Capacitance-Transient Detection of X-Ray Absorption Fine Structure: A Possible Tool to Analyze the Structure of Deep-Level Centers?

摘要

The well-established technique of x-ray absorption fine structure (XAFS) near the absorption edge of inner atomic shells reveals valuable information on the microscopic structure of atoms in the solid. Unfortunately, the x-ray absorption of highly diluted defect atoms in the host vanishes in the absorption background from elastic scattering and the absorption from other elements or atomic shells. Recently, a scheme of capacitance-detection for the x-ray absorption fine structure (XAFS) has been reported [M. Ishii, Phys. Rev. B 65, 085310 (2002).] The XAFS signal is detected by the capacitance changes in a Schottky diode. This detection method claimes to give a site-selectivity of the XAFS signal, which would allow detection of the x-ray absorption of only one specific defect site by blocking all other contributions from other sites. In addition to the proposed measurement of the steady-state capacitance we introduce the new detection of capacitance transients. The variation of the characteristic time constant with x-ray absorption gives the XAFS spectrum. Special attention is given to the question if XAFS detected by capacitance can selectively detect strongly diluted defects. The XAFS at the K-edge of arsenic is measured on GaAs Schottky-diodes containing EL2 defect centers. We will give an in-depth discussion of the results and arguments presented by Ishii and co-workers and show that there is at present no evidence for a site-selectivity of capacitance-XAFS.