Accuracy evaluation in temperature-dependent EXAFS measurements of CdTe

Abstract

The evaluation of uncertainty in temperature-dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K-edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest-neighbours parameters: bond thermal expansion, parallel and perpendicular mean-square relative displacements and the third cumulant. Different causes of uncertainty, a comparison of experimental results with theoretical models, the difference between EXAFS and crystallographic thermal expansions and the meaning of the third cumulant are discussed.

Local vibrational properties of GaAs studied by extended X-ray absorption fine structure

“…The corresponding quantities from Ga EXAFS exhibit a regular temperature dependence. A similar effect, although much weaker, was observed for CdTe; 12,18 in that case, the discrepancy between the results from Cd and Te EXAFS was consistent with the uncertainty bars. In the present case of GaAs the discrepancy is significantly larger than the uncertainty bars.…”

“…16,18 For each of the two sets of measurements (Ga and As edges), the values of the photoelectron wave-vector k were calculated with respect to an edge energy conventionally set at the maximum of the first derivative of one of the low-temperature spectra, chosen as reference. The energy axes of all other spectra were then shifted to achieve the best superposition with the reference spectrum in the edge region and the edge energy of each spectrum was determined accordingly.…”

“…18 The case of GaAs suggests, however, that such a check should be considered quite cautiously even for relatively distant edges when resolutions (and accuracies) better than 10 −3 Å are sought for relative distances.…”

“…The difference between the EXAFS distance and the crystallographic distance at T = 0 K is about 1.6 × 10 −3 Å, intermediate between the values 1.5 × 10 −3 and 1.8 × 10 −3 Å found for Ge and CdTe, respectively. 18 Again at T = 0 K one can evaluate u = R c − R 0 1.8 × 10 −3 Å.…”

“…14 Recently, an investigation based on EXAFS measurements at both Cd and Te K edges in CdTe has shown that a very good agreement can be obtained also for the first and third cumulants and for the perpendicular MSRD. 12,18 The case of GaAs is, however, technically more delicate: while the energy difference between the Cd and Te K edges is 5100 eV, the energy difference between the Ga and As K edges is 1500 eV, corresponding to a photo-electron wavevector range of about 19.3 Å −1 . It is of interest to evaluate the influence of the residual Ga EXAFS on the structural parameters determined from the As EXAFS.…”