Residence sites of fluorine in crystalline silicon and highly oriented pyrolytic graphite

Abstract

Measurements of the electric field gradient (efg) at F impurity sites in crystalline silicon and highly oriented pyrolytic graphite (HOPG) have been compared with cluster model calculations using both Hartree-Fock (HF) and density functional theory (DFT) formalisms. The technique of time-differential perturbed angular distributions of γ-ravs was employed to derive the efg parameters following implantation of¹⁹F via the¹⁹F(p, p′)¹⁹F^* reaction. For the case of HOPG the DFT method gave closer agreement with the experimental values of ¦V _zz¦=3.24(14)×10²²V/m² andη=0.16(3), yieldingV _zz=−3.09×10²²V/m²andη=0.13 for¹⁹F at a site between the layers with point group symmetry C_2h and inter-layer spacingd=3.70 Å. For¹⁹F implantation in silicon three sites were found corresponding to quadrupole frequencies 23.2(3) MHz, 35.2(3) MHz and 37.1(5) MHz. Both HF and DFT calculations are consistent with the assignment of interstitial antibonding and bond centre sites for the 23.2 and 35.2 MHz, respectively. In the former case, the F atom is located 1.81 Å along a <111> direction from a silicon atom; in the latter situation the Si-Si bond length is found to expand by 1.02 Å from its normal lattice value. It is speculated that the third interaction, which occurs at only the 10% level, possibly arises from sites associated with a defect or other impurity. To achieve a reduction in cluster size, the completion of dangling bonds with atoms other than hydrogen was investigated. The results were found to be comparable.