Extended X-Ray Absorption Fine Structure Studies of Impurities in Semiconductors

ABSTRACTWe discuss Extended X-ray Absorption Fine Structure (EXAFS) experiments on impurities in semiconductors. The local structure of the impurity site is determined for the first, second and third neighbor shells. These studies are carried out on absorption edges in the soft x-ray region using a novel fluorescence detection scheme which reveals improved detection sensitivity when compared with more standard electron Auger yield methods. The higher detection sensitivity allows structural studies at atomic densities as low as 1018 at/cm3 and this technique is used to study the local structure of P and S impurities in GaAs and of S in Alx Gal−x, As at concentrations of 1019–1020 at/cm3. The P atoms are substitutional on As sites, and a breathing relaxation of the P first shell Ga atoms is responsible for a P-Ga distance of 2.38Å, which is 0.07Å shorter than the As-Ga distance in GaAs(2.45Å). No detectable relaxation is observed in the P second and third atomic shells. In S-doped GaAs we find two different populations of S-Ga bonds with nearly equal intensity and both with S located on the As sub-lattice. These findings indicate two different configurations of substitutional S in GaAs. The coexistence of two similarly populated, charge compensating configurations is the first structural evidence which allows us to explain the observation that in n-doped GaAs the electrical activity of donors is lower than the atomic concentration. Finally, we present experimental results on S impurities in AlxGal−x, As (0.2<x<0.5). In this system, at the Al concentrations we studied, the S impurities are bound only to Al and not to Ga atoms. We report the formation of an extended complex which is discussed in connection with the structural identification of DX centers in these materials.