RESIDENCE SITES OF FLUORINE IN CRYSTALLINE SILICON AND HIGHLY ORIENTED PYROLYTIC-GRAPHITE

Measurements of the electric field gradient (efg) at F impurity sites in crystalline silicon and highly oriented pyrolytic graphite (HOPG) h ave been compared with cluster model calculations using both Hartree-F ock (HF) and density functional theory (DFT) formalisms. The technique of time-differential perturbed angular distributions of gamma-rays wa s employed to derive the efg parameters following implantation of F-19 via the F-19(p, p')F-19() reaction. For the case of HOPG the DFT met hod gave closer agreement with the experimental values of \V-zz\ = 3.2 4(14) x 10(22) V/m(2) and eta = 0.16(3), yielding V-zz = -3.09 x 10(22 ) V/m(2) and eta = 0.13 for F-19 at a site between the layers with poi nt group symmetry C-2h and inter-layer spacing d = 3.70 Angstrom. For F-19 implantation in silicon three sites were found corresponding to q uadrupole frequencies 23.2(3) MHz, 35.2(3) MHz and 37.1(5) MHz. Both H F and DFT calculations are consistent with the assignment of interstit ial antibonding and bond centre sites for the 23.2 and 35.2 MHz, respe ctively. In the former case, the F atom is located 1.81 Angstrom along a <111> direction from a silicon atom; in the latter situation the Si -Si bond length is found to expand by 1.02 Angstrom from its normal la ttice value. It is speculated that the third interaction, which occurs at only the 10% level, possibly arises from sites associated with a d efect or other impurity. To achieve a reduction in cluster size, the c ompletion of dangling bonds with atoms other than hydrogen was investi gated. The results were found to be comparable.