much cheaper energetically to bend the bonds than it is to stretch them [27]. Our FEFF analysis described below shows that only the local structure model is consistent with the DAFS measurements.

Note that for simple tetragonal distortions, DAFS provides essentially a complete structural determination without a full crystallographic analysis since its diffraction signals provide the average layer spacings and its fine structure signals provide the average bond lengths. Analogous separate x-ray diffraction and XAFS measurements were shown previously to determine the detailed unit cell structure of tetragonally distorted strained layer GeSi samples [27].