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Nuclear quadrupole interactions of ¹⁹F in graphite, silicon, germanium and gallium arsenide Surono, Djoko


Measurements of nuclear quadrupole interactions of 19F* in graphite, silicon, germanium and gallium arsenide have been carried out; the electric field gradients (efgs) at F impurity sites 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-rays (TDPAD) was employed using the 19F(p,p')l9F* reaction to derive the efg parameters following the implantation into the hosts of 19F* in its isomeric state at 197 keV ( Iπ= 5/2+, T1/2=85 ns). For the case of highly oriented pyrolytic graphite (HOPG) the principal components of the electric field gradient and its asymmetry parameter were found to be | Vzz |=3.24(14) x 10[22] V/m² and η=0.16(3), respectively, in close agreement with the DFT predictions of Vzz=-3.09xl0[22] V/m² and η=0.13 obtained for 19F at a site midway between the layers with point group symmetry C2/1 and inter-layer spacing d=3.70 A. As a test of shell model theories an attempt was made to detect the nuclear quadrupole moment Q of the first excited state of 22Na* (E=583 keV, Iπ = 1+, T1/2=243 ns). In this case the 19F(α,η)22Na* reaction was used to implant 22Na* nuclei into HOPG. However, no quadrupole interaction was detected, leading to the conclusion that |QVzz| < 8.7(8) x 10[20] bV/m². Taking (Q=0.06 barns as the best theoretical estimate for the 583 keV state, we find that |VZZ| must be less than 8.7(8) x10[20] V/m². In the 19F 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 first two interactions, with HF predictions of 23.4 MHz and 35.0 MHz, respectively. In the former case the F atom is located 1.81 A along a (111) direction from a silicon atom; in the latter situation the Si-Si bond length is found to expand by 1.02 A 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. In germanium two quadrupole frequencies were detected, with vQ1 =27.5(3) MHz and vQ2=33.0(4) MHz, while in gallium arsenide only a single interaction was observed, with vQ=27.7(3) MHz. In all cases the asymmetry parameter η was close to or equal to zero. In the case of germanium vQ1 and vQ2 are tentatively ascribed to antibonding and bondcentre sites, respectively. In the antibonding configuration the fluorine is situated at 1.88 A along a (111) direction from a germanium atom. For F at the bond-centre site the Ge-Ge bond length was found to increase by about 1 A from its normal lattice value. In gallium arsenide the single frequency vQ is consistent with DFT calculations yielding vQ=28.0 MHz for F at an intrabond site with the fluorine situated at 1.37 A from the Ga and 2.40 A from the As atom. In most cases the DFT predictions are in better agreement with experiment than those of HF. To achieve a reduction in cluster size, the completion of dangling bonds with atoms other than hydrogen was investigated for different silicon clusters. The results were found to be comparable with those obtained for larger clusters terminated by H atoms. [Scientific formulae used in this abstract could not be reproduced.]

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